US20220077818A1 - Photovoltaic module - Google Patents

Abstract

A photovoltaic module can include a solar cell module including a plurality of solar cells; a converter configured to convert a level of DC power input from the solar cell module; a DC-terminal capacitor configured to store DC power output from the converter; an inverter including a plurality of switching elements and configured to convert DC power from the DC-terminal capacitor into AC power; and a controller configured to control the inverter, in which the converter controls some switching elements among of the plurality of switching elements included in the inverter to perform switching at a third switching frequency, and controls other switching elements among the plurality of switching elements included in the inverter to perform switching at a forth switching frequency higher than the third switching frequency.

Description

CROSS-REFERENCE TO RELATED APPLICATION
• This application is a divisional of U.S. application Ser. No. 16/192,390, filed Nov. 15, 2018, which claims the benefit of priority to Korean Patent Application Nos. 10-2017-0152514 filed in the Republic of Korea on Nov. 15, 2017, 10-2017-0160840 filed in the Republic of Korea on Nov. 28, 2017, and 10-2018-0132375 filed in the Republic of Korea on Oct. 31, 2018, the entireties of all these applications are incorporated herein by reference.
BACKGROUND OF THEINVENTION1. Field of the invention
• The present invention relates to a photovoltaic module, and more specifically, to a photovoltaic module having a power conversion device which can be reduced in size.
2. Description of the Related Art
• A photovoltaic module refers to solar cells connected in series or parallel for photovoltaic power generation.
• A power conversion device of a photovoltaic module may perform maximum power point tracking control, convert DC power from solar cells into AC power, and output AC power. Research into such a power conversion device is conducted in various manners.
SUMMARY OF THE INVENTION
• Therefore, the present invention has been made in view of the above problems, and it is an object of the present invention to provide a photovoltaic module having a power conversion device which can be reduced in size.
• Another object of the preset invention is to provide a photovoltaic module capable of performing power conversion with high voltage step-up and high efficiency.
• Another object of the present invention is to provide a photovoltaic module capable of reducing the size of a converter.
• In accordance with an aspect of the present invention, the above and other objects can be accomplished by the provision of a photovoltaic module including a solar cell module including a plurality of solar cells; a converter to convert the level of DC power input from the solar cell module; a DC-terminal capacitor to store DC power output from the converter; an inverter including a plurality of switching elements and configured to convert DC power from the DC-terminal capacitor into AC power; and a controller to control the inverter, in which the converter controls some of the plurality of switching elements included in the inverter to perform switching at a third switching frequency and controls others to perform switching at a forth switching frequency higher than the third switching frequency.
• The photovoltaic module further comprises a filter for filtering the AC power output from the inverter.
• The filter includes an inductor connected to one of output terminals of the inverter, and a capacitor connected between the inductor and the other output terminal of the inverter.
• The some of the plurality of switching elements and the other switching elements are switching elements of different types.
• The some of the plurality of switching elements include metal-oxide-semiconductor field-effect-transistors (MOSFETs), and the other switching elements include GaN transistors or SiC transistors.
• The controller performs asynchronous PWM control on the inverter.
• The controller includes a current controller for outputting a DC-terminal voltage command based on an output current flowing through the inverter; a voltage command compensator for compensating for a voltage command based on the DC-terminal voltage command and a voltage across both terminals of the DC-terminal capacitor; a low-speed switching driving signal generator for outputting a low-speed switching driving signal at the third switching frequency based on an output value from the voltage command compensator; and a high-speed switching driving signal generator for outputting a high-speed switching driving signal at the fourth switching frequency based on the output value from the voltage command compensator.
• The converter includes a full-bridge switching unit for switching the DC power; a transformer having an input side connected to an output terminal of the full-bridge switching unit; and a synchronous rectification unit connected to an output side of the transformer; and a resonant capacitor and a resonant inductor connected between the transformer and the synchronous rectification unit.
• The full-bridge switching unit includes first and second switching elements connected parallel; and third and fourth switching elements respectively connected in series to the first and second switching elements, in which the input side of the transformer is connected between a first node between the first and second switching elements and a second node between the third and fourth switching elements.
• The controller controls the full-bridge switching unit to enter a buck mode and operate at a first switching frequency when a voltage of the DC-terminal capacitor is greater than or equal to a target voltage, and controls the full-bridge switching unit to enter a boost mode and operate at a second switching frequency lower than the first switching frequency when the voltage of the DC-terminal capacitor is lower than the target voltage.
• The controller controls a phase difference between switching elements in the full-bridge switching unit to increase as a difference between the voltage of the DC-terminal capacitor and the target voltage increases when the voltage of the DC-terminal capacitor is greater than or equal to the target voltage.
• The controller controls turn-on duty of switching elements in the synchronous rectification unit to increase as the difference between the voltage of the DC-terminal capacitor and the target voltage increases when the voltage of the DC-terminal capacitor is lower than the target voltage.
• The inverter includes fifth and eighth switching elements connected in series; and sixth and seventh switching elements connected in series, in which the AC power is output through a fifth node between the fifth and eighth switching elements and a sixth node between the sixth and seventh switching elements.
• The controller controls the fifth and eighth switching elements to operate at the fourth switching frequency and controls the sixth and seventh switching elements to operate at the third switching frequency.
• The controller controls the fifth and eighth switching elements to perform switching according to PWM control while the sixth switching element is turned on and controls the eighth and fifth switching elements to perform switching according to PWM control while the seventh switching element is turned on.
• The synchronous rectification unit includes ninth and tenth switching elements connected in series; and first and second capacitors connected in series, in which the output side of the transformer is connected between a third node between the ninth and tenth switching elements and a fourth node between the first and second capacitors.
• The DC-terminal capacitor includes a film capacitor.
• In accordance with another aspect of the present invention, there is provided a photovoltaic module that includes a solar cell module including a plurality of solar cells; a converter to convert the level of DC power input from the solar cell module; a DC-terminal capacitor to store DC power output from the converter; an inverter including first to fourth switching elements and configured to convert DC power from the DC-terminal capacitor into AC power; and a controller to control the inverter, in which the converter performs asynchronous PWM control on the inverter.
• The controller controls the first and fourth switching elements to perform switching according to PWM control while the second switching is turned on and controls the fourth and first switching elements to perform switching according to PWM control while the third switching element is turned on.
• In accordance with another aspect of the present invention, there is provided a photovoltaic module that includes a solar cell module including a plurality of solar cells; a converter for converting the level of DC power input from the solar cell module; a DC-terminal capacitor for storing DC power output from the converter; and a controller for controlling the converter, in which the converter includes a full-bridge switching unit for switching the DC power; a transformer having an input side connected to an output terminal of the full-bridge switching unit; a synchronous rectification unit connected to an output side of the transformer; and a resonant capacitor and a resonant inductor connected between the transformer and the synchronous rectification unit, and the controller varies a switching frequency of the full-bridge switching unit based on an input voltage of the converter or a voltage of the DC-terminal capacitor.
• The controller controls the full-bridge switching unit and the synchronous rectification unit such that the full-bridge switching unit operates in a buck mode and the full-bridge switching unit and the synchronous rectification unit operate at a first switching frequency when the voltage of the DC-terminal capacitor is greater than or equal to a target voltage, and controls the full-bridge switching unit and the synchronous rectification unit such that the synchronous rectification unit operates in a boost mode and the full-bridge switching unit and the synchronous rectification unit operate at a second switching frequency lower than the first switching frequency when the voltage of the DC-terminal capacitor is lower than the target voltage.
• The controller controls the full-bridge switching unit to operate at a maximum switching frequency and varies a phase difference between switching elements in the full-bridge switching unit in the buck mode.
• The controller controls a phase difference between switching elements in the full-bridge switching unit to increase as a difference between the voltage of the DC- terminal capacitor and the target voltage increases when the voltage of the DC-terminal capacitor is greater than or equal to the target voltage.
• The controller controls the full-bridge switching unit to operate at a minimum switching frequency and varies turn-on duty of switching elements in the synchronous rectification unit in the boost mode.
• The controller controls the turn-on duty of the switching elements in the synchronous rectification unit to increase as the difference between the voltage of the DC-terminal capacitor and the target voltage increases when the voltage of the DC-terminal capacitor is lower than the target voltage.
• The controller includes a ripple compensator for compensating for ripples in of the DC-terminal capacitor based on the detected DC-terminal voltage and the target voltage; and a pulse width modulation (PWM) controller for controlling a pulse width with respect to switching elements in the full-bridge switching unit based on the compensated ripples.
• The controller controls ripples in the voltage of the DC-terminal capacitor to decrease.
• The full-bridge switching unit includes first and second switching elements connected in parallel; and third and fourth switching elements respectively connected in series to the first and second switching elements, in which the input side of the transformer is connected between a first node between the first and second switching elements and a second node between the third and fourth switching elements.
• The controller controls turn-on timing of the fourth and third switching elements to be delayed from turn-on timing of the first and second switching elements in the buck mode.
• The controller controls the delay to increase as the difference between the voltage of the DC-terminal capacitor and the target voltage increases when the voltage of the DC-terminal capacitor is greater than or equal to the target voltage.
• The controller controls the first and fourth switching elements and the second and third switching elements to be alternately turned on in the boost mode.
• The photovoltaic module further includes an inverter for converting DC power from the DC-terminal capacitor into AC power, in which the inverter includes fifth and sixth switching elements connected in series; and seventh and eighth switching elements connected in series, in which the AC power is output through a fifth node between the fifth and sixth switching elements and a sixth node between the seventh and eighth switching elements.
• The synchronous rectification unit includes ninth and tenth switching elements connected in series; and first and second capacitors connected in series, in which the output side of the transformer is connected between a third node between the ninth and tenth switching elements and a fourth node between the first and second capacitors.
• The controller controls turn-on duty of the ninth and tenth switching elements to increase as the difference between the voltage of the DC-terminal capacitor and the target voltage increases when the voltage of the DC-terminal capacitor is lower than the target voltage.
• The switching frequency of the full-bridge switching unit is higher than a system frequency.
• In accordance with another aspect of the present invention, there is provided a photovoltaic module including a solar cell module including a plurality of solar cells; a converter for converting the level of DC power input from the solar cell module; a DC-terminal capacitor for storing DC power output from the converter; and a controller for controlling the converter, in which the converter comprises a full-bridge switching unit for switching the DC power; a transformer having an input side connected to an output terminal of the full-bridge switching unit; a synchronous rectification unit connected to an output side of the transformer; and a resonant capacitor and a resonant inductor connected between the transformer and the synchronous rectification unit, and in which the controller controls the full-bridge switching unit or the synchronous rectification unit to operate in a buck mode or a boost mode depending on a voltage level of the DC-terminal capacitor.
BRIEF DESCRIPTION OF THE DRAWINGS
• The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:
• FIG. 1A is a diagram showing an example of a photovoltaic system including a photovoltaic module according to an embodiment of the present invention;
• FIG. 1B is a diagram showing another example of a photovoltaic system including a photovoltaic module according to an embodiment of the present invention;
• FIG. 2 is a diagram showing an internal circuit of the junction box in the photovoltaic module ofFIG. 1A orFIG. 1B according to an embodiment of the present invention;
• FIGS. 3A to 3B are a circuit diagram of a power conversion device included in a photovoltaic module related to the present invention;
• FIG. 4 is a circuit diagram of a power conversion device in the photovoltaic module according to an embodiment of the present invention;
• FIGS. 5 to 7 are diagrams referred to illustrating the power conversion device ofFIG. 4 according to an embodiment of the present invention;
• FIG. 8 is a flowchart illustrating a method of operating the photovoltaic module according to an embodiment of the present invention;
• FIGS. 9 to 14B are diagrams referred to illustrating the power conversion device ofFIG. 4 according to embodiments of the present invention; and
• FIG. 15 is an exploded perspective view of a solar cell module ofFIG. 1A orFIG. 1B according to an embodiment of the present invention.
DETAILED DESCRIPTION OF THE EMBODIMENTS
• The present invention proposes a method for reducing ripples in current input to a converter in a photovoltaic module.
• The present invention will be described in more detail with reference to the drawings.
• The suffixes “module” and “unit” of elements herein are used for convenience of description and do not have any distinguishable meanings or functions. Accordingly, the suffixes “module” and “unit” can be used interchangeably.
• FIG. 1A is a diagram showing an example of a photovoltaic system including a photovoltaic module according to an embodiment of the present invention.
• Referring toFIG. 1A, aphotovoltaic system10aaccording to an embodiment of the present invention may include aphotovoltaic module50 and agateway80.
• Thephotovoltaic module50 may integrally include asolar cell module100 and ajunction box200 including a power conversion device (500 inFIG. 4) which converts DC power in the solar cell module and outputs the converted power.
• Although thejunction box200 is attached to the rear side of thesolar cell module100 in the figure, the present invention is not limited thereto. Thejunction box200 can be separate from thesolar cell module100.
• A cable oln for supplying AC power output from thejunction box200 to agrid90 may be electrically connected to an output terminal of thejunction box200.
• Thegateway80 can be positioned between one or more power conversion devices and thegrid90. Thegateway80 can detect an alternating current (AC) io and an AC voltage vo output from thephotovoltaic module50 through the cable oln. Thegateway80 may output a power factor adjustment signal for power factor adjustment based on a phase difference between the AC io and the AC voltage vo output from thephotovoltaic module50. Thus, thegateway80 and thephotovoltaic module50 can perform power line communication (PLC) using a cable323 (e.g., seeFIG. 2).
• The power conversion device (500 inFIG. 4) included in thephotovoltaic module50 may convert DC power output from thesolar cell module100 into AC power and output the AC power. Thus, the power conversion device (500 inFIG. 4) in thephotovoltaic module50 may include a converter (530 inFIG. 6) and an inverter (540 inFIG. 4).
• The power converter (500 inFIG. 4) can be referred to as a micro-inverter. Accordingly, the micro-inverter can include a converter (530 inFIG. 4) and an inverter (540 inFIG. 4).
• In embodiments of the present invention, a 2-stage power conversion device, which converts the level of DC power output from thesolar cell module100 through theconverter530, is included in the power conversion device (500 inFIG. 4) or the micro-inverter, and then performs AC power conversion through theinverter540 is described.
• The present invention proposes a method for performing power conversion with high voltage step-up and high efficiency through theconverter530.
• Thus, thephotovoltaic module50 according to an embodiment of the present invention can include thesolar cell module100, theconverter530, and acontroller550. Thephotovoltaic module50 according to an embodiment of the present invention may further include theinverter540.
• Theconverter530 in thepower conversion device500 according to an embodiment of the present invention can include a full-bridge switching unit532 that switches DC power, atransformer536 having an input side connected to an output terminal of the full-bridge switching unit532, asynchronous rectification unit538 connected to an output side of thetransformer536, and a resonant capacitor Cr and a resonant inductor Lr which are connected between thetransformer536 and thesynchronous rectification unit538. Thecontroller550 can perform power conversion with high voltage step-up and high efficiency by varying a switching frequency of the full-bridge switching unit532 and thesynchronous rectification unit538 which is a half-bridge switching unit based on the input voltage of theconverter530 or the voltage of a DC-terminal capacitor C.
• Particularly, thecontroller550 can control phase shift of the full-bridge switching unit532 such that the full-bridge switching unit532 operates in a buck mode and control the full-bridge switching unit532 and thesynchronous rectification unit538 to operate at a first switching frequency when the voltage of the DC-terminal capacitor C is greater than or equal to a target voltage, and can control duty of thesynchronous rectification unit538 which is a half-bridge switching unit such that thesynchronous rectification unit538 operates in a boost mode and the full-bridge switching unit532 and thesynchronous rectification unit538 operate at a second switching frequency lower than the first switching frequency when the voltage of the DC-terminal capacitor C is lower than the target voltage, thereby performing power conversion with high voltage step-up and high efficiency.
• Here, the first and second switching frequencies can be much higher than a system frequency, and thus the sizes of circuit elements in theconverter530 can be reduced.
• Particularly, the turn ratio of thetransformer536 can be reduced and thus the size of thetransformer536 can be decreased. Consequently, the size of theconverter530 used in thephotovoltaic module50 can be reduced.
• Control is performed such that ripples in the voltage of the DC-terminal capacitor C are reduced, and thus a film capacitor instead of an electrolytic capacitor can be used as the DC-terminal capacitor C. Accordingly, the size of the DC-terminal capacitor C can be reduced.
• When the voltage of the DC-terminal capacitor is greater than or equal to the target voltage, the full-bridge switching unit532 can be controlled to enter the buck mode in which a phase difference between switching elements in the full-bridge switching unit532 increases as the difference between the voltage of the DC-terminal capacitor and the target voltage increases such that the voltage of the DC-terminal capacitor follows the target voltage.
• When the voltage of the DC-terminal capacitor is lower than the target voltage, thesynchronous rectification unit538 is controlled to enter the boost mode in which turn-on duty of switching elements in thesynchronous rectification unit538 increases as the difference between the voltage of the DC-terminal capacitor and the target voltage increases such that the voltage of the DC-terminal capacitor follows the target voltage.
• In addition, theconverter530 in thepower conversion device500 according to another embodiment of the present invention can include the full-bridge switching unit532 which switches DC power, thetransformer536 having an input side connected to the output terminal of the full-bridge switching unit532, thesynchronous rectification unit538 connected to the output side of thetransformer536, and the resonant capacitor Cr and the resonant inductor Lr connected between thetransformer536 and thesynchronous rectification unit538. Thecontroller550 can control the full-bridge switching unit532 to operate in the buck mode or boost mode depending on the voltage level of the DC-terminal capacitor C, thereby performing power conversion with high voltage step-up and high efficiency.
• Theinverter540 in thepower conversion device500 according to an embodiment of the present invention includes a plurality of switching elements S1 to S4 and converts DC power from a DC-terminal capacitor C into AC power. Some of the plurality of switching elements S1 to S4 can perform switching at a third switching frequency and other switching elements can perform switching at a fourth switching frequency.
• That is, thecontroller550 can control some of the switching elements S1 to S4 to perform switching at the third switching frequency and control the other switching elements to perform switching at the fourth switching frequency.
• Here, the third switching frequency can correspond to a system frequency and the fourth switching frequency can be higher than the system frequency.
• Accordingly, high-speed switching is performed with respect to some switching elements in theinverter540 and thus, not only the size of theinverter540 but also the sizes of other circuit elements in the power conversion device can be reduced according to a high switching frequency.
• The other switching elements among the switching elements S1 to S4 can include GaN transistors or SiC transistors and thus reverse recovery loss during high-speed switching can be reduced.
• Further, afilter570 provided at output terminals of theinverter540 includes an inductor connected to one of the output terminals of theinductor540 and a capacitor connected between the inductor and the other output terminal of theinverter540. Accordingly, a common mode voltage at the output terminals of theinverter540 can be reduced. Furthermore, a harmonic component THD of output current can be reduced.
• Thecontroller550 can perform power conversion with high voltage step-up and high efficiency by varying a switching frequency of a full-bridge switching unit532 based on the input voltage of theconverter530 or the voltage of the DC-terminal capacitor C.
• Particularly, thecontroller550 can control the full-bridge switching unit532 to enter a buck mode and operate at a first switching frequency when the voltage of the DC-terminal capacitor C is greater than or equal to a target voltage, and thecontroller550 can control the full-bridge switching unit532 to enter a boost mode and operate at a second switching frequency lower than the first switching frequency when the voltage of the DC-terminal capacitor C is lower than the target voltage, thereby performing power conversion with high voltage step-up and high efficiency.
• Here, the first and second switching frequencies can be much higher than the system frequency. Accordingly, the sizes of circuit elements in theconverter530 can be reduce.
• Particularly, the turn ratio of atransformer536 can be reduced, and thus the size of thetransformer536 can be decreased. Consequently, the size of theconverter530 used in thephotovoltaic module50 can be reduced.
• Control is performed such that ripples in the voltage of the DC-terminal capacitor C are reduced, and thus a film capacitor instead of an electrolytic capacitor can be used as the DC-terminal capacitor C. Accordingly, the size of the DC-terminal capacitor C can be reduced.
• When the voltage of the DC-terminal capacitor is greater than or equal to the target voltage, the full-bridge switching unit532 can be controlled to enter the buck mode in which a phase difference between switching elements in the full-bridge switching unit532 increases as the difference between the voltage of the DC-terminal capacitor and the target voltage increases such that the voltage of the DC-terminal capacitor follows the target voltage.
• When the voltage of the DC-terminal capacitor is lower than the target voltage, asynchronous rectification unit538 can be controlled to enter the boost mode in which turn-on duty of switching elements in thesynchronous rectification unit538 increases as the difference between the voltage of the DC-terminal capacitor and the target voltage increases such that the voltage of the DC-terminal capacitor follows the target voltage.
• In addition, theconverter540 in thepower conversion device500 according to another embodiment of the present invention is driven according to asynchronous PWM. Accordingly, the sizes of circuit elements in the power conversion device can be reduced.
• FIG. 1B is a diagram showing another example of a photovoltaic system including a photovoltaic module according to an embodiment of the present invention.
• Referring toFIG. 1B, aphotovoltaic system10baccording to an embodiment of the present invention can include a plurality ofphotovoltaic modules50a,50b,. . . ,50nand thegateway80.
• Thephotovoltaic system10bofFIG. 1B differs from thephotovoltaic system10aofFIG. 1A in that thephotovoltaic modules50a,50b,. . . ,50nare connected in parallel.
• Thephotovoltaic modules50a,50b,. . . ,50ncan respectively include solar cell modules100a,100b,. . . ,100nandjunction boxes200a,200b,. . . ,200nincluding circuit elements for converting DC power in the solar cell modules and outputting the converted power.
• Although thejunction boxes200a,200b,. . . ,200nare respectively attached to the rear sides of the solar cell modules100a,100b,. . . ,100nin the figure, the present invention is not limited thereto. Thejunction boxes200a,200b,. . . ,200ncan be separate from the solar cell modules100a,100b,. . . ,100n.
• Further,cables31a,31b,. . . , oln for supplying AC power output from thejunction boxes200a,200b,. . . ,200nto thegrid90 can be electrically connected to output terminals of thejunction boxes200a,200b,. . . ,200n,respectively.
• As described above with reference toFIG. 1A, theconverter530 in thepower conversion device500 included in each of thephotovoltaic modules50a,50b,50ncan include a full-bridge switching unit532 which switches DC power, atransformer536 having an input side connected to an output terminal of the full-bridge switching unit532, asynchronous rectification unit538 connected to an output side of thetransformer536, and a resonant capacitor Cr and a resonant inductor Lr which are connected between thetransformer536 and thesynchronous rectification unit538. Thecontroller550 can perform power conversion with high voltage step-up and high efficiency by varying a switching frequency of the full-bridge switching unit532 and thesynchronous rectification unit538 which is a half-bridge switching unit based on the input voltage of theconverter530 or the voltage of a DC-terminal capacitor C.
• In theinverter540 included in thepower conversion device500 included in each of thephotovoltaic modules50a,50b, . . .50nshown inFIG. 1B, some of the switching elements S1 to S4 can perform switching at the third switching frequency and other switching elements can perform switching at the fourth frequency higher than the third switching frequency, as described above with reference toFIG. 1A. Accordingly, the size of the power conversion device can be reduced.
• Particularly, since switching is performed at the third switching frequency corresponding to the system frequency and the fourth switching frequency higher than the third switching frequency, a high-speed switching effect is obtained and thus, not only the size of theinverter540 but also the sizes of circuit elements in the power conversion device including theinverter540 can be reduced.
• FIG. 2 is a diagram showing an internal circuit of the junction box in the photovoltaic module ofFIG. 1A orFIG. 1B.
• Referring to the figure, thejunction box200 can convert DC power from thesolar cell module100 and output the converted power.
• Particularly, thejunction box200 according to an embodiment of the present invention can include the power conversion device (500 inFIG. 4) for outputting AC power.
• Thus, thejunction box200 can include theconverter530, theinverter540 and thecontroller550 for controlling the same.
• In addition, thejunction box200 can further include abypass diode unit510 for bypass, acapacitor unit520 for storing DC power, and afilter570 for filtering output AC power.
• Thejunction box200 can further include acommunication unit580 for communication with theexternal gateway80.
• In addition, thejunction box200 can further include an input current detector A, an input voltage detector B, a converter output current detector C, a converter output voltage detector D, an inverter output current detector E and an inverter output voltage detector F.
• Thecontroller550 can control theconverter530, theinverter540 and thecommunication unit580.
• Thebypass diode unit510 can include the bypass diodes Dc, Db and Da arranged between the first to fourth conductive lines of thesolar cell module100. Here, the number of bypass diodes is one or more, preferably, one less than the number of conductive lines.
• The bypass diodes Dc, Db and Da receive photovoltaic DC power from thesolar cell module100, particularly, from the first to fourth conductive lines in thesolar cell module100. When a reverse voltage is generated in DC power from at least one of the first to fourth conductive lines, the bypass diodes Dc, Db and Da can bypass the DC power.
• DC power which has passed through thebypass diode unit510 can be input to thecapacitor unit520.
• Thecapacitor unit520 can store the DC power input through thesolar cell module100 and thebypass diode unit510.
• Although the figure shows that thecapacitor unit520 includes a plurality of capacitors Ca, Cb and Cc connected in parallel, a plurality of capacitors can be connected in series and parallel, or connected in series to a ground terminal. Alternatively, thecapacitor unit520 can include only one capacitor.
• Theconverter530 can convert the level of an input voltage from thesolar cell module100, which has passed through thebypass diode unit510 and thecapacitor unit520. Particularly, theconverter530 can perform power conversion using DC power stored in thecapacitor unit520.
• Theconverter530 according to an embodiment of the present invention will be described in more detail with reference toFIG. 4.
• Switching elements in theconverter530 can be turned on/off based on a converter switching control signal from thecontroller550. Accordingly, level-converted DC power can be output.
• Theinverter540 can convert the DC power converted by theconverter530 into AC power.
• The figure shows a full-bridge inverter. That is, upper arm switching elements S1 and S3 connected in series and lower arm switching elements S2 and S4 connected in series are paired, and the two pairs of upper and lower arm switching elements S1, S2, S3 and S4 are connected in parallel. A diode can be connected in anti-parallel with each switching element S1 to S4.
• The switching elements S1 to S4 in theinverter540 can be turned on/off based on an inverter switching control signal from thecontroller550. Accordingly, AC power having a predetermined frequency can be output. Desirably, AC power having the same frequency (e.g., about 60 Hz or 50 Hz) as the AC frequency of the grid is output.
• The capacitor C can be disposed between theconverter530 and theinverter540. The capacitor C can store the DC power having the level converted by theconverter530. Both terminals of the capacitor C can be referred to as DC terminals and thus the capacitor C can be referred to as a DC-terminal capacitor.
• The input current detector A can detect input current ic1 supplied from thesolar cell module100 to thecapacitor unit520.
• The input voltage detector B can detect an input voltage Vc1 supplied from thesolar cell module100 to thecapacitor unit520. Here, the input voltage Vc1 can be the same as the voltage stored in thecapacitor unit520.
• The detected input current ic1 and input voltage vc1 can be input to thecontroller550.
• The converter output current detector C detects output current ic2 from theconverter530, that is, DC-terminal current, and the converter output voltage detector D detects an output voltage vc2 from theconverter530, that is, a DC-terminal voltage. The detected output current ic2 and output voltage vc2 can be input to thecontroller550.
• The inverter output current detector E detects current ic3 output from theinverter540 and the inverter output voltage detector F detects a voltage vc3 output from theinverter540. The detected current ic3 and voltage vc3 are input to thecontroller550.
• Thecontroller550 may output control signals for controlling the switching elements of theconverter530. Particularly, thecontroller550 may output a turn-on timing signal of the switching elements included in theconverter530 based on at least one of the detected input current ic1, input voltage vc1, output current ic2, output voltage vc2, output current ic3 and output voltage vc3.
• Further, thecontroller550 can output inverter control signals for controlling the switching elements S1 to S4 of theinverter540. Particularly, thecontroller550 can output a turn-on timing signal of the switching elements S1 to S4 of theinverter540 based on at least one of the detected input current ic1, input voltage yc1, output current ic2, output voltage vc2, output current ic3 or output voltage vc3.
• Further, thecontroller550 can calculate a maximum power point with respect to thesolar cell module100 and control theconverter530 to output DC power corresponding to maximum power according thereto.
• Thecommunication unit580 can perform communication with thegateway80. For example, thecommunication unit580 can exchange data with thegateway80 through power line communication.
• Thecommunication unit580 can transmit current information, voltage information and power information of thephotovoltaic module50 to thegateway80.
• Thefilter570 can be disposed at the output terminals of theinverter540. In addition, thefilter570 can include a plurality of passive elements and adjust a phase difference between an AC io and an AC voltage vo output from theinverter540 based on at least some of the plurality of passive elements.
• FIGS. 3A and 3B are circuit diagrams of power conversion devices included in a photovoltaic module related to the present invention.
• Apower conversion device500xincluded in the photovoltaic module shown inFIG. 3A includes a full-bridge switching unit532x,atransformer536xand asynchronous rectification unit538x.
• In thesynchronous rectification unit538xincluded in thepower conversion device500xof the photovoltaic module, a diode D1 and a switching element Q5 are connected in series, a diode D2 and a switching element Q6 are connected in series, and the two pairs of the diodes and switching elements are connected in parallel.
• According to thepower conversion device500xincluded in the photovoltaic module ofFIG. 3A, the turn ratio of thetransformer536xneeds to be considerably high, approximately 1:12, when high voltage step-up is required according to variation in an input voltage Vpv. That is, thetransformer536xhaving a high turn ratio needs to be used. Furthermore, an additional leakage inductor is required. Accordingly, the size of thetransformer536xincreases, thus increasing the volume of thepower conversion device500x.
• Apower conversion device500yin the photovoltaic module shown inFIG. 3B includes a full-bridge switching unit532y,atransformer536y,asynchronous rectification unit538y,a resonant capacitor Cr and a resonant inductor Lr between the full-bridge switching unit532yand thetransformer536y.
• According to thepower conversion device500yincluded in the photovoltaic module ofFIG. 3B, thetransformer536yneeds to have a high turn ratio although the resonant capacitor Cr and the resonant inductor Lr are provided at the primary side of thetransformer536y.Accordingly, the size of thetransformer536yincreases, thus increasing the volume of thepower conversion device500y.
• The present invention proposes a method for reducing the size of the transformer while performing power conversion with high voltage step-up and high efficiency. Particularly, the present invention proposes a method for reducing the sizes of circuit elements in the converter by increasing a switching frequency of the full-bridge switching unit532 and thesynchronous rectification unit538 which is a half-bridge switching unit. This will be described with reference toFIG. 4 and the following figures.
• FIG. 4 is a circuit diagram of the power conversion device in the photovoltaic module according to an embodiment of the present invention andFIGS. 5 to 7 are diagrams referred to illustrating the power conversion device ofFIG. 4.
• Referring to the figures, thepower conversion device500 in thephotovoltaic module100 according to an embodiment of the present invention can include thebypass diode unit510, thecapacitor unit520, thecontroller550, thecommunication unit580, the input current detector A, the input voltage detector B, the converter output current detector C, the converter output voltage detector D, the inverter output current detector E and the inverter output voltage detector F, which are shown inFIG. 2, in addition to theconverter530 and theinverter540.
• In addition, to reduce electromagnetic noise, thefilter570 for filtering AC power output from theinverter540 can be provided at the output terminals of theinverter540.
• Thefilter570 can include first and second inductor L1 and L2 connected to both output terminals of theinverter530, respectively, and a capacitor C4 connected between the first inductor L1 and the second inductor L2.
• Accordingly, thefilter570 is realized in an asymmetrical form in consideration of theinverter540 operating according to asynchronous PWM control, and thus a common mode voltage at the output terminals of theinverter540 can be reduced and a harmonic component THD of output current can be reduced.
• The following description focuses on theconverter530 shown inFIG. 4.
• Thepower conversion device500 in thephotovoltaic module50 according to an embodiment of the present invention can include theconverter530 which converts the level of DC power input from thesolar cell module100, and the DC-terminal capacitor C which stores DC power output from theconverter530.
• Thepower conversion device500 in thephotovoltaic module100 according to an embodiment of the present invention can further include theinverter540, which converts the DC power from the DC-terminal capacitor C into AC power.
• Theconverter530 according to an embodiment of the present invention can include the full-bridge switching unit532 which switches DC power, thetransformer536 having the input side connected to the output terminal of the full-bridge switching unit532, thesynchronous rectification unit538 connected to the output side of thetransformer536, the resonant capacitor Cr and the resonant inductor Lr which are connected between thetransformer536 and thesynchronous rectification unit538.
• Particularly, ripples in input current can be reduced according to resonance of the resonant capacitor Cr, the resonant inductor Lr and thetransformer536.
• Switching elements Q1 to Q4 in the full-bridge switching unit532 can perform zero-voltage switching ZVS and zero-current switching ZCS according to the resonant capacitor Cr and the resonant inductor Lr.
• As shown, the full-bridge switching unit532 can include the first and second switching elements Q1 and Q2 connected in series, and the third and fourth switching elements Q3 and Q4 respectively connected in parallel with the first and second switching elements Q1 and Q2.
• In addition, the input terminals na and nb of thetransformer536 can be connected between a first node n1 between the first and second switching elements Q1 and Q2 and a second node n2 between the third and fourth switching elements Q3 and Q4.
• Theinverter540 can include fifth and sixth switching elements S1 and S2 connected in series, and seventh and eighth switching elements S3 and S4 connected in series.
• AC power can be output through a fifth node n5 between the fifth and sixth switching elements S1 and S2 and a sixth node n6 between seventh and eighth switching elements S3 and S4.
• As shown, thesynchronous rectification unit538 can include ninth and tenth switching elements Q9 and Q10 connected in series, and first and second capacitors C1 and C2 connected in series.
• Here, the ninth and tenth switching elements Q9 and Q10 can be connected in parallel with the first and second capacitors C1 and C2.
• The output side of thetransformer536 can be connected between a third node n3 between the ninth and tenth switching elements Q9 and Q10 and a fourth node n4 between the first and second capacitors C1 and C2.
• In addition, thesynchronous rectification unit538 is configured in a half bridge form and thus can be referred to as a half-bridge switching unit.
• Thesynchronous rectification unit538 amplifies an input voltage twice and outputs the amplified voltage and thus can be referred to as a voltage doubler.
• Thecontroller550 can control theconverter530 and theinverter540 together. Particularly, thecontroller550 can output a control signal Sfb to the full-bridge switching unit532 included in theconverter530 for maximum power point tracking control.
• Thecontroller550 can output a control signal Shb to thesynchronous rectification unit538 in order to control the same. Further, thecontroller550 can output a control signal Sic to theinverter540 in order to control the same.
• Thecontroller550 can vary the switching frequency of the full-bridge switching unit532 based on the input voltage of theconverter530 or the voltage of the DC-terminal capacitor C. Specifically, thecontroller550 can control the full-bridge switching unit532 to operate in the buck mode or boost mode depending on the voltage level of the DC-terminal capacitor C.
• Thecontroller550 may control the full-bridge switching unit532 to operate in the buck mode when the voltage of the DC-terminal capacitor C is greater than or equal to a target voltage, and thecontroller550 can control thesynchronous rectification unit538 of half-bridge switching unit to operate in the boost mode when the voltage of the DC-terminal capacitor C is lower than the target voltage.
• Thecontroller550 can control the full-bridge switching unit532 to enter the buck mode and operate at a first switching frequency (1/Tsa=Fsa inFIG. 5) when the voltage of the DC-terminal capacitor C is greater than or equal to the target voltage, and can control thesynchronous rectification unit538 of half-bridge switching unit to operate in the boost mode, and can control the full-bridge switching unit532 and thesynchronous rectification unit538 to operate at a second switching frequency (1/Tsb=Fsb inFIG. 6) lower than the first switching frequency (1/Tsa=Fsa inFIG. 5) when the voltage of the DC-terminal capacitor C is lower than the target voltage.
• It is desirable that the switching frequency of the full-bridge switching unit532 be higher than a system frequency.
• For example, the first switching frequency can be approximately 135 kHz and the second switching frequency can be approximately 90 kHz. Accordingly, high-speed switching is performed and thus, the sizes of the circuit elements in theconverters530 can be reduced. Particularly, the size of thetransformer536 can be reduced.
• Thecontroller550 can control ripples in the voltage of the DC-terminal capacitor C to be reduced through the buck mode or boost mode.
• Further, thecontroller550 can control some of the switching elements S1 to S4 in theinverter540 to perform switching at the third switching frequency, and control other switching elements to perform switching at the fourth switching frequency higher than the third switching frequency.
• That is, thecontroller550 can perform asynchronous PWM control for theinverter540.
• Here, the third switching frequency corresponds to the system frequency and the fourth switching frequency is higher than the third switching frequency, and thus theinverter540 can perform high-speed switching. Accordingly, the sizes of circuit elements in the power conversion device can be reduced, thus decreasing the size of the power conversion device.
• Thecontroller550 can control the fifth and sixth switching elements S1 and S2 to operate at the fourth switching frequency and control the seventh and eighth switching elements S3 and S4 to operate at the third switching frequency.
• Further, thecontroller550 can control the fifth and sixth switching elements S1 and S2 to perform switching according to PWM control while the seventh switching element S3 is turned on, and control the sixth and fifth switching elements S2 and S1 to perform switching according to PWM control while the eighth switching element S4 is turned on.
• In addition, it is desirable that some S3 and S4 of the switching elements S1 to S4 included in theinverter540 and others S1 and S2 of the switching elements S1 to S4 be switching elements of different types.
• The switching elements S1 and S2 among the switching elements S1 to S4 in theinverter540, for example, switching elements performing high-speed switching, can include GaN transistors or SiC transistors. Accordingly, reverse recovery loss during high-speed switching can be reduced.
• The switching elements S3 and S4 among the switching elements S1 to S4 in theinverter540, for example, switching elements performing low-speed switching, can include metal-oxide-semiconductor field-effect-transistors (MOSFETs).
• FIG. 5 is a diagram referred to illustrating a situation in which the fullbridge switching unit532 operates in the buck mode.
• FIG. 5(a) shows a waveform Vdca of the DC-terminal voltage which is the voltage of the DC-terminal capacitor C.
• FIG. 5(b) shows switching control signals SQ1 and SQ4 applied to the gates of the first switching element Q1 and the fourth switching element Q4.
• FIG. 5(c) shows switching control signals SQ2 and SQ3 applied to the gates of the second switching element Q2 and the third switching element Q3.
• FIG. 5(d) shows a voltage waveform VQ4 and a current waveform IQ4 applied to the fourth switching element Q4.
• In the buck mode, the first and fourth switching elements Q1 and Q4 are not alternately turned on and the second and third switching elements Q2 and Q3 are not alternately turned on in the full-bridge switching unit532 and turn-on periods thereof can partially overlap according to phase shift as shown.
• That is, a phase difference between the first switching element Q1 and the fourth switching element Q4 is not fixed to 180 degrees and phases or turn-on timing can be varied according to phase shift.
• The figure shows that the phase difference between the first switching element Q1 and the fourth element Q4 is DLa.
• Thecontroller550 may control the full-bridge switching unit532 to operate at a maximum switching frequency and vary the phase difference DLa between switching elements in the full-bridge switching unit532 in the buck mode.
• When the voltage of the DC-terminal capacitor C is greater than or equal to a target voltage, thecontroller550 can control the phase difference DLa between switching elements in the full-bridge switching unit532 to increase as the difference between the voltage of the DC-terminal capacitor C and the target voltage increases.
• Particularly, thecontroller550 can control the phase difference DLa between the first switching element Q1 and the fourth switching element Q4 to increase as the difference between the voltage of the DC-terminal capacitor C and the target voltage increases.
• Thecontroller550 can control turn-on timing of the fourth and third switching elements Q4 and Q3 in the full-bridge switching unit532 to be delayed from turn-on timing of the first and second switching elements Q1 and Q2 in the buck mode. Accordingly, the DC-terminal voltage Vda can be varied.
• For example, when the first and fourth switching elements Q1 and Q4 are turned on, current flows and thus the resonant capacitor Cr and the resonant inductor Lr resonate.
• Thereafter, when the fourth switching element A4 is turned off and the third switching element Q3 is turned on, the current flowing through thetransformer536 decreases to the ground GND or zero, theconverter530 operates in a discontinue mode (DCM) and a secondary switch may perform zero-current switching (ZCS).
• The switching elements Q9 and Q10 in thesynchronous rectification unit538 can be switched in synchronization with the first and second switching elements Q1 and Q2 in the full-bridge switching unit532.
• Thecontroller550 can control turn-on timing delay to increase as the difference between the voltage of the DC-terminal capacitor C and the target voltage increases when the voltage of the DC-terminal capacitor C is greater than or equal to the target voltage.
• Accordingly, the difference between the voltage of the DC-terminal capacitor C and the target voltage can be reduced, and thus the DC-terminal voltage waveform Vdca having little ripples, as shown inFIG. 5(a), can be output.
• At time Ta and time Tb, zero-voltage turn-on switching705aand705band zero-voltage turn-off switching705aand705bof the switching elements in the full-bridge switching unit532 are performed. Accordingly, power conversion with high voltage step-up and high efficiency can be performed.
• FIG. 6 is a diagram referred to illustrating a situation in which the full-bridge switching unit532 and thesynchronous rectification unit538 operates in the boost mode.
• FIG. 6(a) shows a waveform Vdcb of the DC-terminal voltage which is the voltage of the DC-terminal capacitor C.
• FIG. 6(b) shows the switching control signals SQ1 and SQ4 applied to the gates of the first switching element Q1 and the fourth switching element Q4.
• FIG. 6(c) shows the switching control signals SQ2 and SQ3 applied to the gates of the second switching element Q2 and the third switching element Q3.
• FIG. 6(d) shows switching control signals SQ9 and SQ10 applied to the gates of the ninth switching element Q9 and the tenth switching element Q10 in thesynchronous rectification unit538.
• FIG. 6(e) shows the voltage waveform VQ4 and the current waveform IQ4 applied to the fourth switching element Q4.
• In the boost mode, thecontroller550 can control the first and fourth switching elements Q1 and Q4 and the second and third switching elements Q2 and Q3 in the full-bridge switching unit532 to be alternately turned on, as shown inFIGS. 6(b) and 6(c).
• Thecontroller550 may control the full-bridge switching unit532 to operate at a minimum switching frequency and vary turn-on duty (e.g., the duty cycle) of the switching elements in thesynchronous rectification unit538 in the boost mode.FIG. 6(d) shows that the turn-on duty is DLb.
• For example, the ninth and tenth switching elements Q9 and Q10 in thesynchronous rectification unit538 are turned on with the duty thereof varying while the first and fourth switching elements Q1 and Q4 and the second and third switching elements Q2 and Q3 are alternately turned on.
• When the ninth and tenth switching elements Q9 and Q10 in thesynchronous rectification unit538 are turned on, energy is charged in the resonant inductor Lr. Accordingly, boosting is performed.
• Thecontroller550 can control the turn-on duty DLb of the ninth and tenth switching elements Q9 and Q10 in thesynchronous rectification unit538 to increase as the difference between the voltage of the DC-terminal capacitor C and the target voltage increases when the voltage of the DC-terminal capacitor C is lower than the target voltage.
• Further, thecontroller550 can control turn-on duty of switching elements in thesynchronous rectification unit538 to increase as the difference between the voltage of the DC-terminal capacitor C and the target voltage increases when the voltage of the DC-terminal capacitor C is lower than the target voltage.
• Accordingly, the difference between the voltage of the DC-terminal capacitor C and the target voltage can be reduced, and thus the DC-terminal voltage waveform Vdca with little ripples, as shown inFIG. 6(a), can be output.
• At time T1 and time T2, zero-voltage turn-on switching715aand715band zero-voltage turn-off switching715aand715bof the switching elements in the full-bridge switching unit532 are performed. Accordingly, power conversion with high voltage step-up and high efficiency can be performed.
• FIG. 7 is a block diagram of thecontroller550 of thepower conversion device500 according to an embodiment of the present invention.
• Referring to the figure, thecontroller550 can receive the input voltage Vc1 from the input voltage detector B and the DC-terminal voltage Vdc from the DC-terminal voltage detector D and control the full-bridge switching unit532 to operate in the buck mode or the boost mode.
• Particularly, thecontroller550 can control the full-bridge switching unit532 to operate in the buck mode or control thesynchronous rectification unit538 to operate in the boost mode depending on the voltage level of the DC-terminal capacitor C.
• Specifically, thecontroller550 can control the full-bridge switching unit532 to operate in the buck mode and control the full-bridge switching unit532 and thesynchronous rectification unit538 to operate at the first switching frequency when the voltage of the DC-terminal capacitor C is greater than or equal to a target voltage, and can control thesynchronous rectification unit538 to operate in the boost mode and control the full-bridge switching unit532 and thesynchronous rectification unit538 to operate at the second switching frequency lower than the first switching frequency when the voltage of the DC-terminal capacitor C is lower than the target voltage.
• Thecontroller550 can include aripple compensator910 for compensating for ripples of the DC-terminal capacitor C based on the detected DC-terminal voltage and the target voltage, and a pulse width modulation (PWM)controller920 for controlling a pulse width with respect to the switching elements in the full-bridge switching unit532.
• For example, theripple compensator910 can determine that ripples increase as the difference between the detected DC-terminal voltage and the target voltage increases and compensate for ripples such that the ripples decrease.
• ThePWM controller920 can set a phase shift value of the full-bridge switching unit532 in the buck mode or turn-on duty (e.g., duty cycle) of the switching elements in thesynchronous rectification unit538 in the boost mode based on the compensated ripples.
• Accordingly, thecontroller550 can output the control signal Sfb to the full-bridge switching unit532 in theconverter530 and output the control signal Shb to thesynchronous rectification unit538 to control thesynchronous rectification unit538.
• Further, thecontroller550 can control the full-bridge rectification unit532 to operate in the buck mode or boost mode depending on the level of the input voltage Vc1 or Vpv.
• Specifically, thecontroller550 can control the full-bridge switching unit532 to operate in the buck mode and control the full-bridge switching unit532 and thesynchronous rectification unit538 to operate at the first switching frequency when the input voltage Vc1 or Vpv is greater than or equal to a reference voltage, and can control thesynchronous rectification unit538 to operate in the boost mode and control the full-bridge switching unit532 and thesynchronous rectification unit538 to operate at the second switching frequency lower than the first switching frequency when the input voltage Vc1 or Vpv is lower than the reference voltage.
• FIG. 8 is a flowchart illustrating a method of operating the photovoltaic module according to an embodiment of the present invention.
• Referring to the figure, the input voltage detector B and the DC-terminal voltage detector D in theconverter530 respectively detect the input voltage Vc1 and the DC-terminal voltage Vdc (S1010).
• Then, thecontroller550 receives the input voltage Vc1 from the input voltage detector B and the DC-terminal voltage Vdc from the DC-terminal voltage detector D, selects a switching frequency (S1020), and determines whether to control the full-bridge switching unit532 to operate in the buck mode (S1025).
• For example, thecontroller550 can control the full-bridge switching unit532 to operate in the buck mode when the voltage of the DC-terminal capacitor C is greater than or equal to a target voltage (S1030). Here, the switching frequency of the full-bridge switching unit532 and thesynchronous rectification unit538, which is a half-bridge switching unit, can be the first switching frequency (e.g., approximately 135 kHz).
• When the voltage of the DC-terminal capacitor C is lower than the target voltage, thecontroller550 can control thesynchronous rectification unit538 to operate in the boost mode (S1035). Here, the switching frequency of the full-bridge switching unit532 and thesynchronous rectification unit538, which is a half-bridge switching unit, can be the second switching frequency (e.g., approximately 90 kHz) lower than the first switching frequency (e.g., approximately 135 kHz).
• Description of operations in the buck mode and boost mode are omitted, since the operations have been described with reference toFIGS. 4 to 7.
• Subsequently, thecontroller550 calculates a phase shift of the full-bridge switching unit532 or turn-on duty of thesynchronous rectification unit538 according to the buck mode or the boost mode (S1040).
• Then, thecontroller550 can output the control signal Sfb to the full-bridge switching unit532 in theconverter530 and output the control signal Shb to thesynchronous rectification unit538 in order to control thesynchronous rectification unit538 based on the calculated phase shift or the calculated duty (e.g., duty cycle).
• Accordingly, ripples in the DC-terminal voltage decrease, and thus a film capacitor instead of an electrolytic capacitor having large capacity can be used as the DC-terminal capacitor C. Therefore, the size of the DC-terminal capacitor C can be reduced.
• FIG. 9 is a block diagram of thecontroller550 in thepower conversion device500 according to an embodiment of the present invention.
• Referring toFIG. 9, thecontroller550 can include acalculation unit705 for calculating a difference between an output current command value iacr and an output current ic3 flowing through theinverter540 for low-speed switching and high-speed switching of theinverter540, acurrent controller710 for outputting a DC-terminal voltage command based on the difference, avoltage command compensator720 for compensating for a voltage command based on the DC-terminal voltage command and the voltage across the DC-terminal capacitor C, a low-speed switch drivingsignal generator730 for outputting a low-speed switching driving signal at the third switching frequency based on an output value from thevoltage command compensator720, and a high-speed switch drivingsignal generator740 for outputting a high-speed switching driving signal at the fourth switching frequency based on the output value from thevoltage command compensator720.
• That is, a switching control signal for operating the seventh and eighth switching elements S3 and S4 in theinverter540 can be output through the low-speed switch drivingsignal generator730 and a switching control signal for operating the fifth and sixth switching elements S1 and S2 in theinverter540 can be output through the high-speed switch drivingsignal generator740.
• FIG. 10 is a diagram showing driving signals applied to the gates of the fifth to eighth switching elements S1 to S4 during one cycle of an output current Vac output from theinverter540.
• During a positive half cycle of the output current Vac output from theinverter540, as shown inFIG. 10(a), a driving signal SS3 applied to the seventh switching element S3 has a high level and thus the seventh switching element S3 can be continuously turned on as shown inFIG. 10(d).
• Thecontroller550 can control the fifth switching element S1 and the sixth switching element S2 to perform switching according to PWM control while the seventh switching element S3 is turned on.
• Here, since the fifth switching element S1 and the sixth switching element S2 complementarily operate, a driving signal applied to the sixth switching element S2 can be a driving signal SS2 for complementary PWM when a driving signal applied to the fifth switching element S1 is a driving signal SS1 for control PWM, as shown.
• The seventh switching element S3 and the eighth switching element S4 also complementarily operate, and thus the eighth switching element S4 is turned off while the seventh switching element S3 is turned on.
• When the fifth switching element S1 is turned on and the sixth switching element S2 is turned off while the fifth and sixth switching elements S1 and S2 perform PWM switching during the positive half cycle of the output current Vac, a current path Ipath1 through the fifth switching element S1 and the eighth switching element S4 can be generated, as shown inFIG. 11A.
• When the sixth switching element S2 is turned on and the fifth switching element S1 is turned off while the fifth and sixth switching elements S1 and S2 perform PWM switching during the positive half cycle of the output current Vac, a current path Ipath2 through the eighth switching element S4 and the sixth switching element S2 can be generated, as shown inFIG. 11A.
• During a negative half cycle of the output current Vac output from theinverter540, as shown inFIG. 10(a), a driving signal SS4 applied to the eighth switching element S4 has a high level and thus eighth switching element S4 can be continuously turned on as shown inFIG. 10(e).
• Thecontroller550 can control the sixth switching element S2 and the fifth switching element S1 to perform switching according to PWM control while eighth switching element S4 is turned on.
• Here, since the fifth switching element S1 and the sixth switching element S2 complementarily operate, the driving signal applied to the fifth switching element S1 can be a driving signal SS1 for complementary PWM when the driving signal applied to the sixth switching element S2 is a driving signal SS2 for control PWM, as shown.
• The seventh switching element S3 and the eighth switching element S4 also complementarily operate, and thus the seventh switching element S3 is turned off while the eighth switching element S4 is turned on.
• When the sixth switching element S2 is turned on and the fifth switching element S1 is turned off while the fifth and sixth switching elements S1 and S2 perform PWM switching during the negative half cycle of the output current Vac, a current path Ipath3 through the seventh switching element S3 and the sixth switching element S2 can be generated, as shown inFIG. 11B.
• When the fifth switching element S1 is turned on and the sixth switching element S2 is turned off while the fifth and sixth switching elements S1 and S2 perform PWM switching during the negative half cycle of the output current Vac, a current path Ipath4 through the seventh switching element S3 and the fifth switching element S1 can be generated, as shown inFIG. 11B.
• FIG. 12A is a diagram showing aninverter540mand afilter570mof apower conversion device500mcompared with the present invention.
• Theinverter540mshown inFIG. 12A is similar to that of the present application but thefilter570mdiffers from that of the present application in that thefilter570mis configured in a symmetrical form.
• That is, thefilter570mshown inFIG. 12A can include first and second inductors Lm1 and Lm2 respectively provided at both terminals of theinverter540m,and a capacitor Cm connected between the first and second inductors Lm1 and Lm2.
• A first leg can include the fifth switching element S1 and the sixth switching element S2 of the plurality of switching elements S1 to S4 of theinverter540, and a second leg can include the seventh and eighth switching elements S3 and S4 of the plurality of switching elements S1 to S4 of theinverter540.
• Here, a switching frequency of the first leg can be different from a switching frequency of the second leg.
• In theinverter540 according to an embodiment of the present invention, some legs (the seventh and eighth switching elements S3 and S4) perform low-speed switching and other legs (the fifth and sixth switching elements S1 and S2) perform high-speed switching according to asynchronous PWM, as described above. Accordingly, an output current waveform Iaca and a common mode voltage waveform Vfda as shown inFIG. 12B can appear when a symmetrical filter such as thefilter570mofFIG. 12A is used.
• Particularly, it can be known from the common mode voltage waveform Vfda that a common mode voltage considerably increases.
• To solve such a problem, the present invention uses theasymmetrical filter570 corresponding to theasynchronous inverter540.
• FIG. 13A illustrates theinverter540 and thefilter570 in thepower conversion device500 according to an embodiment of the present invention.
• Thefilter570 according to an embodiment of the present invention can include an inductor Lf connected to one of the output terminals of theinverter540, and a capacitor Cf connected between the inductor Lf and the other output terminal of theinverter540.
• Particularly, since the inductor Lf is connected to only one of the output terminals of theinverter540, a common mode voltage caused by theinverter540 which asynchronously operates according to high-speed switching and low-speed switching can be considerably reduced.
• FIG. 13B shows an output current waveform Iacb and a common mode voltage waveform Vfdb according to theinverter540 and thefilter570 shown inFIG. 13A. It can be known from the figure that the common mode voltage can be considerably reduced according to theinverter540 and thefilter570 shown inFIG. 13A.
• In addition, thecontroller550 according to an embodiment of the present invention controls ripples in the voltage of the DC-terminal capacitor to decrease.
• FIG. 14A shows a system output current Iacn when control for decreasing ripples in the voltage of the DC-terminal capacitor is not performed.
• Referring toFIG. 14A, it can be known that output current is distorted due to harmonic components THD in the system output current Iacn.
• FIG. 14B shows a DC-terminal voltage waveform Vdca, a system output voltage waveform Vaca, and a system output current waveform Iaca when control for decreasing ripples in the voltage of the DC-terminal capacitor is performed.
• Referring toFIG. 14B, it can be known that the DC-terminal voltage waveform Vdca has little ripples and the system output voltage waveform Vaca and the system output current waveform Iaca are hardly distorted. That is, it can be known that harmonic components THD in the system output current Iacn are removed.
• As described inFIGS. 9 to 14B, the low-speed switching and the high-speed switching of theinverter540 are operated based on the dc voltage, so that the converter in thepower conversion device500 according to an embodiment of the present invention is not limited to theconverter530 described inFIGS. 4 to 8.
• In addition, theconverter530 in thepower conversion device500 according to an embodiment of the present invention can include a full-bridge switching unit532 which switches DC power, atransformer536 having an input side connected to an output terminal of the full-bridge switching unit532, asynchronous rectification unit538 connected to an output side of thetransformer536, and a resonant capacitor Cr and a resonant inductor Lr which are connected between thetransformer536 and thesynchronous rectification unit538. Thecontroller550 may perform power conversion with high voltage step-up and high efficiency by varying a switching frequency of the full-bridge switching unit532 and thesynchronous rectification unit538 which is a half-bridge switching unit based on the input voltage of theconverter530 or the voltage of a DC-terminal capacitor C.
• Accordingly, the efficiency of theinverter540 can be further improved bypower conversion apparatus500 combining theconverter530 described inFIGS. 4 to 8 with theinverter540 described inFIGS. 9 to 14B.
• FIG. 15 is an exploded perspective view of the solar cell module ofFIG. 1A orFIG. 1B.
• Referring toFIG. 15, thesolar cell module100 ofFIG. 2 can include a plurality ofsolar cells130. In addition, thesolar cell module100 may further include afirst sealant120 and asecond sealant150 provided on the upper surface and the lower surface of thesolar cells130, arear substrate110 provided under thefirst sealant120, and afront substrate160 provided on thesecond sealant150.
• Thesolar cell130 is a semiconductor device which converts solar energy into electric energy and can be a silicon solar cell, a compound semiconductor solar cell, a tandem solar cell, a dye-sensitized solar cell, a CdTe solar cell, a CIGS solar cell or a thin film solar cell.
• Thesolar cell130 is formed on a light-receiving surface to which sunlight is input and a rear surface opposite the light-receiving surface. For example, thesolar cell130 can include a first conductivity type silicon substrate, a second conductivity type semiconductor layer which is formed on the silicon substrate and has a conductivity type opposite the first conductivity type, an antireflection film which includes at least one opening for partially exposing the second conductivity type semiconductor layer and is formed on the second conductivity type semiconductor layer, a front electrode contacting a portion of the second conductivity type semiconductor layer exposed through the at least one opening, and a rear electrode formed on the rear side of the silicon substrate.
• Thesolar cells130 can be electrically connected in series or parallel, or in serial-parallel. Specifically, the plurality ofsolar cells130 can be electrically connected through theribbon133. Theribbon133 can be attached to the front electrode formed on the light-receiving surface of asolar cell130 and a rear electrode formed on the rear side of a neighboringsolar cell130. The figure shows that theribbon133 is formed in two lines and thesolar cells130 are connected in a row through theribbon133 to form asolar cell string140.
• Thus, six strings140a,140b,140c,140d,140eand140fare formed and each string can include ten solar cells, as described above with reference toFIG. 2.
• Therear substrate110 is a back sheet and serves to execute waterproofing, insulation and sunblocking functions. Therear substrate110 may be a Tedlar/PET/Tedlar (TPT) type but the present invention is not limited thereto. In addition, although therear substrate110 is rectangular inFIG. 15, therear substrate110 can be manufactured in various forms such as a circle and a semicircle according to environment in which thesolar cell module100 is installed.
• Thefirst sealant120 can be attached to therear substrate110 having the same size as therear substrate110, and a plurality ofsolar cells130 can be arranged in several rows on thefirst sealant120.
• Thesecond sealant150 is positioned on thesolar cells130 and attached to thefirst sealant120 through lamination.
• Here, thefirst sealant120 and thesecond sealant150 are used to chemically connect elements of the solar cells. Various materials such as ethylene vinyl acetate (EVA) film can be used as thefirst sealant120 and thesecond sealant150.
• Thefront substrate160 is positioned on thesecond sealant150 such that sunlight is transmitted through thefront substrate160. It is desirable that thefront substrate160 be tempered glass in order to protect thesolar cells130 from external impact. It is more desirable that thefront substrate160 be low-iron tempered glass in order to prevent reflection of sunlight and to improve transmissivity of sunlight.
• As is apparent from the above description, according to an embodiment of the present invention, a photovoltaic module includes a solar cell module including a plurality of solar cells, a converter to convert the level of DC power input from the solar cell module, a DC-terminal capacitor to store DC power output from the converter, an inverter including a plurality of switching elements and configured to convert DC power from the DC-terminal capacitor into AC power, and a controller to control the inverter. The converter can control some of the plurality of switching elements included in the inverter to perform switching at a third switching frequency and control others to perform switching at a fourth switching frequency higher than the first switching frequency, thereby reducing the size of a power conversion device.
• Particularly, switching is performed at the third switching frequency corresponding to a system frequency and the fourth switching frequency higher than the third switching frequency to obtain high-speed switching effect. Accordingly, the sizes of circuit elements in the power conversion device including the inverter can be reduced.
• The other switching elements among the plurality of switching elements may include GaN transistors or SiC transistors, and thus reverse recovery loss during high-speed switching can be reduced.
• A filter provided at output terminals of the inverter includes an inductor connected to one of the output terminals of the inverter and a capacitor connected between the inductor and the other output terminal of the inverter. Accordingly, a common mode voltage at the output terminals of the inverter can be reduced. In addition, harmonic components (THD) of an output current can be reduced.
• The controller can perform power conversion with high voltage step-up and high efficiency by varying a switching frequency of a full-bridge switching unit based on the input voltage of the converter or the voltage of the DC-terminal capacitor.
• Particularly, the controller can control the full-bridge switching unit to enter a buck mode and operate at a first switching frequency when the voltage of the DC-terminal capacitor is greater than or equal to a target voltage and control the full-bridge switching unit to enter a boost mode and operate at a second switching frequency lower than the first switching frequency when the voltage of the DC-terminal capacitor is lower than the target voltage, thereby performing power conversion with high voltage step-up and high efficiency.
• Here, the first and second switching frequencies can be switching frequencies that are much higher than the system frequency, and thus the sizes of circuit elements in the converter can be reduced.
• Particularly, the turn ratio of a transformer can be reduced, thus decreasing the size of the transformer. Consequently, the converter used in the photovoltaic module can be reduced in size.
• Further, ripples in the voltage of the DC-terminal capacitor are controlled to decrease, and thus a film capacitor instead of an electrolyte capacitor can be used as the DC-terminal capacitor. Accordingly, the size of the DC-terminal capacitor can be reduced.
• When the voltage of the DC-terminal capacitor is greater than or equal to the target voltage, the full-bridge switching unit is controlled to enter the buck mode in which a phase difference between switching elements in the full-bridge switching unit increases as the difference between the voltage of the DC-terminal capacitor and the target voltage increases such that the voltage of the DC-terminal capacitor follows the target voltage.
• When the voltage of the DC-terminal capacitor is lower than the target voltage, the synchronous rectification unit is controlled to enter the boost mode in which a turn-on duty of switching elements in the synchronous rectification unit increases as the difference between the voltage of the DC-terminal capacitor and the target voltage increases such that the voltage of the DC-terminal capacitor follows the target voltage.
• A photovoltaic module according to another embodiment of the present invention includes a solar cell module including a plurality of solar cells, a converter to convert the level of DC power input from the solar cell module, a DC-terminal capacitor to store DC power output from the converter, an inverter including first to fourth switching elements and configured to convert DC power from the DC-terminal capacitor into AC power, and a controller to control the inverter. The converter can perform asynchronous PWM control on the inverter to reduce the size of the power conversion device.
• Particularly, switching is performed at the third switching frequency corresponding to the system frequency and the fourth switching frequency higher than the third switching frequency to obtain high-speed switching effect. Accordingly, the sizes of circuit elements in the power conversion device including the inverter can be reduced.
• A photovoltaic module according to another embodiment of the present invention includes a solar cell module including a plurality of solar cells, a converter for converting the level of DC power input from the solar cell module, a DC-terminal capacitor for storing DC power output from the converter, and a controller for controlling the converter. The converter includes a full-bridge switching unit for switching the DC power, a transformer having an input side connected to an output terminal of the full-bridge switching unit, a synchronous rectification unit connected to an output side of the transformer, and a resonant capacitor and a resonant inductor connected between the transformer and the synchronous rectification unit, and the controller varies a switching frequency of the full-bridge switching unit based on an input voltage of the converter or a voltage of the DC-terminal capacitor, thereby performing power conversion with high voltage step-up and high efficiency.
• Particularly, the controller controls the full-bridge switching unit to enter a buck mode such that the full-bridge switching unit and the synchronous rectification unit operate at a first switching frequency when the voltage of the DC-terminal capacitor is greater than or equal to a target voltage, and controls the synchronous rectification unit to enter a boost mode such that the full-bridge switching unit and the synchronous rectification unit operate at a second switching frequency lower than the first switching frequency when the voltage of the DC-terminal capacitor is lower than the target voltage. Accordingly, power conversion with high voltage step-up and high efficiency can be performed.
• Here, the first and second switching frequencies can be switching frequencies that are much higher than a system frequency, and thus the sizes of circuit elements in the converter can be reduced.
• Particularly, the turn ratio of the transformer can be reduced and thus the size of the transformer can be decreased. As a result, the size of the converter used for the photovoltaic module can be reduced.
• Further, ripples in the voltage of the DC-terminal capacitor are controlled to decrease, and thus a film capacitor instead of an electrolyte capacitor can be used as the DC-terminal capacitor. Accordingly, the size of the DC-terminal capacitor can be reduced.
• When the voltage of the DC-terminal capacitor is greater than or equal to the target voltage, the full-bridge switching unit is controlled to enter the buck mode in which a phase difference between switching elements in the full-bridge switching unit increases as the difference between the voltage of the DC-terminal capacitor and the target voltage increases such that the voltage of the DC-terminal capacitor follows the target voltage.
• When the voltage of the DC-terminal capacitor is lower than the target voltage, the synchronous rectification unit is controlled to enter the boost mode in which a turn-on duty of switching elements in the synchronous rectification unit increases as the difference between the voltage of the DC-terminal capacitor and the target voltage increases such that the voltage of the DC-terminal capacitor follows the target voltage.
• A photovoltaic module according to another embodiment of the present invention includes a solar cell module including a plurality of solar cells, a converter for converting the level of DC power input from the solar cell module, a DC-terminal capacitor for storing DC power output from the converter, and a controller for controlling the converter. The converter includes a full-bridge switching unit for switching the DC power, a transformer having an input side connected to an output terminal of the full-bridge switching unit, a synchronous rectification unit connected to an output side of the transformer, and a resonant capacitor and a resonant inductor connected between the transformer and the synchronous rectification unit, and the controller controls the full-bridge switching unit or the synchronous rectification unit to operate in a buck mode or a boost mode depending on a voltage level of the DC-terminal capacitor, thereby performing power conversion with high voltage step-up and high efficiency.
• Particularly, in the photovoltaic module according to another embodiment of the present invention, the full-bridge switching unit is controlled to operate in the buck mode and the full-bridge switching unit and the synchronous rectification unit are controlled to operate at the first switching frequency when the voltage of the DC-terminal capacitor is greater than or equal to a target voltage, and the synchronous rectification unit is controlled to operate in the boost mode and the full-bridge switching unit and the synchronous rectification unit are controlled to operate at the second switching frequency lower than the first switching frequency when the voltage of the DC-terminal capacitor is lower than the target voltage. Accordingly, power conversion with high voltage step-up and high efficiency can be performed.
• The photovoltaic module according to an embodiment of the present invention is not limited to the above-described embodiments and all or some of the embodiments can be selectively combined such that the embodiments can be modified in various manners.
• Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.

Claims (20)

What is claimed is:

1. A photovoltaic module comprising:

a solar cell module including a plurality of solar cells;

a converter configured to convert a level of direct current (DC) power input from the solar cell module;

a DC-terminal capacitor configured to store DC power output from the converter;

an inverter having a plurality of switching elements including first, second, third and fourth switching elements, the inverter being configured to convert DC power from the DC-terminal capacitor into alternating current (AC) power; and

a controller configured to perform asynchronous pulse width modulation (PWM) control on the inverter.

2. The photovoltaic module according toclaim 1, wherein the controller is further configured to:

control the first and fourth switching elements to perform switching according to the PWM control while the second switching is turned on, and

control the fourth and first switching elements to perform switching according to the PWM control while the third switching element is turned on.

3. The photovoltaic module according toclaim 1, wherein the inverter further includes:

a first leg including some switching elements among the plurality of switching elements in the inverter, and

a second leg including other switching elements among the plurality of switching elements in the inverter,

wherein a switching frequency of the first leg is different than a switching frequency of the second leg.

4. The photovoltaic module according toclaim 3, wherein the some switching elements in the first leg of the inverter and the other switching elements in the second leg of the inverter are of different types.

5. The photovoltaic module according toclaim 3, wherein the some switching elements in the first leg of the inverter include GaN transistors or SiC transistors, and the other switching elements in the second leg of the inverter include metal-oxide-semiconductor field-effect-transistors (MOSFETs).

6. The photovoltaic module according toclaim 1, further comprising:

a filter configured to filter the AC power output from the inverter.

7. The photovoltaic module according toclaim 6, wherein the filter includes:

an inductor connected to an output terminal of the inverter; and

a capacitor connected between the inductor and another output terminal of the inverter.

8. The photovoltaic module according toclaim 1, wherein the converter further includes:

a transformer having an input side connected to an output terminal of the full-bridge switching part;

a synchronous rectification part connected to an output side of the transformer;

a resonant capacitor connected between the transformer and the synchronous rectification part; and

a resonant inductor connected between the transformer and the synchronous rectification part.

9. The photovoltaic module according toclaim 8, wherein the full-bridge switching part includes:

fifth and sixth switching elements connected in parallel; and

seventh and eighth switching elements respectively connected in series to the fifth and sixth switching elements,

wherein the input side of the transformer is connected between a first node located between the first and second switching elements and a second node located between the seventh and eighth switching elements.

10. The photovoltaic module according toclaim 9, wherein the controller is further configured to:

control the full-bridge switching part to enter a buck mode and operate at a third switching frequency when a voltage of the DC-terminal capacitor is greater than or equal to a target voltage, and

control the full-bridge switching part to enter a boost mode and operate at a fourth switching frequency lower than the third switching frequency when the voltage of the DC-terminal capacitor is lower than the target voltage.

11. The photovoltaic module according toclaim 8, wherein the controller is further configured to:

increase a phase difference between switching elements in the full-bridge switching part as a difference between a voltage of the DC-terminal capacitor and the target voltage increases, when the voltage of the DC-terminal capacitor is greater than or equal to the target voltage.

12. The photovoltaic module according toclaim 8, wherein the controller is further configured to:

increase a turn-on duty of switching elements in the synchronous rectification part as a difference between a voltage of the DC-terminal capacitor and the target voltage increases, when the voltage of the DC-terminal capacitor is lower than the target voltage.

13. The photovoltaic module according toclaim 8, wherein the synchronous rectification part includes:

ninth and tenth switching elements connected in series; and

first and second capacitors connected in series,

wherein the output side of the transformer is connected between a third node located between the ninth and tenth switching elements and a fourth node located between the first and second capacitors.

14. The photovoltaic module according toclaim 1, wherein the inverter further includes:

the first and forth switching elements connected in series; and

the second and third switching elements connected in series,

wherein the AC power is output through a node located between the first and forth switching elements and a node located between the second and third switching elements.

15. The photovoltaic module according toclaim 14, wherein the controller is further configured to:

control the second and third switching elements to operate at a first switching frequency and control the first and forth switching elements to operate at a second switching frequency lower than the first switching frequency.

16. The photovoltaic module according toclaim 14, wherein the controller is further configured to:

control the first and forth switching elements to perform switching according to PWM control while the second switching element is turned on, and

control the second and third switching elements to perform switching according to PWM control while the third switching element is turned on.

17. The photovoltaic module according toclaim 1, wherein the DC-terminal capacitor includes a film capacitor.

18. A photovoltaic module comprising:

a solar cell module including a plurality of solar cells;

a converter configured to convert a level of direct current (DC) power input from the solar cell module;

a DC-terminal capacitor configured to store DC power output from the converter;

an inverter including a plurality of switching elements and configured to convert DC power from the DC-terminal capacitor into alternating current (AC) power; and

a controller configured to control the inverter,

wherein the converter includes a full-bridge switching part configured to switch the DC power,

wherein the controller is further configured to:

control the full-bridge switching part to enter a buck mode and operate at a first switching frequency when a voltage of the DC-terminal capacitor is greater than or equal to a target voltage, and

control the full-bridge switching part to enter a boost mode and operate at a second switching frequency lower than the first switching frequency when the voltage of the DC-terminal capacitor is lower than the target voltage, and

wherein the converter is further configured to:

control some switching elements among the plurality of switching elements in the inverter to perform switching at a third switching frequency, and

control other switching elements among the plurality of switching elements in the inverter to perform switching at a forth switching frequency higher than the third switching frequency.

19. The photovoltaic module according toclaim 18, wherein the controller includes:

a current controller configured to output a DC-terminal voltage command based on an output current flowing through the inverter;

a voltage command compensator configured to compensate for a voltage command based on the DC-terminal voltage command and a voltage across both terminals of the DC-terminal capacitor;

a low-speed switching driving signal generator configured to output a low-speed switching driving signal at the third switching frequency based on an output value from the voltage command compensator; and

a high-speed switching driving signal generator configured to output a high-speed switching driving signal at the forth switching frequency based on the output value from the voltage command compensator.

20. The photovoltaic module according toclaim 18, wherein the some switching elements in the converter and the other switching elements in the converter are of different types of transistors.

Images