Control circuit of flyback converter and flyback converter

Abstract

The utility model provides a flyback converter control circuit and the flyback converter, comprising a reference voltage generation circuit, which generates a reference voltage according to the adjustment time, the longer the adjustment time is, the smaller the reference voltage is, and the adjustment time is in the time period from the rectification tube follow current end moment to the main power tube turn-off moment; the comparator is used for comparing a current sampling signal representing the current of the main power tube with the reference voltage to generate a comparison signal; and when the current sampling signal reaches the reference voltage, the comparison signal controls the main power tube to be turned off. According to the utility model, primary and secondary side communication and reliable control are realized without additional isolation devices, the cost is low, the condition that primary and secondary sides are conducted at the same time is avoided, and the system reliability is high.

Description

Technical Field

[0001] The utility model relates to the field of power electronics, in particular to a control circuit of a flyback converter and the flyback converter. Background Art

[0002] For the flyback converter in discontinuous working mode, in the absence of additional primary-to-secondary isolation devices to communicate and transmit control signals, the primary controller of the flyback converter needs to control the peak current by identifying the switching information of the secondary controller. Traditional applications control the peak current by setting the frequency curve. However, this design will cause the system to have a large peak current when running at low frequency, which will increase the conduction loss and seriously affect the low-voltage heavy-load efficiency. Summary of the invention

[0003] The purpose of the utility model is to provide a control circuit of a flyback converter and the flyback converter. The flyback converter of the utility model can enable the primary side and the secondary side to communicate and perform primary side peak current control without setting an isolation communication device, while avoiding the possibility of the primary and secondary sides being turned on at the same time.

[0004] The utility model also provides a control circuit of a flyback converter, wherein the flyback converter comprises a main power tube, a rectifier tube and a transformer, wherein the main power tube is connected to the primary winding of the transformer, and the rectifier tube is connected to the secondary winding of the transformer, and comprises:

[0005] A reference voltage generating circuit generates a reference voltage according to an adjustment time, wherein the longer the adjustment time is, the smaller the reference voltage is; and the adjustment time is within a time period from when the rectifier tube freewheeling ends to when the main power tube is turned off;

[0006] A comparator compares a current sampling signal representing the current of the main power tube with the reference voltage to generate a comparison signal; when the current sampling signal reaches the reference voltage, the comparison signal controls the main power tube to turn off.

[0007] Optionally, the reference voltage generating circuit is configured such that, as the adjustment time increases, the reference voltage decreases in a stepwise manner or linearly.

[0008] Optionally, the reference voltage generating circuit is configured to set the reference voltage at a minimum reference voltage value when the adjustment time increases to a threshold time.

[0009] Optionally, the reference voltage generating circuit includes:

[0010] A charging circuit, wherein the charging circuit is charged to generate a first voltage during the adjustment time;

[0011] A subtractor is used to obtain the reference voltage by subtracting the reference voltage from the first voltage.

[0012] Optionally, the charging circuit includes a first capacitor, a first switch and a first current source, the first current source is connected to the first capacitor, and the first switch is connected to the first capacitor in parallel; during the adjustment time, the first switch is disconnected, and the first current source charges the first capacitor to generate the first voltage.

[0013] Optionally, the reference voltage generating circuit further includes a first detection circuit and a second detection circuit.

[0014] The first detection circuit is used to detect the starting time of the adjustment time;

[0015] The second detection circuit is used to detect the end time of the adjustment time;

[0016] A trigger, whose set end is connected to the output end of the first detection circuit, whose reset end is connected to the output end of the second detection circuit, and whose output end outputs a switch control signal for controlling the charging state of the charging circuit; when the first detection circuit detects the starting moment, the trigger is set, and the switch control signal is invalid, so as to control the discharge of the charging circuit; when the second detection circuit detects the ending moment, the switch control signal is valid, so as to control the charging circuit to charge.

[0017] Optionally, the first detection circuit detects the end time of the freewheeling of the rectifier tube, or obtains the start time of the adjustment time after detecting that the freewheeling of the rectifier tube ends for a period of time.

[0018] Optionally, the second detection circuit detects the shutdown moment of the main power tube, or detects the restarting moment of the rectifier tube, or detects the end moment of the restarting of the rectifier tube to obtain the end moment of the adjustment time.

[0019] The present invention also provides a flyback converter, comprising a main power tube, a rectifier tube and a transformer, wherein the main power tube is connected to the primary winding of the transformer, the rectifier tube is connected to the secondary winding of the transformer, and further comprising:

[0020] A primary side controller, used to control the switch state of the main power tube, including any one of the control circuits described above;

[0021] The secondary side controller is used to control the switch state of the rectifier tube, and control the rectifier tube to be turned on for the second time after the rectifier tube continues to flow.

[0022] Optionally, also include,

[0023] an auxiliary winding coupled to one of the windings of the transformer;

[0024] A voltage detection circuit, used for detecting the voltage on the auxiliary winding to obtain a voltage detection signal;

[0025] A current sampling circuit is used to detect the current on the main power tube to obtain a current sampling signal;

[0026] The primary side controller receives the voltage sampling signal and the current sampling signal, and controls the main power tube to be turned on at zero voltage according to the voltage sampling signal and the current sampling signal after the rectifier tube is turned on for the second time.

[0027] Compared with the prior art, the utility model has the following advantages: after the freewheeling of the secondary rectifier tube ends, the adjustment time is obtained, the reference voltage is set according to the adjustment time, and when the current sampling signal representing the current on the main power tube reaches the reference voltage, the main power tube is turned off to achieve peak current control. The peak current control scheme of the utility model does not require additional isolation communication devices to achieve primary and secondary side communication, greatly reducing system costs, and at the same time, it can avoid the possibility of simultaneous conduction of the primary and secondary sides in the working mechanism, thereby improving the reliability of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 It is a schematic diagram of a flyback converter without isolated communication;

[0029] Figure 2 is a conventional switching frequency and peak current curve;

[0030] Figure 3 is another conventional switching frequency and peak current curve;

[0031] Figure 4 It is a working waveform diagram of a flyback converter under traditional peak current control;

[0032] Figure 5 A curve diagram of adjustment time and peak current of the utility model;

[0033] Figure 6 Another curve diagram of adjustment time and peak current of the utility model;

[0034] Figure 7 This is a working waveform diagram of the flyback converter under peak current control of the utility model;

[0035] Figure 8 This is a block diagram of the primary side controller of the flyback converter of the utility model. DETAILED DESCRIPTION

[0036] The preferred embodiments of the present invention are described in detail below in conjunction with the accompanying drawings, but the present invention is not limited to these embodiments. The present invention covers any substitution, modification, equivalent method and scheme made within the spirit and scope of the present invention.

[0037] In order to enable the public to have a thorough understanding of the present invention, specific details are described in detail in the following preferred embodiments of the present invention, and those skilled in the art can fully understand the present invention without these detailed descriptions.

[0038] The present invention is described in more detail in the following paragraphs by way of example with reference to the accompanying drawings. It should be noted that the accompanying drawings are all simplified and in non-precise proportions to facilitate and clearly illustrate the purpose of the embodiments of the present invention.

[0039] like Figure 1As shown, a schematic diagram of a flyback converter without isolated communication is shown, including a rectifier circuit 00, a transformer T1, a main power tube Q, an absorption circuit U0, a rectifier tube Q2, an auxiliary winding Nx, a primary controller U2 and a secondary controller U1. The AC input voltage AC generates a bus voltage VIN after passing through the rectifier circuit 00. The transformer T is used to convert the bus voltage VIN into an output voltage VOUT of the flyback converter; the main power tube Q1 is connected to the primary winding Np of the transformer T, and the main power tube Q1 has a parasitic capacitor C1; The absorption circuit U0 is connected in parallel with both ends of the primary winding T1 to absorb the resonant voltage generated on the parasitic capacitor C1; the rectifier tube Q2 is connected to the secondary winding Ns of the transformer T, and the rectifier tube Q2 has a parasitic capacitor C2; the auxiliary winding Nx is coupled with the primary winding Np, and the voltage on the auxiliary winding is detected by a voltage detection circuit composed of two voltage-dividing resistors R1 and R2 to obtain a voltage detection signal VS. The primary controller U2 is used to control the switching state of the main power tube Q1, and the secondary controller U1 is used to control the switching state of the rectifier tube Q2. In the primary controller U2, pin 2 receives the voltage sampling signal VS on the auxiliary winding Nx, pin 8 is connected to the bus voltage VIN for generating a high-voltage power supply signal HV, pin 5 receives the current sampling signal VCS generated on the sampling resistor Rcs, the sampling resistor Rcs is connected in series with the main power tube Q1, the current sampling circuit includes the sampling resistor Rcs, and is used to detect the current flowing through the power tube Q1 to obtain the current sampling signal VCS, pin 4 is connected to the reference ground, and pin 6 is connected to the control end of the main power tube Q1, and is used to generate a drive signal DRV to drive the main power tube Q1 on and off. In the secondary controller U1, pin 1 is connected to the high-potential output end of the flyback converter to receive the output voltage VOUT, pin 2 receives the output feedback signal FB for generating a compensation voltage COMP, pin 8 is connected to the common connection end of the rectifier tube Q2 and the secondary winding Ns, and receives the switch node voltage SW generated by the connection end, pin 5 is connected to the system ground, and pin 7 is connected to the control end of the rectifier tube Q2, and is used to generate a drive signal GT to drive the rectifier tube Q2 on and off. The primary and secondary sides of the flyback converter have no isolated communication device (capacitor isolation, optocoupler isolation, transformer isolation, etc.), and the primary side controller U2 detects the switch state of the rectifier tube Q2 according to the voltage on the auxiliary winding Nx or the drain voltage of the main power tube Q1, and controls the switch state of the main power tube Q1 according to the detected switch state of the rectifier tube Q2 and the current sampling signal CS, thereby realizing the non-isolated communication control of the primary and secondary sides of the flyback converter. The flyback converter of the utility model is not limited to Figure 1 The structure of the example, other flyback converters such as a flyback converter with an isolated communication device, an asymmetric flyback converter or an active clamp flyback converter all belong to the flyback converter described in the present invention.

[0040] like Figure 2As shown, a conventional switching frequency and peak current curve diagram is illustrated, and the curve diagram corresponds to a peak current control scheme of a flyback converter with an isolated communication device. In the figure, Tsw is the switching period of the flyback converter, the switching frequency fsw=1 / Tsw, Ipk represents the peak current on the main power tube Q1, and COMP is the compensation voltage (obtained according to the output feedback signal FB, representing the load / power size). The curve diagram illustrates the control of the peak current Ipk according to the switching period Tsw (switching frequency 1 / Tsw) and the compensation voltage COMP. When COMP is less than COMP1, the switching period Tsw is Tsw1, the flyback converter operates at the minimum switching frequency, and the peak current Ipk is maintained at the minimum value Ipk_min; when the switching period Tsw decreases from Tsw1 to Tsw2, the peak current Ipk increases as Tsw decreases; when the switching period Tsw reaches Tsw2, that is, when the switching frequency of the flyback converter reaches the maximum switching frequency, the peak current Ipk cannot be further controlled according to the switching frequency fsw, and the peak current Ipk is further controlled according to the compensation voltage COMP. The larger the compensation voltage COMP, the larger the peak current Ipk, until the peak current Ipk reaches the maximum value Ipk_max. According to the curve chart, when the load / output power increases, the compensation voltage COMP increases, the flyback converter enters the BCM (critical conduction) working mode from the DCM (discontinuous conduction) working mode, and after the switching frequency fsw reaches the maximum switching frequency, the peak current Ipk can no longer be controlled to increase further according to the switching frequency fsw. It is necessary to transmit the compensation voltage COMP to the primary side controller U2 through the isolation communication device to further control the peak current, and the isolation communication device cannot be saved.

[0041] like Figure 3 As shown, another conventional switching frequency and peak current curve diagram is shown, which corresponds to a peak current control scheme of a flyback converter without an isolated communication device. The meanings of the parameters Tsw, COMP, and Ipk_min in the curve diagram refer to the above Figure 2The description of each parameter in the curve is not repeated here. When the compensation voltage COMP is less than COMP1, the flyback converter operates at the minimum switching frequency 1 / Tsw; when the compensation voltage COMP is greater than COMP1 and less than COMP2, as the compensation voltage COMP increases, the switching frequency 1 / Tsw increases, and the peak current Ipk_min increases with the increase of the switching frequency 1 / Tsw. When the compensation voltage COMP is less than COMP2, the flyback converter operates in DCM mode; when the compensation voltage COMP is greater than COMP2, the operating frequency of the flyback converter will no longer be controlled by the pre-set frequency curve (that is, as the compensation voltage COMP continues to increase, the switching period Tsw will not continue to drop to Tsw2 as expected), but the switching frequency 1 / Tsw is set according to the turn-off time of the main power tube Q1 set by the system. As shown in the figure, when the compensation voltage COMP reaches COMP2, or at the compensation voltage COMP, the switching frequency 1 / Tsw is set according to the turn-off time of the main power tube Q1 set by the system. When the voltage COMP continues to increase to a value greater than COMP2 (refer to the dotted part corresponding to the Tsw curve), according to the shutdown moment of the main power tube Q1 set by the system, the switching period Tsw reaches the minimum switching period, the switching frequency 1 / Tsw reaches the maximum switching frequency, and the system enters the BCM working mode; therefore, after the compensation voltage COMP reaches COMP2, as the compensation voltage COMP continues to increase, the peak current Ipk reaches Ipk_max and cannot further increase with the switching frequency 1 / Tsw. Limited by this working mode, at a lower switching frequency (that is, when the compensation voltage COMP reaches COMP2 in the figure), the peak current Ipk needs to be increased to the maximum value to ensure that the output power meets the requirements, but this will further increase the effective value of the current under low-voltage input and high-power output, resulting in increased conduction losses and low working efficiency under low-voltage and heavy loads.

[0042] like Figure 4 As shown, a working waveform diagram of a flyback converter under conventional peak current control is illustrated, in which Vswp represents the drain voltage of the main power tube Q1, and Vsws represents the drain voltage of the rectifier tube Q2; VGS2_SR represents the driving signal for driving the rectifier tube Q2 to be turned on for the first time in a switching cycle. During the initial turning-on of the rectifier tube Q2, a freewheeling current is generated on the secondary winding Ns and the rectifier tube Q2; VGS2_ZVS represents the driving signal for driving the rectifier tube Q2 to be turned on for the second time / again in a switching cycle, which is used to achieve zero voltage turn-on (ZVS) of the main power tube Q1; the current sampling signal VCS represents the current flowing through the main power tube Q1 (the excitation current of the transformer T), VCS_REF represents the current valley reference signal, and the reference voltage VREF represents the current peak reference signal; VGS_Q1 represents the driving signal of the main power tube Q1.

[0043] When VGS_Q1 is low, the main power tube Q1 is driven to turn off. After the main power tube Q1 is turned off, after the dead time, when VGS2_SR becomes high, the rectifier tube Q2 is driven to turn on for the first time. After the rectifier tube Q2 is turned on for a period of time, when VGS2_ZVS becomes high, the rectifier tube Q2 is driven to turn on for the second time / again. After the rectifier tube Q2 is turned on for the second time / again, when the primary side controller U2 detects that VCS reaches VCS_REF, the main power tube Q1 is controlled to turn on with zero voltage after a period of time, and VCS gradually increases; the cycle timing is performed from the time when the main power tube Q1 starts to turn off, and the corresponding reference voltage VREF is generated according to the timing time. As the timing time increases, the reference voltage VREF gradually decreases from the set value. When VCS reaches VREF, the main power tube Q1 is controlled to turn off, the cycle timing ends, and the timing starts again. The frequency curve corresponding to this control method is as follows Figure 3 As shown, as the load increases, the operating frequency of the system is determined by the intermittent moment. In order to ensure that the output power meets the output requirements, the peak current of the main power tube Q1 needs to rise to the maximum value at a lower operating frequency, but this will increase the effective value of the current under low-voltage input and heavy load, resulting in increased conduction losses and low efficiency.

[0044] like Figure 5 , 6 As shown, two curves of adjustment time and peak current of the utility model are respectively illustrated. The adjustment time Tdon is the time between the end of the rectifier tube Q2 freewheeling / a period of time after the end of the freewheeling to the time when the main power tube Q1 is turned off / the time when the rectifier tube Q2 is turned on again / the time when the rectifier tube Q2 is turned on again, and the peak current Ipk is set according to the adjustment time Tdon. Figure 5 , 6 When the compensation voltage COMP is less than COMP1, the adjustment time Tdon is maintained at the maximum adjustment time, and the peak current Ipk on the main power tube Q1 is set to be maintained at the minimum value Ipk_min; when the compensation voltage COMP is greater than COMP1, as the compensation voltage COMP increases, the adjustment time Tdon is set to gradually decrease, and Ipk also gradually decreases. When the compensation voltage COMP is greater than COMP3, the adjustment time Tdon is set to be maintained at the minimum adjustment time, and the peak current Ipk reaches the maximum peak current Ipk_max. Figure 5 and Figure 6 Two ways of changing the adjustment time Tdon curve are illustrated respectively. When the compensation voltage COMP is greater than COMP1 and less than COMP3, the adjustment time Tdon follows the compensation voltage COMP in a continuous linear change and a step-by-step linear change respectively. In addition, the adjustment time Tdon can also change in a step-by-step manner with the compensation voltage COMP. Figure 5 and Figure 6As shown in the figure, the utility model obtains a compensation voltage COMP by detecting an output feedback signal FB on the secondary side, and the secondary side controller U1 controls the intermittent time of the flyback converter according to the compensation voltage COMP to determine the turn-on time of the main power tube Q1, and the primary side controller U2 obtains the adjustment time Tdon according to the voltage on the auxiliary winding or the drain voltage of the main power tube, and controls the turn-off time of the main power tube Q1 according to the adjustment time Tdon to control the size of the primary side peak current, thereby realizing the primary side peak current control without isolation communication between the primary and secondary sides.

[0045] like Figure 7 As shown, the working waveform of the flyback converter under the peak current control of the utility model is shown. Figure 4 The same parts will not be described in detail. After the freewheeling of the primary winding ends, that is, after a period of time after the freewheeling of the rectifier tube Q2 ends / the initial opening ends, the timing starts. As the timing time increases, the reference voltage VREF is set to gradually decrease from the set value. When the main power tube Q1 is turned on, the current sampling signal VCS on the main power tube Q1 gradually increases. When VCS reaches VREF, the main power tube Q1 is turned off, and the reference voltage VREF returns to the set value. The figure illustrates three embodiments of timing end. In the cycle T1 embodiment, when the current sampling signal VCS reaches the reference voltage VREF, the timing ends; in the cycle T2 embodiment, when the rectifier tube Q2 is turned on again, the timing ends, and the reference voltage VREF remains unchanged until the current sampling signal VCS reaches the reference voltage VREF; in the cycle T3 embodiment, when the rectifier tube Q2 is turned on again, the timing ends, and the reference voltage VREF remains unchanged until the current sampling signal VCS reaches the reference voltage VREF. The utility model can also start timing immediately after the rectifier tube Q1 continues to flow, that is, the reference voltage VREF can also start to decrease immediately after the rectifier tube Q1 continues to flow, but the reference voltage VREF is usually set to start decreasing after a period of time after the rectifier tube Q1 continues to flow, because the transformer primary excitation (i.e., the transformer energy storage) requires a period of time. The reference voltage VREF in the utility model only needs to be restored to the set value before entering the adjustment time Tdon. In the figure, the reference voltage VREF is restored to the set value when the main power tube Q1 is turned off, which is an optional embodiment. In the utility model, as the adjustment time Tdon increases, the reference voltage VREF decreases; in the figure, VREF decreases linearly as Tdon increases, which is an optional embodiment; in other embodiments, as Tdon increases, VREF can also be reduced in a step-by-step manner, a segmented linear manner, etc. The utility model controls the primary peak current according to the adjustment time Tdon, and there is no need to transmit the communication signal through the communication isolation device. This peak current control method can make the low input voltage work at a high frequency stage when it is overloaded, reducing the conduction loss and improving the system efficiency.

[0046] like Figure 8As shown, a block diagram of the primary side controller of the flyback converter of the utility model is illustrated, including a control circuit 00, a zero voltage turn-on control circuit 03 and a first trigger 04. The control circuit 00 is used to control the peak current of the main power tube Q1, and the zero voltage turn-on control circuit 03 is used to control the main power tube Q1 to achieve zero voltage turn-on. The control circuit 00 includes a reference voltage generating circuit 01 and a first comparator 02. The reference voltage generating circuit 01 is used to generate a reference voltage VREF according to the adjustment time Tdon. The first comparator 02 receives a current sampling signal VCS at its in-phase input terminal and a reference voltage VREF at its inverting input terminal, and its output terminal is connected to the reset terminal R of the first trigger 04; the input terminal of the zero voltage turn-on control circuit 03 receives the current sampling signal VCS and the voltage sampling signal VS representing the auxiliary winding voltage respectively, and its output terminal is connected to the set terminal S of the first trigger 04. The output terminal Q of the first trigger 04 is connected to the control terminal of the main power tube Q1, and the first trigger 04 outputs a driving signal VGS1 for driving the main power tube Q1 on and off. When the current sampling signal VCS rises to the reference voltage VREF, the first comparator 02 outputs a high-level signal to the reset terminal R of the first trigger 04, and the output terminal S of the first trigger 04 outputs a low-level reset signal to control the main power tube Q1 to turn off; when the voltage detection signal VS on the auxiliary winding Nx passes through zero, the current sampling signal VCS of the main power tube Q1 is less than zero for a period of time, and the zero voltage conduction control circuit 03 outputs a high-level signal to the set terminal S of the first trigger 04, and the output terminal S of the first trigger 04 outputs a high-level set signal to control the main power tube Q1 to turn on with zero voltage. The reference voltage generating circuit 01 includes a first detection circuit 101, a second detection circuit 102, a second trigger 103, a charging circuit 104 and a subtractor 105. The charging circuit 104 includes a first switch S1, a first capacitor C1 and a first current source I1. The first current source I1 is connected to the first capacitor C1. The first switch S1 and the first capacitor C1 are connected in parallel. When the first switch S1 is turned on, the voltage on the first capacitor C1 is reset to zero voltage. When the first switch S1 is turned off, the first current source I1 charges the first capacitor C1. The charging voltage generated on the first capacitor C1 is VREF2. The first detection circuit 101 receives a voltage detection signal VS, and detects the first turn-off moment (freewheeling end moment) of the rectifier tube Q2 in a switching cycle according to the voltage detection signal VS. The start moment of the adjustment time Tdon is obtained when the rectifier tube Q2 is turned off for the first time or after a period of time after the first turn-off.

[0047] —outputs a high-level control signal to the set terminal S of the second trigger 103, and the output terminal Q of the trigger 103 outputs a low-level trigger signal to control the first switch S1 to turn off, the first capacitor C1 is charged, and the charging voltage VREF2 generated on the first capacitor C1 gradually increases. The subtractor 104 is used to subtract the set reference value VREF1 from the charging voltage VREF2 to obtain the reference voltage VREF; the second detection circuit 102 receives the driving signal VGS1 of the main power tube, detects the turn-on time of the main power tube Q1 according to the driving signal VGS1 to obtain the end time of the adjustment time Tdon, and outputs a high-level signal to the first detection circuit 102 after the main power tube Q1 is turned on.

[0048] —The reset terminal of the second trigger 103, the output terminal Q of the second trigger outputs a high-level trigger signal to control the first switch S1 to be turned on, the first capacitor C1 stops charging, the voltage VREF2 on the first capacitor C1 is reset to zero voltage, and the reference voltage VREF is restored to the set reference value VREF1; in this example, the charging time of the first capacitor C1 is the time period between the start time and the end time of the adjustment time Tdon, that is, the adjustment time Tdon, that is, Figure 7 The timing time indicated in FIG. In other embodiments, the second detection circuit 102 can also obtain the end time of the adjustment time Tdon by detecting the second (again) turn-on time or the second (again) turn-on end time of the rectifier tube Q2 to control the turn-on of the first switch Q1. The primary side control block diagram of the utility model can complete the peak current control without receiving the communication signal from the secondary side through the isolation communication device. Under the condition of low voltage input and high power output, the flyback converter can work in the high frequency stage, reduce the conduction loss, and improve the system efficiency.

[0049] Although the embodiments are described and illustrated separately above, some common technologies are involved. It is the opinion of ordinary technicians in this field that the embodiments can be replaced and integrated. If the content is not clearly recorded in one of the embodiments, reference can be made to another recorded embodiment.

[0050] The above-described implementation methods do not constitute a limitation on the protection scope of the technical solution. Any modification, equivalent replacement and improvement made within the spirit and principle of the above-described implementation methods shall be included in the protection scope of the technical solution.

Claims

1. A control circuit of a flyback converter, the flyback converter comprising a main power tube, a rectifier tube and a transformer, the main power tube is connected to the primary winding of the transformer, the rectifier tube is connected to the secondary winding of the transformer, characterized in that: include, A reference voltage generating circuit generates a reference voltage according to an adjustment time, wherein the longer the adjustment time is, the smaller the reference voltage is; The adjustment time is within the time period from the end of the rectifier tube freewheeling to the turn-off time of the main power tube; A comparator compares a current sampling signal representing the current of the main power tube with the reference voltage to generate a comparison signal; when the current sampling signal reaches the reference voltage, the comparison signal controls the main power tube to turn off.

2. The control circuit according to claim 1, characterized in that: The reference voltage generating circuit is configured such that, as the adjustment time increases, the reference voltage decreases in a stepwise manner or linearly.

3. The control circuit according to claim 1, characterized in that: The reference voltage generating circuit is configured to set the reference voltage at a minimum reference voltage value when the adjustment time increases to a threshold time.

4. The control circuit according to claim 1, characterized in that: The reference voltage generating circuit comprises: A charging circuit, wherein the charging circuit is charged to generate a first voltage during the adjustment time; A subtractor subtracts a reference voltage from the first voltage to obtain the reference voltage.

5. The control circuit according to claim 4, characterized in that: The charging circuit includes a first capacitor, a first switch and a first current source, the first current source is connected to the first capacitor, and the first switch is connected to the first capacitor in parallel; during the adjustment time, the first switch is disconnected, and the first current source charges the first capacitor to generate the first voltage.

6. The control circuit according to claim 4, characterized in that: The reference voltage generating circuit further includes a first detection circuit and a second detection circuit. The first detection circuit is used to detect the starting time of the adjustment time; The second detection circuit is used to detect the end time of the adjustment time; A trigger, whose set end is connected to the output end of the first detection circuit, whose reset end is connected to the output end of the second detection circuit, and whose output end outputs a switch control signal for controlling the charging state of the charging circuit; when the first detection circuit detects the starting moment, the trigger is set, and the switch control signal is invalid, so as to control the discharge of the charging circuit; when the second detection circuit detects the ending moment, the switch control signal is valid, so as to control the charging circuit to charge.

7. The control circuit according to claim 6, characterized in that: The first detection circuit detects the end time of the freewheeling of the rectifier tube, or detects a period of time after the freewheeling of the rectifier tube ends, and obtains the start time of the adjustment time.

8. The control circuit according to claim 6, characterized in that: The second detection circuit detects the turn-off moment of the main power tube, or detects the turn-on moment of the rectifier tube, or detects the end moment of the turn-on of the rectifier tube, and obtains the end moment of the adjustment time.

9. A flyback converter, comprising a main power tube, a rectifier tube and a transformer, wherein the main power tube is connected to the primary winding of the transformer, the rectifier tube is connected to the secondary winding of the transformer, and further comprising: A primary side controller, used to control the switch state of the main power tube, comprising the control circuit according to any one of claims 1 to 8; The secondary side controller is used to control the switch state of the rectifier tube, and control the rectifier tube to be turned on for the second time after the rectifier tube continues to flow.

10. The flyback converter according to claim 9, characterized in that: Also includes, an auxiliary winding coupled to one of the windings of the transformer; A voltage detection circuit, used for detecting the voltage on the auxiliary winding to obtain a voltage detection signal; A current sampling circuit is used to detect the current on the main power tube to obtain a current sampling signal; The primary side controller receives the voltage detection signal and the current sampling signal, and controls the main power tube to be turned on at zero voltage according to the voltage detection signal and the current sampling signal after the rectifier tube is turned on for the second time.

Images