Control Method of Switching Circuit, Control Circuit and Switching Circuit

Abstract

The present invention discloses a control method, a control circuit and a switching circuit for a switching circuit. When each switching cycle of the switching circuit is in the critical conduction mode or the continuous conduction mode, as the load power decreases, it enters the hiccup mode, and N consecutive switching cycles are in the critical conduction mode, and then one switching cycle is in the discontinuous conduction mode. The N consecutive switching cycles in the critical conduction mode plus one switching cycle in the discontinuous conduction mode form a hiccup cycle; as the load power further decreases, each switching cycle is in the discontinuous conduction mode, where N is a natural number greater than or equal to 1.

Description

Technical Field

[0001] The present invention relates to the field of power electronics technology, and particularly relates to a control method for a switching circuit, a control circuit and a switching circuit. Background Art

[0002] In a switching circuit, when the load power decreases, it will enter the discontinuous conduction mode from the critical conduction mode. For example, when the peak inductor current is less than a certain value, or the switching frequency is higher than a certain value, it will enter the discontinuous conduction mode from the critical conduction mode. When directly switching from the critical conduction mode to the discontinuous conduction mode, at the beginning of entering the discontinuous conduction mode, due to the high frequency, the switching loss is large, which affects the system efficiency. Therefore, how to maintain a high system efficiency when entering the discontinuous conduction mode at a high switching frequency is an urgent problem to be solved in the switching circuit. Summary of the Invention

[0003] In view of this, the purpose of the present invention is to provide a control method for a switching circuit, a control circuit and a switching circuit, so as to solve the problems of large switching loss and low efficiency in the discontinuous mode during high-frequency applications in the prior art.

[0004] The present invention provides a control method for a switching circuit. When each switching cycle of the switching circuit is in the critical conduction mode or the continuous conduction mode, as the load power decreases, it enters the hiccup mode, and N consecutive switching cycles are in the critical conduction mode, and then one switching cycle is in the discontinuous conduction mode. The N consecutive switching cycles in the critical conduction mode plus one switching cycle in the discontinuous conduction mode are the hiccup cycle; when the load power further decreases, each switching cycle is in the discontinuous conduction mode, where N is a natural number greater than or equal to 1.

[0005] Optionally, the switching circuit is an active clamp flyback circuit. The active clamp flyback circuit includes a main switching transistor and a clamp switching transistor. When in the hiccup mode or / and the critical conduction mode, the clamp switching transistor conducts after the main switching transistor turns off, and by adjusting the turn-off time of the clamp switching transistor, the drain voltage of the main switching transistor can oscillate to close to zero voltage;

[0006] Or in the discontinuous mode, the clamp switching transistor conducts after the main switching transistor turns off, and by adjusting the turn-off time of the clamp switching transistor, the clamp switching transistor turns off when the inductor current is close to zero;

[0007] Or in the hiccup mode, after the (N + 1)th conduction of the main switching transistor, the clamp switching transistor does not conduct.

[0008] Optionally, when in the hiccup mode, if the hiccup cycle is greater than the first hiccup cycle threshold, it switches to each switching cycle being in the discontinuous conduction mode;

[0009] Or when the turn-off time of the main switch in the discontinuous conduction mode of the hiccup mode is greater than the first time threshold, or when the time when the inductor current is zero is greater than the fifth time threshold, then it switches to the discontinuous conduction mode for each switching cycle.

[0010] Optionally, when it is in the discontinuous conduction mode for each switching cycle, when the switching cycle is less than the second cycle threshold, or when the turn-off time of the main switch is less than the third time threshold, or when the time when the inductor current is zero is less than the sixth time threshold, then it switches to the hiccup mode.

[0011] Optionally, when the time when both the main switch and the clamping switch are turned off in the discontinuous conduction mode of the hiccup mode is greater than the second time threshold, then it switches to the discontinuous conduction mode for each switching cycle.

[0012] Optionally, when it is in the discontinuous conduction mode for each switching cycle, and the time when both the main switch and the clamping switch are turned off is less than the fourth time threshold, then it switches to the hiccup mode.

[0013] Optionally, when each switching cycle of the switching circuit is in the critical conduction mode, if the peak inductor current is less than the first current threshold or the compensation voltage is less than the first voltage threshold, it switches to the hiccup mode, and the peak inductor current in the hiccup mode is greater than the first current threshold; the output voltage or output current is operationally amplified with a reference value to obtain the compensation voltage.

[0014] Optionally, when each switching cycle of the switching circuit is in the critical conduction mode, and the total time of continuous M switching cycles is less than the seventh time, it switches to the hiccup mode or the discontinuous conduction mode.

[0015] Optionally, when switching from the critical conduction mode to the hiccup mode or to the discontinuous conduction mode, in the critical conduction mode, the clamping switch conducts after the main switch is turned off. By adjusting the turn-off moment of the clamping switch, the drain voltage of the main switch can be oscillated to approach zero voltage or the clamping switch is turned off when the inductor current is close to zero, and then it switches to the hiccup mode or to the discontinuous conduction mode;

[0016] Or when switching from the critical conduction mode to the hiccup mode or to the discontinuous conduction mode, the clamping switch is turned off.

[0017] Optionally, the main switch conducts when the drain voltage of the main switch is less than the bus voltage;

[0018] Or starting from the conduction of the main switch in the previous cycle, when the timing reaches the ninth time, the main switch conducts, and it switches to the hiccup mode or to the discontinuous conduction mode;

[0019] Or start timing from the conduction of the main switching tube in the previous cycle. When the timing reaches the ninth time, the clamping switching tube conducts for the first conduction time. When the conduction time of the clamping switching tube is the first conduction time, the clamping switching tube turns off, and the main switching tube conducts, switching to the hiccup mode or the discontinuous conduction mode.

[0020] Optionally, when each switching cycle is in the discontinuous conduction mode and / or when in the hiccup mode, start timing from the conduction of the clamping switching tube when the inductor current is discontinuous. When the conduction time of the clamping switching tube is the first conduction time, the clamping switching tube turns off, and the main switching tube conducts when the drain-source voltage oscillates near zero voltage; when the conduction time of the main switching tube reaches the third time or the inductor current reaches the third current threshold, the main switching tube turns off. When the timing reaches the second time, the clamping switching tube conducts and starts timing again.

[0021] Optionally, when in the discontinuous conduction mode and the load power continues to decrease and / or when in the hiccup mode, start timing from the conduction of the main switching tube. When the conduction time of the main switching tube reaches the third time or the inductor current reaches the third current threshold, the main switching tube turns off, and the clamping switching tube conducts. When the inductor current is near zero current, the clamping switching tube turns off. When the timing reaches the second time, the main switching tube conducts and starts timing again.

[0022] The present invention also provides a control circuit for a switching circuit. When the control circuit controls each switching cycle of the switching circuit to be in the critical conduction mode or the continuous conduction mode, as the load power decreases, it enters the hiccup mode, with N consecutive switching cycles in the critical conduction mode, and then one switching cycle in the discontinuous conduction mode. The N consecutive switching cycles in the critical conduction mode plus one switching cycle in the discontinuous conduction mode form a hiccup cycle; as the load power further decreases, the control circuit controls each switching cycle to be in the discontinuous conduction mode, where N is a natural number greater than or equal to 1.

[0023] Optionally, the control circuit includes a power detection circuit, a mode selection circuit, and a main switching tube control circuit. The power detection circuit obtains a value representing the load power by detecting the peak value of the inductor current and / or the hiccup cycle and / or the switching cycle and / or the turn-off time of the main switching tube and / or the discontinuous time of the inductor current and / or the compensation voltage. The mode selection circuit receives the output voltage of the power detection circuit and controls different operating modes according to the output voltage of the power detection circuit. The main switching tube control circuit receives the output voltage of the mode selection circuit and controls the conduction and turn-off of the main switching tube according to the output voltage of the mode selection circuit and the compensation voltage; the output voltage or output current is amplified by an operation with a reference value to obtain the compensation voltage.

[0024] Optionally, the switching circuit is an active clamp flyback circuit. The active clamp flyback circuit includes a main switching transistor and a clamp switching transistor. When in the hiccup mode or / and the critical conduction mode, the control circuit controls the clamp switching transistor to conduct after the main switching transistor is turned off. The control circuit adjusts the turn-off time of the clamp switching transistor so that the drain voltage of the main switching transistor can oscillate to near zero voltage;

[0025] Or in the discontinuous mode, the control circuit controls the clamp switching transistor to conduct after the main switching transistor is turned off, and by adjusting the turn-off time of the clamp switching transistor, the clamp switching transistor is turned off when the inductor current is close to zero;

[0026] Or in the hiccup mode, after the (N + 1)-th conduction of the main switching transistor, the control circuit controls the clamp switching transistor not to conduct.

[0027] Optionally, the control circuit further includes a clamp switching transistor control circuit. The clamp switching transistor control circuit receives the output voltage of the mode selection circuit and controls the clamp switching transistor according to the output voltage of the mode selection circuit.

[0028] Optionally, when in the hiccup mode, when the power detection circuit detects that the hiccup period is greater than the first hiccup period threshold, the mode selection circuit switches the mode to the discontinuous conduction mode for each switching cycle;

[0029] Or in the switching cycle of the discontinuous conduction mode in the hiccup mode, when the power detection circuit detects that the turn-off time of the main switching transistor is greater than the first time threshold or the time when the inductor current is zero is greater than the fifth time threshold, the mode selection circuit switches the mode to the discontinuous conduction mode for each switching cycle.

[0030] Optionally, in the switching cycle of the discontinuous conduction mode in the hiccup mode, when the power detection circuit detects that the time when both the main switching transistor and the clamp switching transistor are turned off is greater than the second time threshold, the mode selection circuit switches the mode to the discontinuous conduction mode for each switching cycle.

[0031] Optionally, when each switching cycle of the switching circuit is in the critical conduction mode, if the power detection circuit detects that the peak value of the inductor current is less than the first current threshold or the compensation voltage is less than the first voltage threshold, the mode selection circuit switches the mode to the hiccup mode, and the peak value of the inductor current in the hiccup mode is greater than the first current threshold; the output voltage or output current is arithmetically amplified with a reference value to obtain the compensation voltage.

[0032] Optionally, when each switching period of the switching circuit is in the critical conduction mode, and the power detection circuit detects that the time of continuous M switching periods is less than the seventh time, the mode selection circuit switches the mode to the hiccup mode or the discontinuous conduction mode.

[0033] Optionally, when switching from the critical conduction mode to the hiccup mode or to the discontinuous conduction mode, in the critical conduction mode, the clamping switch transistor conducts after the main switch transistor is turned off, and the control circuit adjusts the turn-off time of the clamping switch transistor so that the drain voltage of the main switch transistor can oscillate to near zero voltage or the clamping switch transistor is turned off when the inductor current approaches zero, and the mode selection circuit switches the mode to the hiccup mode or to the discontinuous conduction mode;

[0034] Or when the mode selection circuit switches the mode from the critical conduction mode to the hiccup mode or to the discontinuous conduction mode, the clamping switch transistor control circuit controls the clamping switch transistor to turn off.

[0035] Optionally, when each switching period is in the discontinuous conduction mode and / or when in the hiccup mode, the clamping switch transistor control circuit starts timing from when the clamping switch transistor conducts when the inductor current is discontinuous. When the conduction time of the clamping switch transistor is the first conduction time, the clamping switch transistor control circuit controls the clamping switch transistor to turn off, and the main switch transistor conducts when the drain-source voltage oscillates near zero voltage; when the conduction time of the main switch transistor reaches the third time or the inductor current reaches the third current threshold, the main switch transistor control circuit controls the main switch transistor to turn off, and when the timing reaches the second time, the clamping switch transistor control circuit controls the clamping switch transistor to conduct and starts timing again.

[0036] The present invention also provides a switching circuit.

[0037] Adopting the circuit structure and method of the present invention, compared with the prior art, it has the following advantages: the switching loss of the switching circuit is low and the conversion efficiency is high in the critical conduction mode for each cycle, the hiccup mode, and the discontinuous conduction mode for each cycle. Description of the Drawings

[0038] Figure 1 For the switching signal LI of the main switch transistor, the inductor current i L and the drain voltage V of the main switch transistor SW of the switching circuit when the switching circuit enters the critical conduction mode for each switching period from the hiccup mode;

[0039] Figure 2 For the switching signal LI of the main switch transistor, the inductor current i L and the drain voltage V of the main switch transistor SWWaveform;

[0040] Figure 3 is the circuit schematic diagram of the active clamp flyback circuit;

[0041] Figure 4 is the waveforms of the main switch transistor switch signal LI, the clamp switch transistor switch signal HI, the inductor current i LM and the drain voltage V of the main switch transistor SW when the active clamp flyback circuit enters the critical conduction mode in each switching cycle from the hiccup mode;

[0042] Figure 5 is the waveforms of the main switch transistor switch signal LI, the clamp switch transistor switch signal HI, the inductor current i LM and the drain voltage V of the main switch transistor SW when the active clamp flyback circuit enters the hiccup mode from the discontinuous conduction mode in each switching cycle;

[0043] Figure 6 is the waveforms of the main switch transistor switch signal LI, the clamp switch transistor switch signal HI, the inductor current i LM and the drain voltage V of the main switch transistor SW when the active clamp flyback circuit is in the discontinuous conduction mode in each switching cycle in one embodiment;

[0044] Figure 7 is the waveforms of the main switch transistor switch signal LI, the clamp switch transistor switch signal HI, the inductor current i LM and the drain voltage V of the main switch transistor SW when the active clamp flyback circuit is in the hiccup mode in another embodiment;

[0045] Figure 8 is the waveforms of the main switch transistor switch signal LI, the clamp switch transistor switch signal HI, the inductor current i LM and the drain voltage V of the main switch transistor SW when the active clamp flyback circuit is in the discontinuous conduction mode in each switching cycle in another embodiment;

[0046] Figure 9 is the waveforms of the main switch transistor switch signal LI, the clamp switch transistor switch signal HI, the inductor current i LM and the drain voltage V of the main switch transistor SW when the active clamp flyback circuit enters the discontinuous conduction mode from the critical conduction mode in one embodiment;

[0047] Figure 10 is the waveforms of the main switch transistor switch signal LI, the clamp switch transistor switch signal HI, the inductor current i LMand the waveform of the drain voltage V of the main switching transistor SW of the waveform;

[0048] Figure 11 For the active clamp flyback circuit in another embodiment, the switching signal LI of the main switching transistor, the switching signal HI of the clamp switching transistor, the inductor current i LM and the waveform of the drain voltage V of the main switching transistor SW of the waveform;

[0049] Figure 12 For the active clamp flyback circuit in another embodiment, the switching signal LI of the main switching transistor, the switching signal HI of the clamp switching transistor, the inductor current i LM and the waveform of the drain voltage V of the main switching transistor SW of the waveform;

[0050] Figure 13 For the active clamp flyback circuit in yet another embodiment, the switching signal LI of the main switching transistor, the switching signal HI of the clamp switching transistor, the inductor current i LM and the waveform of the drain voltage V of the main switching transistor SW of the waveform;

[0051] Figure 14 For the active clamp flyback circuit in yet another embodiment, the switching signal LI of the main switching transistor, the switching signal HI of the clamp switching transistor, the inductor current i LM and the waveform of the drain voltage V of the main switching transistor SW of the waveform;

[0052] Figure 15 For the main circuit diagram and the control circuit diagram when the switching circuit is an active clamp flyback circuit;

[0053] Figure 16 For the block diagram of an embodiment of the control circuit 200;

[0054] Figure 17 For the block diagram of another embodiment of the control circuit 200. Detailed Embodiments

[0055] The following describes in detail the preferred embodiments of the present invention with reference to the accompanying drawings, but the present invention is not limited to these embodiments. The present invention encompasses any alternatives, modifications, equivalent methods, and solutions within the spirit and scope of the present invention.

[0056] 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.

[0057] The present invention will be described more specifically by way of example with reference to the accompanying drawings in the following paragraphs. It should be noted that the accompanying drawings are all in a relatively simplified form and use non-precise scales, only for the purpose of conveniently and clearly assisting in explaining the embodiments of the present invention.

[0058] The present invention discloses a control method for a switching circuit. When each switching cycle of the switching circuit is in the critical conduction mode or the continuous conduction mode, as the load power decreases, it enters the hiccup mode, and for N consecutive switching cycles, it is in the critical conduction mode, and then for one switching cycle, it is in the discontinuous conduction mode. The N consecutive switching cycles in the critical conduction mode plus one switching cycle in the discontinuous conduction mode form a hiccup cycle; when the load power further decreases, each switching cycle is in the discontinuous conduction mode, where N is a natural number greater than or equal to 1.

[0059] Please refer to Figure 1 and Figure 2 as shown, Figure 1 are the switching signal LI of the main switch tube, the inductor current i L and the drain voltage V SW of the main switch tube when the switching circuit enters the critical conduction mode for each switching cycle from the hiccup mode; Figure 2 are the switching signal LI of the main switch tube, the inductor current i L and the drain voltage V SW of the main switch tube when entering the hiccup mode from the discontinuous conduction mode for each switching cycle. In the Figure 1 and Figure 2 hiccup cycle, N = 2. It should be noted that when the switching circuit is a circuit with a transformer, such as a flyback circuit, the inductor current is the magnetizing inductor current.

[0060] It should be noted that it is not necessarily sampling the load power to obtain the load power. It can be using the peak current of the main switch tube, or the switching cycle, or the compensation voltage, or other quantities in the circuit to obtain the load power. In a specific circuit, N is a fixed value and does not change with the change of the load.

[0061] The switching circuit of the present invention has low switching losses and high conversion efficiency in the critical conduction mode, hiccup mode, and discontinuous conduction mode in each cycle. Moreover, it does not switch back and forth near the switching point, and the feedback loop is simple. When in the hiccup mode, if the hiccup period is greater than the first hiccup period threshold, it switches to the discontinuous conduction mode in each switching cycle; when in the discontinuous conduction mode in each switching cycle, if the switching cycle is less than the second cycle threshold, it switches to the hiccup mode. When there is no hysteresis in the switching between the hiccup mode and the discontinuous conduction mode in each switching cycle, the first hiccup period threshold is N + 1 times the second cycle threshold. When there is hysteresis in the switching from the hiccup mode to the discontinuous conduction mode in each switching cycle, the first hiccup period threshold is greater than N + 1 times the second cycle threshold. When there is hysteresis in the switching from the hiccup mode to the discontinuous conduction mode in each switching cycle, it does not switch back and forth between the two modes.

[0062] In addition to judging the hiccup period for switching from the hiccup mode to the discontinuous conduction mode in each switching cycle, the following method can also be used: when the off-time of the main switching tube in the switching cycle of the discontinuous conduction mode in the hiccup mode is greater than the first time threshold, it switches to the discontinuous conduction mode in each switching cycle. When in the discontinuous conduction mode in each switching cycle, if the off-time of the main switching tube is less than the third time threshold, it switches to the hiccup mode. When there is no hysteresis in the switching between the hiccup mode and the discontinuous conduction mode in each switching cycle, the first time threshold is N + 1 times the third time threshold. When there is hysteresis in the switching from the hiccup mode to the discontinuous conduction mode in each switching cycle, the first time threshold is greater than N + 1 times the third time threshold.

[0063] Another way to switch from the hiccup mode to the discontinuous conduction mode in each switching cycle is: when the time when the inductor current is zero in the switching cycle of the discontinuous conduction mode in the hiccup mode is greater than the fifth time threshold, it switches to the discontinuous conduction mode in each switching cycle. When in the discontinuous conduction mode in each switching cycle, if the time when the inductor current is zero is less than the sixth time threshold, it switches to the hiccup mode. When there is no hysteresis in the switching between the hiccup mode and the discontinuous conduction mode in each switching cycle, the fifth time threshold is N + 1 times the sixth time threshold. When there is hysteresis in the switching from the hiccup mode to the discontinuous conduction mode in each switching cycle, the fifth time threshold is greater than N + 1 times the sixth time threshold.

[0064] The above three methods respectively use the switching cycle, the off-time of the main switching tube, and the time when the inductor current is zero to characterize the load power as the condition for switching the working mode. The present invention is not limited to using the above two methods, and various other methods can also be used to characterize the load power as the condition for switching the working mode.

[0065] When each switching cycle of the switching circuit is in the critical conduction mode, the peak inductor current is less than the first current threshold, and it switches to the hiccup mode, and the peak inductor current in the hiccup mode is greater than the first current threshold. When in the hiccup mode, if the peak inductor current is greater than the second current threshold, it switches to the critical conduction mode for each switching cycle, and the peak inductor current in the critical conduction mode for each switching cycle is less than the second current threshold. When there is hysteresis in the switching, the second current threshold is greater than the first current threshold.

[0066] In another embodiment, when each switching cycle of the switching circuit is in the critical conduction mode and the compensation voltage is less than the first voltage threshold, it switches to the hiccup mode; when in the hiccup mode, if the compensation voltage is greater than the second voltage threshold, it switches to the critical conduction mode for each switching cycle. The output voltage or output current is operationally amplified with a reference value to obtain the compensation voltage. When there is hysteresis in the switching, the second voltage threshold is greater than the first voltage threshold.

[0067] In yet another embodiment, when each switching cycle of the switching circuit is in the critical conduction mode and the total time of consecutive M switching cycles is less than the seventh time, it switches to the hiccup mode or the discontinuous conduction mode, where M is a natural number. Preferably, M = N + 1, and the seventh time is M times the shortest switching cycle limit of the switching circuit corresponding to the respective compensation voltage. When in the critical conduction mode or the continuous conduction mode, there are different shortest switching cycle limits, that is, the highest switching frequency limits, under different compensation voltages.

[0068] In one embodiment, please refer to Figure 3 As shown, the switching circuit is an active clamp flyback circuit. The active clamp flyback circuit includes a flyback circuit and a clamping circuit. The flyback circuit includes a transformer T1, a main switch tube M1, a freewheeling diode D1 or a synchronous rectifier. The clamping circuit includes a first capacitor CSA and a clamping switch tube MSA. The first capacitor CSA and the clamping switch tube MSA are connected in series. One end of the first capacitor CSA is connected to the input high voltage terminal Vin, and one end of the clamping switch tube MSA is connected to the common terminal of the main switch tube M1 and the primary winding of the transformer. When each switching cycle of the active clamp flyback circuit is in the critical conduction mode or the continuous conduction mode, as the load power decreases, it enters the hiccup mode, with consecutive N switching cycles in the critical conduction mode, followed by one switching cycle in the discontinuous conduction mode. The sum of consecutive N switching cycles in the critical conduction mode and one switching cycle in the discontinuous conduction mode is the hiccup period; as the load power further decreases, each switching cycle is in the discontinuous conduction mode, where N is a natural number greater than or equal to 1.

[0069] Please refer to Figure 4 、 Figure 5 、 Figure 6 As shown,Figure 4 For the active clamp flyback circuit, the switching signal LI of the main switch tube, the switching signal HI of the clamp switch tube, the inductor current i LM and the drain voltage V of the main switch tube SW in the waveform when entering the critical conduction mode in each switching cycle from the hiccup mode; Figure 5 For the active clamp flyback circuit, the switching signal LI of the main switch tube, the switching signal HI of the clamp switch tube, the inductor current i LM and the drain voltage V of the main switch tube SW in the waveform when entering the hiccup mode from the discontinuous conduction mode in each switching cycle; Figure 6 For the active clamp flyback circuit, in one embodiment, the switching signal LI of the main switch tube, the switching signal HI of the clamp switch tube, the inductor current i LM and the drain voltage V of the main switch tube SW in the waveform when in the discontinuous conduction mode in each switching cycle.

[0070] In the flyback circuit, the exciting inductor current i LM is equivalent to the inductor current i of the switching circuit L . Therefore, the exciting inductor current in the flyback circuit will be expressed by the inductor current later.

[0071] When in the hiccup mode, the clamp switch tube conducts after the main switch tube is turned off. By adjusting the turn-off time of the clamp switch tube, the drain voltage of the main switch tube can oscillate to close to zero voltage. It should be noted that in some embodiments, the drain voltage of the main switch tube reaches zero, and in some cases, the drain voltage of the main switch is greater than zero and close to zero. For example, in one embodiment, if the input voltage is the voltage after rectifying the grid voltage, it may be set to adjust the turn-off time of the clamp switch tube so that the drain voltage of the main switch tube can oscillate to 30V - 60V.

[0072] In another implementation, when in the hiccup mode, in the 1st - Nth switching cycles of the critical conduction mode, the clamp switch tube conducts after the main switch tube is turned off. By adjusting the turn-off time of the clamp switch tube, the drain voltage of the main switch tube can oscillate to close to zero voltage; when in the hiccup mode, in the switching cycles of the discontinuous conduction mode and when each switching cycle is in the discontinuous conduction mode, the clamp switch tube conducts after the main switch tube is turned off, and when the inductor current is near zero current, the clamp switch tube turns off. It should be noted that generally, it is set that the clamp switch tube turns off when the inductor current is zero, but due to system errors, when the clamp switch tube turns off, the inductor current will be near zero current. In Figures 4 - 6 , the conduction of the dotted part of the clamp switch tube can be non-conductive, which is not described here, and the operation of the dotted part will be described later.

[0073] In yet another embodiment, as Figure 7 shown, in an embodiment of the active clamp flyback circuit, the switching signals LI of the main switch, the switching signal HI of the clamp switch, the inductor current i LM and the drain voltage V SW of the main switch in the hiccup mode. When in the hiccup mode, in the 1st to Nth switching cycles of the critical conduction mode, the clamp switch conducts after the main switch is turned off. By adjusting the turn-off time of the clamp switch, the drain voltage of the main switch can oscillate to near zero voltage; in the (N + 1)th switching cycle of the hiccup mode, the clamp switch does not conduct after the main switch is turned off. In Figure 7 , the conduction of the dotted part of the clamp switch can be non-conductive, which is not described here and will be described later.

[0074] The switching between the critical conduction mode, the hiccup mode and the discontinuous conduction mode in each switching cycle in the switching circuit is also applicable to the active clamp flyback circuit and will not be elaborated here.

[0075] When the time when both the main switch and the clamp switch are turned off in the switching cycle of the discontinuous conduction mode in the hiccup mode is greater than the second time threshold, it switches to the discontinuous conduction mode in each switching cycle; when in the discontinuous conduction mode in each switching cycle, the time when both the main switch and the clamp switch are turned off is less than the fourth time threshold, it switches to the hiccup mode. When there is no hysteresis in the switching between the hiccup mode and the discontinuous conduction mode in each switching cycle, the second time threshold is N + 1 times the fourth time threshold. When there is hysteresis in the switching from the hiccup mode to the discontinuous conduction mode in each switching cycle, the second time threshold is greater than N + 1 times the fourth time threshold.

[0076] To further improve the efficiency of the active clamp flyback circuit, when each switching cycle is in the discontinuous conduction mode or / and when in the hiccup mode, starting from the conduction of the clamp switch when the inductor current is discontinuous, when the conduction time of the clamp switch is the first conduction time, the clamp switch is turned off, and the main switch conducts when the drain-source voltage oscillates near zero voltage; when the conduction time of the main switch reaches the third time or the inductor current reaches the third current threshold, the main switch is turned off, and when the timing reaches the second time, the clamp switch conducts and the timing starts again.

[0077] When each switching cycle is in the discontinuous conduction mode, please refer to Figure 6 shown, in one switching cycle, the clamp switch conducts twice, one conduction is at Figure 5In one case, it is a solid line, and in another conduction, it is a dotted line. The timing starts from the rising edge of the dotted line of the switching signal HI of the clamping switch tube. When the timing reaches the second time, the clamping switch tube conducts again and the timing restarts. The second time is equal to the switching period.

[0078] When in the hiccup mode, please refer to Figure 4 As shown, in one hiccup cycle, the clamping switch tube conducts N + 2 times. Among them, N + 1 times of the clamping switch tube conduction occur after the main switch tube is turned off. One conduction of the clamping switch tube represented by a dotted line occurs before the first conduction of the main switch tube in the hiccup cycle. That is, the timing starts from the conduction of the clamping switch tube represented by the dotted line. When the timing reaches the second time, the clamping switch tube conducts again and the timing restarts. The second time is equal to the hiccup cycle. Also, after the (N + 1)th conduction of the main switch tube, the clamping switch tube conducts. The clamping switch tube can be turned off when the inductor current is close to zero, or when the inductor current is negative and the drain-source voltage of the main switch tube is close to zero voltage. It should be noted that generally, by adjusting the turn-off moment of the clamping switch tube, the main switch tube is turned on when the drain-source voltage is close to zero voltage.

[0079] Another implementation is that when in the hiccup mode, please refer to Figure 7 As shown, in one hiccup cycle, the clamping switch tube conducts N + 1 times. Among them, N times of the clamping switch tube conduction occur after the 1st to Nth main switch tubes are turned off. After the (N + 1)th conduction of the main switch tube, the clamping switch tube may not conduct. One conduction of the clamping switch tube represented by a dotted line occurs before the first conduction of the main switch tube in the hiccup cycle. That is, the timing starts from the conduction of the clamping switch tube represented by the dotted line. When the timing reaches the second time, the clamping switch tube conducts again and the timing restarts. The second time is equal to the hiccup cycle.

[0080] When in the discontinuous conduction mode and the load power continues to decrease, it can continue to maintain Figure 6 the working mode, or in one switching period, the clamping switch tube conducts only once. Please refer to Figure 8 As shown, when the active clamp flyback circuit is in the discontinuous conduction mode and the load power continues to decrease, each switching period is the main switch tube switching signal LI, the clamping switch tube switching signal HI, the inductor current i LM and the drain voltage of the main switch tube V SWThe waveform. When in discontinuous conduction mode and the load power continues to decrease, starting from the conduction of the main switch, when the conduction time of the main switch reaches the third time or the inductor current reaches the third current threshold, the main switch turns off, the clamping switch turns on. When the inductor current is near zero current, the clamping switch turns off. When the timing reaches the second time, the main switch turns on. The second time is equal to the switching period.

[0081] It should be noted that generally, by adjusting the turn-off moment of the clamping switch, the clamping switch can be turned off when the inductor current is near zero current. In this way, without detecting the inductor current, it can be achieved that the clamping switch turns off when the inductor current is close to zero. The specific method is to start timing after the clamping switch turns off. After timing for the eighth time, the drain-source voltage of the main switch is close to the bus voltage. Please refer to Figure 3 As shown, the bus voltage is the input voltage Vin. Generally, the output feedback voltage can be obtained by detecting the output feedback voltage. When the output feedback voltage crosses zero, it indicates that the drain-source voltage of the main switch is close to the bus voltage. The output voltage is divided to obtain the output feedback voltage. The eighth time is generally 1 / 4 of the resonant period. Considering some delays in general systems, the eighth time is slightly greater than 1 / 4 of the resonant period. The resonant period is the resonant period formed by the resonance of the transformer excitation inductor and the sum of the output capacitances of the main switch and the clamping switch. Another method to determine the turn-off of the clamping switch when the inductor current is near zero is to start timing after the clamping switch turns off. When timing reaches 1 / 2 of the resonant period, the drain-source voltage of the main switch is close to the valley.

[0082] In one embodiment, when each switching period of the switching circuit is in critical conduction mode and the time of continuous M switching periods is less than the seventh time, it switches to the hiccup mode or discontinuous conduction mode. M is a natural number. Commonly used values of M are 2 to 5.

[0083] When switching from the critical conduction mode to the hiccup mode or to the discontinuous conduction mode, in the critical conduction mode, the clamping switch turns on after the main switch turns off. By adjusting the turn-off moment of the clamping switch, the drain voltage of the main switch can oscillate to close to zero voltage or the clamping switch can turn off when the inductor current is near zero current, and then switch to the hiccup mode or the discontinuous conduction mode.

[0084] In another embodiment, when switching from the critical conduction mode to the hiccup mode or to the discontinuous conduction mode, the clamping switch turns off.

[0085] Please refer to Figures 9 - 14 As shown, for the main switch switching signal LI, the clamping switch switching signal HI, and the inductor current i when switching from the critical conduction mode to the hiccup mode or to the discontinuous conduction mode LMand the drain voltage V of the main switching transistor SW waveform.

[0086] It should be noted that the time of the mode switching point is not limited to Figures 9 - 14 the mode switching point indicated by the arrow in, as long as it is determined that mode switching is required, the specific point at which the switching occurs can be adjusted according to the actual situation.

[0087] When switching from the critical conduction mode to the hiccup mode or to the discontinuous conduction mode, the main switching transistor conducts when the drain voltage of the main switching transistor is less than the bus voltage; the main switching transistor conducts when the drain voltage of the main switching transistor is less than the bus voltage, indicating that the drain voltage of the main switching transistor is near the valley.

[0088] In another embodiment, when switching from the critical conduction mode to the hiccup mode or to the discontinuous conduction mode, the clamping switch transistor turns off when the inductor current approaches zero current or is negative current; starting from the conduction of the main switching transistor in the previous cycle, when the timing reaches the ninth time, the main switching transistor conducts, switching to the hiccup mode or to the discontinuous conduction mode. Please refer to Figure 11 shown, for the switching signal LI of the main switching transistor, the switching signal HI of the clamping switch transistor, the inductor current i LM and the drain voltage V of the main switching transistor SW waveform. The ninth time is the switching period. Under different set switching periods, after the clamping switch transistor turns off, the oscillation time of the drain voltage V SW of the main switching transistor will also be different. Similarly, Figure 13 for the switching signal LI of the main switching transistor, the switching signal HI of the clamping switch transistor, the inductor current i LM and the drain voltage V of the main switching transistor SW waveform.

[0089] Please refer to Figure 12 shown, in another embodiment, when switching from the critical conduction mode to the hiccup mode or to the discontinuous conduction mode, starting from the conduction of the main switching transistor in the previous cycle, when the timing reaches the ninth time, the clamping switch transistor conducts for the first conduction time. When the conduction time of the clamping switch transistor is the first conduction time, the clamping switch transistor turns off and the main switching transistor conducts, switching to the hiccup mode or to the discontinuous conduction mode. After the timing reaches the ninth time, the clamping switch transistor conducts, as shown by the dotted line behind the arrow, and then the clamping switch transistor turns off and the main switching transistor conducts again. Similarly, Figure 14 for the switching signal LI of the main switching transistor, the switching signal HI of the clamping switch transistor, the inductor current i LM and the drain voltage V of the main switching transistor SW waveform.

[0090] The present invention also discloses a control circuit for a switching circuit. When each switching period of the control circuit controls the switching circuit to be in the critical conduction mode or the continuous conduction mode, as the load power decreases, it enters the hiccup mode, and is in the critical conduction mode for N consecutive switching periods, and then one switching period is in the discontinuous conduction mode. The N consecutive switching periods in the critical conduction mode plus one switching period in the discontinuous conduction mode form a hiccup period. As the load power further decreases, the control circuit controls each switching period to be in the discontinuous conduction mode, where N is a natural number greater than or equal to 1. Please refer to Figure 15 As shown, it is the main circuit diagram and the control circuit diagram when the switching circuit is an active clamp flyback circuit.

[0091] Please refer to Figure 16 As shown, the control circuit includes a power detection circuit 210, a mode selection circuit 220, and a main switch control circuit 230. The power detection circuit 210 obtains a value representing the load power by detecting the peak value of the inductor current or / and the hiccup period or / and the switching period or / and the turn-off time of the main switch or / and the compensation voltage. The mode selection circuit 220 receives the output voltage of the power detection circuit 210 and controls different operating modes according to the output voltage of the power detection circuit 210. The main switch control circuit 230 receives the output voltage of the mode selection circuit and controls the conduction and turn-off of the main switch according to the output voltage of the mode selection circuit and the compensation voltage. The output voltage or output current is operated and amplified with a reference value to obtain the compensation voltage. In various operating modes, the main switch control circuit 230 controls the conduction and turn-off of the main switch according to the compensation voltage or / and the output feedback voltage or / and the inductor current or / and the discontinuous time of the inductor current and other quantities. The mode selection circuit 220 outputs representations of different operating modes. In different operating modes, the main switch control circuit 230 will interpret the compensation voltage or / and the output feedback voltage or / and the inductor current or / and the discontinuous time of the inductor current and other quantities differently. For example, in the critical conduction mode for each switching period, the conduction moment of the main switch is determined by the zero-crossing point of the inductor current, and the compensation voltage determines the peak value of the inductor current. In the hiccup mode, there is no conduction of the clamping switch before the first conduction of the main switch. The first conduction moment of the main switch is determined by the hiccup period, and the subsequent N times of conduction of the main switch are determined by the zero-crossing point of the inductor current. In the discontinuous conduction mode, when the clamping switch conducts only once in a switching period, the conduction moment of the main switch is determined by the switching frequency.

[0092] When in the hiccup mode, when the power detection circuit detects that the hiccup period is greater than the first hiccup period threshold, the mode selection circuit switches the mode to the discontinuous conduction mode for each switching cycle; when in the discontinuous conduction mode for each switching cycle, when the power detection circuit detects that the switching cycle is less than the second period threshold, the mode selection circuit switches the mode to the hiccup mode. When there is no hysteresis in the switching between the hiccup mode and the discontinuous conduction mode for each switching cycle, the first hiccup period threshold is N + 1 times the second period threshold. When there is hysteresis in the switching from the hiccup mode to the discontinuous conduction mode for each switching cycle, the first hiccup period threshold is greater than N + 1 times the second period threshold. When there is hysteresis in the switching from the hiccup mode to the discontinuous conduction mode for each switching cycle, there will be no back-and-forth switching between the two modes.

[0093] In addition to the power detection circuit detecting the hiccup period, the switching from the hiccup mode to the discontinuous conduction mode for each switching cycle can also be done in the following way: when in the hiccup mode, when the power detection circuit detects that the off-time of the main switch tube in the switching cycle of the discontinuous conduction mode is greater than the first time threshold, the mode selection circuit switches the mode to the discontinuous conduction mode for each switching cycle. When in the discontinuous conduction mode for each switching cycle, when the power detection circuit detects that the off-time of the main switch tube is less than the third time threshold, the mode selection circuit switches the mode to the hiccup mode. When there is no hysteresis in the switching between the hiccup mode and the discontinuous conduction mode for each switching cycle, the first time threshold is N + 1 times the third time threshold. When there is hysteresis in the switching from the hiccup mode to the discontinuous conduction mode for each switching cycle, the first time threshold is greater than N + 1 times the third time threshold.

[0094] Another way to switch from the hiccup mode to the discontinuous conduction mode for each switching cycle is: when in the hiccup mode, when the time when the inductor current is zero in the switching cycle of the discontinuous conduction mode is greater than the fifth time threshold, the mode selection circuit switches the mode to the discontinuous conduction mode for each switching cycle. When in the discontinuous conduction mode for each switching cycle, when the power detection circuit detects that the time when the inductor current is zero is less than the sixth time threshold, the mode selection circuit switches the mode to the hiccup mode. When there is no hysteresis in the switching between the hiccup mode and the discontinuous conduction mode for each switching cycle, the fifth time threshold is N + 1 times the sixth time threshold. When there is hysteresis in the switching from the hiccup mode to the discontinuous conduction mode for each switching cycle, the fifth time threshold is greater than N + 1 times the sixth time threshold.

[0095] When each switching cycle of the switching circuit is in the critical conduction mode, when the power detection circuit detects that the peak inductor current is less than the first current threshold, the mode selection circuit switches the operating mode to the hiccup mode, and the peak inductor current in the hiccup mode is greater than the first current threshold. When in the hiccup mode, if the power detection circuit detects that the peak inductor current is greater than the second current threshold, the mode selection circuit switches the mode to the critical conduction mode for each switching cycle, and the peak inductor current in the critical conduction mode for each switching cycle is less than the second current threshold. When there is hysteresis in the switching, the second current threshold is greater than the first current threshold.

[0096] In another embodiment, when each switching cycle of the switching circuit is in the critical conduction mode, and the power detection circuit detects that the compensation voltage is less than the first voltage threshold, the mode selection circuit switches the mode to the hiccup mode; when in the hiccup mode, if the power detection circuit detects that the compensation voltage is greater than the second voltage threshold, the mode selection circuit switches the mode to the critical conduction mode for each switching cycle. The output voltage or output current is operationally amplified with a reference value to obtain the compensation voltage. When there is hysteresis in the switching, the second voltage threshold is greater than the first voltage threshold.

[0097] In one embodiment, the switching circuit is an active-clamp flyback circuit. The active-clamp flyback circuit includes a main switch transistor and a clamp switch transistor. When in the hiccup mode, the control circuit controls the clamp switch transistor to conduct after the main switch transistor turns off and turn off when the inductor current is less than zero, so that the drain voltage of the main switch transistor can oscillate to near zero voltage. Please refer to Figure 15 As shown, it is the main circuit diagram and control circuit diagram when the switching circuit is an active-clamp flyback circuit.

[0098] In another implementation manner, when in the hiccup mode, in the 1st to Nth switching cycles in the critical conduction mode, the control circuit controls the clamp switch transistor to conduct after the main switch transistor turns off and turn off when the inductor current is less than zero, so that the drain voltage of the main switch transistor can oscillate to near zero voltage; when in the hiccup mode, in the switching cycle of the discontinuous conduction mode and when each switching cycle is in the discontinuous conduction mode, the control circuit controls the clamp switch transistor to conduct after the main switch transistor turns off, and when the inductor current is near zero current, the control circuit controls the clamp switch transistor to turn off.

[0099] In yet another implementation manner, as Figure 7 shown, for an active-clamp flyback circuit in one embodiment, the switching signal LI of the main switch transistor, the switching signal HI of the clamp switch transistor, the inductor current i LM and the drain voltage V SWThe waveform. When in the hiccup mode, in the 1st to Nth switching cycles of the critical conduction mode, the control circuit controls the clamping switch transistor to conduct after the main switch transistor is turned off and turn off when the inductor current is less than zero, so that the drain voltage of the main switch transistor can oscillate to near zero voltage; in the (N + 1)th switching cycle of the hiccup mode, the control circuit controls the clamping switch transistor not to conduct after the main switch transistor is turned off.

[0100] Please refer to Figure 16 As shown, the control circuit further includes a clamping switch transistor control circuit 240. The clamping switch transistor control circuit 240 receives the output voltage of the mode selection circuit 220 and controls the clamping switch transistor according to the output voltage of the mode selection circuit 220.

[0101] The switching between the critical conduction mode, the hiccup mode, and the discontinuous conduction mode in each switching cycle of the switching circuit is also applicable to the active clamp flyback circuit and will not be elaborated here.

[0102] When in the switching cycle of the discontinuous conduction mode in the hiccup mode, if the power detection circuit detects that the time when both the main switch transistor and the clamping switch transistor are turned off is greater than the second time threshold, the mode selection circuit switches the mode to the discontinuous conduction mode in each switching cycle.

[0103] When each switching cycle of the switching circuit is in the critical conduction mode, if the power detection circuit detects that the peak value of the inductor current is less than the first current threshold or the compensation voltage is less than the first voltage threshold, the mode selection circuit switches the mode to the hiccup mode, and the peak value of the inductor current in the hiccup mode is greater than the first current threshold; the output voltage or output current is amplified by arithmetic operation with the reference value to obtain the compensation voltage.

[0104] In one embodiment, when each switching cycle of the switching circuit is in the critical conduction mode, if the power detection circuit detects that the time of continuous M switching cycles is less than the seventh time, the mode selection circuit switches the mode to the hiccup mode or the discontinuous conduction mode. M is a natural number.

[0105] In one embodiment, when switching from the critical conduction mode to the hiccup mode or to the discontinuous conduction mode, in the critical conduction mode, the clamping switch transistor conducts after the main switch transistor is turned off. The control circuit adjusts the turn-off moment of the clamping switch transistor so that the drain voltage of the main switch transistor can oscillate to near zero voltage or the clamping switch transistor turns off when the inductor current is close to zero, and the mode selection circuit switches the mode to the hiccup mode or to the discontinuous conduction mode.

[0106] In another embodiment, when the mode selection circuit switches the mode from the critical conduction mode to the hiccup mode or to the discontinuous conduction mode, the clamp switch transistor control circuit controls the clamp switch transistor to turn off.

[0107] When each switching cycle is in the discontinuous conduction mode and / or when in the hiccup mode, the clamp switch transistor control circuit starts timing from the conduction of the clamp switch transistor when the inductor current is discontinuous. When the conduction time of the clamp switch transistor is the first conduction time, the clamp switch transistor control circuit controls the clamp switch transistor to turn off, so that the drain-source voltage of the main switch transistor oscillates to zero voltage, and the main switch transistor control circuit controls the main switch transistor to turn on at zero voltage; when the conduction time of the main switch transistor reaches the third time or the inductor current reaches the third current threshold, the main switch transistor control circuit controls the main switch transistor to turn off, the clamp switch transistor control circuit controls the clamp switch transistor to turn on, when the inductor current is near zero current, the clamp switch transistor control circuit controls the clamp switch transistor to turn off, and when the timing reaches the second time, the clamp switch transistor control circuit controls the clamp switch transistor to turn on and starts timing again.

[0108] Although the embodiments are separately described and elaborated above, for the parts involving common technologies, in the view of those of ordinary skill in the art, substitutions and integrations can be made between the embodiments. For the content not clearly recorded in one of the embodiments, reference can be made to the other embodiment with records.

[0109] The above-described embodiments do not constitute a limitation on the protection scope of the technical solution. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the above embodiments shall be included within the protection scope of the technical solution.

Claims

1. A control method for a switching circuit, where the switching circuit is a flyback circuit. When each switching cycle of the switching circuit is in the critical conduction mode or the continuous conduction mode, as the load power decreases, it enters the hiccup mode. For N consecutive switching cycles in the critical conduction mode, followed by one switching cycle in the discontinuous conduction mode, the sum of N consecutive switching cycles in the critical conduction mode and one switching cycle in the discontinuous conduction mode is the hiccup cycle. As the load power further decreases, each switching cycle is in the discontinuous conduction mode. Among them, N is a natural number greater than or equal to 1.

2. The control method for the switching circuit according to claim 1, characterized in that: the switching circuit is an active-clamp flyback circuit, the active-clamp flyback circuit includes a flyback circuit and a clamping circuit, the flyback circuit includes a main switch tube and a transformer, the clamping circuit includes a first capacitor and a clamping switch tube, the first capacitor and the clamping switch tube are connected in series between the input high-voltage terminal and the common terminal of the main switch tube and the primary winding of the transformer. When in the hiccup mode or / and the critical conduction mode, the clamping switch tube conducts after the main switch tube turns off. By adjusting the turn-off time of the clamping switch tube, the drain voltage of the main switch tube can oscillate to near zero voltage; or in the discontinuous mode, the clamping switch tube conducts after the main switch tube turns off. By adjusting the turn-off time of the clamping switch tube, the clamping switch tube turns off when the inductor current approaches zero; or in the hiccup mode, after the (N + 1)th conduction of the main switch tube, the clamping switch tube does not conduct.

3. The control method for the switching circuit according to claim 1 or 2, characterized in that: when in the hiccup mode, if the hiccup cycle is greater than the first hiccup cycle threshold, it switches to each switching cycle being in the discontinuous conduction mode; or when the turn-off time of the main switch tube in the switching cycle of the discontinuous conduction mode in the hiccup mode is greater than the first time threshold or the time when the inductor current is zero is greater than the fifth time threshold, it switches to each switching cycle being in the discontinuous conduction mode.

4. The control method for the switching circuit according to claim 3, characterized in that: when each switching cycle is in the discontinuous conduction mode, if the switching cycle is less than the second cycle threshold, or the turn-off time of the main switch tube is less than the third time threshold, or the time when the inductor current is zero is less than the sixth time threshold, it switches to the hiccup mode.

5. The control method for the switching circuit according to claim 2, characterized in that: when the time when both the main switch tube and the clamping switch tube are off in the switching cycle of the discontinuous conduction mode in the hiccup mode is greater than the second time threshold, it switches to each switching cycle being in the discontinuous conduction mode.

6. The control method for the switching circuit according to claim 5, characterized in that: when each switching cycle is in the discontinuous conduction mode, and the time when both the main switch tube and the clamping switch tube are off is less than the fourth time threshold, it switches to the hiccup mode.

7. The control method for the switching circuit according to claim 1 or 2, characterized in that: When each switching cycle of the switching circuit is in the critical conduction mode, the peak inductor current is less than the first current threshold or the compensation voltage is less than the first voltage threshold, it switches to the hiccup mode, and the peak inductor current in the hiccup mode is greater than the first current threshold; The output voltage or output current is amplified by an operational amplifier with a reference value to obtain the compensation voltage.

8. The control method of the switching circuit according to claim 2, characterized in that: When each switching cycle of the switching circuit is in the critical conduction mode and the total time of consecutive M switching cycles is less than the seventh time, it switches to the hiccup mode or the discontinuous conduction mode, where M is a natural number.

9. The control method of the switching circuit according to claim 8, characterized in that: When switching from the critical conduction mode to the hiccup mode or to the discontinuous conduction mode, in the critical conduction mode, the clamping switch transistor conducts after the main switch transistor turns off. By adjusting the turn-off time of the clamping switch transistor, the drain voltage of the main switch transistor can oscillate to near zero voltage or the clamping switch transistor turns off when the inductor current approaches zero, and then it switches to the hiccup mode or the discontinuous conduction mode; Or when switching from the critical conduction mode to the hiccup mode or to the discontinuous conduction mode, the clamping switch transistor turns off.

10. The control method of the switching circuit according to claim 9, characterized in that: The main switch transistor conducts when the drain voltage of the main switch transistor is less than the bus voltage; Or starting from the conduction of the main switch transistor in the previous cycle, when the timing reaches the ninth time, the main switch transistor conducts, and it switches to the hiccup mode or the discontinuous conduction mode; Or starting from the conduction of the main switch transistor in the previous cycle, when the timing reaches the ninth time, the clamping switch transistor conducts for the first conduction time. When the conduction time of the clamping switch transistor is the first conduction time, the clamping switch transistor turns off and the main switch transistor conducts, and it switches to the hiccup mode or the discontinuous conduction mode.

11. The control method of the switching circuit according to claim 2, characterized in that: When each switching cycle is in the discontinuous conduction mode or / and when in the hiccup mode, starting from the conduction of the clamping switch transistor when the inductor current is discontinuous, when the conduction time of the clamping switch transistor is the first conduction time, the clamping switch transistor turns off, and the main switch transistor conducts when the drain-source voltage oscillates near zero voltage; when the conduction time of the main switch transistor reaches the third time or the inductor current reaches the third current threshold, the main switch transistor turns off. When the timing reaches the second time, the clamping switch transistor conducts and the timing restarts.

12. The control method of the switching circuit according to claim 2, characterized in that: When in the discontinuous conduction mode and the load power continues to decrease or / and when in the hiccup mode, starting from the conduction of the main switch transistor, when the conduction time of the main switch transistor reaches the third time or the inductor current reaches the third current threshold, the main switch transistor turns off and the clamping switch transistor conducts. When the inductor current is near zero current, the clamping switch transistor turns off. When the timing reaches the second time, the main switch transistor conducts and the timing restarts.

13. A control circuit for a switching circuit, where the switching circuit is a flyback circuit. When each switching period of the control circuit controls the switching circuit to be in the critical conduction mode or the continuous conduction mode, as the load power decreases, it enters the hiccup mode, and for N consecutive switching periods, it is in the critical conduction mode, and then for one switching period, it is in the discontinuous conduction mode. The sum of N consecutive switching periods in the critical conduction mode and one switching period in the discontinuous conduction mode is the hiccup period. As the load power further decreases, the control circuit controls each switching period to be in the discontinuous conduction mode. Wherein, N is a natural number greater than or equal to 1.

14. The control circuit for the switching circuit according to claim 13, characterized in that: The control circuit includes a power detection circuit, a mode selection circuit, and a main switch control circuit. The power detection circuit obtains a value representing the load power by detecting the peak inductor current or / and the hiccup period or / and the switching period or / and the main switch turn-off time or / and the inductor current discontinuous time or / and the compensation voltage. The mode selection circuit receives the output voltage of the power detection circuit and controls different operating modes according to the output voltage of the power detection circuit. The main switch control circuit receives the output voltage of the mode selection circuit and controls the conduction and turn-off of the main switch according to the output voltage of the mode selection circuit and the compensation voltage; The output voltage or output current is subjected to operational amplification with a reference value to obtain the compensation voltage.

15. The control circuit for the switching circuit according to claim 14, characterized in that: The switching circuit is an active clamp flyback circuit, which includes a flyback circuit and a clamping circuit. The flyback circuit includes a main switch and a transformer. The clamping circuit includes a first capacitor and a clamping switch. The first capacitor and the clamping switch are connected in series between the input high voltage terminal and the common terminal of the main switch and the primary winding of the transformer. When in the hiccup mode or / and the critical conduction mode, the control circuit controls the clamping switch to conduct after the main switch turns off. The control circuit adjusts the turn-off moment of the clamping switch so that the drain voltage of the main switch can oscillate to near zero voltage; Or in the discontinuous mode, the control circuit controls the clamping switch to conduct after the main switch turns off, and by adjusting the turn-off moment of the clamping switch, the clamping switch turns off when the inductor current approaches zero; Or in the hiccup mode, after the (N + 1)th conduction of the main switch, the control circuit controls the clamping switch not to conduct.

16. The control circuit for the switching circuit according to claim 15, characterized in that: The control circuit further includes a clamping switch control circuit. The clamping switch control circuit receives the output voltage of the mode selection circuit and controls the clamping switch according to the output voltage of the mode selection circuit.

17. The control circuit for the switching circuit according to claim 15 or 16, characterized in that: When in the hiccup mode, when the power detection circuit detects that the hiccup period is greater than the first hiccup period threshold, the mode selection circuit switches the mode to the discontinuous conduction mode for each switching cycle; Or when in the switching cycle of the discontinuous conduction mode in the hiccup mode, when the power detection circuit detects that the turn-off time of the main switching transistor is greater than the first time threshold or the time when the inductor current is zero is greater than the fifth time threshold, the mode selection circuit switches the mode to the discontinuous conduction mode for each switching cycle.

18. The control circuit of the switching circuit according to claim 16, characterized in that: When in the switching cycle of the discontinuous conduction mode in the hiccup mode, when the power detection circuit detects that the time when both the main switching transistor and the clamping switching transistor are turned off is greater than the second time threshold, the mode selection circuit switches the mode to the discontinuous conduction mode for each switching cycle.

19. The control circuit of the switching circuit according to claim 16, characterized in that: When each switching cycle of the switching circuit is in the critical conduction mode, when the power detection circuit detects that the peak value of the inductor current is less than the first current threshold or the compensation voltage is less than the first voltage threshold, the mode selection circuit switches the mode to the hiccup mode, and the peak value of the inductor current in the hiccup mode is greater than the first current threshold; The output voltage or the output current is arithmetically amplified with a reference value to obtain the compensation voltage.

20. The control circuit of the switching circuit according to claim 16, characterized in that: When each switching cycle of the switching circuit is in the critical conduction mode, when the power detection circuit detects that the time of continuous M switching cycles is less than the seventh time, the mode selection circuit switches the mode to the hiccup mode or the discontinuous conduction mode, where M is a natural number.

21. The control circuit of the switching circuit according to claim 20, characterized in that: When switching from the critical conduction mode to the hiccup mode or switching to the discontinuous conduction mode, in the critical conduction mode, the clamping switching transistor conducts after the main switching transistor is turned off, and the control circuit adjusts the turn-off moment of the clamping switching transistor so that the drain voltage of the main switching transistor can oscillate to near zero voltage or the clamping switching transistor is turned off when the inductor current is close to zero, and the mode selection circuit switches the mode to the hiccup mode or switches to the discontinuous conduction mode; Or when the mode selection circuit switches the mode from the critical conduction mode to the hiccup mode or switches to the discontinuous conduction mode, the clamping switching transistor control circuit controls the clamping switching transistor to turn off.

22. The control circuit of the switching circuit according to claim 16, characterized in that: When each switching cycle is in the discontinuous conduction mode or / and when in the hiccup mode, the clamping switch tube control circuit starts timing from the conduction of the clamping switch tube when the inductor current is discontinuous. When the conduction time of the clamping switch tube is the first conduction time, the clamping switch tube control circuit controls the clamping switch tube to turn off, and the main switch tube turns on when the drain-source voltage oscillates near zero voltage; when the conduction time of the main switch tube reaches the third time or the inductor current reaches the third current threshold, the main switch tube control circuit controls the main switch tube to turn off. When the timing reaches the second time, the clamping switch tube control circuit controls the clamping switch tube to turn on and starts timing again.

23. A switching circuit, characterized in that: it includes the control circuit of the switching circuit according to any one of claims 13 to 22, or adopts the control method of the switching circuit according to any one of claims 1 to 12.

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