Flyback converter and control device and control method thereof

Abstract

The invention discloses a flyback converter and a control device and a control method thereof. The control method comprises the following steps: acquiring a sampling signal representing current flowing through a main switch tube; determining whether the sampling signal satisfies a preset condition based on the plurality of negative reference thresholds, and providing an indication signal; the plurality of negative reference threshold values at least comprise a first threshold value and a second threshold value, the first threshold value is smaller than the second threshold value, the first threshold value and the second threshold value are both smaller than zero, the first time period is a time period when the sampling signal is smaller than or equal to the first threshold value, and the second time period is a time period when the sampling signal is smaller than or equal to zero. The second time period is a time period when the sampling signal is less than or equal to a second threshold value, and when the first time period is not less than the first preset time and the second time period is not less than the second preset time, determining that the sampling signal satisfies a preset condition. Through the arrangement, the reliability of negative current sampling can be improved, and the pulse width of the synchronous rectifier tube can be reasonably designed to optimize the efficiency.

Description

Technical Field

[0001] The present invention relates to the field of electronic power technology, and more specifically, to a flyback converter and a control device and a control method thereof. Background Art

[0002] Flyback converter is widely used in AC-DC and DC-DC conversion, and provides insulation isolation between the input stage and the output stage. It is a type of switching power supply. As a commonly used type of flyback converter, feedback elements such as optocouplers may not be set between the primary and secondary sides. This type of flyback converter includes a primary coil and a secondary coil. The primary coil is connected to a main switch tube, and the secondary coil is connected to a synchronous rectifier tube. The flyback converter provides energy to the load by turning on and off the main power switch tube. Therefore, the switching control of the main power switch tube is the control core of the flyback converter.

[0003] For the above-mentioned type of flyback converter, in order to achieve the zero voltage switch (Zero Voltage Switch, ZVS) of the primary main switch tube, the synchronous rectifier tube is delayed to turn off for a certain time or the synchronous rectifier tube is turned on for a certain pulse width twice before the main switch tube is turned on, and the transformer excitation inductance is additionally reversely excited. After the shutdown, this energy is used to achieve the zero voltage switch of the main switch tube. At the same time, in order to achieve communication between the primary coil and the secondary coil, the negative excitation sampling signal generated in the primary coil after the synchronous rectifier tube is turned off is usually used directly to transmit signals such as frequency. For example, at the beginning of each cycle, the synchronous rectifier tube is first controlled to turn on with a small pulse. When the small pulse is turned off, a corresponding negative current is generated on the primary side. The sampling of the negative current through the primary circuit sampling resistor generates corresponding negative pressure information to achieve the zero voltage switch of the main switch tube. At the same time, the primary circuit turns on the main switch tube after identifying the negative pressure information. However, due to the small amplitude of the negative current information, the negative pressure information identified by the primary circuit has reliability and noise immunity problems.

[0004] Therefore, it is desired to provide an improved flyback converter to solve the above problems. Summary of the invention

[0005] In view of the above problems, an object of the present invention is to provide a flyback converter and a control device and a control method thereof, so as to improve the reliability of negative current information sampling.

[0006] According to a first aspect of the present invention, a control method for a flyback converter is provided, wherein the flyback converter comprises a main switch tube located in a primary circuit and a synchronous rectifier tube located in a secondary circuit, and the control method comprises:

[0007] Acquiring a sampling signal representing a current flowing through the main switch tube;

[0008] Determining whether the sampled signal meets a preset condition based on a plurality of negative reference thresholds, and providing an indication signal; and

[0009] Control the main switch tube according to the indication signal,

[0010] The multiple negative reference thresholds include at least a first threshold and a second threshold, the first threshold is less than the second threshold, and both the first threshold and the second threshold are less than zero.

[0011] The first time period is a time period when the sampling signal is less than or equal to the first threshold, and the second time period is a time period when the sampling signal is less than or equal to the second threshold.

[0012] When the first time period is not less than a first preset time, and the second time period is not less than a second preset time, the sampling signal meets the preset condition.

[0013] Optionally, judging whether the sampled signal meets a preset condition based on a plurality of negative reference thresholds and providing an indication signal comprises:

[0014] comparing the sampled signal with the plurality of negative reference thresholds;

[0015] obtaining at least a portion of the first time period and the second time period; and

[0016] providing the indication signal,

[0017] When the sampling signal is less than or equal to the first threshold, timing is started to obtain at least a part of the first time period.

[0018] When at least a portion of the first time period reaches the first preset time, the sampling signal is compared with the second threshold value,

[0019] When the sampling signal is less than or equal to the second threshold, start timing to obtain at least a part of the second time period,

[0020] When at least a part of the second time period reaches a third preset time, a valid indication signal is provided to control the main switch tube to be turned on,

[0021] The second preset time is the sum of the first preset time and the third preset time.

[0022] Optionally, judging whether the sampled signal meets a preset condition based on a plurality of negative reference thresholds and providing an indication signal comprises:

[0023] comparing the sampled signal with the plurality of negative reference thresholds;

[0024] acquiring the first time period and the second time period; and

[0025] providing the indication signal,

[0026] When the sampling signal is less than or equal to the first threshold, timing is started to obtain the first time period, and when the first time period reaches the first preset time, timing of the first time period is stopped.

[0027] When the sampling signal is less than or equal to the second threshold, start timing to obtain the second time period, and when the second time period reaches the second preset time, stop timing the second time period.

[0028] When the first time period reaches the first preset time and the second time period reaches the second preset time, a valid indication signal is provided to control the main switch tube to be turned on.

[0029] Optionally, before obtaining the sampling signal of the current flowing through the main switch tube, the method further includes: judging whether the synchronous rectifier tube is in a predetermined state according to the sampling voltage of the main switch tube, wherein the predetermined state is a state where the synchronous rectifier tube is turned on for a predetermined period of time before the main switch tube is turned on and then turned off to realize the control of the main switch tube to be turned on at zero voltage,

[0030] When the synchronous rectifier tube is in a predetermined state, the current flowing through the main switch tube starts to be sampled.

[0031] Optionally, the sampled voltage represents the drain-source voltage of the main switch tube, or the flyback converter further includes an auxiliary winding located in the primary circuit, and the sampled voltage represents the voltage on the auxiliary winding.

[0032] Optionally, judging whether the synchronous rectifier tube is in a predetermined state according to the sampled voltage of the main switch tube includes:

[0033] Detecting the slope of the sampling voltage of the main switch tube;

[0034] When the absolute value of the slope is greater than / equal to a predetermined slope, detecting a falling edge of the sampling voltage;

[0035] When the falling edge is detected, determining whether the sampled voltage is less than a threshold voltage;

[0036] When the sampling voltage is less than the threshold voltage, it is determined that the synchronous rectifier is in a predetermined state.

[0037] Optionally, judging whether the synchronous rectifier tube is in a predetermined state according to the sampled voltage of the main switch tube includes:

[0038] Determining whether the volt-second product of the portion of the sampled voltage that is greater than or equal to the threshold voltage is greater than / equal to a predetermined volt-second product;

[0039] When the volt-second product of the portion of the sampled voltage that is greater than or equal to the threshold voltage is greater than / equal to a predetermined volt-second product, detecting a falling edge of the sampled voltage;

[0040] When the falling edge is detected, determining whether the sampled voltage is less than a threshold voltage;

[0041] When the sampling voltage is less than the threshold voltage, it is determined that the synchronous rectifier is in a predetermined state.

[0042] Optionally, when the sampling voltage is the drain-source voltage of the main switch tube, the threshold voltage is the voltage value of the input signal of the flyback converter; or

[0043] When the sampled voltage is the voltage on the auxiliary winding, the threshold voltage is zero.

[0044] Optionally, before determining whether the volt-second product of the portion of the sampled voltage that is greater than or equal to the threshold voltage is greater than / equal to a predetermined volt-second product, the following operations are performed:

[0045] When a zero-point signal representing the zero-crossing point of the sampling signal is detected, the volt-second product judgment unit is triggered to calculate the volt-second product of the portion of the sampling voltage that is greater than or equal to the threshold voltage.

[0046] Optionally, when the sampled voltage represents the voltage on the auxiliary winding, the voltage representing the auxiliary winding during the conduction period of the synchronous rectifier is recorded as the first voltage, and when it is detected that the sampled voltage is slightly less than the first voltage, a zero-point signal representing the zero-crossing point of the sampled signal is detected;

[0047] When the sampling voltage represents the drain-source voltage of the main switch tube, the voltage representing the drain-source voltage of the main switch tube during the conduction period of the synchronous rectifier tube is the second voltage. When it is detected that the sampling voltage is slightly smaller than the second voltage, a zero point signal representing the zero crossing point of the sampling signal is detected.

[0048] Optionally, when the synchronous rectifier is in a predetermined state, it is determined whether the sampling signal meets a preset condition within a predetermined time period; if it is not detected that the sampling signal meets the preset condition within the predetermined time period, the slope detection or the volt-second product detection is re-enabled.

[0049] Optionally, when the sampling voltage is the drain-source voltage of the main switch tube, when the sampling voltage rises above the input signal, the slope detection or the volt-second product detection is re-enabled;

[0050] When the sampled voltage is the voltage on the auxiliary winding, the slope detection or the volt-second product detection is re-enabled when the sampled voltage rises and crosses zero.

[0051] The present invention further provides a control device for a flyback converter, based on which the control method described above is implemented, the control device comprises:

[0052] A sampling module, used to obtain a sampling signal representing the current flowing through the main switch tube;

[0053] A judging module, which judges whether the sampled signal meets a preset condition based on a plurality of negative reference thresholds, and provides an indication signal; and

[0054] A control module controls the main switch tube according to the indication signal,

[0055] The multiple negative reference thresholds include at least a first threshold and a second threshold, the first threshold is less than the second threshold, and both the first threshold and the second threshold are less than zero.

[0056] The first time period is a time period when the sampling signal is less than or equal to the first threshold, and the second time period is a time period when the sampling signal is less than or equal to the second threshold.

[0057] When the first time period is not less than a first preset time, and the second time period is not less than a second preset time, the sampling signal meets the preset condition.

[0058] The present invention also provides a flyback converter, comprising:

[0059] A main switch tube, a transformer and a synchronous rectifier tube, the transformer includes a primary coil and a secondary coil, the main switch tube is connected to the primary coil, and the synchronous rectifier tube is connected to the secondary coil; and

[0060] Based on the control method described above, control is performed to determine whether the main switch is turned on.

[0061] The flyback converter and its control device and control method provided by the present invention can effectively avoid noise interference and improve the reliability of negative current sampling of the main switch tube by setting multiple negative reference thresholds and multiple preset times; further, under the premise of ensuring the reliability of negative current sampling, the changing negative current waveform sampling is matched, and compared with the setting of a single threshold, the pulse width of the synchronous rectifier tube can be reasonably designed to optimize efficiency.

[0062] In order to solve the problem of malfunction of the main switch tube under interference and avoid the main switch tube and the synchronous rectifier tube being connected in common, the flyback converter and its control device and control method add volt-second product detection or slope judgment before negative current detection, which can avoid malfunction of the main switch tube and improve the robustness of the flyback converter. BRIEF DESCRIPTION OF THE DRAWINGS

[0063] The above and other objects, features and advantages of the present invention will become more apparent through the following description of the embodiments of the present invention with reference to the accompanying drawings, in which:

[0064] Figure 1 A circuit diagram of a conventional flyback converter is shown;

[0065] Figure 2 Shown according to Figure 1 A working waveform diagram of a flyback converter;

[0066] Figure 3 Shown according to Figure 1 Another working waveform diagram of the flyback converter;

[0067] Figure 4 A schematic diagram of a flyback converter according to an embodiment of the present invention is shown;

[0068] Figure 5 A working waveform diagram of a flyback converter according to an embodiment of the present invention is shown;

[0069] Figure 6 A flow chart of a control method according to a first embodiment of the present invention is shown;

[0070] Figure 7 A flow chart of a control method according to a second embodiment of the present invention is shown;

[0071] Figure 8 A flow chart of a control method according to a third embodiment of the present invention is shown. DETAILED DESCRIPTION

[0072] The present invention will be described in more detail below with reference to the accompanying drawings. In each of the accompanying drawings, the same elements are represented by similar reference numerals. For the sake of clarity, the various parts in the accompanying drawings are not drawn to scale. In addition, some well-known parts may not be shown in the drawings.

[0073] Many specific details of the present invention are described below, such as device structure, materials, dimensions, processing technology and techniques, so as to more clearly understand the present invention. However, as those skilled in the art will appreciate, the present invention may be implemented without following these specific details.

[0074] It should be understood that the connection / coupling of A and B in the embodiment of the present application means that A and B can be connected in series or in parallel, or A and B are connected through other devices, and the embodiment of the present application is not limited to this.

[0075] The embodiments of the flyback converter and its control device and control method provided by the present application will be described below in conjunction with the accompanying drawings.

[0076] Figure 1 A circuit diagram of a conventional flyback converter is shown.

[0077] like Figure 1 As shown, the flyback converter 100 includes a primary circuit 110 and a secondary circuit 120. The primary circuit 110 mainly includes a primary coil N P And the main switch tube Q P The secondary circuit 120 includes the secondary coil N S , synchronous rectifier Q SR And output capacitor Co. Primary coil N P and secondary coil N S There is a magnetic core in between to form a transformer.

[0078] In order to realize the primary side main switch tube Q P Zero Voltage Switch (ZVS) is a synchronous rectifier tube Q SR Delay the shutdown for a certain time or the synchronous rectifier Q SR In the main switch tube Q P Before turning on, a certain pulse width is applied twice to reversely excite the transformer magnetizing inductance. After turning off, this energy is used to realize the main switch tube Q P Zero voltage switching.

[0079] Specifically, Figure 2 Shown according to Figure 1 A working waveform diagram of a flyback converter; Figure 3 Shown according to Figure 1 Another working waveform of the flyback converter. Figure 2 and Figure 3 In, V GS_P Represents the gate voltage of the main switch tube, V DS_P represents the drain-source voltage of the main switch tube, Vin represents the input signal (for example, a DC signal obtained by rectifying an AC signal), I P Represents the current flowing through the main switch tube, I SR Represents the current flowing through the synchronous rectifier, V DS_SR Represents the drain-source voltage of the synchronous rectifier, V GS_SR Represents the gate voltage of the synchronous rectifier.

[0080] like Figure 2 As shown, in order to realize the primary main switch tube Q P Zero voltage switching, synchronous rectifier Q SR In the main switch tube Q P Before opening, a certain pulse width t will be opened twice w That is, during the time period t2-t3, the gate voltage V GS_SR Pull up a certain pulse width t w , the transformer excitation inductance is additionally reversely excited, and after shutdown, this energy is used to realize the main switch tube Q P Zero voltage switching.

[0081] like Figure 3 As shown, in order to realize the primary main switch tube Q P Zero voltage switching, synchronous rectifier Q SR Delay shutdown for a certain time t w That is, during the time period t1-t2, the gate voltage V GS_SR Pull up a certain pulse width t w , the transformer excitation inductance is additionally reversely excited, and after shutdown, this energy is used to realize the main switch tube Q P Zero voltage switching.

[0082] Figure 4 A schematic diagram of a flyback converter according to an embodiment of the present invention is shown; Figure 5 A working waveform diagram of a flyback converter according to an embodiment of the present invention is shown.

[0083] like Figure 4 As shown, the flyback converter 200 includes a primary circuit 210, a secondary circuit 220 and a control device 230. The main components of the primary circuit 210 include a primary coil N P And the main switch tube Q P The secondary circuit 120 includes the secondary coil N S , synchronous rectifier Q SR And output capacitor Co. Primary coil N P and secondary coil N S There is a magnetic core between them to form a transformer. Optionally, the primary circuit 210 also includes an auxiliary winding N aux , in order to obtain the voltage VS.

[0084] The operation of the flyback converter 200 is divided into a main switch on stage and a main switch off stage. P When conducting, the primary coil N P Directly connected to the input voltage. Primary coil N P The current in the transformer and the magnetic field in the transformer core increase, storing energy in the core. SThe voltage generated in the synchronous rectifier Q SR The load is powered by the capacitor. P When turned off, the primary coil N P The current in the secondary coil is 0, and a forward voltage is induced on the secondary coil. At this time, the synchronous rectifier Q SR When it is turned on, the energy stored in the core is transferred to the output capacitor Co and the load.

[0085] In the embodiment of the present invention, the control device 230 includes a current sampling module 231 , a judgment module 232 and a control module 233 . Optionally, the control device 230 also includes an enabling module 234 .

[0086] The current sampling module 231 is connected to the main switch tube Q P , used to obtain the characteristic of the current flowing through the main switch tube Q P The judging module 232 is connected to the current sampling module 231, and judges whether the sampling signal meets the preset conditions based on multiple negative reference thresholds, and provides an indication signal; the control module 233 is connected to the judging module 232, and is used to control the main switch tube Q according to the indication signal. P The sampling signal, for example, flows through the main switch tube Q P The current can also be the sampling resistor R CS The voltage across the two ends can represent the current flowing through the main switch tube Q P The current flowing through the main switch tube Q P The electrical signal of the current.

[0087] exist Figure 5 In the figure, VS represents the voltage on the auxiliary winding, and the meanings of the other signals can be found in Figure 2 and Figure 3 The description in , will not be repeated here. Among them, Figure 5 and Figure 2 The conduction timing of the synchronous rectifier in the circuit is the same, that is, the ZVS of the main switch tube is achieved by the secondary conduction of the synchronous rectifier. Figure 5 The working principle of the present invention is further explained.

[0088] like Figure 5 As shown, the multiple negative reference thresholds include at least a first threshold Ineg_1 and a second threshold Ineg_2, and the first threshold Ineg_1 is less than the second threshold Ineg_2; wherein the reference threshold here can be a voltage reference threshold or a current reference threshold, which can be set according to different application scenarios, for example, for Figure 4 Through the sampling resistor R CS When a sampling signal is obtained, the reference threshold can be designed as a voltage signal.

[0089] In the embodiment of the present invention, the first time period is defined as a time period when the sampling signal is less than or equal to the first threshold value Ineg_1, and the second time period is defined as a time period when the sampling signal is less than or equal to the second threshold value Ineg_2. When the first time period is not less than the first preset time, and the second time period is not less than the second preset time, the judgment module 232 determines that the sampling signal meets the preset condition.

[0090] like Figure 5 As shown, the first time period includes at least time period td1, and time period td1 starts timing when it is detected that the sampling signal is less than or equal to the first threshold value Ineg_1, and ends timing after time period td1 reaches the first preset time; the second time period includes at least time period td2, and time period td2 starts timing when time period td1 reaches the first preset time and the sampling signal is detected to be less than or equal to the first threshold value Ineg_2, and ends timing after time period td2 reaches the third preset time. In this embodiment, the second preset time is the sum of the first preset time and the third preset time. Therefore, when time period td1 reaches the first preset time and time period td2 reaches the third preset time, it can be deduced that the sum of time periods td1 and td2 reaches the second preset time, that is, the sampling signal meets the preset condition.

[0091] In actual operation, the complete values ​​of the first time period and the second time period can be calculated, but in order to improve efficiency, as another embodiment of the present invention, the next step can also be executed after the first time period reaches the first preset time and the second time period reaches the second preset time.

[0092] In an embodiment of the present invention, a lower first threshold value Ineg_1 is set to detect whether the time when the sampling signal is lower than the first threshold value Ineg_1 reaches a first preset time. This method can appropriately increase the threshold value relative to a single threshold value, reduce negative excitation energy, and improve system efficiency; a higher second threshold value Ineg_2 is set to detect whether the sampling signal is lower than the second threshold value Ineg_2 for a second preset time. When it is detected that the sampling signal meets the above preset conditions, the main switch tube will be controlled to turn on. This method can well match the problem of the negative sampling signal gradually increasing during action.

[0093] In some embodiments, the current sampling module 231 detects the current flowing through the main switch tube QP to obtain a sampling signal. Once the sampling signal is lower than the first threshold value Ineg_1, timing starts. When the timing time reaches the first preset time, it is determined whether the sampling signal remains lower than the second threshold value Ineg_2. If the sampling signal remains lower than the second threshold value Ineg_2, the timing is restarted. When the timing time reaches the third preset time, it is determined that the sampling signal meets the preset condition.

[0094] In other embodiments, the current sampling module 231 detects the current flowing through the main switch tube QP to obtain a sampling signal. Once the sampling signal is lower than the first threshold value Ineg_1, timing starts. When the timing time reaches the first preset time, timing of the first time period is stopped. At the same time, when the sampling signal is less than or equal to the second threshold value Ineg_2, timing will also start. When the timing time reaches the second preset time, timing of the second time period is stopped. When the first time period reaches the first preset time and the second time period reaches the second preset time, it is determined that the sampling signal meets the preset condition.

[0095] It should be understood that the first threshold, the second threshold, the first preset time, the second preset time and the third preset time in the embodiment of the present invention can be set according to actual circuit requirements.

[0096] As an example, the judgment module 232 includes: a comparison unit, which is used to compare the sampling signal with multiple negative reference thresholds; a timing unit, which is used to obtain at least a part of the first time period and the second time period; and an indication unit, which is used to provide an indication signal. For example, when the comparison unit detects that the sampling signal is less than or equal to the first threshold, the timing unit is triggered to start timing to obtain at least a part of the first time period. When at least a part of the first time period reaches the first preset time, the comparison unit is triggered to compare the sampling signal with the second threshold. When the comparison unit detects that the sampling signal is less than or equal to the second threshold, the timing unit is triggered to start timing to obtain at least a part of the second time period. When at least a part of the second time period reaches the third preset time, the indication unit is triggered to provide a valid indication signal to control the main switch tube to be turned on. For another example, when the comparison unit detects that the sampling signal is less than or equal to the first threshold, the timing unit is triggered to start timing to obtain a first time period. At the same time, when the comparison unit detects that the sampling signal is less than or equal to the second threshold, the timing unit is triggered to start timing to obtain a second time period. When the first time period reaches the first preset time, the timing of the first time period is stopped. When the second time period reaches the second preset time, the timing of the second time period is stopped. When the first time period reaches the first preset time and the second time period reaches the second preset time, the indication unit is triggered to provide a valid indication signal to control the main switch tube to be turned on.

[0097] In some other embodiments, an enabling module 234 is added to prevent the primary circuit 210 from malfunctioning and avoid the primary and secondary circuits from being in common. For example, the enabling module 234 is used to determine the synchronous rectifier Q according to the corresponding sampling voltage. SR Is it a predetermined state? The predetermined state means that before the main switch tube is turned on, the synchronous rectifier tube Q SR Predetermined conduction time period t w After that, it is turned off to realize the state of zero voltage conduction of the main switch tube. SRWhen the synchronous rectifier tube QP is in the predetermined state, the enabling module 234 triggers the current sampling module 231 to start sampling the current flowing through the main switch tube QP. This solution can further improve the anti-interference ability of the circuit and prevent the main switch tube from being mis-turned on. Optionally, the current sampling module 231 continuously detects the current flowing through the main switch tube QP to obtain a sampling signal. SR When the state is predetermined, the enabling module 234 triggers the determining module 232 to process the sampling signal.

[0098] In some embodiments, the enabling module 234 includes a slope determination unit and an enabling unit. The slope determination unit is used to determine the synchronous rectifier Q according to the slope of the corresponding sampled voltage. SR Whether it is a predetermined state; the enabling unit is used to SR When the absolute value of the slope of the sampled voltage is greater than / equal to the predetermined slope, the sampled voltage has a falling edge, and the sampled voltage is less than the corresponding threshold, the slope judgment unit determines that the synchronous rectifier tube Q SR Specifically, the slope judgment unit first detects the slope of the sampled voltage; when the absolute value of the slope of the sampled voltage is greater than / equal to the predetermined slope, the falling edge of the sampled voltage is detected; when the falling edge is detected, it is determined whether the sampled voltage is less than the threshold voltage. When the sampled voltage is a voltage signal VS representing the auxiliary winding ( Figure 4 The threshold voltage is zero, and when the sampling voltage is the drain-source voltage of the main switch tube V DS_P When the sampling voltage is less than the threshold voltage, the synchronous rectifier tube Q SR The sampling voltage can also be the drain-source voltage V representing the main switch tube. DS_P A voltage is generated, and the threshold voltage can be changed accordingly.

[0099] It should be noted that although the above description of the working principle of the present invention is based on Figure 2 The working waveform in the present invention is introduced, but the core idea of ​​the present invention is also applicable to Figure 3 The waveform in FIG. 1 is also applicable to the solution of achieving ZVS of the main switch tube by extending the conduction time of the synchronous rectifier tube; it will not be repeated here.

[0100] In some other embodiments, the enabling module 234 includes a zero point detection unit, a volt-second product judgment unit and an enabling unit. The zero point detection unit is used to trigger the volt-second product judgment unit to calculate the volt-second product of the sampled voltage when a zero point signal representing the zero point of the sampled signal is detected; the volt-second product judgment unit is used to judge the synchronous rectifier tube Q according to the volt-second product of the sampled voltage. SR Whether it is a predetermined state; the enabling unit is used toSR When the sampling voltage is greater than or equal to the threshold voltage, the volt-second product is greater than / equal to the predetermined volt-second product, the sampling voltage has a falling edge, and the sampling voltage is less than the corresponding threshold value, the volt-second product judgment unit determines that the synchronous rectifier tube Q SR Specifically, the sampling signal can represent the current I flowing through the main switch tube. P , the zero-point detection unit detects the zero-point signal of the sampling signal (that is, the point where the current flowing through the main switch tube is zero). In actual operation, the zero-point of the sampling signal is difficult to detect. The zero-point signal can be detected based on the comparison of the sampling voltage with the corresponding reference voltage. When the sampling voltage represents the voltage on the auxiliary winding, the voltage representing the auxiliary winding during the conduction period of the synchronous rectifier tube is recorded as the first voltage, and the reference voltage at this time is a value slightly smaller than the first voltage; when the sampling voltage represents the drain-source voltage of the main switch tube, the voltage representing the drain-source voltage of the main switch tube during the conduction period of the synchronous rectifier tube is the second voltage, and the reference voltage at this time is a value slightly smaller than the second voltage. When it is detected that the sampling voltage is slightly less than the corresponding reference voltage, the zero-point signal representing the zero-point of the sampling signal is detected. The specific values ​​of the first voltage and the second voltage can be set by those skilled in the art according to actual conditions, as long as the zero-point signal can be identified. When a zero-point signal is detected, the volt-second product judgment unit is triggered to calculate the volt-second product of the sampling voltage, which refers to the volt-second product when the sampling voltage is greater than or equal to the threshold voltage, that is, to detect whether the sampling voltage is greater than or equal to the threshold voltage. When the sampling voltage is greater than or equal to the threshold voltage, the volt-second product of the sampling voltage is calculated; the volt-second product judgment unit compares the calculated volt-second product with the predetermined volt-second product to determine whether the volt-second product of the sampling voltage is greater than / equal to the predetermined volt-second product; when the volt-second product of the sampling voltage is greater than / equal to the predetermined volt-second product, the falling edge of the sampling voltage is detected; when the falling edge is detected, it is determined whether the sampling voltage is less than the threshold voltage; when the sampling voltage is less than the threshold voltage, the synchronous rectifier tube Q is determined. SR It is in the scheduled state.

[0101] It should be understood that the enabling module in the above solution can be used in flyback converters of various working modes, for example, it can be applied to Figure 2 and Figure 3 The flyback converter is shown in the working mode. Figure 3 When the flyback converter is in the working mode shown in FIG. 1 , if the enabling module adopts the volt-second product detection method described above, the synchronous rectifier tube needs to be turned off briefly at time t1; in addition, when the enabling module is applied to Figure 2 When the flyback converter is in the working mode shown in the figure, if the enabling module adopts the volt-second product detection method, the zero point detection unit can also be omitted.

[0102] In addition, in some embodiments, a detection time period t representing the detection validity period may be set. wait , the first time period td1 and the second time period td2 are both within the detection time period t wait Detection time period t wait It is the synchronous rectifier Q SR The time period set after the predetermined state is met can be set according to the actual situation. wait , when it is detected that the sampling signal does not meet the preset conditions, the slope detection or volt-second product detection will be re-enabled. In addition, when the voltage on the auxiliary winding rises through zero or the drain-source voltage of the main switch rises over Vin, or the voltage value representing the drain-source voltage of the main switch rises over the corresponding threshold, the slope detection or volt-second product detection will be re-enabled.

[0103] Some examples of the flyback converter according to the embodiment of the present invention are described above. However, the embodiment of the present invention is not limited thereto and there may be other extensions and deformations.

[0104] For example, it should be understood that the threshold voltage in the aforementioned embodiment may be a reference ground voltage, or may be replaced by other non-zero reference potentials (having a positive voltage amplitude or a negative voltage amplitude) or a reference signal with controlled variation.

[0105] For another example, the coil inductance and capacitance provided in the embodiments of the present application may be lumped parameter capacitor elements and inductor elements, or may be other equivalent elements with similar functions to capacitors and inductors. The equivalent structures described here may be, for example but not limited to, microstrip lines, varactors, conductor structures with certain patterns, and the like that may provide inductive impedance and / or capacitive impedance.

[0106] This article also provides a control method for a flyback converter, which can achieve all the functions described above.

[0107] Specifically, Figure 6 A flow chart of a control method according to a first embodiment of the present invention is shown.

[0108] like Figure 6 As shown, the control method includes steps S101 to S104.

[0109] In step S101, it is determined whether the synchronous rectifier is in a predetermined state according to the sampled voltage. This step is an optional step, which is used to prevent malfunction of the primary circuit and avoid the primary and secondary circuits from being connected. For example, it is determined whether the synchronous rectifier is in a predetermined state according to the sampled voltage. The predetermined state means that before the main switch tube is turned on, the synchronous rectifier tube Q SR Predetermined conduction time period t wAfter that, the main switch tube is turned off to realize the state of zero voltage conduction of the control main switch tube. When the synchronous rectifier tube is in a predetermined state, the next step is executed. This step can improve the anti-interference ability of the circuit and prevent the main switch tube from being mis-turned on. In the present invention, the sampled voltage represents the voltage on the auxiliary winding or the voltage between the drain and the source on the main switch tube.

[0110] As an example, in this step, when the absolute value of the slope of the sampled voltage is greater than / equal to the predetermined slope, the sampled voltage has a falling edge, and the sampled voltage is less than the threshold voltage, the slope judgment module determines that the synchronous rectifier is in a predetermined state. Specifically, the slope of the sampled voltage of the main switch tube is detected; when the slope is greater than / equal to the predetermined slope, the falling edge of the sampled voltage is detected; when the falling edge is detected, it is determined whether the sampled voltage is less than the threshold voltage; when the sampled voltage is less than the threshold voltage, it is determined that the synchronous rectifier is in a predetermined state.

[0111] As another example, in this step, when the volt-second product of the portion of the sampled voltage greater than or equal to the threshold voltage is greater than / equal to a predetermined volt-second product, the sampled voltage has a falling edge, and the sampled voltage is less than the threshold voltage, the volt-second product judgment unit determines that the synchronous rectifier is in a predetermined state. Specifically, when a zero-point signal is detected, a volt-second product detection is performed on the sampled voltage; it is determined whether the volt-second product of the portion of the sampled voltage greater than or equal to the threshold voltage is greater than / equal to a predetermined volt-second product; when the volt-second product of the portion of the sampled voltage greater than or equal to the corresponding threshold voltage is greater than / equal to a predetermined volt-second product, a falling edge of the sampled voltage is detected; when a falling edge is detected, it is determined whether the sampled voltage is less than the threshold voltage; when the sampled voltage is less than the threshold voltage, it is determined that the synchronous rectifier is in a predetermined state.

[0112] In the above example, when the sampled voltage is the drain-source voltage of the main switch tube, the threshold voltage is the voltage value of the input signal of the flyback converter; when the sampled voltage represents the voltage on the auxiliary winding of the primary circuit, the threshold voltage is zero. The sampled voltage can also represent the drain-source voltage V of the main switch tube. DS_P A voltage is generated, and the threshold voltage can be changed accordingly.

[0113] In step S102, a sampling signal representing the current flowing through the main switch tube is obtained. The sampling signal is, for example, the current flowing through the main switch tube, or the voltage across the sampling resistor (the voltage can represent the current flowing through the main switch tube), or other electrical signals that can represent the current flowing through the main switch tube.

[0114] In step S103, based on multiple negative reference thresholds, it is determined whether the sampling signal meets the preset condition, and an indication signal is provided. In this step, the multiple negative reference thresholds include at least a first threshold and a second threshold, the first threshold is less than the second threshold, the first time period is a time period when the sampling signal is less than or equal to the first threshold, and the second time period is a time period when the sampling signal is less than or equal to the second threshold. When the first time period is not less than the first preset time, and the second time period is not less than the second preset time, it is determined that the sampling signal meets the preset condition.

[0115] Please refer to Figure 5 , the first time period at least includes time period td1, time period td1 starts when the sampling signal is less than or equal to the first threshold value Ineg_1, and ends when time period td1 reaches the first preset time; the second time period at least includes time period td2, time period td2 starts when time period td1 reaches the first preset time and the sampling signal is detected to be less than or equal to the first threshold value Ineg_2, and ends when time period td2 reaches the third preset time. In this embodiment, the second preset time is the sum of the first preset time and the third preset time. Therefore, when time period td1 reaches the first preset time and time period td2 reaches the third preset time, it can be deduced that the sum of time periods td1 and td2 reaches the second preset time, that is, the sampling signal meets the preset condition.

[0116] In actual operation, the complete values ​​of the first time period and the second time period can be calculated, but in order to improve efficiency, the next step can also be performed after the first time period reaches the first preset time and the second time period reaches the second preset time.

[0117] In step S104, the main switch is controlled according to the indication signal. In this step, if the indication signal is valid, it means that the indication signal is generated after the synchronous rectifier is turned off after being turned on for a predetermined period of time, and the turn-on condition of the main switch of the primary circuit is met, and the main switch is controlled to be turned on; if the indication signal is invalid, it means that the turn-on condition of the main switch of the primary circuit is not met, and the main switch is controlled to be turned off.

[0118] Figure 7 A flow chart of a control method according to a second embodiment of the present invention is shown.

[0119] like Figure 7 As shown, the control method includes steps S201-S206, and the control method starts when the main switch tube is in the off state.

[0120] In step S201, the slope of the sampled voltage is detected. Figure 5The sampling voltage (representing the voltage on the auxiliary winding or the voltage between the drain and source of the main switch tube) has different slopes in different stages, but only when the synchronous rectifier tube is in a predetermined state (in a predetermined time period t w When the synchronous rectifier is turned on during the period and then turned off), the absolute value of the slope of the sampled voltage is significantly greater than the absolute value of the slope in other time periods, and the slope must be a negative value. According to the above principle, after the slope of the sampled voltage is detected, steps S202-S204 are executed to further determine whether the synchronous rectifier is in a predetermined state.

[0121] In step S202, it is determined whether the slope is greater than or equal to a predetermined slope (the slope value refers to the size of the absolute value). If the slope is greater than or equal to the predetermined slope, step S203 is executed; if the slope is less than the predetermined slope, the process returns to step S201 to continue detecting the slope of the sampled voltage of the main switch tube.

[0122] In step S203, it is determined whether a falling edge of the sampled voltage is detected. If a falling edge is detected (ie, the sampled voltage is in a falling state at this time), step S204 is executed; if no falling edge is detected, detection is continued.

[0123] In step S204, it is determined whether the sampled voltage is less than the threshold voltage. When the sampled voltage is less than the threshold voltage, it is determined that the synchronous rectifier is in a predetermined state, and step S205 is executed. Otherwise, it is determined that the synchronous rectifier is in an off state, and step S201 is returned to be executed.

[0124] It should be understood that there are many ways to detect the slope, and other slope detection steps can be used to replace steps S201-204.

[0125] In step S205, negative current detection is enabled. If the current of the main switch tube is not continuously monitored, this step is to start detecting the current flowing through the main switch tube; if the current of the main switch tube is continuously monitored, this step is to start judging and processing the sampling signal representing the current flowing through the main switch tube.

[0126] In step S206, it is determined whether the sampling signal meets the preset conditions. In this step, within a predetermined time period, based on multiple negative reference thresholds, it is determined whether the sampling signal meets the preset conditions. The specific determination method can refer to the above description and will not be repeated here. When the sampling signal meets the preset conditions, the main switch tube is turned on. If the sampling signal does not meet the preset conditions, it returns to step S201.

[0127] In this embodiment, the predetermined threshold of the slope is set to distinguish the decline of normal free resonance from the rapid decline after the reverse excitation of the synchronous rectifier tube. After the slope is satisfied, the sampling voltage is lower than the corresponding threshold voltage (that is, the voltage on the auxiliary winding is lower than zero, or the drain-source voltage of the main switch tube is lower than Vin, or the voltage value representing the drain-source voltage of the main switch tube is lower than the corresponding threshold), and the primary negative current detection is enabled within a predetermined time period. If it is detected that the sampling signal meets the preset condition, the primary switch tube is turned on. If not, the slope detection needs to be re-enabled. In addition, if the voltage on the auxiliary winding rises through zero or the drain-source voltage of the main switch tube rises through Vin, or the voltage value representing the drain-source voltage of the main switch rises through the corresponding threshold, the slope detection will also be re-enabled.

[0128] Figure 8 A flow chart of a control method according to a third embodiment of the present invention is shown.

[0129] like Figure 8 As shown, the control method includes steps S301-S308.

[0130] In step S301, a zero-point signal representing the zero-crossing point of the sampling signal is detected. Figure 5 , the zero-point signal corresponds to the effective starting point of the volt-second product of the sampled voltage. The volt-second product is calculated from this starting point, and steps S302-306 are executed to determine whether the synchronous rectifier is in a predetermined state according to the volt-second product.

[0131] In step S302, the sampled voltage is subjected to volt-second product detection. The volt-second product detection refers to calculating the differential product of the time of the sampled voltage and the voltage, corresponding to Figure 5 The voltage VS shaded area, the first shaded area A1 is less than the predetermined volt-second product, and the second shaded area B1 is greater than / equal to the predetermined volt-second product. It should be understood that if the control method is used for Figure 2 The flyback converter in the working mode shown can omit step S301 and directly execute step S302 to perform volt-second product detection on the sampled voltage. In this way, the first volt-second product detected should include the shaded areas A1 and A2, and this volt-second product area is still smaller than the predetermined volt-second product.

[0132] In step S303, it is determined whether the sampled voltage is less than the threshold voltage. This step can filter out the positive part of the sampled voltage (i.e., the part where the sampled voltage is greater than or equal to the threshold voltage); when the sampled voltage is greater than or equal to the threshold voltage, step S304 is executed, and when the sampled voltage is less than the threshold voltage, step S302 is returned to execute.

[0133] In step S304, it is determined whether the volt-second product of the sampled voltage is greater than or equal to a predetermined volt-second product. When it is determined that the volt-second product of the sampled voltage is greater than or equal to the predetermined volt-second product, step S305 is executed, and when it is determined that the volt-second product of the sampled voltage is less than the predetermined volt-second product, the process returns to step S302.

[0134] In step S305, it is determined whether a falling edge of the sampled voltage is detected. If a falling edge is detected, step S306 is executed, and if no falling edge is detected, detection is continued.

[0135] In step S306, it is determined whether the sampled voltage is less than the threshold voltage. When the sampled voltage is less than the threshold voltage, it is determined that the synchronous rectifier is in a predetermined state, and step S307 is executed. Otherwise, the process returns to step S301.

[0136] In step S307, negative sampling signal detection is enabled. If the current of the main switch tube is not continuously monitored, this step is to start detecting the current flowing through the main switch tube; if the current of the main switch tube is continuously monitored, this step is to start judging and processing the sampling signal representing the current flowing through the main switch tube.

[0137] In step S308, it is determined whether the sampling signal meets the preset conditions. In this step, within a predetermined time period, based on multiple negative reference thresholds, it is determined whether the sampling signal meets the preset conditions. The specific determination method can refer to the above description and will not be repeated here. When the sampling signal meets the preset conditions, the main switch tube is turned on. If the sampling signal does not meet the preset conditions, it returns to step S301.

[0138] In this embodiment, the predetermined volt-second product is set to be able to distinguish the volt-second product of normal free resonance from the volt-second product after the synchronous rectifier is reversely excited. After the sampling voltage after the volt-second product is satisfied is lower than the corresponding threshold voltage (i.e., the voltage on the auxiliary winding is lower than zero, or the drain-source voltage of the main switch tube is lower than Vin, or the voltage value of the drain-source voltage of the main switch tube exceeds the corresponding threshold), the primary negative current detection is enabled within a predetermined time period. If it is detected that the sampling signal meets the preset condition, the primary switch tube is turned on. If not, the volt-second product detection needs to be re-enabled. In addition, if the voltage on the auxiliary winding rises through zero or the drain-source voltage of the main switch tube rises through Vin, the volt-second product detection will also be re-enabled.

[0139] In summary, the embodiments of the present invention provide a flyback converter and a control device and a control method thereof. By adding multiple negative reference thresholds and multiple preset time periods, the sampling of the changing negative current waveform is matched while ensuring the reliability of the primary negative current sampling. Compared with the setting of a single threshold, the pulse width of the synchronous rectifier tube can be reasonably designed to optimize the efficiency.

[0140] In some embodiments, in order to solve the problem of false operation of the main switch tube under interference and avoid the main switch tube and the synchronous rectifier tube being connected in common, volt-second product detection or slope judgment is added before negative current detection, which can avoid false operation of the main switch tube and improve the robustness of the flyback converter.

[0141] At the same time, those skilled in the art can realize that, in combination with the structures and methods of the various examples described in the embodiments disclosed herein, different configuration methods or adjustment methods can be used to implement the described functions for each structure or a reasonable deformation of the structure, but such implementation should not be considered to be beyond the scope of the present application. In addition, it should be understood that the connection relationship between the various components of the amplifier in the aforementioned figure in the embodiments of the present application is a schematic example and does not impose any limitation on the embodiments of the present application.

[0142] It should be noted that, in this article, relational terms such as first and second, etc. are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device. In the absence of further restrictions, the elements defined by the statement "comprise a ..." do not exclude the presence of other identical elements in the process, method, article or device including the elements.

[0143] According to the embodiments of the present invention as described above, these embodiments do not describe all the details in detail, nor do they limit the invention to the specific embodiments described. Obviously, many modifications and changes can be made based on the above description. This specification selects and specifically describes these embodiments in order to better explain the principles and practical applications of the present invention, so that those skilled in the art can make good use of the present invention and the modified use based on the present invention. The present invention is limited only by the claims and their full scope and equivalents.

Claims

1. A control method for a flyback converter, wherein the flyback converter comprises a main switch tube located in a primary circuit and a synchronous rectifier tube located in a secondary circuit, and the control method comprises: Acquiring a sampling signal representing a current flowing through the main switch tube; Determining whether the sampled signal meets a preset condition based on a plurality of negative reference thresholds, and providing an indication signal; as well as Control the main switch tube according to the indication signal, The multiple negative reference thresholds include at least a first threshold and a second threshold, the first threshold is less than the second threshold, and both the first threshold and the second threshold are less than zero. The first time period is a time period when the sampling signal is less than or equal to the first threshold, and the second time period is a time period when the sampling signal is less than or equal to the second threshold. When the first time period is not less than a first preset time, and the second time period is not less than a second preset time, the sampling signal meets the preset condition.

2. The control method according to claim 1, wherein: The step of judging whether the sampled signal meets a preset condition based on a plurality of negative reference thresholds and providing an indication signal comprises: comparing the sampled signal with the plurality of negative reference thresholds; obtaining at least a portion of the first time period and the second time period; and providing the indication signal, When the sampling signal is less than or equal to the first threshold, timing is started to obtain at least a part of the first time period. When at least a portion of the first time period reaches the first preset time, the sampling signal is compared with the second threshold value, When the sampling signal is less than or equal to the second threshold, start timing to obtain at least a part of the second time period, When at least a part of the second time period reaches a third preset time, a valid indication signal is provided to control the main switch tube to be turned on, The second preset time is the sum of the first preset time and the third preset time.

3. The control method according to claim 1, wherein: The step of judging whether the sampled signal meets a preset condition based on a plurality of negative reference thresholds and providing an indication signal comprises: comparing the sampled signal with the plurality of negative reference thresholds; acquiring the first time period and the second time period; and providing the indication signal, When the sampling signal is less than or equal to the first threshold, timing is started to obtain the first time period, and when the first time period reaches the first preset time, timing of the first time period is stopped. When the sampling signal is less than or equal to the second threshold, start timing to obtain the second time period, and when the second time period reaches the second preset time, stop timing the second time period. When the first time period reaches the first preset time and the second time period reaches the second preset time, a valid indication signal is provided to control the main switch tube to be turned on.

4. The control method according to claim 1, wherein: Before obtaining the sampling signal of the current flowing through the main switch tube, the method further includes: judging whether the synchronous rectifier tube is in a predetermined state according to the sampling voltage of the main switch tube, wherein the predetermined state is a state where the synchronous rectifier tube is turned on for a predetermined period of time before the main switch tube is turned on, and then turned off to realize the state of controlling the main switch tube to be turned on at zero voltage. When the synchronous rectifier tube is in a predetermined state, the current flowing through the main switch tube starts to be sampled.

5. The control method according to claim 4, wherein: The sampled voltage represents the drain-source voltage of the main switch tube, or the flyback converter further includes an auxiliary winding located in the primary circuit, and the sampled voltage represents the voltage on the auxiliary winding.

6. The control method according to claim 5, wherein: Judging whether the synchronous rectifier tube is in a predetermined state according to the sampled voltage of the main switch tube includes: Detecting the slope of the sampling voltage of the main switch tube; When the absolute value of the slope is greater than / equal to a predetermined slope, detecting a falling edge of the sampling voltage; When the falling edge is detected, determining whether the sampled voltage is less than a threshold voltage; When the sampling voltage is less than the threshold voltage, it is determined that the synchronous rectifier is in a predetermined state.

7. The control method according to claim 5, wherein: Judging whether the synchronous rectifier tube is in a predetermined state according to the sampled voltage of the main switch tube includes: Determining whether the volt-second product of the portion of the sampled voltage that is greater than or equal to the threshold voltage is greater than / equal to a predetermined volt-second product; When the volt-second product of the portion of the sampled voltage that is greater than or equal to the threshold voltage is greater than / equal to a predetermined volt-second product, detecting a falling edge of the sampled voltage; When the falling edge is detected, determining whether the sampled voltage is less than a threshold voltage; When the sampling voltage is less than the threshold voltage, it is determined that the synchronous rectifier is in a predetermined state.

8. The control method according to claim 7, wherein: Before determining whether the volt-second product of the portion of the sampled voltage that is greater than or equal to the threshold voltage is greater than / equal to a predetermined volt-second product, the following operations are performed: When a zero-point signal representing the zero-crossing point of the sampling signal is detected, the volt-second product judgment unit is triggered to calculate the volt-second product of the portion of the sampling voltage that is greater than or equal to the threshold voltage.

9. The control method according to claim 8, wherein: When the sampling voltage represents the voltage on the auxiliary winding, the voltage representing the auxiliary winding during the conduction period of the synchronous rectifier is recorded as the first voltage, and when it is detected that the sampling voltage is slightly less than the first voltage, a zero-point signal representing the zero-crossing point of the sampling signal is detected; When the sampling voltage represents the drain-source voltage of the main switch tube, the voltage representing the drain-source voltage of the main switch tube during the conduction period of the synchronous rectifier tube is the second voltage. When it is detected that the sampling voltage is slightly smaller than the second voltage, a zero point signal representing the zero crossing point of the sampling signal is detected.

10. The control method according to any one of claims 6 to 9, wherein: When the sampling voltage is the drain-source voltage of the main switch tube, the threshold voltage is the voltage value of the input signal of the flyback converter; or When the sampled voltage is the voltage on the auxiliary winding, the threshold voltage is zero.

11. The control method according to claim 6 or 7, wherein: When the synchronous rectifier is in a predetermined state, determining whether the sampling signal meets a preset condition within a predetermined time period; When it is not detected that the sampling signal meets the preset condition within the predetermined time period, the slope detection or the volt-second product detection is re-enabled.

12. The control method according to claim 6 or 7, wherein: When the sampling voltage is the drain-source voltage of the main switch tube, the slope detection or the volt-second product detection is re-enabled when the sampling voltage rises above the input signal; When the sampled voltage is the voltage on the auxiliary winding, the slope detection or the volt-second product detection is re-enabled when the sampled voltage rises and crosses zero.

13. A control device for a flyback converter, based on which the control method according to any one of claims 1 to 12 is implemented, the control device comprising: A sampling module, used to obtain a sampling signal representing the current flowing through the main switch tube; A judgment module, which judges whether the sampled signal meets a preset condition based on a plurality of negative reference thresholds, and provides an indication signal; as well as A control module controls the main switch tube according to the indication signal, The multiple negative reference thresholds include at least a first threshold and a second threshold, the first threshold is less than the second threshold, and both the first threshold and the second threshold are less than zero. The first time period is a time period when the sampling signal is less than or equal to the first threshold, and the second time period is a time period when the sampling signal is less than or equal to the second threshold. When the first time period is not less than a first preset time, and the second time period is not less than a second preset time, the sampling signal meets the preset condition.

14. A flyback converter, comprising: A main switch tube, a transformer and a synchronous rectifier tube, wherein the transformer includes a primary coil and a secondary coil, the main switch tube is connected to the primary coil, and the synchronous rectifier tube is connected to the secondary coil; as well as Whether the main switch tube is turned on is controlled based on the control method according to any one of claims 1 to 12.

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