Disclosure of Invention
Accordingly, the invention and the technical problem solved by the invention are to provide a control method and a device of a flyback converter and the flyback converter, which can reduce the voltage stress of a main switching tube, recover leakage inductance energy, improve efficiency, and have simple structure and low cost.
The invention aims to solve the technical problems that the flyback transformer leakage inductance energy clamping circuit comprises a clamping tube and a clamping capacitor, wherein the clamping tube and the clamping capacitor are connected in series, the positions of two serial devices can be exchanged, the serial devices are connected with a primary winding of a transformer in parallel after being connected in series, the clamping tube provides a channel for absorbing and releasing leakage inductance energy, and the clamping capacitor is a medium for temporarily storing the leakage inductance energy. The clamp tube and the main power switch tube are in non-complementary conduction control, so that the clamp circuit can be suitable for continuous, intermittent and critical working mode flyback circuits, the problems of excessively complex control and high cost of the clamp circuit of the existing flyback transformer are solved, the low cost and high efficiency in high-frequency application occasions can be considered, and the universality and the practicability are stronger. Specifically, the clamp tube is turned on for a short fixed time only after the main power switching tube is turned off, and the time is far less than the turn-off time of the main tube. In the period, the leakage inductance is demagnetized firstly to release energy into the clamping capacitor, and after the leakage inductance is demagnetized, the clamping capacitor reversely performs excitation action on the leakage inductance again, and the energy stored by the clamping capacitor in the excitation period is fed back into the output capacitor through the transformer.
As a first aspect of the present invention, one embodiment of a control method is provided as follows:
a control method is applied to a flyback converter, the flyback converter comprises a primary side circuit, a secondary side circuit, a transformer and a control device, the primary side circuit comprises a main power switching tube, a clamping capacitor and a primary winding of the transformer, the secondary side circuit comprises a rectifying diode and a secondary winding of the transformer, the control device comprises a feedback circuit, a primary side controller and a driver, the control device is configured to execute the control method to control the opening and the closing of the main power switching tube and the clamping tube, the main power switching tube and the clamping tube are controlled to be closed in an initial state of each working cycle of the flyback converter, and the control method comprises the following steps in each working cycle of the flyback converter:
The feedback circuit obtains a first signal representing the magnitude of the output voltage of the flyback converter and outputs the first signal to the primary side controller;
The primary side controller generates a first pulse signal according to the first signal and the second signal, and turns on the main power switch Guan Da for a first duration time so that the primary winding stores energy;
After the first duration, turning off the main power switching tube for a first pause time, such that during the first pause time, energy in the primary winding is released via the secondary winding when the rectifying diode is on, leakage inductance energy of the primary winding being transferred to the clamping capacitor when a body diode of the clamping tube is on;
After the first pause time, the driver turns on the clamping tube for a second duration according to the second pulse signal, so that leakage inductance energy stored in the clamping capacitor is transferred to the secondary side circuit through the transformer and then is output;
after the second duration, the clamp Guan Da is turned off for a second pause time.
Further, the second duration is a fixed time.
Further, the second duration is a few hundred nanoseconds.
As a first aspect of the present invention, another embodiment of the control method is provided as follows:
A control method is applied to a flyback converter, the flyback converter comprises a primary side circuit, a secondary side circuit, a transformer and a control device, the primary side circuit comprises a main power switching tube, a clamping capacitor and a primary winding of the transformer, the secondary side circuit comprises a synchronous rectifying tube and a secondary winding of the transformer, the control device comprises a feedback circuit, a primary side controller, a secondary side controller, an isolation circuit and a driver, the control device is configured to execute the control method to control the opening and the closing of the main power switching tube, the clamping tube and the synchronous rectifying tube, the main power switching tube, the clamping tube and the synchronous rectifying tube are controlled to be closed in an initial state of each working cycle of the flyback converter, and the control method comprises the following steps of:
The feedback circuit obtains a third signal representing the output voltage of the flyback converter and outputs the third signal to the primary side controller, and the primary side controller obtains a fourth signal representing the flowing current of the source electrode of the main power switching tube;
the primary side controller generates a third pulse signal according to the third signal and the fourth signal, and turns on the main power switching tube for a third duration time so that the primary winding stores energy;
After the third duration, turning off the main power switching tube for a third pause time;
After the third pause time, the secondary side controller generates a fourth pulse signal according to the fifth signal, turns on the synchronous rectifying tube for a fourth duration time, so that energy in the primary winding is released through the secondary winding when the synchronous rectifying tube is conducted, and leakage inductance energy of the primary winding is transferred to the clamping capacitor when a body diode of the clamping tube is conducted during the fourth duration time;
after the fourth duration, the driver turns on the clamping tube for the fifth duration according to the fifth pulse signal, so that leakage inductance energy stored in the clamping capacitor is transferred to the secondary side circuit through the transformer and then is output;
after the fifth duration, the clamp Guan Da is turned off for a fourth dwell time.
Further, the secondary side controller enables the secondary side controller to acquire a fifth signal representing the drain-source voltage of the synchronous rectifier by acquiring an enabling signal, and enables the secondary side controller to generate a fifth pulse signal based on a falling edge signal of the fourth pulse signal.
Further, the enable signal is generated a period of time before the rising edge of the third pulse signal.
Further, when the fifth signal reaches a first threshold value, the fourth pulse signal is generated, the synchronous rectifying tube is turned on, the fourth duration starts, when the fifth signal reaches a second threshold value, the fourth pulse signal is stopped, the synchronous rectifying tube is turned off, the fourth duration ends, the first threshold value and the second threshold value are negative threshold values, and the absolute value of the first threshold value is larger than the absolute value of the second threshold value.
Further, the fifth duration is a fixed time.
Further, the fifth duration is a few hundred nanoseconds.
As a second aspect of the present invention, one embodiment of the control device provided is as follows:
A control device is applied to a flyback converter, the flyback converter comprises a primary side circuit, a secondary side circuit, a transformer and the control device, the primary side circuit comprises a main power switching tube, a clamping capacitor and a primary winding of the transformer, the secondary side circuit comprises a rectifying diode and a secondary winding of the transformer, the control device comprises a feedback circuit, a primary side controller and a driver, the control device is configured to control the on and off of the main power switching tube and the clamping tube, the main power switching tube and the clamping tube are controlled to be turned off in the initial state of each working period of the flyback converter, and the control device performs the following actions in each working period of the flyback converter:
The feedback circuit obtains a first signal representing the magnitude of the output voltage of the flyback converter and outputs the first signal to the primary side controller;
The primary side controller generates a first pulse signal according to the first signal and the second signal, and turns on the main power switch Guan Da for a first duration time so that the primary winding stores energy;
After the first duration, turning off the main power switching tube for a first pause time, such that during the first pause time, energy in the primary winding is released via the secondary winding when the rectifying diode is on, leakage inductance energy of the primary winding being transferred to the clamping capacitor when a body diode of the clamping tube is on;
After the first pause time, the driver turns on the clamping tube for a second duration according to the second pulse signal, so that leakage inductance energy stored in the clamping capacitor is transferred to the secondary side circuit through the transformer and then is output;
after the second duration, the clamp Guan Da is turned off for a second pause time.
Further, the second duration is a fixed time.
Further, the second duration is a few hundred nanoseconds.
As a second aspect of the present invention, another embodiment of the control device is provided as follows:
A control device is applied to a flyback converter, the flyback converter comprises a primary side circuit, a secondary side circuit, a transformer and a control device, the primary side circuit comprises a main power switching tube, a clamping capacitor and a primary winding of the transformer, the secondary side circuit comprises a synchronous rectifying tube and a secondary winding of the transformer, the control device comprises a feedback circuit, a primary side controller, a secondary side controller, an isolation circuit and a driver, the control device is configured to control the on and off of the main power switching tube and the clamping tube, the main power switching tube and the clamping tube are controlled to be turned off in an initial state of each working period of the flyback converter, and the control device performs the following actions in each working period of the flyback converter:
The feedback circuit obtains a third signal representing the output voltage of the flyback converter and outputs the third signal to the primary side controller, and the primary side controller obtains a fourth signal representing the flowing current of the source electrode of the main power switching tube;
the primary side controller generates a third pulse signal according to the third signal and the fourth signal, and turns on the main power switching tube for a third duration time so that the primary winding stores energy;
After the third duration, turning off the main power switching tube for a third pause time;
After the third pause time, the secondary side controller generates a fourth pulse signal according to the fifth signal, turns on the synchronous rectifying tube for a fourth duration time, so that energy in the primary winding is released through the secondary winding when the synchronous rectifying tube is conducted, and leakage inductance energy of the primary winding is transferred to the clamping capacitor when a body diode of the clamping tube is conducted during the fourth duration time;
after the fourth duration, the driver turns on the clamping tube for the fifth duration according to the fifth pulse signal, so that leakage inductance energy stored in the clamping capacitor is transferred to the secondary side circuit through the transformer and then is output;
after the fifth duration, the clamp Guan Da is turned off for a fourth dwell time.
Further, the secondary side controller enables the secondary side controller to acquire a fifth signal representing the drain-source voltage of the synchronous rectifier by acquiring an enabling signal, and enables the secondary side controller to generate a fifth pulse signal based on a falling edge signal of the fourth pulse signal.
Further, the enable signal is generated a period of time before the rising edge of the third pulse signal.
Further, when the fifth signal reaches a first threshold value, the fourth pulse signal is generated, the synchronous rectifying tube is turned on, the fourth duration starts, when the fifth signal reaches a second threshold value, the fourth pulse signal is stopped, the synchronous rectifying tube is turned off, the fourth duration ends, the first threshold value and the second threshold value are negative threshold values, and the absolute value of the first threshold value is larger than the absolute value of the second threshold value.
Further, the fifth duration is a fixed time.
Further, the fifth duration is a few hundred nanoseconds.
As a third aspect of the present invention, an embodiment of a flyback converter is provided as follows:
A flyback converter comprises a primary side circuit, a secondary side circuit, a transformer and a control device, wherein the primary side circuit comprises a main power switching tube, a clamping capacitor and a primary winding of the transformer, the secondary side circuit comprises a rectifying diode or a synchronous rectifying tube and a secondary winding of the transformer, the control device is any one embodiment of the control device when the secondary side circuit comprises the rectifying diode and the secondary winding of the transformer, and the control device is any one embodiment of the control device when the secondary side circuit comprises the synchronous rectifying tube and the secondary winding of the transformer. According to the control method and the control device, the clamp tube is conducted for a period of time after the main tube is turned off, leakage inductance energy is fed back to the main power circuit, and the turn-off time is before the excitation inductance demagnetization is finished or the main tube is turned on again. In particular, compared with the prior art, the invention has at least the following technical advantages:
1. The clamping circuit is simple, the reliability is high, meanwhile, the clamping capacitance is small, and the clamping tube can select a switching tube with the characteristic of small current specification, so that the flyback converter can realize the effects of volume reduction and cost reduction;
2. The clamping circuit suppresses voltage spikes when the switching tube is turned off, and simultaneously recovers part of leakage inductance energy, so that the efficiency of the flyback converter is improved, and the EMI performance is improved;
3. the leakage inductance demagnetizing time is longer when the load is larger, so that the pulse width of the clamping tube can be adjusted based on load compensation, and the leakage inductance energy recovery effect is better realized.
4. The control method and the control device are suitable for flyback circuits in various working modes, and have stronger universality and practicability.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
Detailed Description
It should be noted that, without conflict, the embodiments of the present application and features of the embodiments may be combined with each other. The application will be described in detail below with reference to the drawings in connection with embodiments.
In order that those skilled in the art will better understand the present application, a technical solution in the embodiments of the present application will be clearly and completely described below with reference to the accompanying drawings in which it is apparent that the described embodiments are only some embodiments of the present application, not all embodiments. All other embodiments, which can be made by those skilled in the art based on the embodiments of the present application without making any inventive effort, shall fall within the scope of the present application.
It should be noted that the terms "first," "second," and the like in the description and the claims of the present application and the above figures are used for distinguishing between similar objects and not necessarily for describing a particular sequential or chronological order. It is to be understood that the data so used may be interchanged where appropriate in order to describe the embodiments of the application herein. Furthermore, the terms "comprises," "comprising," and "having," and any variations thereof, are intended to cover a non-exclusive inclusion, such that a process, method, system, article, or apparatus that comprises a list of steps or elements is not necessarily limited to those steps or elements expressly listed but may include other steps or elements not expressly listed or inherent to such process, method, article, or apparatus.
It should be understood that in the description and claims and in the drawings, when a step is described as being continued to another step, the step may be continued directly to the other step or through a third step, and when an element/unit is described as being "continued" to another element/unit, the element/unit may be "directly connected" to the other element/unit or "connected" to the other element/unit through a third element/unit.
Moreover, the drawings of the present disclosure are schematic representations of the present disclosure and are not necessarily drawn to scale. The same reference numerals in the drawings denote the same or similar parts, and thus a repetitive description thereof will be omitted. Some of the block diagrams shown in the figures are functional entities and do not necessarily correspond to physically or logically separate entities. The functional entities may be implemented in software or in one or more hardware modules or integrated circuits or in different networks and/or processor devices and/or micro-control devices.
First embodiment
Fig. 2 (a) is a schematic circuit diagram of a flyback converter applied to the control method according to the first embodiment of the present invention, where the flyback converter of fig. 2 (a) includes a primary side circuit, a secondary side circuit, a transformer T1 and a control device, the primary side circuit includes a main power switching tube Q1, a clamp tube Q2, a clamp capacitor Cr and a primary winding of the transformer T1, the secondary side circuit includes a rectifier diode D1 and a secondary winding of the transformer T1, the control device includes a feedback circuit 242, a primary side controller 240 and a driver 241, and the control device is configured to perform the control method according to the first embodiment of the present invention to control on and off of the main power switching tube Q1 and the clamp tube Q2, and the main power switching tube Q1 and the clamp tube Q2 are controlled to be turned off in an initial state of each working cycle of the flyback converter.
The control method of the embodiment includes the following steps in each working cycle of the flyback converter:
the feedback circuit 242 obtains a first signal representing the magnitude of the flyback converter output voltage Vo and outputs the first signal to the primary side controller 240, and the primary side controller 240 obtains a second signal representing the magnitude of the current flowing through the source electrode of the main power switching tube Q1;
The primary side controller 240 generates a first pulse signal SW1 according to the first signal and the second signal, and turns on the main power switching tube Q1 for a first duration so that the primary winding stores energy;
After a first duration, the main power switching tube Q1 is turned off for a first pause time, so that during the first pause time, energy in the primary winding is released via the secondary winding when the rectifying diode D1 is turned on, and leakage inductance energy of the primary winding is transferred to the clamping capacitor when the body diode of the clamping tube Q2 is turned on;
after the first pause time, the driver 241 turns on the clamp Q2 for a second duration according to the second pulse signal SW2, so that the leakage inductance energy stored in the clamp capacitor Cr is transferred to the secondary side circuit through the transformer T1 and then output;
after the second duration, clamp Q2 is turned off for a second pause time.
The following innovative process for this embodiment is described as follows:
The inventor of the present application found through researches that, when the drain-source voltage of the primary side circuit of the flyback converter with an active clamping function reaches n×vout+vin (where N is the turn ratio of the primary winding and the secondary winding of the transformer, vout is the output voltage of the flyback converter, vin is the input voltage of the flyback converter) due to the existence of the clamping capacitor, the body diode of the clamping switch is turned on, the energy of the leakage inductance is transferred to the clamping capacitor, and finally the drain-source voltage of the primary side circuit reaches n×vout+vin+Δvc (Δvc is the voltage increment value of the clamping capacitor after the energy of the leakage inductance is transferred to the clamping capacitor in each period), since the clamping capacitor stores energy in each period, in order to avoid the breakdown risk of the drain-source voltage of the primary power switch caused by the period-by-period accumulation of the energy of the leakage inductance, the leakage inductance needs to be released in each period.
In the control method in the prior art of fig. 1 (b), during each working period of the flyback converter, the clamp tube assumes two tasks when being conducted, namely, one is to absorb the voltage peak of the drain and source of the main power switch tube and recover the leakage inductance energy to the output end, and the other is to realize zero-voltage conduction of the main power switch tube. Therefore, the conduction time of the clamping tube is designed after the demagnetization of the secondary side winding is finished, so that the voltage on the clamping capacitor carries out reverse excitation on the primary side winding, and zero-voltage switching-on of the main power switching tube is realized. Because the leakage inductance energy is small, when the clamping tube is conducted, the voltage of the clamping capacitor can be quickly reduced, and the primary side winding cannot be continuously and reversely excited by stable voltage. Therefore, the control method of the prior art needs to store more energy with a larger clamp capacitor to maintain a stable clamp capacitor voltage, so that the negative current slope of the primary winding is stable. When the capacitance C of the clamp capacitor is large, the charge current i=c×dv/dt of the clamp capacitor is large, so that a clamp switching tube with a higher current level is required.
In the control method of the prior art shown in fig. 1 (c), in each working period of the flyback converter, the clamp switching tube is conducted to absorb the peak voltage of the drain and source of the main power switching tube and recover the leakage inductance energy to the output end, and the conduction time is proportional to the demagnetizing time, so that the conduction time is long, the effective value current flowing through the clamp switching tube is large, the loss of the clamp loop is large, the recovery effect of the leakage inductance energy is poor, and the efficiency cannot be improved well.
Considering that the value of the switch tube for realizing zero-voltage conduction is not great in a low-power occasion, and the low-power occasion pursues lower cost, a simpler control method is needed, so that the voltage stress of the main power switch tube can be reduced, leakage inductance energy can be recovered, and the efficiency is improved.
Therefore, the control method of the embodiment is proposed, the clamping tube is turned on after the energy in the primary winding is released through the secondary winding and the leakage inductance energy of the primary winding is transferred to the clamping capacitor, namely, the clamping tube is turned on before the demagnetization of the secondary winding is finished, the leakage inductance energy stored in the clamping capacitor is released to the output end of the secondary side circuit, at the moment, the leakage inductance energy is not required to be used for realizing ZVS of the main power switching tube, so that the value of the clamping capacitor only meets the requirement of absorbing the voltage peak of the leakage source electrode of the main power switching tube, the clamping capacitor with larger capacity is not required, and the smaller clamping capacitor has smaller charging and discharging current i=C, dV/dt, so that the clamping tube can select a switching tube with small current specification, and the cost and the volume of the flyback converter are reduced.
It can be seen that although the present embodiment cannot achieve zero-voltage conduction of the main power switch tube, there are great advantages in terms of cost and volume, and the present embodiment is more competitive in the low-power situation than the prior art.
The output power range of the flyback converter is preferably less than 150W, and it should be noted that the power range does not limit the protection scope of the present application, and those skilled in the art may choose whether to use the solutions of this embodiment and other embodiments of the present application according to actual needs.
Fig. 2 (b) is a waveform diagram of the operation of the flyback converter applied in the control method according to the first embodiment of the present invention, and the operation of the flyback converter of fig. 2 (a) is described below with reference to fig. 2 (b):
The first stage (t 0-t 1) is that the primary side controller 240 provides the first pulse signal SW1 to turn on the main power switch tube Q1 to work, the primary side inductive current IL_p flows in the primary winding along the forward direction;
The second stage (T1-T2) is that the main power switch tube Q1 is turned off, the primary side inductance current IL_p rapidly charges the drain-source capacitance Cds of the main power switch tube Q1 to enable the drain-source voltage to rise, when rising to vin+N+Vout, the secondary side rectifying diode D1 is conducted, the transformer T1 starts demagnetizing, the drain-source voltage of the main power switch tube Q1 continues to rise due to the existence of the drain inductance Lk until rising to N+Vout+Vin+DeltaVc (DeltaVc is the voltage increment value of the clamp capacitance after the drain inductance energy of each period is transferred to the clamp capacitance), the body diode of the clamp tube Q2 is conducted to provide a low impedance loop for the drain inductance energy, and the drain inductance energy is stored in the clamp capacitance Cr;
And a third stage (T2-T3) in which the leakage inductance energy is still transferred to the clamp capacitor, and the driver 241 provides a second pulse signal SW3 to turn on the clamp tube Q2, so as to provide a lower impedance loop for the leakage inductance energy, and store the residual leakage inductance energy into the clamp capacitor, and the leakage inductance energy is completely transferred, i.e. the leakage inductance has been demagnetized, and then the clamp capacitor Cr and the leakage inductance resonate, so as to reversely excite the leakage inductance Lk, the excitation voltage is Vc-n×vo, and the leakage inductance energy stored in the clamp capacitor is fed back to the output end through the transformer, so as to realize recovery of the leakage inductance energy, wherein the conduction time of the clamp tube Q2 is a fixed time T1.
The clamping tube Q2 is preferably an enhanced N-channel MOS tube, and a switching tube with smaller current specification and smaller package can be selected compared with the prior art.
The capacitance of the clamp capacitor Cr is smaller than that of the prior art, and is generally between 1 nF and 20 nF.
Preferably, the second duration (i.e. the on time T1 of the clamp Q2) of the present embodiment may be a fixed pulse width, generally hundreds of nanoseconds, so that the leakage inductance energy stored in the clamp capacitor can be transferred to the output end within a resonance period of the leakage inductance Lk and the clamp capacitor Cr, and the current of the resonant circuit and the loss of the leakage inductance energy during the transfer process are reduced as much as possible.
Preferably, the second duration (i.e. the on time T1 of the clamp Q2) of the present embodiment is a pulse width varying with the load, and the larger the load is, the longer the leakage inductance demagnetizing time is, so that the on time of the clamp Q2 can be compensated for the pulse width based on the load, so as to better realize the leakage inductance energy recovery.
Second embodiment
Fig. 3 (a) is a schematic circuit diagram of a flyback converter applied to the control method according to the second embodiment of the present invention, please refer to fig. 3 (a), wherein the flyback converter includes a primary side circuit, a secondary side circuit, a transformer T1 and a control device, the primary side circuit includes a main power switching tube Q1, a clamping tube Q2, a clamping capacitor Cr and a primary winding of the transformer T1, the secondary side circuit includes a synchronous rectifying tube Q3 and a secondary winding of the transformer T1, the control device includes a feedback circuit 242, a primary side controller 240, a secondary side controller 243, an isolation circuit 244 and a driver 241, the control device is configured to execute the control method according to the present embodiment to control the on and off of the main power switching tube Q1, the clamping tube Q2 and the synchronous rectifying tube Q3, and the main power switching tube Q1, the clamping tube Q2 and the synchronous rectifying tube Q3 are controlled to be turned off in an initial state of each working cycle of the flyback converter.
The control method of the embodiment includes the following steps in each working cycle of the flyback converter:
The feedback circuit 242 obtains a third signal representing the magnitude of the flyback converter output voltage Vo and outputs the third signal to the primary side controller 240, the primary side controller 240 obtains a fourth signal representing the magnitude of the current flowing through the source electrode of the main power switching tube Q1, and the secondary side controller 243 obtains a fifth signal representing the drain-source voltage of the synchronous rectifying tube Q3;
the primary side controller 240 generates a third pulse signal SW1 according to the third signal and the fourth signal, and turns on the main power switching tube Q1 for a third duration time, so that the primary winding stores energy;
After a third duration, turning off the main power switching transistor Q1 for a third pause time;
after the third pause time, the secondary side controller 243 generates a fourth pulse signal SR according to the fifth signal, turns on the synchronous rectifier Q3 for a fourth duration, so that energy in the primary winding is released via the secondary winding when the synchronous rectifier Q3 is turned on, and so that leakage inductance energy of the primary winding is transferred to the clamp capacitor Cr when the body diode of the clamp transistor Q2 is turned on during the fourth duration;
After the fourth duration, the driver 241 turns on the clamp Q2 for a fifth duration according to the fifth pulse signal, so that the leakage inductance energy stored in the clamp capacitor Cr is transferred to the secondary side circuit through the transformer T1 and then output;
after the fifth duration, clamp Q2 is turned off for a fourth pause time.
The control method of the present embodiment is the same as the control method of the first embodiment, and is different in that the secondary side circuit of the flyback converter applied in the present embodiment adopts a synchronous rectification scheme, so that the efficiency of the flyback converter can be further improved, but the synchronous rectification tube also needs to be controlled, so that the control method is more complex than the first embodiment, and a person skilled in the art can select whether the secondary side circuit adopts diode rectification or synchronous rectification tube rectification according to the actual situation.
Fig. 3 (b) is a waveform diagram of the operation of the flyback converter applied in the control method according to the second embodiment of the present invention, and the operation of the flyback converter of fig. 3 (a) is described below with reference to fig. 3 (b):
The first stage (t 0-t 1) is that the primary side controller 240 provides the first pulse signal SW1 to turn on the main power switch tube Q1 to work, the primary side inductive current IL_p flows in the primary winding along the forward direction;
The second stage (T1-T2) is that the main power switch tube Q1 is turned off, the primary side inductance current IL_p rapidly charges the drain-source capacitance Cds of the main power switch tube Q1 to enable the drain-source voltage to rise, when rising to vin+N+Vout, the secondary side controller 243 gives out a third pulse signal SR to control the synchronous rectifier tube Q3 to be conducted, the transformer T1 starts to demagnetize, due to the existence of leakage inductance Lk, the leakage inductance energy enables the drain-source voltage of the main power switch tube Q1 to continue rising until rising to N+Vout+Vin+DeltaVc (DeltaVc is the voltage increasing value of the clamp capacitance after transferring the leakage inductance energy to the clamp capacitance in each period), the body diode of the clamp tube Q2 is conducted to provide a low impedance loop for the leakage inductance energy, and most of the leakage inductance energy is stored in the clamp capacitance Cr;
And a third stage (T2-T3) in which the leakage inductance is demagnetized, the energy is stored in the clamp capacitor Cr, the transformer T1 is in the demagnetizing stage until the time T3, the secondary side controller 243 stops providing the third pulse signal SR when the transformer T1 is about to demagnetize, the synchronous rectifier Q3 is turned off, a falling edge signal is generated when the third pulse signal SR is stopped being provided, the falling edge signal is processed by the secondary side controller 243 and transmitted to the driver 241 through the isolation circuit to generate the second pulse signal SW2, the clamp transistor Q2 is controlled to be turned on for a period of time T1, and the transmission of the leakage inductance energy stored in the clamp capacitor Cr to the secondary side output end is completed during the period.
Further, the secondary side controller 243 in this embodiment enables the secondary side controller 243 to obtain a fifth signal representing the drain-source voltage of the synchronous rectifier Q3 by obtaining the enable signal, and enables the secondary side controller 243 to generate the fifth pulse signal SW2 based on the falling edge signal of the fourth pulse signal SR.
Further, the enable signal is generated a period of time before the rising edge of the third pulse signal SW1, so that the fifth pulse signal SW2 is guaranteed to be forcibly turned off when being continued to the next enable signal in the current continuous mode, and the phenomenon that the main power switch tube and the synchronous rectifier tube are damaged together due to the fact that the enable signal and the third pulse signal SW1 exist simultaneously is avoided.
Further, when the fifth signal reaches the first threshold, the fourth pulse signal SR is generated, the synchronous rectifying tube Q3 is turned on, the fourth duration starts, when the fifth signal reaches the second threshold, the fourth pulse signal SR is stopped, the synchronous rectifying tube Q3 is turned off, the fourth duration ends, the first threshold and the second threshold are negative thresholds, the absolute value of the first threshold is larger than the absolute value of the second threshold, and therefore the synchronous rectifying tube is turned off before demagnetization ends, and conditions are created for the generation of the fifth pulse signal SW 2.
Likewise, in this embodiment, the clamp Q2 is preferably an enhanced N-channel MOS transistor, and a switching transistor with smaller current specification and smaller package can be selected as compared with the prior art.
Similarly, the capacitance of the clamp capacitor Cr in this embodiment is smaller than that of the prior art, and is generally between 1 nF and 20 nF.
Preferably, the isolation circuit may be capacitive isolation or other isolation means.
Preferably, the end time of the fifth duration (i.e., the off time of the clamp Q2) is near the time when the secondary inductor current il_s drops to 0, so as to avoid the problem of loss caused by resonance of the exciting inductor and the clamp capacitor after the demagnetization is completed.
Preferably, the fifth duration (i.e. the on time T1 of the clamp Q2) of the present embodiment may be a fixed pulse width, generally hundreds of nanoseconds, so that the leakage inductance energy stored in the clamp capacitor can be transferred to the output end within the resonance period of the leakage inductance Lk and the clamp capacitor Cr, thereby minimizing the effective value current of the resonant circuit and reducing the loss in the leakage inductance energy transfer process.
Third embodiment
The present embodiment provides a first control device for use in the flyback converter of fig. 2 (a), the control device comprising a feedback circuit 242, a primary side controller 240 and a driver 241, the control device of the present embodiment being configured to perform any one of the embodiments of the control method of the first embodiment in each duty cycle of the flyback converter of fig. 2 (a).
The beneficial effects of each specific implementation of the embodiment are identical to those of the corresponding implementation of the first embodiment, and are not repeated here.
Fourth embodiment
This embodiment provides a second control device for use in the flyback converter of fig. 3 (a), the control device comprising a feedback circuit 242, a primary side controller 240, a secondary side controller 243, an isolation circuit 244 and a driver 241, the control device of this embodiment being configured to perform any particular implementation of the control method of the second embodiment in each operating cycle of the flyback converter of fig. 3 (a).
The beneficial effects of each specific implementation of the embodiment are consistent with those of the corresponding implementation of the second embodiment, and are not repeated here.
Fifth embodiment
The embodiment provides a flyback converter which comprises a primary side circuit, a secondary side circuit, a transformer and a control device, wherein the primary side circuit comprises a primary power switching tube, a clamping capacitor and a primary winding of the transformer, the secondary side circuit comprises a rectifying diode or a synchronous rectifying tube and a secondary winding of the transformer, the control device is any specific implementation mode of the third control device when the secondary side circuit comprises the rectifying diode and the secondary winding of the transformer, and the control device is any specific implementation mode of the control device of the fourth embodiment when the primary side circuit comprises the synchronous rectifying tube and the secondary winding of the transformer.
The beneficial effects of each specific implementation manner of this embodiment are indirectly consistent with the beneficial effects of the corresponding implementation manners of the first embodiment and the second embodiment, and are not described herein.
In light of the foregoing, it will be evident to those skilled in the art that various modifications, substitutions and alterations can be made hereto without departing from the inventive concept as defined by the appended claims.