Control method and control circuit of half-bridge flyback converter, chip and power supply equipment

Abstract

The embodiment of the invention provides a control method and control circuit of a half-bridge flyback converter, a chip and power supply equipment. The control method comprises the steps that when the half-bridge flyback converter works in a free resonance state, in a first switching period, the second primary switching tube is controlled to be switched on so that the half-bridge flyback converter can exit from the free resonance state, after a preset switching-on duration, the second primary switching tube is controlled to be switched off so that excitation current can be negative current, and the half-bridge flyback converter can be switched off. And the half-bridge flyback converter enters the free resonance state again, and the first primary switch tube is controlled to be switched on at the moment which is separated from the switching-off moment of the second primary switch tube by odd number times of the free resonance half cycle, so that the valley bottom of the first primary switch tube is switched on, the efficiency of the half-bridge flyback converter can be improved, and the service life of the half-bridge flyback converter is prolonged. And the peak stress when the secondary switch tube is turned off can be reduced, so that the EMI of the half-bridge flyback converter is improved.

Description

Technical Field

[0001] Embodiments of the present disclosure relate to the field of integrated circuit technology, and in particular to a control method, a control circuit, a chip, and a power supply device of a half-bridge flyback converter. Background Art

[0002] In the control scheme of the half-bridge flyback converter, the protocol controller transmits a feedback signal, i.e., output power information, to the primary controller through an isolation optocoupler. The primary controller controls the on and off of the first primary switch tube and the second primary switch tube in the half-bridge flyback converter according to the optocoupler feedback signal, wherein the first primary switch tube is connected in series with the primary winding, and the second primary switch tube is connected in parallel with the primary winding.

[0003] Usually under light load, the half-bridge flyback converter enters DCM (Discontinue Conduction Mode) to reduce the switching frequency, thereby achieving the purpose of reducing switching losses. However, the insufficient current stored in the excitation inductor makes it impossible for the first primary switch tube to achieve zero voltage switching (Zero Voltage Switching, ZVS). If the first primary switch tube is forcibly turned on, the peak stress when the secondary switch tube is turned off will be higher, thereby worsening the (Electromagnetic Interference, EMI) of the half-bridge flyback converter. Summary of the invention

[0004] The present invention provides a control method, circuit, chip and power supply device of a half-bridge flyback converter, which can improve the efficiency of the half-bridge flyback converter and reduce the peak stress when the secondary switch tube is turned off, thereby improving the EMI of the half-bridge flyback converter.

[0005] In a first aspect, the present disclosure provides a control method for a half-bridge flyback converter, wherein the half-bridge flyback converter comprises: a primary power circuit and a secondary power circuit coupled to the primary power circuit, wherein the primary power circuit comprises a primary winding and a first primary switch tube and a second primary switch tube connected in series between a voltage input terminal and ground, and the secondary power circuit comprises a secondary winding and a secondary switch tube connected to the secondary winding.

[0006] The control method comprises:

[0007] When the half-bridge flyback converter operates in a free resonance state, in a first switching cycle, the second primary switch tube is controlled to be turned on so that the half-bridge flyback converter exits the free resonance state; after a preset turn-on time, the second primary switch tube is controlled to be turned off so that the excitation current is a negative current, and the half-bridge flyback converter enters the free resonance state again; at a moment that is M / 2 free resonance cycles apart from the turn-off moment of the second primary switch tube, the first primary switch tube is controlled to be turned on, where M is an odd number greater than or equal to 1.

[0008] In some embodiments of the present disclosure, M is less than or equal to 5.

[0009] In some embodiments of the present disclosure, before controlling the second primary switch tube to be turned on in the first switching cycle, the method further includes:

[0010] In the first switching cycle, the second primary switch tube is controlled to be turned on for a first time period so that the excitation current is zero.

[0011] In some embodiments of the present disclosure, before the first switching cycle, the half-bridge flyback converter continuously operates in a critical conduction mode (CrM) for N second switching cycles, where N is an integer greater than or equal to 1.

[0012] Before controlling the second primary switch tube to be turned on in the first switching cycle, the method further includes:

[0013] In the first switching cycle, the second primary switch tube is controlled to be turned off.

[0014] In some embodiments of the present disclosure, before the first switching cycle, the half-bridge flyback converter continuously operates for N second switching cycles under CrM, where N is an integer greater than or equal to 1.

[0015] Before controlling the second primary switch tube to be turned on in the first switching cycle, the method further includes:

[0016] In the second switching cycle, the second primary switch tube is controlled to be turned on for a second time period so that the excitation current is a negative current.

[0017] In some embodiments of the present disclosure, in the first switching cycle, controlling the second primary switch tube to be turned on includes:

[0018] The sampling point voltage of the half-bridge flyback converter is sampled, and the turn-on time of the second primary switch tube is determined according to the valley value of the sampling point sampling voltage in the free resonance state.

[0019] In a second aspect, the present disclosure provides a control circuit of a half-bridge flyback converter, wherein the half-bridge flyback converter comprises: a primary power circuit and a secondary power circuit coupled to the primary power circuit, wherein the primary power circuit comprises a primary winding and a first primary switch tube and a second primary switch tube connected in series between a voltage input terminal and ground, and the secondary power circuit comprises a secondary winding and a secondary switch tube connected to the secondary winding.

[0020] The control circuit comprises a primary control unit.

[0021] The primary control unit is configured to, when the half-bridge flyback converter operates in a free resonance state, control the second primary switch tube to be turned on in a first switching cycle so that the half-bridge flyback converter exits the free resonance state; after a preset turn-on time, control the second primary switch tube to be turned off so that the excitation current is a negative current, and the half-bridge flyback converter enters the free resonance state again; and control the first primary switch tube to be turned on at a moment that is M / 2 free resonance cycles apart from the turn-off moment of the second primary switch tube, where M is an odd number greater than or equal to 1.

[0022] In some embodiments of the present disclosure, M is less than or equal to 5.

[0023] In some embodiments of the present disclosure, the primary control unit is further configured to, in the first switching cycle, control the second primary switch tube to be turned on for a first time period so that the excitation current is zero.

[0024] In some embodiments of the present disclosure, before the first switching cycle, the half-bridge flyback converter continuously operates for N second switching cycles under CrM, where N is an integer greater than or equal to 1.

[0025] The primary control unit is further configured to control the second primary switch tube to be turned off during the first switching cycle.

[0026] In some embodiments of the present disclosure, before the first switching cycle, the half-bridge flyback converter continuously operates for N second switching cycles under CrM, where N is an integer greater than or equal to 1.

[0027] The primary control unit is further configured to, in the second switching period, control the second primary switch tube to be turned on for a second time period so that the excitation current is a negative current.

[0028] In some embodiments of the present disclosure, the control circuit further includes a primary sampling unit.

[0029] The primary sampling unit is configured to sample the sampling point voltage of the half-bridge flyback converter, and the primary control unit is further configured to determine the turn-on time of the second primary switch tube according to the valley value of the sampling point sampling voltage in the free resonance state.

[0030] In a third aspect, the present disclosure provides a chip for executing the steps of any method provided in the first aspect.

[0031] In a fourth aspect, the present disclosure provides a power supply device, including a half-bridge flyback converter and a control circuit, wherein the control circuit is used to execute the steps of any method provided in the first aspect.

[0032] The technical solution disclosed in the present invention provides a control method for a half-bridge flyback converter. When the half-bridge flyback converter operates in a free resonance state, the second primary switch tube is controlled to be turned on in the first switching cycle to make the half-bridge flyback converter exit the free resonance state. After a preset turn-on time, the second primary switch tube is controlled to be turned off to make the excitation current negative current, and the half-bridge flyback converter enters the free resonance state again. At a moment that is an odd multiple of the free resonance half cycle apart from the turn-off moment of the second primary switch tube, the first primary switch tube is controlled to be turned on to achieve valley turning-on of the first primary switch tube. This can not only improve the efficiency of the half-bridge flyback converter, but also reduce the peak stress when the secondary switch tube is turned off, thereby improving the EMI of the half-bridge flyback converter. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following briefly introduces the drawings required for describing the embodiments. It should be noted that the drawings described below only relate to some embodiments of the present disclosure, rather than limiting the present disclosure, wherein:

[0034] Figure 1 The present invention is a schematic diagram of the structure of a controller of a half-bridge flyback converter provided in the prior art.

[0035] Figure 2 The present invention provides a schematic diagram of operating waveforms of a half-bridge flyback converter provided by the prior art.

[0036] Figure 3 A schematic structural diagram of a control circuit of a half-bridge flyback converter provided in an embodiment of the present disclosure.

[0037] Figure 4 A schematic structural diagram of another control circuit of a half-bridge flyback converter provided in an embodiment of the present disclosure.

[0038] Figure 5 A flow chart of a control method provided in an embodiment of the present disclosure.

[0039] Figure 6 A schematic diagram of operating waveforms of a half-bridge flyback converter provided in an embodiment of the present disclosure.

[0040] Figure 7 A schematic diagram of operating waveforms of another half-bridge flyback converter provided in an embodiment of the present disclosure.

[0041] Figure 8 A schematic diagram of operating waveforms of another half-bridge flyback converter provided in an embodiment of the present disclosure. DETAILED DESCRIPTION

[0042] In order to make the purpose, technical solution and advantages of the embodiments of the present disclosure clearer, the technical solution of the embodiments of the present disclosure will be clearly and completely described below in conjunction with the accompanying drawings. Obviously, the described embodiments are part of the embodiments of the present disclosure, rather than all of the embodiments. Based on the described embodiments of the present disclosure, all other embodiments obtained by those skilled in the art without creative work also fall within the scope of protection of the present disclosure.

[0043] Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by those skilled in the art to which the disclosed subject matter belongs. It will be further understood that terms such as those defined in commonly used dictionaries should be interpreted as having a meaning consistent with their meaning in the context of the specification and the relevant art, and will not be interpreted in an idealized or overly formal form unless otherwise explicitly defined herein. As used herein, a statement that two or more parts are "connected" together shall mean that the parts are joined together directly or through one or more intermediate components.

[0044] Reference to "embodiments" in this disclosure means that a particular feature, structure, or characteristic described in conjunction with the embodiments may be included in at least one embodiment of the present application. The appearance of the phrase "embodiments" in various places in the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment that is mutually exclusive with other embodiments. It is explicitly and implicitly understood by those skilled in the art that the embodiments described in this disclosure may be combined with other embodiments.

[0045] In addition, the terms "first", "second", etc. in the specification and claims of the present disclosure or the above-mentioned drawings are used to distinguish different objects rather than to describe a specific order, and may explicitly or implicitly include one or more of the features.

[0046] The term "and / or" in this disclosure is only a description of the association relationship of associated objects, indicating that three relationships may exist. For example, A and / or B can represent: A exists, A and B exist at the same time, and B exists. In addition, the character " / " in this article generally indicates that the associated objects before and after are in an "or" relationship.

[0047] In the description of the present disclosure, unless otherwise specified, "plurality" and "at least two" mean more than two (including two), and similarly, "multiple groups" and "at least two groups" mean more than two groups (including two).

[0048] In order to enable those skilled in the art to better understand the solution of the present application, the technical solution in the embodiments of the present application will be clearly and completely described below in conjunction with the accompanying drawings.

[0049] Figure 1 A schematic diagram of the structure of a controller of a half-bridge flyback converter is provided in the prior art. Figure 1 As shown, the half-bridge flyback converter 10 includes an input capacitor Cin, a first primary switch tube Q1, a second primary switch tube Q2, a resonant inductor Lr, a primary winding Np, a resonant capacitor Cr and a sampling resistor Rs.

[0050] Among them, the first electrode plate of the input capacitor Cin is connected to the voltage input end and the first end of the first primary switch tube Q1, the second end of the first primary switch tube Q1 is connected to the first end of the resonant inductor Lr and the first end of the second primary switch tube Q2, the second end of the resonant inductor Lr is connected to the first end of the sampling resistor Rs and the second end of the second primary switch tube Q2 through the primary winding Np and the resonant capacitor Cr, and the second end of the sampling resistor Rs and the second electrode plate of the input capacitor Cin are connected to the power ground PGND.

[0051] The half-bridge flyback converter 10 further includes an auxiliary winding Naux, a first auxiliary sampling resistor R1 and a second auxiliary sampling resistor R2, wherein a first end of the auxiliary winding Naux is connected to a first end of the first auxiliary sampling resistor R1, a second end of the auxiliary winding Naux is connected to a power ground PGND, and a second end of the first auxiliary sampling resistor R1 is connected to a power ground PGND through a second auxiliary sampling resistor R2.

[0052] The half-bridge flyback converter 10 also includes a secondary winding Ns, an output capacitor Cout and a secondary switch tube Q3, wherein a first end of the secondary winding Ns is connected to a voltage output end and a first plate of the output capacitor Cout, a second end of the secondary winding Ns is connected to a first end of the secondary switch tube Q3, and a second end of the secondary switch tube Q3 and a second plate of the output capacitor Cout are connected to a signal ground SGND.

[0053] The controller 20 includes a primary controller 21, a protocol controller 22, a secondary synchronous rectification controller 23 and an isolation optocoupler 24. The first input end of the primary controller 21 is connected to the connection point of the first auxiliary sampling resistor R1 and the second auxiliary sampling resistor R2, the second input end of the primary controller 21 is connected to the output end of the isolation optocoupler 24, and the two output ends of the primary controller 21 are respectively connected to the control end of the first primary switch tube Q1 and the control end of the second primary switch tube Q2.

[0054] The first output end of the secondary synchronous rectification controller 23 is connected to the control end of the secondary switch tube Q3, the second output end of the secondary synchronous rectification controller 23 is connected to the input end of the protocol controller 22, and the output end of the protocol controller 22 is connected to the input end of the isolation optocoupler 24. The isolation optocoupler 24 includes a light emitting diode and a phototransistor, the positive electrode of the light emitting diode is connected to the output end of the protocol controller 22, the negative electrode of the light emitting diode is connected to the signal ground SGND, the first end of the phototransistor is connected to the second input end of the primary controller 21, and the second end of the phototransistor is connected to the power ground PGND.

[0055] The secondary synchronous rectification controller 23 can control the on and off of the secondary switch tube Q3, and generate a feedback signal reflecting the on and off of the secondary switch tube Q3. The secondary synchronous rectification controller 23 can transmit the feedback signal to the protocol controller 22. The protocol controller 22 transmits the feedback signal, that is, the output power information, to the primary controller 21 through the isolation optocoupler 24. The primary controller 21 can control the on and off of the first primary switch tube Q1 and the second primary switch tube Q2 according to the optocoupler feedback signal.

[0056] Figure 2 A schematic diagram of a working waveform of a half-bridge flyback converter provided by the prior art is shown in FIG. Figure 2 As shown, the half-bridge flyback converter 10 enters DCM under light load. At time T, the primary controller 21 maintains the second primary control signal VQ2 at a low level to keep the second primary switch tube Q2 turned off, and the primary controller 21 pulls up the first primary control signal VQ1 to turn on the first primary switch tube Q1.

[0057] However, due to insufficient current storage in the primary winding Np, the zero-voltage turn-on of the first primary switch tube Q1 cannot be achieved at time T. Forcibly turning on the first primary switch tube Q1 at time T will result in reduced efficiency of the half-bridge flyback converter 10. In addition, a spike will be generated at the drain of the secondary switch tube Q3, that is, the drain voltage Vsdr of the secondary switch tube Q3 has a spike at time T, resulting in a higher spike stress when the secondary switch tube Q3 is turned off, thereby worsening the EMI of the half-bridge flyback converter 10.

[0058] In view of this, the present disclosure provides a control method for a half-bridge flyback converter. When the half-bridge flyback converter operates in a free resonance state, the second primary switch tube is controlled to be turned on in a first switching cycle to make the half-bridge flyback converter exit the free resonance state. After a preset turn-on time, the second primary switch tube is controlled to be turned off to make the excitation current a negative current, and the half-bridge flyback converter enters the free resonance state again. At a moment that is an odd multiple of the free resonance half cycle apart from the turn-off moment of the second primary switch tube, the first primary switch tube is controlled to be turned on to achieve valley turning-on of the first primary switch tube. This can not only improve the efficiency of the half-bridge flyback converter, but also reduce the peak stress when the secondary switch tube is turned off, thereby improving the EMI of the half-bridge flyback converter.

[0059] The technical solution is described in detail below with reference to several specific embodiments.

[0060] Figure 3 A schematic diagram of a control circuit of a half-bridge flyback converter provided in an embodiment of the present disclosure is shown in FIG. Figure 3 As shown, the half-bridge flyback converter 100 includes a primary power circuit 110 and a secondary power circuit 120, and the primary power circuit 110 is coupled to the secondary power circuit 120. The primary power circuit 110 includes a first primary switch tube Q1, a second primary switch tube Q2 and a primary winding Np, and the first primary switch tube Q1 and the second primary switch tube Q2 are connected in series between the voltage input terminal and the power ground PGND.

[0061] For example, Figure 3 As shown, the primary power circuit 110 also includes a resonant inductor Lr, a resonant capacitor Cr, an input capacitor Cin and a sampling resistor Rs. The first plate of the input capacitor Cin is a voltage input terminal, and the second plate of the input capacitor Cin is connected to the power ground PGND. The voltage input terminal is connected to the power ground PGND through the first primary switch tube Q1 and the second primary switch tube Q2 in sequence, and the connection point of the first primary switch tube Q1 and the second primary switch tube Q2 is connected to the power ground PGND through the resonant inductor Lr, the primary winding Np, the resonant capacitor Cr and the sampling resistor Rs in sequence.

[0062] In other embodiments, the voltage input terminal is connected to the power ground PGND through the first primary switch tube Q1, the second primary switch tube Q2 and the sampling resistor Rs in sequence, and the connection point of the first primary switch tube Q1 and the second primary switch tube Q2 is connected to the voltage input terminal through the resonant inductor Lr, the primary winding Np and the resonant capacitor Cr in sequence, such as Figure 4 As shown, Figure 4 A schematic structural diagram of another control circuit of a half-bridge flyback converter provided in an embodiment of the present disclosure.

[0063] Continue to see Figure 3 and Figure 4As shown, the secondary power circuit 120 includes a secondary winding Ns, an output capacitor Cout and a secondary switch tube Q3, the first plate of the output capacitor Cout is a voltage output end, the second plate of the output capacitor Cout is connected to the signal ground SGND, and the secondary winding Ns is connected to the secondary switch tube Q3.

[0064] Exemplarily, a first end of the secondary winding Ns is connected to a first plate of the output capacitor Cout, and a second end of the secondary winding Ns is connected to the signal ground SGND through the secondary switch tube Q3. Figure 3 In other embodiments, the first end of the secondary winding Ns is connected to the voltage output end through the secondary switch tube Q3, and the second end of the secondary winding Ns is connected to the second plate of the output capacitor Cout. Figure 4 shown.

[0065] Continue to see Figure 3 and Figure 4 As shown, the primary power circuit 110 also includes an auxiliary winding Naux, a first auxiliary sampling resistor R1 and a second auxiliary sampling resistor R2. The first end of the auxiliary winding Naux is connected to the power ground PGND through the first auxiliary sampling resistor R1 and the second auxiliary sampling resistor R2 in sequence, and the second end of the auxiliary winding Naux is connected to the power ground PGND.

[0066] The three output terminals of the control circuit 200 are respectively connected to the control terminals of the first primary switch tube Q1 , the second primary switch tube Q2 and the secondary switch tube Q3 to control the first primary switch tube Q1 , the second primary switch tube Q2 and the secondary switch tube Q3 .

[0067] The control circuit 200 is used to execute the steps of the control method provided by the present disclosure. The specific control method is described in detail below.

[0068] Figure 5 A flow chart of a control method provided by an embodiment of the present disclosure is shown in FIG. Figure 5 As shown, the specific steps of the control method include:

[0069] S101, when the half-bridge flyback converter operates in a free resonance state, in a first switching cycle, the second primary switch tube is controlled to be turned on, so that the half-bridge flyback converter exits the free resonance state.

[0070] For example, Figure 3 and Figure 4As shown, the control circuit 200 includes a primary control module 210 and a secondary control module 220. The primary control module 210 includes a first primary drive unit 211, a second primary drive unit 212 and a primary control unit 213. For example, the primary control unit 213 is an asymmetric half-bridge (Asymmerical Half-Bridge, AHB) controller. The first output end of the primary control unit 213 is connected to the control end of the first primary switch tube Q1 through the first primary drive unit 211, and the second output end of the primary control unit 213 is connected to the control end of the second primary switch tube Q2 through the second primary drive unit 212.

[0071] The secondary control module 220 includes a secondary driving unit 221 and a secondary control unit 222 . For example, the secondary control unit 222 is a synchronous rectification (SR) controller. The output end of the secondary control unit 222 is connected to the control end of the secondary switch tube Q3 through the secondary driving unit 221 .

[0072] Figure 6 A schematic diagram of operating waveforms of a half-bridge flyback converter provided in an embodiment of the present disclosure is shown in FIG. Figure 6 As shown, the half-bridge flyback converter 100 continuously operates in DCM for multiple first switching cycles. At time T1 of the first switching cycle, the first primary control signal VQ1, the second primary control signal VQ2 and the secondary control signal VQ3 maintain a low level. The primary control unit 213 controls the first primary drive unit 211 to keep the first primary switch tube Q1 turned off according to the first primary control signal VQ1, and controls the second primary drive unit 212 to keep the second primary switch tube Q2 turned off according to the second primary control signal VQ2. The secondary control unit 222 controls the secondary drive unit 221 to keep the secondary switch tube Q3 turned off according to the secondary control signal VQ3.

[0073] Obviously, the half-bridge flyback converter 100 is in a free resonance state, and the first primary switch tube Q1 is not turned on. At this time, the primary control unit 213 pulls up the second primary control signal VQ2, and controls the second primary drive unit 330 to turn on the second primary switch tube Q2 according to the second primary control signal VQ2. The drain voltage Vsrd of the secondary switch tube Q3 is pulled down to zero, and the half-bridge flyback converter 100 exits the free resonance state.

[0074] S102, after a preset conduction time, the second primary switch tube is controlled to be turned off, so that the excitation current is a negative current, and the half-bridge flyback converter enters a free resonance state again.

[0075] For example, Figure 6As shown, after the moment T1 of the first switching cycle and at the moment T2 which is separated from the moment T1 by a preset time length Ton, the first primary control signal VQ1 and the secondary control signal VQ3 maintain a low level, the primary control unit 213 controls the first primary driving unit 211 to keep the first primary switch tube Q1 turned off according to the first primary control signal VQ1, and the secondary control unit 222 controls the secondary driving unit 221 to turn off the secondary switch tube Q3 according to the secondary control signal VQ3.

[0076] At this time, the excitation current Imag is a negative current, the primary control unit 213 pulls down the second primary control signal VQ2, and controls the second primary driving unit 212 to turn off the second primary switch tube Q2 according to the second primary control signal VQ2, and the half-bridge flyback converter 100 enters the free resonance state again, and the conduction time of the second primary switch tube Q2 is the preset time Ton.

[0077] S103, controlling the first primary switch tube to be turned on at a time after the second primary switch tube is turned off and at a time interval of M / 2 free resonance cycles from the turn-off time.

[0078] Here, M is an odd number greater than or equal to 1.

[0079] For example, Figure 6 As shown, after the moment T2 of the first switching cycle and at the moment T0 which is 3 / 2 free resonance cycles apart from the moment T2, the drain voltage Vsrd of the secondary switch tube Q3 reaches a peak value, and the voltage across the first primary switch tube Q1 reaches a valley value. The second primary control signal VQ2 and the secondary control signal VQ3 maintain a low level, and the primary control unit 213 controls the second primary drive unit 212 to keep the second primary switch tube Q2 turned off according to the second primary control signal VQ2, and the secondary control unit 222 controls the secondary drive unit 221 to keep the secondary switch tube Q3 turned off according to the secondary control signal VQ3.

[0080] At this time, the primary control unit 213 pulls up the first primary control signal VQ1 and controls the first primary driving unit 211 to turn on the first primary switch tube Q1 according to the first primary control signal VQ1, so as to achieve valley turning on of the first primary switch tube Q1.

[0081] It should be noted that Figure 6 Only the case of M=3 is shown as an example, that is, when the half-bridge flyback converter 100 enters the free resonance state again after the second switch tube Q2 is turned on for a period of time, the first primary switch tube Q1 is turned on at a time interval of three times half of the free resonance cycle from the start time of the free resonance state to achieve valley switching. In practical applications, M can also be 1, 5 or an odd number greater than 5, and the present disclosure does not make specific restrictions on this.

[0082] In summary, when the half-bridge flyback converter 100 operates in the first switching cycle of DCM before the first primary switch tube Q1 is turned on, the half-bridge flyback converter 100 operates in a free resonance state. In the first switching cycle, the second primary switch tube Q2 is controlled to be turned on so that the half-bridge flyback converter 100 exits the free resonance state. The second primary switch tube Q2 is controlled to be turned on for a fixed time and then turned off, which can generate a negative excitation current Imag and the half-bridge flyback converter 100 enters the free resonance state again. At an odd multiple of the free resonance half cycle, the first primary switch tube Q1 is controlled to be turned on to achieve the valley turning on of the first primary switch tube Q1. The valley turning on of the first primary switch tube Q1 can not only improve the efficiency of the half-bridge flyback converter 100, but also reduce the peak stress when the secondary switch tube Q3 is turned off, thereby improving the EMI of the half-bridge flyback converter 100.

[0083] In some embodiments, M is less than or equal to 5.

[0084] For example, with the free resonance of the half-bridge flyback converter 100, the valley value of the voltage across the first primary switch tube Q1 becomes larger and larger. Therefore, by setting M to be less than or equal to 5, the first primary switch tube Q1 can be turned on when the valley value is small, thereby ensuring the valley bottom turning-on effect of the first primary switch tube Q1.

[0085] For example, M=1, when the half-bridge flyback converter 100 enters the free resonance state again after the second switch tube Q2 is turned on for a period of time, the first primary switch tube Q1 is turned on at a time interval of half a free resonance cycle from the start time of the free resonance state to achieve valley switching.

[0086] M=3, when the half-bridge flyback converter 100 enters the free resonance state again after the second switch tube Q2 is turned on for a period of time, the first primary switch tube Q1 is turned on at a time interval of three times of half a free resonance period from the start time of the free resonance state to achieve valley switching.

[0087] M=5, when the half-bridge flyback converter 100 enters the free resonance state again after the second switch tube Q2 is turned on for a period of time, the first primary switch tube Q1 is turned on at a time interval of five times of half a free resonance period from the start time of the free resonance state to achieve valley switching.

[0088] In some embodiments, the specific steps of the control method before S101 further include:

[0089] S201, in a first switching cycle, controlling the second primary switch tube to be turned on for a first time period so that the excitation current is zero.

[0090] For example, Figure 7 A schematic diagram of another operating waveform of a half-bridge flyback converter provided in an embodiment of the present disclosure is shown in FIG. Figure 7 As shown, the half-bridge flyback converter 100 continuously operates in CrM for two second switching cycles, and then operates in DCM for one first switching cycle.

[0091] like Figure 6 and Figure 7 As shown, at time T3 of the first switching period, the first primary control signal VQ1 maintains a low level, and the primary control unit 213 controls the first primary driving unit 211 to keep the first primary switch tube Q1 turned off according to the first primary control signal VQ1.

[0092] At this time, the primary control unit 213 pulls up the second primary control signal VQ2, and controls the second primary drive unit 212 to turn on the second primary switch tube Q2 according to the second primary control signal VQ2, and the secondary control unit 222 pulls up the secondary control signal VQ3, and controls the secondary drive unit 221 to turn on the secondary switch tube Q3 according to the secondary control signal VQ3. The resonant capacitor Cr and the resonant inductor Lr begin to resonate, the primary power circuit 110 transfers energy to the secondary power circuit 120, and the excitation current Imag decreases linearly.

[0093] After moment T3 of the first switching cycle and at moment T4 which is separated from moment T3 by a first time length Ton1, the excitation current Imag is zero, the primary control unit 213 pulls down the second primary control signal VQ2, and controls the second primary driving unit 212 to turn off the second primary switch tube Q2 according to the second primary control signal VQ2.

[0094] At this time, the first primary control signal VQ1 and the secondary control signal VQ3 maintain a low level, the primary control unit 213 controls the first primary drive unit 211 to keep the first primary switch tube Q1 turned off according to the first primary control signal VQ1, and the secondary control unit 222 controls the secondary drive unit 221 to keep the secondary switch tube Q3 turned off according to the secondary control signal VQ3, and the half-bridge flyback converter 100 enters a free resonance state.

[0095] It should be noted that Figure 7 It is only exemplified that the half-bridge flyback converter 100 works in CrM for two consecutive second switching cycles and then works in DCM for one first switching cycle. In practical applications, it is also possible to work in CrM for one second switching cycle and then work in DCM for one first switching cycle, or to work in CrM for three or more consecutive second switching cycles and then work in DCM for one first switching cycle, which is not specifically limited in the present disclosure.

[0096] In some embodiments, before the first switching cycle, the half-bridge flyback converter 100 continuously operates for N second switching cycles under CrM, where N is an integer greater than or equal to 1, and the specific steps of the control method before S101 further include:

[0097] S201', in the first switching cycle, controlling the second primary switch tube to be turned off.

[0098] For example, Figure 8 A schematic diagram of operating waveforms of another half-bridge flyback converter provided in an embodiment of the present disclosure is shown in FIG. Figure 8 As shown, the half-bridge flyback converter 100 continuously operates in CrM for two second switching cycles and then operates in DCM for one first switching cycle.

[0099] At time T3 of the first switching cycle, the secondary control unit 222 pulls up the secondary control signal VQ3 until the resonant current Ires is zero and then pulls down the secondary control signal VQ3. The secondary control unit 222 can control the secondary driving unit 221 to turn on the secondary switch tube Q3 at time T3 according to the secondary control signal VQ3, and turn off the secondary switch tube Q3 when the resonant current Ires is zero.

[0100] From the moment T3 of the first switching cycle to the end moment of the first switching cycle, the first primary control signal VQ1 and the second primary control signal VQ2 maintain a low level, and the primary control unit 213 controls the first primary driving unit 211 to keep the first primary switch tube Q1 turned off according to the first primary control signal VQ1, and controls the second primary driving unit 212 to keep the second primary switch tube Q2 turned off according to the second primary control signal VQ2.

[0101] In this way, after the resonant current Ires passes through zero, since the second primary switch tube Q2 is turned off, the resonant inductor Lr and the resonant capacitor Cr no longer resonate, and the excitation current Imag decreases linearly through the secondary switch tube Q3 until the excitation current Imag decreases to zero, and the half-bridge flyback converter 100 enters a free resonance state.

[0102] It should be noted that Figure 8 Only the case of N=2 is exemplified, that is, the half-bridge flyback converter 100 works for two consecutive second switching cycles under CrM and then works for one first switching cycle under DCM. In practical applications, N may also be 1, that is, the half-bridge flyback converter 100 works for one second switching cycle under CrM and then works for one first switching cycle under DCM, or N may also be an integer greater than or equal to 3, which is not specifically limited in the present disclosure.

[0103] In some embodiments, before the first switching cycle, the half-bridge flyback converter 100 continuously operates for N second switching cycles under CrM, and the specific steps of the control method before S101 further include:

[0104] S301, in a second switching cycle, controlling the second primary switch tube to be turned on for a second time period so that the excitation current is a negative current.

[0105] For example, Figure 7 and Figure 8 As shown, at time T3 ′ of the second switching period, the first primary control signal VQ1 maintains a low level, and the primary control unit 213 controls the first primary driving unit 211 to keep the first primary switch Q1 turned off according to the first primary control signal VQ1 .

[0106] At this time, the primary control unit 213 pulls up the second primary control signal VQ2, and controls the second primary drive unit 212 to turn on the second primary switch tube Q2 according to the second primary control signal VQ2, and the secondary control unit 222 pulls up the secondary control signal VQ3, and controls the secondary drive unit 221 to turn on the secondary switch tube Q3 according to the secondary control signal VQ3. The resonant capacitor Cr and the resonant inductor Lr begin to resonate, the primary power circuit 110 transfers energy to the secondary power circuit 120, and the excitation current Imag decreases linearly.

[0107] After the moment T3' of the second switching cycle, at the moment T4' which is separated from the moment T3' by the second time length Ton2, the second time length Ton2 is greater than the first time length Ton1, and the excitation current Imag is a negative current. At this time, the primary control unit 213 pulls down the second primary control signal VQ2, and controls the second primary driving unit 212 to turn off the second primary switch tube Q2 according to the second primary control signal VQ2.

[0108] At the same time, the first primary control signal VQ1 and the secondary control signal VQ3 are maintained at a low level. The primary control unit 213 controls the first primary driving unit 211 to keep the first primary switch tube Q1 turned off according to the first primary control signal VQ1, and the secondary control unit 222 controls the secondary driving unit 221 to keep the secondary switch tube Q3 turned off according to the secondary control signal VQ3.

[0109] After the moment T4' of the second switching cycle and at a moment separated from the moment T4' by a preset dead time, the primary control unit 213 pulls up the first primary control signal VQ1, and controls the first primary driving unit 211 to turn on the first primary switch tube Q1 according to the first primary control signal VQ1, thereby achieving zero voltage turn-on of the first primary switch tube Q1.

[0110] It should be noted that S301 can be executed before S201, and the working process of the half-bridge flyback converter 100 is as follows: Figure 7 Alternatively, S301 may be performed before S201′, and the working process of the half-bridge flyback converter 100 is as follows: Figure 8 shown.

[0111] In some embodiments, the specific steps of the control method before S201 further include:

[0112] In the first switching cycle, the first primary switch tube is controlled to be turned on for a third period of time so that the excitation current increases from zero to a preset value; after the first primary switch tube is turned off and at a moment separated by a dead time from the turning-off moment of the first primary switch tube, the second primary switch tube is controlled to be turned on.

[0113] For example, Figure 6 As shown, at time T5 of the first switching cycle, the second primary control signal VQ2 and the secondary control signal VQ3 maintain a low level, the primary control unit 213 controls the second primary drive unit 212 to keep the second primary switch tube Q2 turned off according to the second primary control signal VQ2, and the secondary control unit 222 controls the secondary drive unit 221 to keep the secondary switch tube Q3 turned off according to the secondary control signal VQ3.

[0114] At this time, the primary control unit 213 pulls up the first primary control signal VQ1 , and controls the first primary driving unit 211 to turn on the first primary switch Q1 according to the first primary control signal VQ1 , and the excitation current Imag is zero and starts to increase.

[0115] When the excitation current Imag increases to a preset value, the time difference between the moment when the excitation current Imag is the preset value and T5 is a third time length. At this time, the primary control unit 213 pulls down the first primary control signal VQ1, and controls the first primary driving unit 211 to turn off the first primary switch tube Q1 according to the first primary control signal VQ1.

[0116] At the same time, the second primary control signal VQ2 and the secondary control signal VQ3 maintain a low level, and the primary control unit 213 controls the second primary driving unit 212 to keep the second primary switch tube Q2 turned off according to the second primary control signal VQ2, and the secondary control unit 222 controls the secondary driving unit 221 to keep the secondary switch tube Q3 turned off according to the secondary control signal VQ3.

[0117] After the first primary switch tube Q1 is turned off, at time T3 which is a dead time apart from the turn-off time of the first primary switch tube Q1, the primary control unit 213 pulls up the second primary control signal VQ2, and controls the second primary driving unit 212 to turn on the second primary switch tube Q2 according to the second primary control signal VQ2.

[0118] At the same time, the first primary control signal VQ1 and the secondary control signal VQ3 maintain a low level, the primary control unit 213 controls the first primary driving unit 211 to keep the first primary switch tube Q1 turned off according to the first primary control signal VQ1, and the secondary control unit 222 controls the secondary driving unit 221 to keep the secondary switch tube Q3 turned off according to the secondary control signal VQ3.

[0119] In some embodiments, the specific steps of the control method before S301 further include:

[0120] In the second switching cycle, the first primary switch tube is controlled to be turned on for a third period of time so that the excitation current increases from zero to a preset value; after the first primary switch tube is turned off and at a moment separated by a dead time from the turning-off moment of the first primary switch tube, the second primary switch tube is controlled to be turned on.

[0121] For example, Figure 7 and Figure 8 As shown, at time T5' of the second switching cycle, the second primary control signal VQ2 and the secondary control signal VQ3 maintain a low level, the primary control unit 213 controls the second primary drive unit 212 to keep the second primary switch tube Q2 turned off according to the second primary control signal VQ2, and the secondary control unit 222 controls the secondary drive unit 221 to keep the secondary switch tube Q3 turned off according to the secondary control signal VQ3.

[0122] At this time, the primary control unit 213 pulls up the first primary control signal VQ1 , and controls the first primary driving unit 211 to turn on the first primary switch Q1 according to the first primary control signal VQ1 , and the excitation current Imag is zero and starts to increase.

[0123] When the excitation current Imag increases to the preset value, the time difference between the moment when the excitation current Imag is the preset value and T5' is the third time length. At this time, the primary control unit 213 pulls down the first primary control signal VQ1, and controls the first primary driving unit 211 to turn off the first primary switch tube Q1 according to the first primary control signal VQ1.

[0124] At the same time, the second primary control signal VQ2 and the secondary control signal VQ3 maintain a low level, and the primary control unit 213 controls the second primary driving unit 212 to keep the second primary switch tube Q2 turned off according to the second primary control signal VQ2, and the secondary control unit 222 controls the secondary driving unit 221 to keep the secondary switch tube Q3 turned off according to the secondary control signal VQ3.

[0125] After the first primary switch Q1 is turned off, at time T3' which is a dead time from the turn-off time of the first primary switch Q1, the primary control unit 213 pulls up the second primary control signal VQ2, and controls the second primary driving unit 212 to turn on the second primary switch Q2 according to the second primary control signal VQ2.

[0126] At the same time, the first primary control signal VQ1 and the secondary control signal VQ3 maintain a low level, the primary control unit 213 controls the first primary driving unit 211 to keep the first primary switch tube Q1 turned off according to the first primary control signal VQ1, and the secondary control unit 222 controls the secondary driving unit 221 to keep the secondary switch tube Q3 turned off according to the secondary control signal VQ3.

[0127] In some embodiments, the specific steps of the control method before S101 further include:

[0128] S401, sampling a sampling point voltage of a half-bridge flyback converter.

[0129] For example, Figure 3 and Figure 4 As shown, the primary control module 210 also includes a first primary sampling unit 214, the input end of the first primary sampling unit 214 is connected to the connection point of the first auxiliary sampling resistor R1 and the second auxiliary sampling resistor R2, then the connection point is the first sampling point, and the sampling voltage of the connection point is the first sampling point sampling voltage.

[0130] In other embodiments, the primary control module 210 further includes a second primary sampling unit 215, the input end of the second primary sampling unit 215 is connected to the connection point of the first primary switch tube Q1 and the second primary switch tube Q2, then the connection point is the second sampling point, that is, the midpoint of the half bridge, and the sampling voltage of the connection point is the second sampling point sampling voltage, that is, the midpoint voltage of the half bridge. Alternatively, the input end of the second primary sampling unit 215 is connected to the connection point of the sampling resistor Rs and the resonant capacitor Cr, then the connection point is the third sampling point, and the sampling voltage of the connection point is the third sampling point sampling voltage, or the input end of the second primary sampling unit 215 is connected to the connection point of the sampling resistor Rs and the second primary switch tube Q2, then the connection point is the fourth sampling point, and the sampling voltage of the connection point is the fourth sampling point sampling voltage.

[0131] It should be noted that the embodiments of the present disclosure are only described by taking the sampling point as the midpoint of the half bridge and the sampling point voltage as the midpoint voltage of the half bridge as an example.

[0132] The primary sampling unit can continuously sample the half-bridge midpoint voltage of the half-bridge flyback converter 100 to obtain the half-bridge midpoint sampling voltage. The half-bridge midpoint voltage oscillates and changes in the free resonance state, and the corresponding half-bridge midpoint sampling voltage oscillates and changes. The primary control unit 213 can determine the half-bridge midpoint sampling voltage information in the free resonance state based on the oscillation change of the half-bridge midpoint voltage in the free resonance state.

[0133] S402, determining a turn-on time of the second primary switch tube according to a valley value of a sampling voltage at a sampling point in a free resonance state.

[0134] Exemplarily, the half-bridge midpoint sampling voltage information may include the half-bridge midpoint sampling voltage change rate and the half-bridge midpoint sampling voltage change trend. The primary control unit 213 may determine the valley value of the half-bridge midpoint sampling voltage according to the half-bridge midpoint sampling voltage change rate and the half-bridge midpoint sampling voltage change trend in the free resonance state. For example, when the half-bridge midpoint sampling voltage change trend in the free resonance state is decreasing, and the half-bridge midpoint sampling voltage change rate is approximately zero, it can be determined that the half-bridge midpoint sampling voltage has reached the valley value.

[0135] Alternatively, the half-bridge midpoint sampling voltage information may include the lowest valley voltage of each oscillation cycle, and the lowest valley voltage of the current oscillation cycle is compared with the lowest valley voltage of the previous oscillation cycle to determine whether the lowest valley voltage of the current oscillation cycle is the valley value of the half-bridge midpoint sampling voltage. When it is determined that the lowest valley voltage of the current oscillation cycle is not the valley value of the half-bridge midpoint sampling voltage, as time goes by, the current oscillation cycle is updated to the next oscillation cycle, and the previous oscillation cycle is updated to the current oscillation cycle, and the above determination steps are repeated until it is determined that the lowest valley voltage of the current oscillation cycle is the valley value of the half-bridge midpoint sampling voltage.

[0136] The primary control unit 213 determines the moment corresponding to the valley value of the half-bridge midpoint sampling voltage as the turn-on moment of the second primary switch tube Q2, and at the turn-on moment of the second primary switch tube Q2, pulls up the second primary control signal VQ2, and controls the second primary driving unit 212 to turn on the second primary switch tube Q2 according to the second primary control signal VQ2.

[0137] The control circuit 200 provided in the present disclosure includes a primary control module 210, a first output end of the primary control module 210 is connected to the control end of the first primary switch tube Q1, a second output end of the primary control module 210 is connected to the control end of the second primary switch tube Q2, and the primary control module 210 includes a primary control unit 213.

[0138] The primary control unit 213 is configured to, when the half-bridge flyback converter 100 operates in a free resonance state, control the second primary switch tube Q2 to be turned on in a first switching cycle so that the half-bridge flyback converter 100 exits the free resonance state; after a preset turn-on time, control the second primary switch tube Q2 to be turned off so that the excitation current Imag is a negative current, and the half-bridge flyback converter 100 enters the free resonance state again; and control the first primary switch tube Q1 to be turned on at a moment M / 2 free resonance cycles apart from the turn-off moment of the second primary switch tube Q2, where M is an odd number greater than or equal to 1.

[0139] In some embodiments, M is less than or equal to 5.

[0140] In some embodiments, the primary control unit 213 is further configured to, in the first switching cycle, control the second primary switch tube Q2 to be turned on for a first duration Ton1 so that the excitation current Imag is zero.

[0141] In some embodiments, before the first switching cycle, the half-bridge flyback converter 100 continuously operates at CrM for N second switching cycles, where N is an integer greater than or equal to 1.

[0142] The primary control unit 213 is further configured to control the second primary switch tube Q2 to be turned off in the first cycle.

[0143] In some embodiments, before the first switching cycle, the half-bridge flyback converter 100 continuously operates at CrM for N second switching cycles, where N is an integer greater than or equal to 1.

[0144] The primary control unit 213 is further configured to control the second primary switch tube Q2 to be turned on for a second time period in the second switching cycle, so that the excitation current Imag is a negative current.

[0145] In some embodiments, see Figure 3 and Figure 4 The primary control module 210 further includes a first primary sampling unit 214 , an input end of the first primary sampling unit 214 is connected to a connection point between the first auxiliary sampling resistor R1 and the second auxiliary sampling resistor R2 , ie, a first sampling point.

[0146] In some embodiments, see Figure 3 and Figure 4 The primary control module 210 also includes a second primary sampling unit 215, the input end of the second primary sampling unit 215 is connected to the connection point of the first primary switch tube Q1 and the second primary switch tube Q2, that is, the midpoint of the half bridge, or the connection point of the sampling resistor Rs and the resonant capacitor Cr, that is, the third sampling point, or the connection point of the sampling resistor Rs and the second primary switch tube Q2, that is, the fourth sampling point.

[0147] Based on the above embodiment, the primary sampling unit is configured to sample the sampling point voltage of the half-bridge flyback converter 100. The primary control unit 213 is also configured to determine the turn-on time of the second primary switch tube Q2 according to the valley value of the sampling point sampling voltage in the free resonance state.

[0148] In some embodiments, see Figure 3 and Figure 4 The primary control module 210 further includes a first primary driving unit 211 , a second primary driving unit 212 , a first primary sampling unit 214 and a second primary sampling unit 215 .

[0149] The input end of the first primary sampling unit 214 is connected to the connection point of the first auxiliary sampling resistor R1 and the second auxiliary sampling resistor R2, the output end of the first primary sampling unit 214 is connected to the first input end of the primary control unit 213, the first output end of the primary control unit 213 is connected to the control end of the first primary switch tube Q1 through the first primary driving unit 211, and the second output end of the primary control unit 213 is connected to the control end of the second primary switch tube Q2 through the second primary driving unit 212.

[0150] For example, Figure 3 As shown, the output end of the second primary sampling unit 215 is connected to the second input end of the primary control unit 213, and the input end of the second primary sampling unit 215 is connected to the connection point of the sampling resistor Rs and the resonant capacitor Cr. Figure 4 As shown, the output end of the second primary sampling unit 215 is connected to the second input end of the primary control unit 213 , and the input end of the second primary sampling unit 215 is connected to the connection point of the sampling resistor Rs and the second primary switch tube Q2 .

[0151] In some embodiments, see Figure 3 and Figure 4 The control circuit 200 further includes a secondary control module 220 , and the secondary control module 220 includes a secondary driving unit 221 , a secondary sampling unit 223 and a secondary control unit 222 .

[0152] The output end of the secondary control unit 222 is connected to the control end of the secondary switch tube Q3 through the secondary driving unit 221 , the input end of the secondary sampling unit 223 is connected to the drain of the secondary switch tube Q3 , and the output end of the secondary sampling unit 223 is connected to the input end of the secondary control unit 222 .

[0153] The control circuit 200 provided in the embodiment of the present disclosure is used to execute the steps of any of the above-mentioned method embodiments, has functional modules corresponding to the method embodiments, and has the beneficial effects of the method embodiments, which will not be repeated here.

[0154] The present disclosure also provides a chip for executing the steps of any of the above method embodiments, having functional modules corresponding to the method embodiments, and having the beneficial effects of the method embodiments, which will not be described in detail here.

[0155] The present disclosure also provides a power supply device, including a half-bridge flyback converter 100 and a control circuit 200, wherein the control circuit 200 is used to execute the steps of any of the above-mentioned method embodiments. The power supply device has functional modules corresponding to the method embodiments and has the beneficial effects of the method embodiments, which will not be repeated here.

[0156] Unless the context clearly indicates otherwise, the singular form of the words used herein and in the appended claims includes the plural form and vice versa. Thus, when referring to the singular, the plural form of the corresponding term is generally included. Similarly, the words "comprise" and "include" are to be interpreted as inclusive rather than exclusive. Likewise, the terms "include" and "or" should be interpreted as inclusive, unless such interpretation is expressly prohibited herein. Where the term "example" is used herein, the "example" is merely illustrative and should not be considered exclusive or comprehensive.

[0157] Several embodiments of the present disclosure are described in detail above, but it is obvious that those skilled in the art can make various modifications and variations to the embodiments of the present disclosure without departing from the spirit and scope of the present disclosure. The protection scope of the present disclosure is defined by the attached claims.

Claims

1. A control method for a half-bridge flyback converter, characterized in that: The half-bridge flyback converter comprises: a primary power circuit and a secondary power circuit coupled to the primary power circuit, wherein the primary power circuit comprises a primary winding and a first primary switch tube and a second primary switch tube connected in series between a voltage input terminal and a ground, and the secondary power circuit comprises a secondary winding and a secondary switch tube connected to the secondary winding; The control method comprises: When the half-bridge flyback converter operates in a free resonance state, in a first switching cycle, the second primary switch tube is controlled to be turned on, so that the half-bridge flyback converter exits the free resonance state; After a preset conduction time, the second primary switch tube is controlled to be turned off so that the excitation current is a negative current, and the half-bridge flyback converter enters the free resonance state again; At a time that is M / 2 free resonance cycles apart from a turn-off time of the second primary switch tube, the first primary switch tube is controlled to be turned on, where M is an odd number greater than or equal to 1.

2. The control method according to claim 1, characterized in that: M is less than or equal to 5.

3. The control method according to claim 1, characterized in that: Before controlling the second primary switch tube to be turned on in the first switching cycle, the method further includes: In the first switching cycle, the second primary switch tube is controlled to be turned on for a first time period so that the excitation current is zero.

4. The control method according to claim 1, characterized in that: Before the first switching cycle, the half-bridge flyback converter continuously operates in a critical conduction mode (CrM) for N second switching cycles, where N is an integer greater than or equal to 1; Before controlling the second primary switch tube to be turned on in the first switching cycle, the method further includes: In the first switching cycle, the second primary switch tube is controlled to be turned off.

5. The control method according to claim 3 or 4, characterized in that: Before the first switching cycle, the half-bridge flyback converter continuously operates for N second switching cycles under CrM, where N is an integer greater than or equal to 1; Before controlling the second primary switch tube to be turned on in the first switching cycle, the method further includes: In the second switching cycle, the second primary switch tube is controlled to be turned on for a second time period so that the excitation current is a negative current.

6. The control method according to any one of claims 1 to 4, characterized in that: Before controlling the second primary switch tube to be turned on in the first switching cycle, the method further includes: Sampling a sampling point voltage of the half-bridge flyback converter; The turn-on time of the second primary switch tube is determined according to the valley value of the sampling voltage at the sampling point in the free resonance state.

7. A control circuit for a half-bridge flyback converter, characterized in that: The half-bridge flyback converter comprises: a primary power circuit and a secondary power circuit coupled to the primary power circuit, wherein the primary power circuit comprises a primary winding and a first primary switch tube and a second primary switch tube connected in series between a voltage input terminal and a ground, and the secondary power circuit comprises a secondary winding and a secondary switch tube connected to the secondary winding; The control circuit includes a primary control unit; The primary control unit is configured to, when the half-bridge flyback converter operates in a free resonance state, control the second primary switch tube to be turned on in a first switching cycle so that the half-bridge flyback converter exits the free resonance state; after a preset turn-on time, control the second primary switch tube to be turned off so that the excitation current is a negative current, and the half-bridge flyback converter enters the free resonance state again; and control the first primary switch tube to be turned on at a moment that is M / 2 free resonance cycles apart from the turn-off moment of the second primary switch tube, where M is an odd number greater than or equal to 1.

8. The control circuit according to claim 7, characterized in that: M is less than or equal to 5.

9. The control circuit according to claim 7, characterized in that: The primary control unit is further configured to, in the first switching cycle, control the second primary switch tube to be turned on for a first time period so that the excitation current is zero.

10. The control circuit according to claim 7, characterized in that: Before the first switching cycle, the half-bridge flyback converter continuously operates in a critical conduction mode (CrM) for N second switching cycles, where N is an integer greater than or equal to 1; The primary control unit is further configured to control the second primary switch tube to be turned off during the first switching cycle.

11. The control circuit according to claim 9 or 10, characterized in that: Before the first switching cycle, the half-bridge flyback converter continuously operates for N second switching cycles under CrM, where N is an integer greater than or equal to 1; The primary control unit is further configured to, in the second switching period, control the second primary switch tube to be turned on for a second time period so that the excitation current is a negative current.

12. The control circuit according to any one of claims 7 to 10, characterized in that: The control circuit also includes a primary sampling unit; The primary sampling unit is configured to sample a sampling point voltage of the half-bridge flyback converter; The primary control unit is further configured to determine a turn-on time of the second primary switch tube according to a valley value of a sampling voltage at a sampling point in the free resonance state.

13. A chip, characterized in that: Used to perform the steps of the method according to any one of claims 1 to 6.

14. A power supply device, characterized in that: The invention comprises a half-bridge flyback converter and a control circuit, wherein the control circuit is used to execute the steps of the method according to any one of claims 1 to 6.

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