CN115021577A

CN115021577A - Control method, control device and flyback converter

Info

Publication number: CN115021577A
Application number: CN202210717937.5A
Authority: CN
Inventors: 邓世源; 李健
Original assignee: Mornsun Guangzhou Science and Technology Ltd
Current assignee: Mornsun Guangzhou Science and Technology Ltd
Priority date: 2022-06-23
Filing date: 2022-06-23
Publication date: 2022-09-06

Abstract

The invention discloses a control method, a control device and a flyback converter, wherein the control method comprises the following steps in each working period in sequence: switching on the main power switch tube for a first duration so that the primary winding stores energy; switching on the rectifier for a second duration such that energy stored in the primary winding is released via the secondary winding while transferring leakage inductance energy of the primary winding to the clamping capacitor; switching on the clamping switch tube for a third duration time, so that the leakage inductance energy stored in the clamping capacitor is transferred to the secondary side circuit through the transformer and then is output; turning on the rectifier tube for a fourth duration again to enable the main power switch tube to be conducted before the negative current reaches zero; wherein there is a time interval between the first duration and the second duration, the fourth duration and the third duration, the fourth duration and the first duration of the next duty cycle. The invention can reduce the capacitance value of the clamping capacitor, thereby reducing the cost of the clamping switch tube.

Description

Control method, control device and flyback converter

Technical Field

The invention relates to the field of switching converters, in particular to a control method, a control device and a flyback converter.

Background

In the field of medium and small power switching power supplies, a flyback converter is the most popular circuit topology, but with the development of high frequency and small size of a switching power supply, the loss of the flyback converter is more and more emphasized, especially the switching loss of a primary side power switching tube, and the leakage source of the primary side power switching tube needs to bear a voltage peak due to the existence of leakage inductance of the flyback converter, so that a corresponding clamping circuit needs to be added in the flyback converter to limit the voltage peak of the primary side power switching tube, a common clamping circuit is an RCD clamping circuit, however, clamping circuits such as an RCD clamping circuit, an LCD clamping circuit and the like are lossy absorption, which further causes the performance reduction of the flyback converter, and hinders the high frequency development of the flyback converter.

In order to further improve the operating frequency of the flyback converter and reduce the switching loss, a secondary active clamp flyback converter is proposed in the art, fig. 1a is a schematic diagram of a conventional secondary active clamp flyback converter circuit, and fig. 1b is a timing diagram of the secondary active clamp flyback converter of fig. 1 a. The secondary active clamping flyback converter generates reverse current on the secondary side by prolonging the conduction time of the secondary side synchronous rectifier tube after the demagnetization of the transformer is finished, and the primary side can also generate negative current after the secondary side synchronous rectifier tube is closed, so that zero voltage switching-on (ZVS) of the primary side power switching tube can be realized.

Although the secondary active clamping flyback converter can realize zero voltage switching-on of the primary side power switch tube, the primary side power switch tube still needs the RCD clamping circuit to limit the voltage peak of the drain electrode and the source electrode of the primary side power switch tube, the control method is complex, frequency conversion control is needed to realize zero voltage switching-on of the primary side power switch tube in a wide input voltage range and a load range, and higher frequency is needed in light load at high voltage, so that the iron loss of a transformer and the clamping loss of the RCD are increased.

If the primary active clamp flyback converter appears, fig. 2a is a schematic diagram of a conventional primary active clamp flyback converter circuit, and fig. 2b is a timing diagram of the primary active clamp flyback converter of fig. 2 a. The primary active clamping flyback converter realizes zero voltage switching-on of a main power switch tube by switching on a clamping switch tube for a period of time before the main power switch tube is switched on, and limits a drain-source voltage peak of the main power switch tube through a clamping capacitor, thereby saving RCD clamping.

Disclosure of Invention

Therefore, the technical problem to be solved by the present invention is to provide a control method, a control device and a flyback converter, which can not only realize the zero voltage switching of the main tube, but also recover the leakage inductance energy, and the present invention can reduce the capacitance value of the clamp capacitor, thereby reducing the cost of the clamp switch tube.

As a first aspect of the present invention, there is provided an embodiment of a control method as follows:

a control method is applied to a flyback converter, and 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 switch tube, a clamping capacitor and a primary winding of the transformer; the secondary side circuit comprises a rectifier tube and a secondary winding of the transformer; the control device comprises a primary side controller, a secondary side controller, an isolation circuit and control logic; the primary side controller is used for controlling the conduction and the disconnection of the main power switch tube and the clamping switch tube, the secondary side controller is used for controlling the conduction and the disconnection of the rectifier tube, the isolation circuit is used for bidirectionally transmitting a synchronous signal between the primary side controller and the secondary side controller, and the control logic is used for executing the control method; the main power switch tube, the clamping switch tube and the rectifying tube are turned off at the initial state of each working period of the flyback converter; the control method comprises the following steps in each working period of the flyback converter in sequence:

turning on the main power switch for a first duration such that the primary winding stores energy;

switching on the rectifier for a second duration such that energy stored in the primary winding is released via the secondary winding while leakage energy from the primary winding is transferred to the clamping capacitor;

turning on the clamping switch tube for a third duration, so that the leakage inductance energy stored in the clamping capacitor is transferred to the secondary side circuit through the transformer and then is output;

turning on the rectifier tube for a fourth duration again to enable the main power switch tube to be conducted before negative current reaches zero;

wherein there is a first time interval between the first duration and the second duration, a second time interval between the fourth duration and the third duration, and a third time interval between the fourth duration and the first duration of the next duty cycle.

Further, the third duration is proportional to the output power of the flyback converter.

Further, the fourth duration begins when the junction capacitance voltage of the main power switch tube resonates to a peak, and the drain-source voltage of the rectifier tube resonates to a corresponding trough.

Further, the control method further includes, in each operating cycle: acquiring an input voltage signal representing the current input voltage of the flyback converter, comparing the input voltage signal with a first threshold, and determining whether to switch on the rectifier tube for a fourth duration again according to a comparison result, specifically:

when the input voltage signal is less than or equal to the first threshold value, the rectifier tube is not turned on again for a fourth duration;

and when the input voltage signal is greater than the first threshold value, turning on the rectifier tube for a fourth duration again.

Further, the determining whether to turn on the rectifier tube again for the fourth duration according to the comparison result is realized by whether the primary side controller transmits a synchronization signal to the secondary side controller, specifically:

when the input voltage signal is less than or equal to the first threshold, the primary side controller transmits a synchronization signal to the secondary side controller so that the secondary side controller does not turn on the rectifier tube again for a fourth duration;

when the input voltage signal is greater than the first threshold, the primary-side controller does not transmit a synchronization signal to the secondary-side controller, such that the secondary-side controller turns on the rectifier tube again for a fourth duration.

As a second aspect of the present invention, there is provided an embodiment of a control apparatus as follows:

a control device is applied to a flyback converter, and 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 switch tube, a clamping capacitor and a primary winding of the transformer; the secondary side circuit comprises a rectifier tube and a secondary winding of the transformer; the main power switch tube, the clamping switch tube and the rectifying tube are turned off at the initial state of each working period of the flyback converter; the control device includes:

a primary side controller configured to control the main power switch tube and the clamping switch tube to be switched on and off;

a secondary side controller configured to control on and off of the rectifier tube;

an isolation circuit configured to bidirectionally transfer a synchronization signal between the primary side controller and the secondary side controller;

and control logic configured to perform the following control actions in turn in each duty cycle of the flyback converter:

switching on the main power switch tube for a first duration so that the primary winding stores energy;

turning on the rectifier for a second duration such that energy stored in the primary winding is released via the secondary winding while leakage energy from the primary winding is transferred to the clamping capacitor;

Further, the control logic is configured wherein the third duration is proportional to the output power of the flyback converter.

Further, the control logic is configured wherein the fourth duration begins when the junction capacitance voltage of the main power switch resonates to a peak when the rectifier drain-source voltage resonates to a corresponding trough.

Further, the control logic is configured wherein the control method further comprises, in each duty cycle: acquiring an input voltage signal representing the current input voltage of the flyback converter, comparing the input voltage signal with a first threshold, and determining whether to switch on the rectifier tube for a fourth duration again according to a comparison result, specifically:

and when the input voltage signal is larger than the first threshold value, turning on the rectifying tube for a fourth duration again.

Further, the control logic is configured wherein the determining whether to reopen the rectifier tube for a fourth duration depending on the comparison result is achieved by whether the primary side controller passes a synchronization signal to the secondary side controller, in particular:

As a third aspect of the present invention, there is provided an embodiment of a flyback converter as follows:

a flyback converter comprising: a primary side circuit, a secondary side circuit, a transformer, and any of the above control devices; the primary side circuit comprises a main power switch tube, a clamping capacitor and a primary winding of the transformer; the secondary side circuit comprises a rectifier tube and a secondary winding of the transformer; the control device is used for controlling the conduction and the disconnection of the main power switch tube, the clamping switch tube and the rectifying tube; the main power switch tube, the clamping switch tube and the rectifying tube are turned off at the initial state of each working period of the flyback converter.

Compared with the prior art, the control method and the control device have the advantages that: the function of absorbing the voltage peak of the drain-source voltage of the main power switch tube and the function of realizing the zero-voltage conduction of the main power switch tube are completely decoupled, the problem of the voltage peak of the drain-source voltage of the main power switch tube in the flyback converter is solved, the influence of the loss of an RCD clamping circuit on the efficiency of the flyback converter is avoided, the capacitance value of a clamping capacitor in an active clamping circuit is smaller than that of a clamping capacitor in a primary active clamping flyback converter, the clamping switch tube with lower cost is convenient to select, meanwhile, the zero-voltage switching-on of the main power switch tube under the full-range input voltage can be realized, and the efficiency of the flyback converter is further improved.

Drawings

Fig. 1a is a schematic diagram of a conventional secondary active clamp flyback converter circuit;

fig. 1b is a timing diagram of the secondary active clamp flyback converter of fig. 1 a;

fig. 2a is a schematic diagram of a conventional primary active clamp flyback converter circuit;

fig. 2b is a timing diagram of the primary active clamp flyback converter of fig. 2 a;

fig. 3 is a schematic circuit diagram of a flyback converter to which the control method of the present invention is applied;

FIG. 4 is a timing chart of the first embodiment of the control method of the present invention;

FIG. 5 is a flowchart of a second embodiment of the control method of the present invention;

fig. 6 is a timing diagram of fig. 5 in the case where Vin is less than or equal to the first threshold.

Detailed Description

It should be noted that the embodiments and features of the embodiments in the present application may be combined with each other without conflict. The present application will be described in detail below with reference to the embodiments with reference to the attached drawings.

In order to make the technical solutions better understood by those skilled in the art, the technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application, and it is obvious that the described embodiments are only partial embodiments of the present application, but not all embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present application.

It should be noted that the terms "first," "second," and the like in the description and claims of this application and in the drawings described above are used for distinguishing between similar elements and not necessarily for describing a particular sequential or chronological order. It should be understood that the data so used may be interchanged under appropriate circumstances such that embodiments of the application described herein may be used. 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 specification, claims and drawings hereof, when a step is described as continuing to another step, that step may continue directly to that other step, or through a third step to that other step; when an element/unit is described as being "connected" to another element/unit, that element/unit may be "directly connected" to that other element/unit, or "connected" to that other element/unit through a third element/unit.

Furthermore, the drawings of the present disclosure are merely schematic representations, 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. These 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.

Fig. 3 is a schematic circuit diagram of a flyback converter to which the control method of the present invention is applied, please refer to fig. 3, in which the flyback converter includes a primary side circuit, a secondary side circuit, a transformer T1 and a control device; the primary side circuit comprises a main power switch tube S1, a clamping switch tube S3, a clamping capacitor C1 and a primary winding of a transformer; the secondary side circuit comprises a rectifier tube S2 and a secondary winding of the transformer; the control means comprises a primary side controller 220, a secondary side controller 211, an isolation circuit 221 and control logic; the primary side controller 220 is used for controlling the on and off of the main power switch tube S1 and the clamp switch tube S3, the secondary side controller 211 is used for controlling the on and off of the rectifier tube, the isolation circuit 221 is used for bidirectionally transmitting a synchronization signal SYNC between the primary side controller 220 and the secondary side controller 211, and a control logic (not shown in fig. 3) is used for executing the control method provided by the invention; the main power switch S1, the clamp switch S3 and the rectifier S2 are turned off at the initial state of each duty cycle of the flyback converter.

Fig. 3 also shows a feedback circuit 223, an output capacitor Co, and a secondary side detection circuit in the secondary side controller 211, which are related to the present invention, and the functions of these circuits are common knowledge of those skilled in the art, and therefore they are not described in detail.

The primary side controller 220 is configured to control the on and off of the main power switch tube S1 and the active clamp switch tube S3 according to an input voltage signal Vin of the flyback converter, a feedback signal FB, a drain-source voltage signal Vds-MP of the main power switch tube, and a synchronization signal SYNC; the secondary side controller 220 is configured to control the on and off of the rectifier tube S2 according to the rectifier tube drain-source voltage signal Vds-SR, and the secondary side controller 220 may be further configured to control the on and off of the rectifier tube S2 according to the synchronization signal SYNC according to actual conditions.

First embodiment

Referring to fig. 4, the timing diagram of the first embodiment of the control method of the present invention includes, in each duty cycle of the flyback converter, in sequence:

turning on the main power switch S1 for a first duration such that the primary winding stores energy;

switching on the rectifier S2 for a second duration such that the energy stored in the primary winding is released via the secondary winding while the leakage energy from the primary winding is transferred to the clamp C1;

the clamping switch tube S3 is switched on for a third duration, so that the leakage inductance energy stored in the clamping capacitor C1 is transferred to a secondary side circuit through the transformer T1 and then is output;

and turning on the rectifier tube S2 for a fourth duration again such that the main power switch tube S1 conducts before the negative current goes to zero;

The present embodiment requires setting the first time interval between the first duration and the second duration, setting the second time interval between the fourth duration and the third duration, and setting the third time interval between the fourth duration and the first duration of the next duty cycle, in order to avoid the main power switch S1, the clamp switch S3 and the rectifier S2 being turned on simultaneously.

The primary side controller 220 of the present embodiment is configured to control the on and off of the main power switch tube S1 and the active clamp switch tube S3 according to the input voltage signal Vin, the feedback signal FB, the drain-source voltage signal Vds-MP of the main power switch tube, and the synchronization signal SYNC of the flyback converter; the secondary side controller 220 is configured to control the turn-on and turn-off of the rectifier S2 based only on the rectifier drain-source voltage signal Vds-SR.

Compared with the existing primary active clamp flyback converter control method shown in fig. 2b, the control method of the embodiment can reduce the capacitance value of the clamp capacitor and reduce the cost of the clamp switch tube, and detailed analysis is as follows:

due to the existence of the clamping capacitor, when the drain-source voltage of the main power switch tube S1 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, and Vin is the input voltage of the flyback converter), the body diode of the clamping switch tube S3 is turned on, and the energy of the leakage inductance is transferred to the clamping capacitor, so as to prevent the drain-source voltage of the main power switch tube S1 from exceeding N × Vout + Vin.

In the control method in the prior art, the clamp switch tube is conducted once in each working period, and the conduction of the clamp switch tube not only needs to realize the function of absorbing the voltage peak of the drain-source electrode of the main power switch tube, but also needs to realize the function of zero-voltage conduction of the main power switch tube. The conduction time of the clamping switch tube is designed after the demagnetization of the secondary side winding is finished, so that the voltage on the clamping capacitor reversely excites the primary side winding, and the second function, namely zero voltage switching-on of the main power switch tube, is realized. Because the leakage inductance energy is small, when the clamping switch tube is switched on, the voltage of the clamping capacitor is quickly reduced, and the primary winding cannot be continuously reversely excited by stable voltage, therefore, the control method in the prior art needs to adopt a large clamping capacitor to store more energy so as to maintain the voltage at two ends of the clamping capacitor to be stable, and thus the negative current slope of the primary winding is stable. In addition, since the charging current i of the clamp capacitor is equal to C × dV/dt, when the capacitance value C of the clamp capacitor is large, the charging current becomes large, and thus a clamp switching tube with a higher current level is required.

In the control method of this embodiment, the method for absorbing the voltage spike of the drain-source voltage of the main power switch tube includes: switching on a clamping switch tube for a third duration before the secondary winding is demagnetized, and releasing leakage inductance energy stored in a clamping capacitor to the output end of the secondary side circuit; the method for realizing the zero voltage conduction of the main power switch tube S1 comprises the following steps: when the rectifier tube S2 is turned on again, the output voltage reversely excites the secondary winding, the rectifier tube S2 is turned off after a fourth duration, the polarity of the secondary winding is reversed after the secondary winding is turned off, and the primary winding generates a negative current which can completely evacuate charges stored in the junction capacitor between the drain and source electrodes of the main power switch tube S1, thereby realizing zero-voltage conduction of the main power switch tube S1. Therefore, in the embodiment, the function of absorbing the voltage spike of the drain-source voltage of the main power switch tube and the function of realizing the zero-voltage conduction of the main power switch tube are completely decoupled, the problem of the voltage spike of the drain-source voltage of the main power switch tube in the flyback converter can be solved, the influence of the loss of the RCD clamping circuit on the efficiency of the flyback converter is avoided, and the capacitance value of the clamping capacitor in the active clamping circuit is smaller than that of the clamping capacitor in the prior art, so that the clamping switch tube with lower cost can be selected conveniently, the zero-voltage switching-on of the main power switch tube under the full-range input voltage can be realized, and the efficiency of the flyback converter is further improved.

In the second duration, the leakage inductance current flowing through the primary winding charges the clamping capacitor C1 through the body diode of the clamping switch tube S3, so that the leakage inductance energy of the primary winding is transferred to the clamping capacitor C1, and in the process of transferring the leakage inductance energy of the primary winding, the voltage of the clamping capacitor C1 reaches the maximum when the current of the primary winding reaches zero, so that the maximum capacitance value of the clamping capacitor can be designed, and specifically, the maximum capacitance value of the clamping capacitor can be determined according to the maximum value of the input voltage and the maximum value of the output voltage of the flyback converter.

Preferably, the third duration is proportional to the output power of the flyback converter, i.e. the larger the output power is, the longer the third duration is, because the larger the output power is, the larger the current of the primary side winding is, and the larger the corresponding leakage inductance energy is, and therefore, the more the leakage inductance energy stored in the clamping capacitor is, and therefore, the longer the third duration is, so that the leakage inductance energy of the clamping capacitor can be completely released.

Preferably, during the third duration, the voltage of the clamping capacitor C1 is gradually decreased until the end of the third duration, the voltage of the clamping capacitor C1 is still greater than the reflected voltage, because when the rectifier S2 is turned on again, the drain-source voltage of the main power switch S1 is equal to N × Vout + Vin, and the voltage of the clamping capacitor C1 is still greater than the reflected voltage, that is, the voltage across the clamping capacitor C1 is greater than N × Vout + Vin, that is, the body diode of the active clamping switch S2 is turned off, so as to avoid the energy at the secondary side output end from being transferred to the clamping capacitor C1, where the reflected voltage is the product of the primary-secondary turn ratio of the transformer and the output voltage of the flyback converter.

Preferably, the second duration ends when the current of the secondary winding decreases to the set forward current value, so that the third duration is still within the demagnetization time of the secondary winding, and the leakage inductance of the clamping capacitor and the primary side is prevented from resonating to a negative half period, so that the energy of the leakage inductance returns to the clamping capacitor again.

Preferably, the fourth duration starts when the junction capacitance voltage of the main power switch S1 resonates to a peak, and at this time, the drain-source voltage of the rectifier S2 resonates to a corresponding valley, so as to turn on the rectifier S2 again when the junction capacitance voltage of the main power switch S1 resonates to a peak, thereby enabling the valley of the rectifier S2 to be turned on and reducing the turn-on loss of the rectifier S2. Specifically, the rectifier tube S2 is turned on again when the junction capacitance voltage of the main power switch tube S1 resonates to a peak, so that the reflected voltage reversely excites the excitation inductance of the primary winding, when the current of the secondary winding reaches a set negative current, the rectifier tube S2 is turned off, at this time, the primary winding generates a negative current, and the main power switch tube S1 is turned on before the negative current reaches zero, thereby realizing zero-voltage turn-on of the main power switch tube S1.

In the following, a detailed analysis is made on the timing chart of the first embodiment of the control method of the present invention shown in fig. 4 with reference to the flyback converter shown in fig. 3, and the following six stages are repeated in each operating cycle of the flyback converter:

first stage (t0-t 1): that is, during the first duration, the primary controller 220 generates the high level driving signal LSGD to control the main power switch S1 to be turned on, the input voltage Vin excites the primary winding, the current IL-P in the primary circuit flows through the transformer T1 and linearly increases, the transformer continuously stores energy, and when the current IL-P in the primary circuit reaches a certain value, the primary controller 220 generates the low level driving signal LSGD to control the main power switch S1 to be turned off.

Second stage (t1-t 2): in a first time interval, the control device keeps the main power switch tube S1, the rectifier tube S2 and the clamp switch tube S3 all turned off, current IL-P in a primary side circuit charges a junction capacitor CdS of the main power switch tube S1, drain-source voltage Vds-MP of the main power switch tube S1 continuously rises, and when the voltage rises to Vin + N Vout, the stage is ended, wherein Vin is input voltage of the flyback converter, Vout is output voltage of the flyback converter, and N is the turn ratio of a primary winding and a secondary winding of the transformer T1.

Third stage (t2-t 3): when the drain-source voltage of the main power switch tube S1 rises to Vin + N × Vout, the drain-source voltage Vds-SR of the rectifier tube S2 drops to zero volts, the first time interval ends, the second time duration is entered, the secondary side controller 211 generates a high level SRGD to control the conduction of the rectifier tube S2, the energy stored in the transformer T1 starts to be transferred to the output end of the flyback converter, that is, the transformer T1 starts to demagnetize, at this time, the current IL-P in the primary side circuit will drop linearly and charge the clamp capacitor C1 through the body diode of the clamp tube S3, and when the current IL-P is zero, the voltage across the clamp capacitor C1 reaches the maximum.

In the fourth stage (t3-t 4): when the current IL-S of the secondary winding decreases to a forward current value, the secondary side controller 211 generates a low level SRGD, controls the rectifier S2 to turn off, and the second duration ends. When the rectifier tube S2 is turned off, the secondary side controller 211 transmits the synchronization signal SYNC to the primary side controller 220, and when the primary side controller 220 receives the synchronization signal SYNC, the primary side controller 220 generates a high level HSGD to control the clamp switch tube S3 to be turned on, and enters a third duration, the leakage inductance energy stored in the clamp capacitor C1 is transferred to the secondary output end through the transformer T1, and the voltage of the clamp capacitor C1 gradually decreases. The conducting time of the clamp switch tube S3 will not exceed half of the resonant period time of the clamp capacitor C1 resonating with the leakage inductor Lk at the longest, and the conducting time is linear with the feedback signal FB obtained by the feedback circuit 223, when the output power is high, the higher the feedback signal FB is, the higher the primary side controller 220 will also correspondingly lengthen the conducting time of the clamp switch tube S3 according to the feedback signal FB, when the conducting time of the clamp switch tube S3 reaches the set conducting time, the primary side controller 220 generates a low level HSGD signal to control the clamp switch tube S3 to turn off, when the clamp switch tube S3 turns off, the drain-source voltage of the main power switch tube S1 is Vin + N Vout, that is, the voltage at two ends of the clamp capacitor C1 is not lower than N Vout.

In the fifth phase t4-t 5: after the clamping switch tube S3 is turned off, and enters a second time interval, the junction capacitor Cds of the main power switch tube S1 resonates with the excitation inductor Lm of the primary winding.

In the sixth phase t5-t 6: when the junction capacitance voltage of the main power switch tube S1 resonates to the Nth wave crest, the drain-source voltage of the rectifier tube S2 resonates to the Nth wave trough, the secondary side controller 211 generates a high-level SRGD signal, controls the rectifier tube S2 to be switched on, enters a fourth duration, the reflected voltage reversely excites the excitation inductance Lm of the primary winding, when the current IL-S of the secondary winding reaches a set negative current, the secondary side controller 211 generates a low-level SRGD signal, controls the rectifier tube S2 to be switched off, enters a third time interval, the current IL-P of the primary winding generates a negative current, the primary side controller 220 generates a high-level LSGD signal before the negative current reaches zero, controls the main power switch tube S1 to be switched on, and accordingly realizes the zero-voltage switching on of the main power switch tube S1;

in the third stage, the voltage of the clamping capacitor C1 reaches the maximum value, and the maximum value exceeds N × Vout, so in the fourth stage, the drain-source voltage of the main power switch S1 suddenly rises when the clamping switch S3 is turned on.

The conducting time of the clamping switch tube S3 is not more than half of the resonant period time of the clamping capacitor C1 and the leakage inductance Lk.

Second embodiment

With respect to the control method of the first embodiment described above, the inventors have observed that when the input voltage is lower than N × Vout, the drain-source voltage of the primary side main power switch tube S1 may naturally reach 0V, and thus zero voltage switching may be achieved, and thus the rectifier tube S2 may not need to be turned on again for the fourth duration.

Fig. 5 is a timing chart of a control method according to a second embodiment of the present invention, referring to fig. 5, the control method of the present embodiment is different from the control method of the first embodiment in that each working cycle further includes: acquiring an input voltage signal representing the current input voltage of the flyback converter, comparing the input voltage signal Vin with a first threshold Vth, and determining whether to turn on the rectifying tube S2 for a fourth duration again according to the comparison result, specifically:

when the input voltage signal Vin is less than or equal to the first threshold Vth, the rectifier tube S2 is not turned on again for a fourth duration;

when the input voltage signal Vin is greater than the first threshold Vth, the rectifying tube S2 is turned on again for a fourth duration.

Wherein the determination of whether to reopen the rectifier tube for the fourth duration according to the comparison result is achieved by whether the primary side controller 220 communicates the synchronization signal SYNC to the secondary side controller 211, specifically;

when the input voltage signal Vin is less than or equal to the first threshold Vth, the primary side controller 220 transmits the synchronization signal SYNC to the secondary side controller 211 so that the secondary side controller 211 does not turn on the rectifying tube S3 again for the fourth duration;

when the input voltage signal Vin is greater than the first threshold Vth, the primary side controller 220 does not transmit the synchronization signal SYNC to the secondary side controller 211, so that the secondary side controller 211 turns on the rectifying tube S3 again for a fourth duration.

Preferably, the input voltage signal Vin is obtained by directly dividing the voltage through a sampling resistor.

When the input voltage signal Vin is greater than the first threshold Vth, the operation timing of the control method of the present embodiment is the same as that of the first embodiment, please refer to fig. 4.

When the input voltage signal Vin is less than or equal to the first threshold Vth, the working timing of the control method of the present embodiment is shown in fig. 6. In this case, the primary side controller 220 is configured to control the on and off of the main power switch S1 and the active clamp switch S3 according to the input voltage signal Vin of the flyback converter, the feedback signal FB, the drain-source voltage signal Vds-MP of the main power switch, and the synchronization signal SYNC; the secondary side controller 220 is configured to control the turn-on and turn-off of the rectifier S2 according to the rectifier drain-source voltage signal Vds-SR and the synchronization signal SYNC.

When the input voltage signal Vin is less than or equal to the first threshold Vth and the first threshold is set to be close to N × Vout, the drain-source voltage Vds-MP of the main power switch tube S1 can be close to zero voltage turn-on even when the voltage Vds-MP is at the valley, and the switching loss of the main power switch tube S1 can be reduced by calculating the formula Vds-MP as Vin-N × Vout.

The timing diagram of fig. 6 is analyzed in detail below in conjunction with the flyback converter of fig. 3, in which the following six phases are repeated in each operating cycle of the flyback converter:

first stage (t0-t 1): the description is not repeated as in the first embodiment.

In the second stage (t1-t 2): the description is not repeated as in the first embodiment.

In the third stage (t2-t 3): the description is not repeated as in the first embodiment.

In the fourth stage (t3-t 4): the difference from the first embodiment is that when the input voltage is equal to or less than the first threshold Vth, the primary side controller 220 transmits the synchronization signal SYNC to the secondary side controller 211 so that the secondary side controller 211 does not turn on the rectifying tube S3 again for the fourth duration.

In the fifth stage (t4-t 5): the description is not repeated as in the first embodiment.

In the sixth stage (t5-t 6): when the primary controller 220 detects that the drain-source voltage Vds-MP of the main power switch tube S1 resonates to the nth valley, the primary controller 220 generates a high-level LSGD signal to control the main power switch tube S1 to be turned on (the next working period has been entered at this time), and the rectifier tube S2 is not turned on again when the secondary controller 211 detects the synchronization signal SYNC signal;

the voltage of the drain-source voltage of the main power switch tube S1 at the valley is Vds-MP-Vin-N Vout, and since Vin is equal to or less than the first threshold Vth, the main power switch tube S1 can also realize zero-voltage turn-on.

Third embodiment

The present embodiment provides a control device, which is applied to the flyback converter shown in fig. 3, wherein the flyback converter includes a primary side circuit, a secondary side circuit, a transformer T1 and the control device of the present embodiment; the primary side circuit comprises a main power switch tube S1, a clamping switch tube S3, a clamping capacitor C1 and a primary winding of a transformer T1; the secondary side circuit comprises a rectifier tube S2 and a secondary winding of a transformer T1; the main power switch tube S1, the clamping switch tube S3 and the rectifying tube S2 are turned off at the initial state of each working period of the flyback converter; the control device of the present embodiment includes:

a primary side controller 220 configured to control the main power switch tube S1 and the clamp switch tube S3 to be turned on and off;

a secondary side controller 211 configured to control on and off of the rectifying tube S2;

an isolation circuit 221 configured to bidirectionally transfer a synchronization signal SYNC between the primary-side controller 220 and the secondary-side controller 211;

and control logic configured to perform the control actions of either of the first embodiment control method embodiments and the second embodiment control method embodiments in each duty cycle.

The advantageous effects of the control device of this embodiment correspond to the specific embodiments in the first embodiment/the second embodiment, and the description will not be repeated.

Fourth embodiment

The present embodiment provides a flyback converter, as shown in fig. 3, including: any one of the primary side circuit, the secondary side circuit, the transformer T1 and the detailed implementation of the control device in the third embodiment; the primary side circuit comprises a main power switch tube S1, a clamping switch tube S3, a clamping capacitor C1 and a primary winding of a transformer T1; the secondary side circuit comprises a rectifier tube S2 and a secondary winding of a transformer T1; the control device is used for controlling the conduction and the disconnection of the main power switch tube S1, the clamping switch tube S3 and the rectifying tube S2; the main power switch S1, the clamp switch S3 and the rectifier S2 are turned off at the initial state of each duty cycle of the flyback converter.

The advantageous effects of the control device of this embodiment indirectly correspond to the specific embodiments in the first embodiment/the second embodiment, and the description will not be repeated.

The foregoing is merely a preferred embodiment of the present invention, and it should be noted that the above-described preferred embodiment should not be construed as limiting the present invention. For those skilled in the art, several equivalent power sources, modifications and decorations can be made without departing from the spirit and scope of the present invention, and these equivalent power sources, modifications and decorations should be regarded as the protection scope of the present invention, and no description is given here, and the protection scope of the present invention should be subject to the scope defined by the claims.

Claims

1. A control method is applied to a flyback converter, and 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 switch tube, a clamping capacitor and a primary winding of the transformer; the secondary side circuit comprises a rectifier tube and a secondary winding of the transformer; the control device comprises a primary side controller, a secondary side controller, an isolation circuit and control logic; the primary side controller is used for controlling the conduction and the disconnection of the main power switch tube and the clamping switch tube, the secondary side controller is used for controlling the conduction and the disconnection of the rectifier tube, the isolation circuit is used for bidirectionally transmitting a synchronous signal between the primary side controller and the secondary side controller, and the control logic is used for executing the control method; the main power switch tube, the clamping switch tube and the rectifying tube are turned off at the initial state of each working period of the flyback converter; the method is characterized in that: the control method comprises the following steps in each working period of the flyback converter in sequence:

2. The control method according to claim 1, characterized in that: the third duration is proportional to the output power of the flyback converter.

3. The control method according to claim 1, characterized in that: the fourth duration begins when the junction capacitance voltage of the main power switch tube resonates to a peak, and at this time, the drain-source voltage of the rectifier tube resonates to a corresponding trough.

4. The control method according to any one of claims 1 to 3, characterized by further comprising, in each duty cycle: acquiring an input voltage signal representing the current input voltage of the flyback converter, comparing the input voltage signal with a first threshold, and determining whether to switch on the rectifier tube for a fourth duration again according to a comparison result, specifically:

5. The control method according to claim 4, characterized in that: the determination of whether to reopen the rectifier tube for the fourth duration according to the comparison result is realized by whether the primary side controller transmits a synchronization signal to the secondary side controller, specifically:

when the input voltage signal is greater than the first threshold, the primary-side controller does not transmit a synchronization signal to the secondary-side controller, so that the secondary-side controller turns on the rectifier tube again for a fourth duration.

6. A control device is applied to a flyback converter, and 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 switch tube, a clamping capacitor and a primary winding of the transformer; the secondary side circuit comprises a rectifier tube and a secondary winding of the transformer; the main power switch tube, the clamping switch tube and the rectifying tube are turned off at the initial state of each working period of the flyback converter; characterized in that the control device comprises:

a primary side controller configured to control the conduction and the disconnection of the main power switch tube and the clamping switch tube;

7. The control device according to claim 6, characterized in that: the control logic is configured wherein the third duration is proportional to the output power of the flyback converter.

8. The control device according to claim 6, characterized in that: the control logic is configured wherein the fourth duration begins when the junction capacitance voltage of the main power switch resonates to a peak when the rectifier drain-source voltage resonates to a corresponding valley.

9. The control apparatus of any of claims 6 to 8, wherein the control logic is configured wherein the control method further comprises, in each duty cycle: acquiring an input voltage signal representing the current input voltage of the flyback converter, comparing the input voltage signal with a first threshold, and determining whether to switch on the rectifier tube for a fourth duration again according to a comparison result, specifically:

when the input voltage signal is smaller than or equal to the first threshold value, the rectifier tube is not turned on again for a fourth duration;

10. The control device according to claim 9, characterized in that: the control logic is configured wherein the determining whether to re-fire the rectifier tube for a fourth duration in dependence on the comparison result is achieved by whether the primary side controller passes a synchronization signal to the secondary side controller, in particular:

when the input voltage signal is less than or equal to the first threshold value, the primary side controller transmits a synchronous signal to the secondary side controller, so that the secondary side controller does not switch on the rectifier tube for a fourth duration again;

11. A flyback converter, comprising: a primary side circuit, a secondary side circuit, a transformer and the control device of any one of claims 6 to 10; the primary side circuit comprises a main power switch tube, a clamping capacitor and a primary winding of the transformer; the secondary side circuit comprises a rectifier tube and a secondary winding of the transformer; the control device is used for controlling the conduction and the disconnection of the main power switch tube, the clamping switch tube and the rectifying tube; the main power switch tube, the clamping switch tube and the rectifying tube are turned off at the initial state of each working period of the flyback converter.