Flyback converter and control circuit and control method thereof

Abstract

The invention provides a flyback converter and a control circuit and a control method thereof. The control method comprises the following steps: acquiring a load state of the flyback converter; when the load state is a light load state, the device works in an intermittent conduction mode; when the load state is a heavy load state, the device works in a critical conduction mode; when the load state is between a light load state and a heavy load state, the hybrid conduction mode works in a hybrid conduction mode, the period of the hybrid conduction mode comprises N switching periods, the first N-1 switching periods work in a critical conduction mode, the Nth switching period works in an intermittent conduction mode, and the Nth switching period works in an intermittent conduction mode; and controlling each switching period in the mixed conduction mode period to work in a critical conduction mode or an intermittent conduction mode according to the set maximum value of the mixed conduction mode period and the set first duty ratio. According to the flyback converter, each switching period in the hybrid conduction mode can be adaptively controlled, so that the system efficiency of the flyback converter under the condition that the load state is between a light load state and a heavy load state is improved.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to the technical field of power electronics, and more particularly, to a flyback converter and a control circuit and a control method thereof. BACKGROUND

[0002] The flyback converter has the characteristics of simple circuit structure, input and output electrical isolation, wide voltage regulation range, and easy multi-output, and is thus suitable for being used as an auxiliary switching power supply in a power electronic device and is widely used in the field.

[0003] The flyback converter provides power for a load through the process of storing and releasing energy. The selection of the working mode depends on the demand of the load and the working condition of the circuit. As the load changes, the flyback converter switches the corresponding working mode to improve the system efficiency. Generally, the flyback converter works in the critical conduction mode when the load is heavy, and works in the discontinuous conduction mode as the load decreases. At present, the second switch of the flyback converter is turned on once more before the first switch of the flyback converter is turned on, so as to realize zero-voltage turn-on in the discontinuous conduction mode and improve the efficiency of the flyback converter. However, because the second switch is turned on once more, the switching loss is increased, so that the efficiency cannot reach the optimum.

[0004] Therefore, it is necessary to provide an improved technical solution to overcome the above technical problems in the prior art. SUMMARY

[0005] Therefore, it is necessary to provide an improved technical solution to overcome the above technical problems in the prior art.

[0006] According to a first aspect of the present disclosure, a control method of a flyback converter is provided, comprising: obtaining a load state of the flyback converter; working in a discontinuous conduction mode when the load state is light load; working in a critical conduction mode when the load state is heavy load; and working in a hybrid conduction mode when the load state is between light load and heavy load, the hybrid conduction mode period including N switching periods, the first N-1 switching periods working in the critical conduction mode, and the Nth switching period working in the discontinuous conduction mode, wherein each switching period in the hybrid conduction mode period works in the critical conduction mode or the discontinuous conduction mode according to a set hybrid conduction mode period maximum value and a set first duty cycle, and N is a positive integer.

[0007] Optionally, the controlling the each switching cycle in the hybrid conduction mode period to operate in the critical conduction mode or the discontinuous conduction mode according to the set maximum value of the hybrid conduction mode period and the set first duty cycle comprises: obtaining a time point at which the magnetizing inductor current of the flyback converter drops to zero in each switching cycle in the hybrid conduction mode period; and when a time length between the start time point of the hybrid conduction mode period and the time point at which the magnetizing inductor current drops to zero in the switching cycle is greater than a time threshold, controlling the switching cycle to operate in the discontinuous conduction mode; the time threshold is equal to one half of a product of the set maximum value of the hybrid conduction mode period and the set first duty cycle.

[0008] Optionally, the controlling the each switching cycle in the hybrid conduction mode period to operate in the critical conduction mode or the discontinuous conduction mode according to the set maximum value of the hybrid conduction mode period and the set first duty cycle comprises: obtaining a time point at which the magnetizing inductor current of the flyback converter drops to zero in each switching cycle in the hybrid conduction mode period; and when a time length between the start time point of the hybrid conduction mode period and the time point at which the magnetizing inductor current drops to zero in the switching cycle is greater than a time threshold, controlling the switching cycle to operate in the discontinuous conduction mode; the time threshold is equal to one half of a product of the set maximum value of the hybrid conduction mode period and the set first duty cycle.

[0009] Optionally, the flyback converter comprises a first switch and a second switch, and the controlling the switching cycle to operate in the critical conduction mode in the hybrid conduction mode comprises: controlling the first switch to be turned on at the beginning of the switching cycle; controlling the second switch to be turned on after the first switch is turned off; and controlling the second switch to be turned off at the time point at which the magnetizing inductor current drops to zero in the switching cycle and delaying a second time.

[0010] Optionally, the flyback converter comprises a first switch and a second switch, and the controlling the switching cycle to operate in the discontinuous conduction mode in the hybrid conduction mode comprises: controlling the first switch to be turned on at the beginning of the switching cycle; controlling the second switch to be turned on after the first switch is turned off; and controlling the second switch to be turned off at the time point at which the magnetizing inductor current drops to zero in the switching cycle and keeping the first switch turned off until the end of the hybrid conduction mode period.

[0011] Optionally, the control method further comprises: obtaining a first time in the hybrid conduction mode period, the first time starting from the start time point of the hybrid conduction mode period and ending at the time point at which the magnetizing inductor current of the flyback converter drops to zero in the Nth switching cycle; and obtaining the hybrid conduction mode period according to the first time and the first duty cycle.

[0012] Optionally, the hybrid conduction mode period is a quotient of the first time and the first duty cycle.

[0013] Optionally, the hybrid conduction mode period is also obtained according to the maximum value of the hybrid conduction mode period, and when the quotient of the first time and the first duty cycle is less than the maximum value of the hybrid conduction mode period, the hybrid conduction mode period is the quotient of the first time and the first duty cycle, otherwise the hybrid conduction mode period is the maximum value of the hybrid conduction mode period.

[0014] Optionally, the method for obtaining the time when the magnetizing inductance current of the flyback converter drops to zero in each switching cycle within the hybrid conduction mode period comprises: performing a voltage-second balance detection in the each switching cycle to obtain a voltage-second balance time, and taking the voltage-second balance time as the time when the magnetizing inductance current drops to zero, wherein the voltage-second balance time represents that the electric charge flowing out of the auxiliary winding during the conduction of the first switch of the flyback converter is equal to the electric charge flowing into the auxiliary winding during the conduction of the second switch of the flyback converter.

[0015] Optionally, the method for obtaining the time when the magnetizing inductance current of the flyback converter drops to zero in the Nth switching cycle comprises: performing a voltage-second balance detection in the Nth switching cycle to obtain a voltage-second balance time, and taking the voltage-second balance time as the time when the magnetizing inductance current drops to zero, wherein the voltage-second balance time represents that the electric charge flowing out of the auxiliary winding during the conduction of the first switch of the flyback converter is equal to the electric charge flowing into the auxiliary winding during the conduction of the second switch of the flyback converter; or detecting the time when the slope of the signal representing the auxiliary winding voltage reaching a slope threshold for the first time, and taking the time as the time when the magnetizing inductance current drops to zero.

[0016] Optionally, the method for obtaining the load state of the flyback converter comprises: generating an error compensation signal according to an output feedback signal of the flyback converter; and obtaining the load state according to the error compensation signal, wherein when the error compensation signal is greater than a first set threshold, the load state is heavy load, when the error compensation signal is less than a second set threshold, the load state is light load, when the error compensation signal is less than the first set threshold and greater than the second set threshold, the load state is between light load and heavy load, and the first set threshold is greater than the second set threshold.

[0017] Optionally, the control method further comprises: in the hybrid conduction mode, setting the first duty cycle according to the error compensation signal, wherein when the error compensation signal is greater than the second set threshold and less than the first set threshold, the first duty cycle increases with the increase of the error compensation signal.

[0018] Optionally, when the error compensation signal is equal to the first set threshold, the first duty cycle has a maximum value, and the maximum value of the first duty cycle is 100%.

[0019] Optionally, the peak current jump increases when the error compensation signal falls below the first set threshold.

[0020] According to a second aspect of the present disclosure, a control circuit of a flyback converter is provided, the flyback converter comprising a first switch and a second switch, the control circuit comprising: a mode control circuit configured to obtain a load state of the flyback converter, control the flyback converter to operate in a discontinuous conduction mode when the load state is a light load, control the flyback converter to operate in a critical conduction mode when the load state is a heavy load, and control the flyback converter to operate in a hybrid conduction mode when the load state is between the light load and the heavy load, the hybrid conduction mode period comprising N switching periods, the first N-1 switching periods operating in the critical conduction mode, and the Nth switching period operating in the discontinuous conduction mode, N being a positive integer, the mode control circuit further configured to control each switching period in the hybrid conduction mode period to operate in the critical conduction mode or the discontinuous conduction mode according to a set maximum value of the hybrid conduction mode period and a set first duty cycle; and a switch control circuit connected to the mode control circuit, configured to control switching states of the first switch and the second switch.

[0021] Optionally, the control circuit further comprises: a current zero-crossing detection circuit configured to obtain a time when an excitation inductor current of the flyback converter falls to zero in each switching period in the hybrid conduction mode period, the mode control circuit configured to control the switching period to operate in the discontinuous conduction mode when a time length between a start time of the hybrid conduction mode period and the time when the excitation inductor current falls to zero in the switching period is greater than a time threshold, the time threshold being equal to one half of a product of the set maximum value of the hybrid conduction mode period and the set first duty cycle.

[0022] Optionally, the control circuit further comprises: an output feedback circuit configured to generate an error compensation signal according to an output feedback signal of the flyback converter; the mode control circuit configured to obtain the load state of the flyback converter according to the error compensation signal, wherein the load state is the heavy load when the error compensation signal is greater than a first set threshold, the load state is the light load when the error compensation signal is less than a second set threshold, and the load state is between the light load and the heavy load when the error compensation signal is less than the first set threshold and greater than the second set threshold, the first set threshold being greater than the second set threshold.

[0023] Optionally, the control circuit further comprises: a period control circuit configured to obtain the hybrid conduction mode period according to a first time and the first duty cycle, the mode control circuit configured to obtain the first time, the first time starting from a start time of the hybrid conduction mode period and ending at a time when an excitation inductor current of the flyback converter falls to zero in the Nth switching period.

[0024] Optionally, the period control circuit comprises: a first capacitor unit comprising a first capacitor, the first capacitor unit generating a first current in the first time and discharging the first capacitor at the beginning of each period and then charging the first capacitor by the first current; a second capacitor unit comprising a second capacitor, the second capacitor unit providing a second current and discharging the second capacitor at the beginning of each period and then charging the second capacitor by the second current; and a comparator generating a period control signal according to a comparison result of a voltage across the first capacitor and a voltage across the second capacitor, the period control signal representing the hybrid conduction mode period or the discontinuous conduction mode period, wherein a quotient of the second current and the first current is the first duty cycle, and a capacitance of the first capacitor is equal to a capacitance of the second capacitor.

[0025] Optionally, the first capacitor unit further comprises: a first resistor; a first current source comprising an eighth switch tube and a ninth switch tube, a control terminal of the eighth switch tube being connected to a first terminal of the eighth switch tube and connected to a power supply voltage via the first resistor, a control terminal of the ninth switch tube being connected to the control terminal of the eighth switch tube, a first terminal of the ninth switch tube being connected to the power supply voltage, a second terminal of the ninth switch tube being connected to a first terminal of the first capacitor, a second terminal of the first capacitor being grounded; a third switch tube, a control terminal of the third switch tube receiving a first control signal, a first terminal of the third switch tube being connected to a second terminal of the eighth switch tube, a second terminal of the third switch tube being grounded; a fourth switch tube, a first terminal of the fourth switch tube being connected to the first terminal of the first capacitor, a second terminal of the fourth switch tube being connected to the second terminal of the first capacitor, a control terminal of the fourth switch tube receiving a second control signal, the second capacitor unit further comprising: a second resistor; a second current source comprising a fifth switch tube and a sixth switch tube, a control terminal of the fifth switch tube being connected to a first terminal of the fifth switch tube and connected to the power supply voltage via the second resistor, a second terminal of the fifth switch tube being grounded, a control terminal of the sixth switch tube being connected to the control terminal of the fifth switch tube, a first terminal of the sixth switch tube being connected to the power supply voltage, a second terminal of the sixth switch tube being connected to a first terminal of the second capacitor, a second terminal of the second capacitor being grounded; a seventh switch tube, a first terminal of the seventh switch tube being connected to the first terminal of the second capacitor, a second terminal of the seventh switch tube being connected to the second terminal of the second capacitor, a control terminal of the seventh switch tube receiving the second control signal, wherein an active state of the first control signal is maintained for the first time, the second control signal is a pulse signal at the beginning of each period, a resistance of the first resistor is equal to a resistance of the second resistor, and a quotient of a mirroring coefficient of the second current source and a mirroring coefficient of the first current source is the first duty cycle.

[0026] According to a third aspect of the present disclosure, there is provided a flyback converter comprising a control circuit of a flyback converter as described above.

[0027] The flyback converter and the control circuit and the control method thereof provided by the present disclosure obtain the load state of the flyback converter, and control the flyback converter to operate in different modes under different load states. In the case where the load state is between light load and heavy load, the flyback converter operates in a hybrid conduction mode, and the hybrid conduction mode period includes N switching periods, the first N-1 switching periods operate in a critical conduction mode, and the Nth switching period operates in a discontinuous conduction mode. The present disclosure controls each switching period in the hybrid conduction mode period to operate in the critical conduction mode or the discontinuous conduction mode by setting the maximum value of the hybrid conduction mode period and the first duty cycle, so as to adaptively control each switching period in the hybrid conduction mode, thereby improving the system efficiency of the flyback converter in the case where the load state is between light load and heavy load.

[0028] Further, the present disclosure increases the peak current mutation when the error compensation signal decreases to the first set threshold, so as to add control hysteresis, and avoid the switching between the critical conduction mode and the hybrid conduction mode. BRIEF DESCRIPTION OF DRAWINGS

[0029] Figure 1 FIG. 1 shows a structure schematic diagram of a flyback converter according to an embodiment of the present disclosure;

[0030] Figure 2 FIG. 2 shows a structure schematic diagram of another flyback converter according to an embodiment of the present disclosure;

[0031] Figure 3 FIG. 3 shows a waveform schematic diagram of a peak current and a first duty cycle in a control circuit of a flyback converter according to an embodiment of the present disclosure;

[0032] Figures 4a-4e FIG. 4 shows waveform schematic diagrams of different hybrid conduction mode periods of a flyback converter in a hybrid conduction mode according to an embodiment of the present disclosure;

[0033] Figure 5 FIG. 5 shows a waveform schematic diagram of a flyback converter in a flyback converter according to an embodiment of the present disclosure;

[0034] Figure 6 FIG. 6 shows a schematic diagram of a period control circuit in a flyback converter according to an embodiment of the present disclosure;

[0035] Figure 7 FIG. 7 shows a waveform schematic diagram of a period control circuit in a flyback converter according to an embodiment of the present disclosure;

[0036] Figure 8 Fig. 1 shows a flow diagram of a control method of a flyback converter according to an embodiment of the present application;

[0037] Figure 9 Fig. 2 shows a waveform diagram of another peak current and a first duty cycle in a control circuit of a flyback converter according to an embodiment of the present application;

[0038] Figure 10 Fig. 3 shows a waveform diagram of another peak current in a control circuit of a flyback converter according to an embodiment of the present application;

[0039] Figure 11 Fig. 4 shows a waveform diagram of another peak current in a control circuit of a flyback converter according to an embodiment of the present application;

[0040] Figure 12 Fig. 5 shows a structure diagram of another flyback converter according to an embodiment of the present application;

[0041] Figure 13 Fig. 6 shows a structure diagram of another flyback converter according to an embodiment of the present application;

[0042] Figure 14 Fig. 7 shows a structure diagram of another flyback converter according to an embodiment of the present application. DETAILED DESCRIPTION

[0043] Various embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. In the various drawings, the same elements are denoted by the same or similar reference numerals.

[0044] Figure 1 Fig. 1 shows a structure diagram of a flyback converter according to an embodiment of the present application. Figure 2 Fig. 2 shows a structure diagram of another flyback converter according to an embodiment of the present application. Figure 3 Fig. 3 shows a waveform diagram of a peak current and a first duty cycle in a control circuit of a flyback converter according to an embodiment of the present application. Figures 4a-4e Fig. 4 shows waveform diagrams of different hybrid conduction mode periods of a flyback converter in a hybrid conduction mode according to an embodiment of the present application. Figure 5 Fig. 5 shows a waveform diagram of a voltage-second balance detection circuit in a flyback converter according to an embodiment of the present application. Figure 6 Fig. 6 shows a schematic diagram of a period control circuit in a flyback converter according to an embodiment of the present application. Figure 7 Fig. 7 shows a waveform diagram of a period control circuit in a flyback converter according to an embodiment of the present application.

[0045] Reference Signs List Figure 1The flyback converter 200 in the embodiment is, for example, an asymmetric half-bridge flyback converter, which includes a transformer T having a primary winding Np and a secondary winding Ns, a first inductor Lk and a capacitor Co on the primary side of the flyback converter 200, and a diode Dl and an output capacitor Cout on the secondary side of the flyback converter 200.

[0046] On the primary side, the second switch Q2 and the first switch Ql are connected in series between the input voltage Vin and the reference ground. In one possible embodiment, the first switch Ql and the second switch Q2 are both NMOS field effect transistors. The first inductor Lk, the primary winding Np of the transformer T and the capacitor Co are connected in series between the drain and the source of the second switch Q2 to form a resonant tank together in the on state of the second switch Q2. The equivalent inductance of the primary winding Np of the transformer T in the resonant tank is the magnetizing inductance. It should be noted that in low power power supply applications, the leakage inductance of the transformer T can be used instead of the first inductor Lk.

[0047] On the secondary side, the diode Dl and the secondary winding Ns of the transformer T are connected in series between the voltage output terminal and the ground. The anode of the diode Dl is connected to the non-identical end of the secondary winding Ns, so as to rectify the induced voltage opposite to the magnetizing voltage of the transformer T to provide a direct current output voltage Vo. The output capacitor Cout is connected between the voltage output terminal and the ground to filter the direct current output voltage Vo to obtain a smooth voltage waveform.

[0048] The flyback converter 200 further includes an auxiliary winding Naux and a sampling terminal VS to obtain a sampling signal Is representing the auxiliary winding voltage Vaux. Exemplarily, the auxiliary winding Naux and the sampling terminal VS further include a resistor RFB1. Further, a switch QlO is connected between the sampling terminal VS and the ground. In other embodiments, a sampling resistor is connected between the source of the first switch Ql and the ground to obtain an inductor current sampling signal.

[0049] The working process of the flyback converter 200 includes: energy storage: when the first switch Ql is turned on, the inductor current sampling signal of the primary winding rises, the transformer T stores energy, and the load is powered by the output capacitor Cout; energy release: when the first switch Ql is turned off, the inductor induced voltage of the primary winding reverses, the diode Dl is turned on, the energy in the transformer T is supplied to the load via the diode Dl, and the output capacitor Cout is charged to supplement the energy just lost.

[0050] The control circuit 100 in the flyback converter 200 is used to obtain the load state of the flyback converter 200, and control the flyback converter 200 to work in different modes according to the load state. The control circuit 100 comprises a mode control circuit 120 and a switch control circuit 130.

[0051] The mode control circuit 120 obtains the load state of the flyback converter, and outputs a mode control signal according to the load state. The mode control signal controls the flyback converter 200 to work in the discontinuous conduction mode when the load state is light load, to work in the critical conduction mode when the load state is heavy load, and to work in the hybrid conduction mode when the load state is between the light load and the heavy load. The hybrid conduction mode cycle Tskip comprises N switch cycles, and N is a positive integer. For example, the first N-1 switch cycles work in the critical conduction mode, and the Nth switch cycle works in the discontinuous conduction mode. The mode control circuit 120 also controls each switch cycle in the hybrid conduction mode cycle to work in the critical conduction mode (BCM) or the discontinuous conduction mode (DCM) according to the set maximum value Tmax of the hybrid conduction mode cycle and the set first duty cycle Duty.

[0052] Referring to Figure 3 , Figures 4a-4e , for example, in the case of heavy load, the flyback converter works in the critical conduction mode (BCM). As the load decreases, in the case between the heavy load and the light load, the flyback converter works in the hybrid conduction mode (SKIP). Further, the hybrid conduction mode cycle Tskip comprises N switch cycles, for example, the first N-1 switch cycles work in the critical conduction mode (BCM), and the Nth switch cycle works in the discontinuous conduction mode (DCM). For example, referring to Figures 4a-4c , the hybrid conduction mode cycle Tskip with N>1 is shown, and the number of switch cycles in the hybrid conduction mode cycle changes as the load changes. As the load decreases, the cycle in which the flyback converter works in the hybrid conduction mode can be as shown in Figure 4a , comprising 3 critical conduction mode switch cycles and 1 discontinuous conduction mode switch cycle, as shown in Figure 4b , comprising 2 critical conduction mode switch cycles and 1 discontinuous conduction mode switch cycle, or as shown in Figure 4c , comprising 1 critical conduction mode switch cycle and 1 discontinuous conduction mode switch cycle. As the load continues to decrease, referring to Figures 4d-4e , the hybrid conduction mode cycle Tskip with N=1 is shown. That is, the cycle in which the flyback converter works in the hybrid conduction mode comprises 1 discontinuous conduction mode switch cycle. For example, as the load decreases, the cycle in which the flyback converter works in the hybrid conduction mode is as shown in Figure 4dThe value shown is less than the maximum period Tmax of the hybrid conduction mode, or it can be as follows: Figure 4e The value shown is equal to the maximum value of the hybrid conduction mode period, Tmax. When the hybrid conduction mode period Tskip increases to the maximum value of the conduction mode period, Tmax, it is forced to enter the next hybrid conduction mode period, Tskip. Then, as the load continues to decrease, under light load conditions, the flyback converter operates in discontinuous conduction mode. For example, under light load conditions, the flyback converter first operates in PWM mode, and the peak current decreases. As the load continues to decrease, the flyback converter operates in Burst mode.

[0053] It should be noted that the maximum period Tmax in the hybrid conduction mode is determined based on the minimum switching frequency allowed by the flyback converter. The minimum switching frequency is set to prevent the flyback converter from entering audio. For example, the minimum switching frequency can be set to 25kHz or 30kHz, or any frequency in between. Implementation of this application is not limited to this.

[0054] The switching control circuit 130 is connected to the mode control circuit 120 to control the switching state of the first switch Q1 and the second switch Q2 of the flyback converter in different modes.

[0055] Furthermore, the control circuit 100 also includes an output feedback circuit 110. The output feedback circuit 110 generates an error compensation signal Vcomp based on the output feedback signal of the flyback converter. For example, the output feedback circuit 110 can generate the error compensation signal Vcomp based on the output feedback signal VFB, which characterizes the output voltage Vo. This output feedback signal VFB can be obtained by detecting the voltage at the common terminal of two resistors connected in series between the output voltage Vo and the secondary ground. It should be noted that this application does not limit the type or acquisition method of the output feedback signal. In other embodiments, the output feedback signal can also be a signal characterizing the output current or the output power, etc.

[0056] Furthermore, the mode control circuit 120 obtains the load state of the flyback converter based on the error compensation signal Vcomp. For example, when the error compensation signal Vcomp is greater than a first set threshold Vcomp1, the load state is heavy load; when the error compensation signal Vcomp is less than a second set threshold Vcomp2, the load state is light load; and when the error compensation signal Vcomp is less than the first set threshold Vcomp1 and greater than the second set threshold Vcomp2, the load is between light load and heavy load. The first set threshold Vcomp1 is greater than the second set threshold Vcomp2.

[0057] Further, the control circuit 100 further comprises a current zero-crossing detection circuit 140 for obtaining a time at which the magnetizing inductor current ILm of the flyback converter drops to zero in each switching period within the hybrid conduction mode period Tskip. The mode control circuit 120 controls the switching period to operate in the discontinuous conduction mode (DCM) when a time duration between the start time of the hybrid conduction mode period Tskip and the time at which the magnetizing inductor current ILm drops to zero in the switching period is greater than a time threshold. The time threshold is for example equal to one half of a product of the maximum value of the hybrid conduction mode period Tmax and the first duty cycle Duty.

[0058] In other embodiments, the mode control circuit 120 controls the switching period to operate in the discontinuous conduction mode (DCM) or the boundary conduction mode (BCM) when a time duration between the start time of the hybrid conduction mode period Tskip and the time at which the magnetizing inductor current ILm drops to zero in the switching period is equal to the time threshold.

[0059] In other embodiments, the mode control circuit 120 controls the switching period to operate in the boundary conduction mode (BCM) when a time duration between the start time of the hybrid conduction mode period Tskip and the time at which the magnetizing inductor current ILm drops to zero in the switching period is less than the time threshold.

[0060] Further, the mode control circuit 120 further obtains a first time Ton from the start time of the hybrid conduction mode period Tskip until the time at which the magnetizing inductor current ILm drops to zero in the Nth switching period, and obtains the hybrid conduction mode period Tskip according to the first time Ton and the first duty cycle Duty. Further, the hybrid conduction mode period Tskip is a quotient of the first time Ton and the first duty cycle Duty.

[0061] In other embodiments, the mode control circuit 120 further obtains the hybrid conduction mode period Tskip according to a maximum value of the hybrid conduction mode period Tmax. When the quotient of the first time Ton and the first duty cycle Duty is less than the maximum value of the hybrid conduction mode period Tmax, the hybrid conduction mode period Tskip is the quotient of the first time Ton and the first duty cycle Duty, otherwise the hybrid conduction mode period Tskip is the maximum value of the hybrid conduction mode period Tmax.

[0062] Exemplarily, the current zero-crossing detection circuit 140 comprises a volt-second balance detection circuit 141, for example. The volt-second balance detection circuit 141 is configured to perform volt-second balance detection in each switching cycle to obtain a volt-second balance time, which is taken as the time when the magnetizing inductor current ILm is detected to drop to zero. The volt-second balance time represents that the electric charge flowing out of the auxiliary winding Naux during the conduction of the first switch Q1 of the flyback converter and the electric charge flowing into the auxiliary winding Naux during the conduction of the second switch Q2 of the flyback converter are equal.

[0063] In other embodiments, referring to Figure 2 , the flyback converter 300 is different from the flyback converter 200 in that the control circuit is different. The control circuit 400 in the flyback converter 300 further comprises a period control circuit 150 on the basis of the control circuit 100, and the current zero-crossing detection circuit 240 further comprises a voltage detection circuit 142 on the basis of the current zero-crossing detection circuit 140.

[0064] Further, the volt-second balance detection circuit 141 performs volt-second balance detection and outputs a volt-second balance signal in the hybrid conduction mode, the volt-second balance signal representing that the electric charge flowing out of the auxiliary winding Naux during the conduction of the first switch Q1 and the electric charge flowing into the auxiliary winding Naux during the conduction of the second switch Q2 are equal in each switching cycle.

[0065] Exemplarily, during the conduction of the first switch Q1, the switch Q10 is controlled to be turned on to sample the current Is flowing out of the auxiliary winding Naux via the sampling terminal VS, and during the conduction of the second switch Q2, the switch Q10 is controlled to be turned on to sample the current Is flowing into the auxiliary winding Naux via the sampling terminal VS. Referring to Figure 5 , based on the charge balance method, when the electric charge Is*t flowing into the auxiliary winding Naux via the sampling terminal VS during the conduction of the second switch Q2 is equal to the electric charge Is*t flowing out of the auxiliary winding Naux via the sampling terminal VS during the conduction of the first switch Q1, the volt-second balance time t2 is obtained, and at this time, the volt-second balance signal is output to indicate that the volt-second balance is reached. In alternative embodiments, the volt-second balance time t2 can be obtained by sampling the voltage Vaux of the auxiliary winding, and then when the volt-second of the area S2 is equal to the volt-second of the area S1.

[0066] Further, in this embodiment, for example, the time when the slope of the first appearance of the negative of the signal Vs representing the voltage Vaux of the auxiliary winding in the Nth switching cycle of the hybrid conduction mode period (this switching cycle works in the discontinuous conduction mode) reaches a slope threshold value is taken as the time when the drain-source voltage Vds1 of the first switch Q1 reaches the inflection point in the DCM mode (t1 in Figures 4a-4e ), and the time when the magnetizing inductor current ILm is detected to drop to zero in the Nth switching cycle is further obtained. Then the first time Ton is directly obtained.

[0067] The period control circuit 150 obtains the hybrid conduction mode period Tskip according to the first time Ton and the first duty ratio Duty. Further, the hybrid conduction mode period Tskip is the quotient of the first time Ton and the first duty ratio Duty.

[0068] Further, referring to Figure 6 、 Figure 7 The period control circuit 150 comprises a first capacitor unit 151, a second capacitor unit 152, and a comparator U3.

[0069] The first capacitor unit 151 generates a first current I1 in the first time Ton, and discharges the first capacitor C1 at the beginning of each period, and then charges the first capacitor C1 through the first current I1. The second capacitor unit 152 provides a second current I2, and discharges the second capacitor C2 at the beginning of each period, and then charges the second capacitor C2 through the second current I2. The comparator U1 generates a period control signal Tctrl according to the comparison result of the voltage across the first capacitor C1 and the voltage across the second capacitor C2, and the period control signal Tctrl represents the switching period of the skip mode or the discontinuous conduction mode. The quotient of the second current I2 and the first current I1 is the first duty ratio Duty, and the capacitance of the first capacitor C1 is equal to the capacitance of the second capacitor C2.

[0070] Exemplarily, the first capacitor unit 151 comprises a first capacitor C1, a first resistor R1, and a first current source U2. Exemplarily, the first current source U2 is a current mirror, which comprises an eighth switch tube and a ninth switch tube. The control terminal of the eighth switch tube is connected to the first terminal thereof via the first resistor R1 and a power supply voltage VCC. The control terminal of the ninth switch tube is connected to the control terminal of the eighth switch tube. The first terminal of the ninth switch tube is connected to the power supply voltage VCC. The second terminal of the ninth switch tube is connected to the first terminal of the first capacitor C1. The second terminal of the first capacitor C1 is grounded. The control terminal of a third switch tube Q3 receives a first control signal Vduty. The first terminal of the third switch tube Q3 is connected to the second terminal of the eighth switch tube. The second terminal of the third switch tube Q3 is grounded. The first terminal of a fourth switch tube Q4 is connected to the first terminal of the first capacitor C1. The second terminal of the fourth switch tube Q4 is connected to the second terminal of the first capacitor C1. The control terminal of the fourth switch tube Q4 receives a second control signal Vpulse.

[0071] The second capacitor unit 152 comprises a second capacitor C2, a second resistor R2, a second current source U3, which is exemplarily a current mirror comprising a fifth switch tube and a sixth switch tube, a control terminal of the fifth switch tube being connected to a first terminal thereof and connected to a power supply voltage VCC via the second resistor R2, a second terminal of the fifth switch tube being grounded, a control terminal of the sixth switch tube being connected to the control terminal of the fifth switch tube, a first terminal of the sixth switch tube being connected to the power supply voltage VCC, a second terminal of the sixth switch tube being connected to a first terminal of the second capacitor C2, a second terminal of the second capacitor C2 being grounded, a first terminal of a seventh switch tube Q7 being connected to the first terminal of the second capacitor C2, a second terminal of the seventh switch tube Q7 being connected to the second terminal of the second capacitor C2, and a control terminal of the seventh switch tube Q7 receiving the second control signal Vpulse.

[0072] wherein the first control signal Vduty has a valid state for a first time ton, the second control signal Vpulse is a pulse signal at the beginning of each period, the first resistor R1 has the same resistance value as the second resistor R2, and a quotient of a current coefficient of the second current source U3 and a current coefficient of the first current source U2 is the first duty ratio Duty. It should be noted that the second current source U3 and the first current source U2 can also be other current control current sources.

[0073] Referring to Figure 6 The second control signal Vpulse is used to clear the voltages of the capacitors C1 and C2 at the beginning of each hybrid conduction mode period. The first capacitor C1 is charged only for the first time ton, and the second capacitor C2 is charged for the whole period. During the whole period, the voltage across the first capacitor C1 is always greater than the voltage across the second capacitor C2, until the voltage across the second capacitor C2 reaches the voltage across the first capacitor C1, and the current period ends.

[0074] Further, the switch control circuit 130 further comprises a parameter setting circuit 160. The parameter setting circuit 160 sets the first duty ratio Duty according to the error compensation signal Vcomp in the hybrid conduction mode and the discontinuous conduction mode. The parameter setting circuit 160 also sets a peak current Vcs according to the error compensation signal Vcomp. The peak current Vcs is used at least for comparison with the inductor current sampling signal, and controls the first switch tube Q1 to be turned off when the inductor current sampling signal reaches the peak current Vcs.

[0075] Exemplarily, referring to Figure 3The Duty curve is formed by a plurality of first duty ratios Duty. The plurality of first duty ratios Duty are set according to different error compensation signals Vcomp. The Vcs curve is formed by a plurality of peak currents. The plurality of peak currents are set according to different error compensation signals Vcomp. The curve L1 is an upper limit of the operating frequency of the flyback converter, and the curve L2 is a lower limit of the operating frequency of the flyback converter. Further, when the error compensation signal Vcomp is less than or equal to a second set threshold Vcomp2, the first duty ratio Duty has a minimum value Duty_min. When the error compensation signal Vcomp is equal to a first set threshold Vcomp1, the first duty ratio Duty has a maximum value. When the error compensation signal Vcomp is greater than the second set threshold Vcomp2 and less than the first set threshold Vcomp1, the first duty ratio Duty increases as the error compensation signal Vcomp increases. The maximum value of the first duty ratio Duty is 100%.

[0076] Exemplarily, the switch control circuit 130 controls the switch period to operate in the critical conduction mode in the hybrid conduction mode includes: controlling the first switch tube Q1 to be turned on at the beginning of the critical conduction mode switch period, controlling the second switch tube Q2 to be turned on after the first switch tube Q1 is turned off, and delaying the control of the second switch tube Q2 to be turned off by a second time tZVS at the moment when the magnetizing inductor current ILm drops to zero (the moment of volt-second balance) in the switch period.

[0077] The switch control circuit 130 controls the switch period to operate in the critical conduction mode in the hybrid conduction mode includes: controlling the first switch tube Q1 to be turned on at the beginning of the critical conduction mode switch period, controlling the second switch tube Q2 to be turned on after the first switch tube Q1 is turned off, and delaying the control of the second switch tube Q2 to be turned off by a second time tZVS at the moment when the magnetizing inductor current ILm drops to zero (the moment of volt-second balance) in the switch period.

[0078] Further, in the hybrid conduction mode, the switch control circuit 130 judges whether the current time is greater than the time threshold (the time when the red dotted line in the figure) at the moment when the magnetizing inductor current ILm drops to zero each time, in the case that the current time is greater than the time threshold, the current time is the first time, and then the switch control circuit 130 controls the second switch tube Q2 to be turned off and keeps the first switch tube Q1 to be turned off. In the case that the current time is less than the time threshold, the switch control circuit 130 delays the second time to turn off the second switch tube Q2, and controls the first switch tube Q1 and the second switch tube Q2 in the flyback converter to be alternately zero-voltage turned on in the next switching period. In the case that the current time is equal to the time threshold, the switch control circuit 130 can delay the second time to turn off the second switch tube Q2, and controls the first switch tube Q1 and the second switch tube Q2 in the flyback converter to work in the discontinuous conduction mode in the next switching period. Or in the case that the current time is equal to the time threshold, the current time is the first time, and then the switch control circuit 130 controls the second switch tube Q2 to be turned off and keeps the first switch tube Q1 to be turned off.

[0079] In the case of the hybrid conduction mode N>1, exemplarily, see Figure 4a In the first three switching periods, the time threshold (the time when the red dotted line in the figure) is not reached each time when the volt-second balance is reached, until the current time is greater than the time threshold (the time when the red dotted line in the figure) when the volt-second balance is reached in the fourth switching period, that is, the time when the volt-second balance signal is output when the volt-second balance is reached in the fourth switching period (or the time when the slope of the signal Vs representing the auxiliary winding voltage Vaux first appears negative and reaches the slope threshold) is the first time ton, and then the switch control circuit 130 controls the second switch tube Q2 to be turned off and keeps the first switch tube Q1 to be turned off for a time Toff, until the next hybrid conduction mode period Tskip starts. Wherein, the time when the next hybrid conduction mode period Tskip starts is determined according to the current hybrid conduction mode period Tskip.

[0080] It should be noted that in the hybrid conduction mode, the switch control circuit 130 controls the first switch tube Q1 and the second switch tube Q2 in the flyback converter to be alternately zero-voltage turned on in the first time ton when each hybrid conduction mode period Tskip starts, controls the first switch tube Q1 to be turned on at the start of each hybrid conduction mode period, and then controls the first switch tube Q1 to be turned off according to the comparison of the inductor current sampling signal representing the current flowing through the first switch tube Q1 and the peak current Vcs to control the peak of the magnetizing inductor current ILm, controls the second switch tube Q2 to be turned on after the first switch tube Q1 is turned off, and delays the first switch tube Q1 to be turned off for the second time tZVS after the volt-second balance is reached. Then continue to control the first switch tube Q1 and the second switch tube Q2 to be alternately zero-voltage turned on.

[0081] In the case of the hybrid conduction mode N=1, referring to Figure 4d At the beginning of the hybrid conduction mode period, the time length between the moment when the first negative slope of the signal Vs representing the auxiliary winding voltage Vaux reaches the slope threshold value and the moment when the voltage-second balance is reached or the signal Vs representing the auxiliary winding voltage Vaux reaches the slope threshold value for the first time (the moment shown by the red dashed line in the figure), i.e., the moment when the output voltage-second balance signal is output or the moment when the first negative slope of the signal Vs representing the auxiliary winding voltage Vaux reaches the slope threshold value for the first time is the first time ton, and then the switching control circuit 130 controls the second switch Q2 to be turned off and keeps the first switch Q1 to be turned off for a time Toff until the next hybrid conduction mode period Tskip starts. The moment when the next hybrid conduction mode period Tskip starts is determined according to the current hybrid conduction mode period Tskip. However, when the hybrid conduction mode period Tskip reaches the maximum hybrid conduction mode period Tmax, referring to Figure 4e After the first time ton, the switching control circuit 130 controls the second switch Q2 to be turned off and keeps the first switch Q1 to be turned off until the next hybrid conduction mode period Tskip starts. The moment when the next hybrid conduction mode period Tskip starts is determined according to the maximum hybrid conduction mode period Tmax.

[0082] It should be noted that in the discontinuous conduction mode switching period of the hybrid conduction mode, the switching control circuit 130 controls the first switch Q1 in the flyback converter to be turned on at the first time Ton at the beginning of the switching period, and controls the second switch Q2 to be turned on after the first switch Q1 is turned off. At the beginning of the switching period, the first switch Q1 is turned on, and then the first switch Q1 is turned off according to the comparison between the inductor current sampling signal and the peak current Vcs, for example, and the second switch Q2 is turned on after the first switch Q1 is turned off.

[0083] The signal Vs representing the auxiliary winding voltage is, for example, a voltage signal.

[0084] Figure 8 A flowchart of a control method of a flyback converter is shown according to an embodiment of the present application. Referring to Figure 8 The control method can be executed in the control circuit 100 or the control circuit 400, for example. The control method comprises the following steps:

[0085] Step S410: obtaining the load state of the flyback converter. Illustratively, the step of obtaining the load state of the flyback converter comprises: generating an error compensation signal according to the output feedback signal of the flyback converter; and obtaining the load state of the flyback converter according to the error compensation signal. Wherein, when the error compensation signal is greater than a first set threshold, the load state is heavy load; when the error compensation signal is less than a second set threshold, the load state is light load; when the error compensation signal is less than the first set threshold and greater than the second set threshold, the load state is between heavy load and light load; the first set threshold is greater than the second set threshold.

[0086] Step S420: working in discontinuous conduction mode when the load state is light load, working in critical conduction mode when the load state is heavy load, and working in hybrid conduction mode when the load state is between light load and heavy load. The hybrid conduction mode period includes N switching periods, the first N-1 switching periods work in critical conduction mode, and the Nth switching period works in discontinuous conduction mode. Wherein, according to the set maximum value of the hybrid conduction mode period and the set first duty cycle, each switching period in the hybrid conduction mode period works in critical conduction mode or discontinuous conduction mode, and N is a positive integer.

[0087] Further, according to the set maximum value of the hybrid conduction mode period and the set first duty cycle, each switching period in the hybrid conduction mode period works in critical conduction mode or discontinuous conduction mode includes: obtaining the time when the excitation inductance current of the flyback converter drops to zero in each switching period in the hybrid conduction mode period; and when the time length between the start time of the hybrid conduction mode period and the time when the excitation inductance current drops to zero in the switching period is greater than a time threshold, the switching period works in discontinuous conduction mode. The time threshold is equal to one half of the product of the maximum value of the hybrid conduction mode period and the first duty cycle.

[0088] Further, it also includes: when the time length between the start time of the hybrid conduction mode period and the time when the excitation inductance current drops to zero in the switching period is equal to the time threshold, the switching period works in critical conduction mode or discontinuous conduction mode; when the time length between the start time of the hybrid conduction mode period and the time when the excitation inductance current drops to zero in the switching period is less than the time threshold, the switching period works in critical conduction mode.

[0089] Further, the flyback converter includes a first switch tube and a second switch tube, and in the hybrid conduction mode, controlling the switching period to work in critical conduction mode includes: controlling the first switch tube to be turned on at the beginning of the switching period; controlling the second switch tube to be turned on after the first switch tube is turned off; and controlling the second switch tube to be turned off at the time when the excitation inductance current drops to zero in the switching period with a second time delay.

[0090] Further, the controlling the switch period to operate in the discontinuous conduction mode in the hybrid conduction mode comprises: controlling the first switch to be turned on at the beginning of the switch period; controlling the second switch to be turned on after the first switch is turned off; and controlling the second switch to be turned off at the moment when the magnetizing inductor current drops to zero in the switch period, and keeping the first switch to be turned off until the end of the hybrid conduction mode period.

[0091] Further, the control method further comprises obtaining a first time in the hybrid conduction mode period, the first time starting from the beginning of the hybrid conduction mode period and ending at the moment when the magnetizing inductor current of the flyback converter drops to zero in the Nth switch period; and obtaining the hybrid conduction mode period according to the first time and the first duty cycle. The hybrid conduction mode period is the quotient of the first time and the first duty cycle.

[0092] Further, the hybrid conduction mode period is further obtained according to a maximum hybrid conduction mode period. When the quotient of the first time and the first duty cycle is less than the maximum hybrid conduction mode period, the hybrid conduction mode period is the quotient of the first time and the first duty cycle, otherwise the hybrid conduction mode period is the maximum hybrid conduction mode period.

[0093] Further, the method for obtaining the moment when the magnetizing inductor current of the flyback converter drops to zero in each switch period in the hybrid conduction mode period comprises: performing a voltage-second balance detection in each switch period to obtain a voltage-second balance moment, and taking the voltage-second balance moment as the moment when the magnetizing inductor current drops to zero, wherein the voltage-second balance moment represents that the electric charge flowing out of the auxiliary winding during the conduction of the first switch of the flyback converter is equal to the electric charge flowing into the auxiliary winding during the conduction of the second switch of the flyback converter.

[0094] Further, the method for obtaining the moment when the magnetizing inductor current of the flyback converter drops to zero in the Nth switch period comprises: performing a voltage-second balance detection in the Nth switch period to obtain a voltage-second balance moment, and taking the voltage-second balance moment as the moment when the magnetizing inductor current drops to zero; or detecting the moment when the slope of a signal representing the voltage of the auxiliary winding reaches a slope threshold for the first time in the Nth switch period, and taking the moment as the moment when the magnetizing inductor current drops to zero.

[0095] Further, the control method further comprises: in the hybrid conduction mode, setting the first duty cycle according to the error compensation signal, wherein when the error compensation signal is greater than a second set threshold and less than a first set threshold, the first duty cycle increases with the increase of the error compensation signal.

[0096] Further, when the error compensation signal is equal to the first set threshold, the first duty cycle has a maximum value, and the maximum value of the first duty cycle is 100%.

[0097] Figure 9Fig. 3 shows a waveform diagram of another peak current in a control circuit of a flyback converter according to an embodiment of the present application. Figure 10 Fig. 4 shows a waveform diagram of yet another peak current in a control circuit of a flyback converter according to an embodiment of the present application. Figure 11 Fig. 5 shows a waveform diagram of yet another peak current in a control circuit of a flyback converter according to an embodiment of the present application.

[0098] Referring to Figure 9 , the control method of the flyback converter provided in the present application increases the control hysteresis in order to avoid switching back and forth between the critical conduction mode and the mixed conduction mode at the critical load point (error compensation signal Vcomp1). Exemplarily, when the error compensation signal decreases to the first set threshold, the peak current Vcs suddenly increases, and then the equivalent switching frequency becomes smaller, and correspondingly, the power suddenly increases.

[0099] Referring to Figure 10 , a frequency curve is shown as the load decreases from high to low, in which the critical conduction mode switches to the mixed conduction mode. Referring to Figure 11 , a frequency curve is shown as the load increases from low to high, in which the mixed conduction mode switches to the critical conduction mode. The dashed line part is the switching point area, and the steady-state operating point will not work in the dashed line area. Taking the frequency curve as the load decreases from high to low, in which the critical conduction mode switches to the mixed conduction mode, as an example, when the error compensation signal Vcomp decreases from high to the error compensation signal Vcomp1, the working frequency will rapidly decrease due to the sudden increase of the peak current, and then the flyback converter will not easily jump after switching from the critical conduction mode to the mixed conduction mode due to slight changes in the error compensation signal.

[0100] The flyback converter and the control circuit and the control method thereof provided in the present disclosure obtain the load state of the flyback converter, and control the flyback converter to work in different modes under different load states. The mixed conduction mode is used when the load state is between light load and heavy load, and the mixed conduction mode period includes N switching periods, and the N switching periods include N-1 switching periods of the critical conduction mode and 1 switching period of the discontinuous conduction mode. The present application controls each switching period in the mixed conduction mode period to work in the critical conduction mode or the discontinuous conduction mode by setting the maximum value of the mixed conduction mode period and the first duty ratio, so as to adaptively control each switching period in the mixed conduction mode, and then improve the system efficiency of the flyback converter when the load state is between light load and heavy load.

[0101] Further, the present application increases the control hysteresis by suddenly increasing the peak current when the error compensation signal decreases to the first set threshold, so as to avoid switching back and forth between the critical conduction mode and the mixed conduction mode.

[0102] Figure 12 Fig. 6 shows a structure schematic diagram of another flyback converter according to an embodiment of the present application. Figure 13 Fig. 6 shows a structure schematic diagram of another flyback converter according to an embodiment of the present application. Figure 14 Fig. 6 shows a structure schematic diagram of another flyback converter according to an embodiment of the present application.

[0103] It should be noted that, Figure 1 and Figure 2 are all examples of asymmetric half-bridge flyback converter with the first switch Q1 located at the low side. In other embodiments, the first switch Q1 can also be located at the high side, for example, as shown in asymmetric half-bridge flyback converter 500. Figure 12 The above embodiments are all described by taking asymmetric half-bridge flyback converter as an example. However, it can be understood that the present application is not limited thereto. Based on similar working principles, the solutions of the above embodiments can also be applied to flyback converters of other topologies, for example, active clamp flyback converter 600 as shown in Figure 13 , for example, zero-voltage switching flyback converter 700 as shown in Figure 14 , and so on. Among them, in Figure 13 , the second switch Q2 is used as a clamp switch, one end of the second switch Q2 is connected with the clamp capacitor C clamp, and the other end of the second switch Q2 is connected with the common end of the first switch Q1 and the primary winding of the transformer. In Figure 14 , the first switch Q1 is connected with the primary winding, and the second switch Q2 is connected with the secondary winding.

[0104] It should be noted that the control circuit in flyback converter 500, flyback converter 600, and flyback converter 700 can be control circuit 100 or control circuit 400.

[0105] The above described embodiments do not constitute a limitation on the protection scope of the technical solutions. Any modification, equivalent replacement, and improvement made within the spirit and principles of the above described embodiments shall be included in the protection scope of the technical solutions.

Claims

1. A control method of a flyback converter, wherein, The control method comprises: obtaining a load state of the flyback converter; working in discontinuous conduction mode (DCM) when the load state is light load; working in critical conduction mode (CRM) when the load state is heavy load; and working in hybrid conduction mode (HCM) when the load state is between light load and heavy load, the HCM period comprising N switching periods, the first N-1 switching periods working in the CRM, and the Nth switching period working in the DCM, wherein each switching period in the HCM period is controlled to work in the CRM or the DCM according to a set maximum HCM period and a set first duty cycle, and N is a positive integer. Controlling each switching period in the HCM period to work in the CRM or the DCM according to a set maximum HCM period and a set first duty cycle comprises:

2. The control method of a flyback converter according to claim 1, wherein, obtaining a time when the magnetizing inductor current of the flyback converter in each switching period in the HCM period drops to zero; and controlling the switching period to work in the DCM when a time length between a start time of the HCM period and the time when the magnetizing inductor current drops to zero is greater than a time threshold. The time threshold is equal to one half of a product of the maximum HCM period and the first duty cycle. The control method further comprises:

3. The control method of a flyback converter according to claim 2, wherein, controlling the switching period to work in the CRM or the DCM when the time length between the start time of the HCM period and the time when the magnetizing inductor current drops to zero is equal to the time threshold; and controlling the switching period to work in the CRM when the time length between the start time of the HCM period and the time when the magnetizing inductor current drops to zero is less than the time threshold. The flyback converter comprises a first switch and a second switch, and controlling the switching period to work in the CRM in the HCM mode comprises:

4. The control method of a flyback converter according to claim 2, wherein, controlling the first switch to be turned on at the beginning of the switching period; controlling the second switch to be turned on after the first switch is turned off; controlling the second switch to be turned off at the time when the magnetizing inductor current drops to zero in the switching period with a second time delay. The flyback converter comprises a first switch and a second switch, and controlling the switching period to work in the DCM in the HCM mode comprises:

5. The control method of a flyback converter according to claim 2, wherein, controlling the first switch to be turned on at the beginning of the switching period; controlling the second switch to be turned on after the first switch is turned off; controlling the second switch to be turned off at the time when the magnetizing inductor current drops to zero in the switching period, and keeping the first switch turned off until the end of the HCM period. The control method further comprises:

6. The control method of a flyback converter according to any one of claims 1 to 5, wherein obtaining a first time in the HCM period, the first time starting from the start time of the HCM period and ending at the time when the magnetizing inductor current of the flyback converter drops to zero in the Nth switching period; and ​ The mixed conduction mode period is obtained according to the first time and the first duty cycle.

7. The control method of a flyback converter according to claim 6, wherein The mixed conduction mode period is the quotient of the first time and the first duty cycle.

8. The control method of a flyback converter according to claim 7, wherein The mixed conduction mode period is also obtained according to the mixed conduction mode period maximum, When the quotient of the first time and the first duty cycle is less than the mixed conduction mode period maximum, the mixed conduction mode period is the quotient of the first time and the first duty cycle, otherwise the mixed conduction mode period is the mixed conduction mode period maximum.

9. The control method of a flyback converter according to claim 2, wherein, The method for obtaining the time when the magnetizing inductance current of the flyback converter drops to zero in each switching cycle within the mixed conduction mode period comprises: In the each switching cycle, a volt-second balance detection is performed to obtain a volt-second balance time, which is taken as the time when the magnetizing inductance current is detected to drop to zero, wherein the volt-second balance time represents that the electric charge flowing out of the auxiliary winding during the conduction of the first switch of the flyback converter is equal to the electric charge flowing into the auxiliary winding during the conduction of the second switch of the flyback converter.

10. The control method of a flyback converter according to claim 6, wherein, The method for obtaining the time when the magnetizing inductance current of the flyback converter drops to zero in the Nth switching cycle comprises: In the Nth switching cycle, a volt-second balance detection is performed to obtain a volt-second balance time, which is taken as the time when the magnetizing inductance current is detected to drop to zero, wherein the volt-second balance time represents that the electric charge flowing out of the auxiliary winding during the conduction of the first switch of the flyback converter is equal to the electric charge flowing into the auxiliary winding during the conduction of the second switch of the flyback converter. Or in the Nth switching cycle, the time when the slope of the signal representing the auxiliary winding voltage first reaches a slope threshold value in a negative direction is detected, which is taken as the time when the magnetizing inductance current is detected to drop to zero.

11. The control method of a flyback converter according to claim 1, wherein, The method for obtaining the load state of the flyback converter comprises: generating an error compensation signal according to the output feedback signal of the flyback converter; and obtaining the load state according to the error compensation signal, wherein when the error compensation signal is greater than a first set threshold value, the load state is heavy load, when the error compensation signal is less than a second set threshold value, the load state is light load, when the error compensation signal is less than the first set threshold value and greater than the second set threshold value, the load state is between light load and heavy load, and the first set threshold value is greater than the second set threshold value.

12. The control method of a flyback converter according to claim 11, wherein, The control method further comprises: In the mixed conduction mode, the first duty cycle is set according to the error compensation signal, wherein when the error compensation signal is greater than the second set threshold value and less than the first set threshold value, the first duty cycle increases with the increase of the error compensation signal.

13. The control method of a flyback converter according to claim 12, wherein, When the error compensation signal is equal to the first set threshold value, the first duty cycle has a maximum value, and the maximum value of the first duty cycle is 100%.

14. The control method of a flyback converter according to claim 11, wherein, When the error compensation signal drops to the first set threshold value, the peak current mutation increases.

15. A control circuit for a flyback converter, the flyback converter comprising a first switch and a second switch, wherein, The control circuit comprises: a mode control circuit configured to obtain a load state of the flyback converter, control the flyback converter to operate in a discontinuous conduction mode (DCM) when the load state is a light load, control the flyback converter to operate in a critical conduction mode (CRM) when the load state is a heavy load, and control the flyback converter to operate in a hybrid conduction mode (HCM) when the load state is between the light load and the heavy load, the HCM period including N switching periods, the first N-1 switching periods operating in the CRM, and the Nth switching period operating in the DCM, N being a positive integer, and the mode control circuit further configured to control each switching period in the HCM period to operate in the CRM or the DCM according to a maximum value of the HCM period and a first duty cycle, and a switch control circuit connected to the mode control circuit and configured to control switching states of the first switch and the second switch.

16. The control circuit for a flyback converter of claim 15, wherein, Further comprising: a current zero-crossing detection circuit configured to obtain a time when an excitation inductor current of the flyback converter in each switching period in the HCM period drops to zero, the mode control circuit configured to control the switching period to operate in the DCM when a time duration between a start time of the HCM period and the time when the excitation inductor current drops to zero is greater than a time threshold, the time threshold being equal to one half of a product of the maximum value of the HCM period and the first duty cycle.

17. The control circuit for a flyback converter of claim 15, wherein, Further comprising: an output feedback circuit configured to generate an error compensation signal according to an output feedback signal of the flyback converter, the mode control circuit configured to obtain the load state of the flyback converter according to the error compensation signal, wherein the load state is the heavy load when the error compensation signal is greater than a first set threshold, the load state is the light load when the error compensation signal is less than a second set threshold, and the load state is between the light load and the heavy load when the error compensation signal is less than the first set threshold and greater than the second set threshold, the first set threshold being greater than the second set threshold.

18. The control circuit of a flyback converter of any of claims 15-17, wherein, Further comprising: a period control circuit configured to obtain the HCM period according to a first time and the first duty cycle, wherein the mode control circuit obtains the first time, the first time starting from the start time of the HCM period and ending at a time when the excitation inductor current of the flyback converter in the Nth switching period drops to zero.

19. The control circuit for a flyback converter of claim 18, wherein, The period control circuit comprises: a first capacitor unit including a first capacitor, the first capacitor unit generating a first current during the first time and discharging the first capacitor at the beginning of each period and then charging the first capacitor with the first current, a second capacitor unit including a second capacitor, the second capacitor unit providing a second current and discharging the second capacitor at the beginning of each period and then charging the second capacitor with the second current, and a comparator configured to generate a period control signal based on a comparison of a voltage across the first capacitor and a voltage across the second capacitor, the period control signal representing a hybrid conduction mode period or a discontinuous conduction mode period, wherein a quotient of the second current and the first current is the first duty cycle, and a capacitance of the first capacitor is equal to a capacitance of the second capacitor.

20. The control circuit of the flyback converter of claim 19, wherein the first capacitor unit further comprises: a first resistor; a first current source comprising an eighth switch and a ninth switch, a control terminal of the eighth switch being connected to a first terminal of the eighth switch and to a power supply voltage via the first resistor, a control terminal of the ninth switch being connected to the control terminal of the eighth switch, a first terminal of the ninth switch being connected to the power supply voltage, a second terminal of the ninth switch being connected to a first terminal of the first capacitor, a second terminal of the first capacitor being connected to ground; a third switch, a control terminal of the third switch receiving a first control signal, a first terminal of the third switch being connected to a second terminal of the eighth switch, a second terminal of the third switch being connected to ground; a fourth switch, a first terminal of the fourth switch being connected to the first terminal of the first capacitor, a second terminal of the fourth switch being connected to the second terminal of the first capacitor, a control terminal of the fourth switch receiving a second control signal, the second capacitor unit further comprises: a second resistor; a second current source comprising a fifth switch and a sixth switch, a control terminal of the fifth switch being connected to a first terminal of the fifth switch and to the power supply voltage via the second resistor, a second terminal of the fifth switch being connected to ground, a control terminal of the sixth switch being connected to the control terminal of the fifth switch, a first terminal of the sixth switch being connected to the power supply voltage, a second terminal of the sixth switch being connected to a first terminal of the second capacitor, a second terminal of the second capacitor being connected to ground; a seventh switch, a first terminal of the seventh switch being connected to the first terminal of the second capacitor, a second terminal of the seventh switch being connected to the second terminal of the second capacitor, a control terminal of the seventh switch receiving the second control signal, wherein an active state of the first control signal is maintained for the first time, the second control signal is a pulse signal at the beginning of each period, a resistance of the first resistor is equal to a resistance of the second resistor, and a quotient of a mirror ratio of the second current source and the first current source is the first duty cycle.

21. A flyback converter, wherein, A control circuit of a flyback converter as claimed in any one of claims 15 to 20.

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