Disclosure of Invention
The invention aims to overcome the technical defect that the efficiency of the traditional LLC converter is low under the working condition of a wide voltage range, and provides a variable structure LLC converter with a wide output voltage range and a variable structure LLC converter method with the wide output voltage range, which can realize the variable range of twice the output voltage. On the premise of keeping the characteristics of traditional LLC isolation and high power density, the high electric energy conversion efficiency is also ensured.
The technical scheme adopted by the invention is as follows:
A variable structure LLC converter with a wide output voltage range comprises a primary side inverter circuit, a secondary side rectifier circuit, a three-winding transformer for connecting the inverter circuit and the rectifier circuit, and an LC series resonant cavity; the primary side inverter circuit is a full-bridge inverter circuit, and the full-bridge inverter circuit is connected with the LC series resonant cavity and the primary winding of the transformer; the secondary side is a rectifier circuit, wherein two transformer windings respectively form two half-wave rectifier circuits with a diode d1, a diode d2, a capacitor C1 and a capacitor C2, the capacitor C1 is connected in series with the diode d3, the capacitor C2 is connected in series with the diode d4, and two series midpoints are connected through a switch tube S5; the output end of the rectifying circuit at the secondary side is connected with an output inductor and a load.
A variable structure LLC converter with a wide output voltage range comprises a primary side inverter circuit, a secondary side rectifier circuit, a transformer for connecting the inverter circuit and the rectifier circuit, and an LC series resonant cavity; the primary side inverter circuit is a full-bridge inverter circuit, and the full-bridge inverter circuit is connected with the LC series resonant cavity and the primary winding of the transformer; the secondary side is a rectifying circuit, wherein a capacitor C1 and a diode d3 are connected in series to form an output capacitor bridge arm, a capacitor C2 and a diode d4 form another output capacitor bridge arm, the capacitance and the diode position of the two output capacitor bridge arms are opposite, and the two output capacitor bridge arms are connected in parallel and then connected with an output inductor and a load; the diode d1 and the diode d2, the switching tube S5 and the switching tube S6 form a bridge rectifier circuit with a variable structure, wherein the switching tube S5 and the switching tube S6 are connected in series and are connected with the midpoints of two output capacitor bridge arms.
As a further improvement of the invention, the inputs of a plurality of variable structure LLC converters are connected in parallel and the outputs of the variable structure LLC converters are connected in series to form a medium-voltage high-power direct current converter topology.
As a further improvement of the present invention, the expression of the output voltage Vo is:
Vo＝(1+d)V2 (1)
Wherein V2 is the voltage at two ends of the secondary winding of the transformer, and d is the duty ratio of the switching tube S5.
As a further improvement of the present invention, the expression of the output voltage Vo is:
Vo＝2DV2 (2)
Wherein V2 is the voltage at two ends of the secondary winding of the transformer, and D is the duty ratio of the switching tube S5 to the switching tube S6.
As a further improvement of the present invention, the excitation inductance Lm of the transformer satisfies:
Where tdead represents the dead time of the same leg switching device, Cs represents the junction capacitance of the switching device, and fr represents the resonant frequency of the LC series resonant cavity.
As a further improvement of the invention, the resonant capacitance satisfies by the resonant frequency and the transformer leakage inductance:
Wherein fr represents the resonant frequency of the LC series resonant cavity, and Lr represents the leakage inductance value of the transformer.
A method of controlling a wide output voltage range variable structure LLC converter, comprising the steps of:
When the switching tube S5 is turned off, the output capacitors are connected in parallel, the output voltage is equal to the voltage at two ends of the capacitors, when the switching tube S5 is turned on, the two output capacitors are connected in series, the output voltage is equal to the sum of the voltages of the two capacitors, the output voltage is regulated by regulating the duty ratio of the switching device, and the voltage regulating range is 1 to 2 times.
A method of controlling a wide output voltage range variable structure LLC converter, comprising the steps of:
when the switch tube S5,S6 is simultaneously conducted, the two output capacitors are connected in series, the output voltage is equal to the sum of the voltages of the two capacitors, otherwise, the output capacitors are connected in parallel, and the output voltage is equal to the voltages at the two ends of the capacitors; the output voltage is regulated by regulating the duty ratio of the switching device, and the voltage regulating range is 1 to 2 times; by appropriate control, the switching timing of S5 is synchronized with Q1,Q4, and the switching timing of S6 is synchronized with Q2,Q3, so that S5,S6 has a synchronous rectification function.
A control method of a variable structure LLC converter implementing a wider output voltage range, comprising the steps of:
When the primary side switching tubes Q1,Q4 and Q2,Q3 are complementarily conducted at a duty ratio of 50%, the primary side works in a full-bridge inversion mode; when the switching tube Q3 on the primary side is kept off and the switching tube Q4 is kept on, the full-bridge inversion on the primary side can work in a half-bridge mode; the two modes of the half bridge and the full bridge can widen the voltage regulating range of the converter to 1 to 4 times.
Compared with the prior art, the invention has the following advantages:
The topology provided by the invention can be applied to a distributed power generation system and a battery or super capacitor charging system in a medium-high power place, and has the following remarkable advantages: the isolated direct current converter has the advantages that the secondary side adopts a novel structure, the change of the output voltage within the range of 1-2 times can be realized by changing the serial-parallel connection relation of the output capacitor, the isolated direct current converter is suitable for renewable energy power generation and battery charging, the defect of low efficiency of the traditional LLC converter under the working condition of wide voltage range is overcome, and the requirements of wide-range regulation of the output voltage and high transformation ratio of renewable energy power generation during battery/super capacitor charging are met. The converter realizes voltage regulation through the on duty ratio of the secondary side switching device instead of frequency conversion voltage regulation by using the resonant cavity, so that the converter can work at fixed switching frequency, the volumes of passive devices such as a transformer, an inductor and the like are reduced, the power density is improved, and the cost is reduced.
Detailed Description
The present invention will be described in further detail with reference to the drawings and examples, in order to make the objects, technical solutions and advantages of the present invention more apparent. It should be understood that the specific embodiments described herein are for purposes of illustration only and are not intended to limit the present invention, in which case the switching device uses silicon-based MOSFETs.
The invention provides a variable structure LLC converter with a wide output voltage range, wherein the primary side is a traditional full-bridge inverter circuit, the secondary side is a novel rectifying circuit consisting of four diodes, two capacitors and a full-control switching device, and the primary side and the secondary side are connected through a three-winding high-frequency transformer and an LC series resonant cavity. The structure is shown in figure 1.
The primary side switching tube Q1,Q2,Q3,Q4 of the converter forms a full-bridge inverter circuit, and the resonant cavity Lr,Cr is connected in series with the primary winding of the transformer. On the secondary side, two windings of the transformer and the diode d1,d2 respectively form two half-wave rectification, so that the capacitor C1,C2 can be respectively charged in positive and negative half waves; the capacitor C1 and the diode d3 are connected in series to form an arm, the capacitor C2 and the diode d4 also form an arm, the positions of the capacitors and the diodes of the two arms are opposite, and the two arms are connected in parallel to serve as an output capacitor to be connected with an output load; the midpoints of the two output capacitor bridge arms are connected through a switch tube S5, and the design aims at that when S5 is turned off, the two output capacitors are in parallel connection, and the output voltage of the converter is equal to the voltage at the two ends of the capacitors; when S5 is conducted, the two output capacitors are in series connection, the output voltage is equal to the sum of the two capacitor voltages, and the output voltage can be adjusted by adjusting the duty ratio of S5, and the voltage adjusting range is 1 to 2 times. Let V2 be the voltage across the secondary winding of the transformer, d be the on duty cycle of the secondary switching tube S5, then the expression of the output voltage Vo is:
Vo＝(1+d)V2 (1)
Because PWM voltage regulation is adopted to replace variable frequency voltage regulation, the converter can work near a resonance point at a fixed frequency, and the switching frequency is slightly lower than the resonance frequency, so that ZVS can be realized for all switching tubes on the primary side, and ZCS can be realized for the diode d1,d2 on the secondary side. Unlike conventional LLC parameter designs, the magnetizing inductance of the converter of the present invention can be designed to be large enough to reduce the turn-off and turn-on losses of the primary switching device. However, in order to ensure that the switching tubes on the primary side of the converter can realize ZVS, that is, the charging and discharging of junction capacitances of four switching devices on the primary side can be completed in dead time, the excitation inductance Lm of the transformer needs to satisfy:
Where tdead represents the dead time of the same leg switching device, Cs represents the junction capacitance of the switching device, and fr represents the resonant frequency of the LC series resonant cavity. For the design of the series resonant cavity, since the converter of the invention only works near the resonance point, the resonance inductance has no great influence on the operation characteristics of the converter. Therefore, in order to reduce the volume and the cost, the leakage inductance of the transformer is utilized as the resonance inductance, and meanwhile, the resonance capacitance can be calculated through the resonance frequency and the leakage inductance value of the transformer:
Where fr denotes the resonant frequency of the LC series resonant cavity and Lr denotes the transformer leakage inductance value.
The present invention also proposes another variable structure LLC converter with a wide output voltage range, in structure, the primary side and the series resonant cavity are completely identical to those of invention 1, as shown in fig. 2.
On the secondary side, a capacitor C1 and a diode d3 are connected in series to form an arm, a capacitor C2 and a diode d4 also form an arm, the positions of the capacitors and the diodes of the two arms are opposite, and the two arms are connected in parallel to be used as an output capacitor to be connected with an output load; the diode d1,d2 and the body diode of the switch tube S5,S6 form a bridge rectifier with a variable structure, the diode bridge arm of the traditional bridge rectifier is connected to a direct current bus in a connection mode as shown by d1,d2 in the figure, in the invention, the anode of the S5 body diode and the cathode of the S6 body diode are respectively connected with the midpoints of two output capacitor bridge arms, the design aims at that the alternating current measuring direction is positive, a circuit charges a capacitor C1, and conversely, when the current is negative, the circuit charges the capacitor C2, so that the charging of the two capacitors in one switch period is realized; in addition to the rectifying effect, S5,S6 is also used as a switching device connected to the middle point of two output capacitor bridge arms, and the purpose of this design is that when S5,S6 is turned on simultaneously, two output capacitors are in a series connection, and when at least one of them is turned off, two output capacitors are in a parallel connection, so by adjusting the duty ratio of the switching device S5,S6 that is turned on simultaneously, the output voltage gain can be changed from 1 to 2.
The driving signals of the switching tubes of the converter are shown in fig. 3, the primary side switching tubes Q1,Q4 and Q2,Q3 are alternately conducted (dead time is ignored) with a duty ratio of 50%, and the switching frequency is slightly lower than the resonance frequency, so that all the switching tubes on the primary side can realize ZVS, and the secondary side diode d1,d2 can realize ZCS; the duty ratio of the secondary side switching tube S5,S6 is D, (D is more than or equal to 0.5 and less than or equal to 1), the switching time of S5 is controlled to be synchronous with Q1,Q4, and the switching time of S6 is controlled to be synchronous with Q2,Q3, so that S5,S6 has a synchronous rectification function, and the loss of the converter can be reduced. Let V2 be the voltage at both ends of the secondary winding of the transformer, neglect the influence of dead time on the circuit, the expression of output voltage Vo is:
Vo＝2DV2 (4)
The design of the excitation inductance and series resonant cavity parameters in the converter is identical to that described in invention 1.
The primary side of the proposed two variable structure LLC converters can be operated in a half-bridge mode, namely Q3 is kept to be off and Q4 is kept to be on, aiming at places with wider voltage ranges. The voltage regulating range of the converter can be expanded to four times by changing the half-bridge mode and the full-bridge mode.
The invention also provides a part of medium voltage high power places, the variable structure LLC converter shown in fig. 1 and 2 is taken as a sub-module as a whole, a new topology is constructed by adopting a mode of input parallel connection and output serial connection, and the variable structure LLC converter is applied to places of hundreds of kilowatts and megawatts, and the structure is shown in fig. 4-5.
In the scheme of the first mode, the secondary side structure can charge two output capacitors C1,C2 in one switching period; the diode and capacitor series circuit and the structure of the switching tube S5 can enable the output capacitors to form a series connection or parallel connection relation by changing the state of the switching tube, when the switching tube S5 is turned off, the output capacitors are connected in parallel, the output voltage is equal to the voltage at two ends of the capacitor, when the S5 is turned on, the two output capacitors are connected in series, the output voltage is equal to the sum of the two capacitor voltages, the output voltage is regulated by regulating the duty ratio of the switching device, and the voltage regulating range is 1 to 2 times.
In the scheme of the second mode, when the switch tube S5,S6 is simultaneously turned on, two output capacitors are connected in series, the output voltage is equal to the sum of the voltages of the two capacitors, otherwise, the output capacitors are connected in parallel, and the output voltage is equal to the voltages at two ends of the capacitors. The output voltage is regulated by adjusting the duty cycle of the switching device, with a voltage regulation range of 1 to 2 times. By proper control, the switching time of S5 is synchronized with Q1,Q4, and the switching time of S6 is synchronized with Q2,Q3, so that S5,S6 has synchronous rectification function, and the loss of the converter can be reduced.
As a further improvement of the invention, when the switching tube Q3 on the primary side is kept off and Q4 is kept on, the full-bridge inversion on the primary side can work in the half-bridge mode, and the whole voltage regulating range of the converter can be expanded to be 1 to 4 times.
As a further improvement of the invention, a plurality of variable structure LLC converters are used in medium voltage high power locations; and the inputs of the isolation type high-frequency resonant direct current-direct current converters with the wide output voltage range are connected in parallel and the outputs of the isolation type high-frequency resonant direct current-direct current converters with the wide output voltage range are connected in series.
The present invention will be described in detail with reference to specific embodiments and drawings.
Example 1
The topology of the variable structure LLC converter of the present invention is shown in FIG. 1.
The primary side of the full-bridge inverter circuit is formed by a full-bridge inverter circuit, a switching device Q1,Q2 is connected to two ends of an input voltage in series, namely, the drain electrode of Q1 is connected with the positive electrode of a direct current voltage, the source electrode of Q2 is connected with the negative electrode of the direct current voltage, the source electrode of Q1 is connected with the drain electrode of Q2, and the connection modes of Q3,Q4 and Q1,Q2 are consistent. The middle points of the two bridge arms are connected with the transformer through a series resonant cavity, namely, the source electrode of the Q1 is connected with a resonant capacitor, the other end of the capacitor is connected with an inductor, the other end of the inductor is connected with a primary side winding of the transformer, and the other end of the winding is connected with the source electrode of the Q3; the number of primary windings of the transformer is 1, but two secondary windings are arranged, and the two secondary windings respectively form two half-wave rectification with a diode d1,d2 and a capacitor C1,C2, and the specific connection mode is as follows: the homonymous end of the winding 1 is connected with the anode of d1, and the two ends of the capacitor C1 are respectively connected with the cathode of d1 and the other end of the transformer winding 1; similarly, the same-name end of the winding 2 is connected with the cathode of d2, and two ends of the capacitor C2 are respectively connected with the anode of d2 and the other end of the transformer winding 2; the diode d3,d4 is connected in series with the capacitor C1,C2 respectively, namely the cathode of d3 is connected with C1, while the anode is connected with the anode of d2, the anode of d4 is connected with C2, while the cathode is connected with the cathode of d1; finally, the drain of the switch tube S5 is connected to the anode of the diode d4, and the source is connected to the cathode of the diode d3.
In this embodiment, specific parameters of the converter are shown in table 1:
table 1 transducer specific parameters
The converter is simulated by MATLAB/Simulink, and the output voltage of the converter is verified to have 2 times voltage regulating capability.
When the duty cycle of the secondary side switching tube S5 is 0.3, the converter steady-state waveform is as shown in fig. 6. Wherein vAB represents the primary side bridge voltage, ir represents the resonant current, id1,id2 represents the current flowing through the diode d1,d2, vc1,vc2 represents the voltage across the capacitor C1,C2, vs represents the output voltage before the output inductor, io represents the output current, vin represents the input voltage, and vo represents the output voltage. The on duty ratio of the regulator S5 is changed from 0 to 1 in 2ms, the dynamic waveform of the converter is shown in figure 7, and the output voltage is also changed from 100V to 200V, so that the converter has the function of double voltage regulation.
The efficiency curves of the converter under different voltage gains are calculated through simulation, as shown in fig. 8, and the efficiency of the converter is more than 97.5%, wherein only the loss of the switching device is considered.
Example 2
Another variable structure LLC converter topology of the invention is shown in fig. 2.
The primary side structure is exactly the same as that of invention 1. The secondary side is firstly a diode uncontrolled rectification with a variable structure, a diode d1,d2 is connected in series on an output direct current bus, namely, the cathode of d1 is connected with a positive direct current bus, the anode of d2 is connected with a negative direct current bus, and the anode of d1 is connected with the cathode of d2 and is connected with the secondary winding of the transformer; the drain electrode of the switch tube S5 is connected with the source electrode of the S6 and the other end of the secondary winding; the cathode of the diode d3 is connected with the output capacitor C1 on the output direct current bus, the anode of d3 is connected with the cathode of the direct current bus, C1 is connected with the positive direct current bus, in contrast, the anode of the diode d4 is connected with the output capacitor C2 and connected on the output direct current bus in parallel, the cathode of d4 is connected with the positive direct current bus, and C2 is connected with the negative direct current bus; finally, the source of the switching tube S5 is connected to the cathode of d3, and the drain of S6 is connected to the anode of d4.
In this embodiment, the specific parameters of the converter are also as shown in table 1:
the converter was simulated by MATLAB/Simulink, and the steady-state simulation waveform was as shown in fig. 9 when the duty cycle of the secondary side switching tube S5,S6 of the converter was 0.75. When the duty ratio of S5,S6 is increased from 0.5 to 1 in 2ms, the dynamic waveform is as shown in figure 10, the output voltage is also increased from 100V, and finally the output voltage of the converter is 200V in a steady state, which proves that the converter can realize double voltage regulation. And the efficiency curves of the converter at different voltage gains are calculated through simulation as in fig. 11, wherein only the losses of the switching devices are considered.
The foregoing description of the preferred embodiments of the invention is not intended to be limiting, but rather is intended to cover all modifications, equivalents, and alternatives falling within the spirit and principles of the invention.