A single-stage power factor correction control circuit and a switching power supply

Abstract

The present invention discloses a single-stage power factor correction control circuit and a switching power supply. The single-stage power factor correction control circuit includes a grid input Vin, a rectifier bridge DB1, a capacitor C1, a capacitor C2, a switching transistor S1, a switching transistor S2, a switching transistor S3, a diode D1, an inductor L1, a transformer T1, a first output rectification circuit, an output capacitor Co, a power factor correction control circuit, and a resonant control drive circuit. Beneficial effects: The present invention provides a single-stage power factor correction control circuit, which realizes power factor correction within a wide range of input grid voltages, reduces current harmonic distortion, can also limit and stabilize the bus capacitor voltage, and avoids overstress of devices.

Description

Technical Field

[0001] The present invention relates to the field of circuits and switching power supplies, and in particular to a single-stage power factor correction control circuit and a switching power supply. Background Art

[0002] In the field of power supply, high power density, high efficiency and low cost driving power supply are more competitive. Usually, the driving power supply will choose the resonant circuit to achieve the purpose of high power density and high efficiency. The resonant circuit can realize the zero voltage turn-on of two or more switch tubes on the primary side and the zero current turn-off of the secondary side rectifier diode, which can reduce the switching loss of the power supply and improve the efficiency and power density of the power converter. At the same time, in order to improve the power factor, an active PFC power factor correction circuit is often added to the front stage of the resonant circuit, but this will make the circuit complex and costly.

[0003] To this end, the prior art replaces the PFC circuit with a charge pump circuit so that the single-stage resonant circuit meets the power factor requirements. However, the resonant circuit with a charge pump has the following problems: when the circuit is in the working condition where the amplitude of the input voltage changes within a certain range, when the amplitude of the input voltage increases and the energy required by the resonant main circuit remains unchanged, the voltage on the bus capacitor increases immediately; or, when the output power of the resonant main circuit changes within a certain range (that is, the power required by the resonant main circuit changes within a certain range), the output power decreases and the input voltage remains unchanged, the voltage on the bus capacitor increases immediately. If the voltage on the bus capacitor is at a higher amplitude level, the related devices of the subsequent resonant main circuit need to withstand higher voltage pressure. Therefore, when designing the circuit, these devices of the resonant main circuit need to select the withstand voltage performance according to the bus capacitor voltage with the highest amplitude. Devices with high withstand voltage performance are relatively expensive. For circuits that work for a long time at a low-amplitude bus capacitor voltage and occasionally work at a high-amplitude bus capacitor voltage, it is too wasteful to select devices with high withstand voltage performance, but they must be selected, otherwise the devices will be damaged due to withstand voltage problems under high-amplitude bus capacitor voltages.

[0004] At the same time, charge pump PFC, as a passive measure, is difficult to achieve ideal power factor correction within a wider input and load range, and the power factor and harmonic effects are not ideal.

[0005] In view of this, how to stably limit the voltage of the bus capacitor to a certain voltage value, avoid voltage pressure on the device due to excessive bus capacitor voltage, and achieve better power factor correction within a wider input and load range has become a technical problem that needs to be urgently solved by technical personnel in this field.

[0006] Currently, no effective solution has been proposed for the problems in the related technologies. Summary of the invention

[0007] In view of the problems in the related art, the present invention provides a single-stage power factor correction control circuit and a switching power supply to overcome the above-mentioned technical problems existing in the related art.

[0008] Therefore, the specific technical solution adopted by the present invention is as follows:

[0009] According to an aspect of the present invention, there is provided a single-stage power factor correction control circuit, which includes a grid input Vin, a rectifier bridge DB1, a capacitor C1, a capacitor C2, a switching transistor S1, a switching transistor S2, a switching transistor S3, a diode D1, an inductor L1, a transformer T1, a first output rectifier circuit, an output capacitor Co, a power factor correction control circuit, and a resonant control drive circuit;

[0010] Among them, the grid input Vin is connected to the first end and the third end of the rectifier bridge DB1. The second end of the rectifier bridge DB1 is sequentially connected to the positive electrode of the capacitor C1 and the first end of the switching transistor S1. The second end of the switching transistor S1 is sequentially connected to one end of the inductor L1 and the first end of the switching transistor S2. The other end of the inductor L1 is connected to the first input end of the transformer T1. The second input end of the transformer T1 is connected to one end of the capacitor C2. The output end of the transformer T1 is connected in parallel with the first output rectifier circuit. The first output rectifier circuit is connected in parallel with the output capacitor Co. The other end of the capacitor C2 is sequentially connected to the first end of the switching transistor S3, the negative electrode of the diode D1, and the fourth end of the rectifier bridge DB1. The positive electrode of the diode D1 is sequentially connected to the negative electrode of the capacitor C1, the second end of the switching transistor S3, and the second end of the switching transistor S2 and grounded. The third end of the switching transistor S3 is connected to the power factor correction control circuit. The third end of the switching transistor S2 is connected to the second end of the resonant control drive circuit. The third end of the switching transistor S1 is connected to the first end of the resonant control drive circuit;

[0011] Among them, by controlling the on-time of the switching transistor S3, the voltage of the capacitor C1 can be limited and stabilized. When the inductor current of L1 flows in the positive half cycle, by controlling the on-time of the switching transistor S3, the current magnitude flowing through the rectifier bridge DB1 and the input grid Vin to the capacitor C1 can be controlled. The longer the on-time of the switching transistor S3, the smaller the current charging the capacitor C1. On the contrary, the shorter the on-time of the switching transistor S3, the larger the current charging the capacitor C1. When the on-time of the switching transistor S3 is at a certain value, the charging and discharging currents of the capacitor C1 are equal, and the voltage of the capacitor C1 is in a steady state;

[0012] According to the input grid voltage, control and modulate the conduction time or conduction duty cycle of the switching transistor S3, so that the average value of the current flowing through the rectifier bridge DB1 and the grid tracks the input grid voltage in the power frequency cycle, then the power factor correction function can be achieved.

[0013] Furthermore, the capacitor C1 is a polarized capacitor.

[0014] Furthermore, the output capacitor Co is a polarized capacitor.

[0015] Furthermore, when the current of the inductor L1 flows from right to left in the negative half cycle, the diode D1 conducts and the rectifier bridge DB1 is cut off, and the current does not pass through the input grid Vin.

[0016] Furthermore, when the current of the inductor L1 flows from left to right in the positive half cycle, if the switching transistor S3 conducts, the rectifier bridge DB1 is cut off, and the current does not pass through the input grid Vin.

[0017] Furthermore, when the current of the inductor L1 flows from left to right in the positive half cycle, if the switching transistor S3 is turned off, the rectifier bridge DB1 conducts, and the current passes through the input grid Vin.

[0018] Furthermore, the resonant control drive circuit provides energy to the output capacitor Co and the load through the transformer T1 and the first output rectifier circuit.

[0019] Furthermore, the resonant control drive circuit adopts frequency feedback control of the output voltage or output current.

[0020] According to another aspect of the present invention, a switching power supply is provided, which is composed of the above single-stage power factor correction control circuit.

[0021] The beneficial effects of the present invention are as follows:

[0022] (1) The present invention provides a single-stage power factor correction control circuit, which realizes power factor correction within a wide range of input grid voltages, reduces current harmonic distortion, can also limit and stabilize the bus capacitor voltage, and avoids overstress of devices.

[0023] (2) The single-stage power factor correction control circuit of the present invention limits and stabilizes the voltage of capacitor C1 by controlling the conduction time of switch S3, preventing overstress of circuit devices. At the same time, the input grid current is sampled, and through capacitor integration, the peak voltage of the capacitor represents the average current. By adopting the peak control method and controlling the conduction time of switch S3, the input current tracks the input grid voltage, realizing the power factor correction function. The single-stage power factor correction circuit of the present invention has a simple circuit and convenient control. Compared with the two-stage circuit, the circuit cost is lower. Compared with the passive charge pump PFC, it can well balance the bus capacitance voltage and power factor and can be applied to a wide range of input and output load ranges. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required in the embodiments. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0025] Figure 1 is a schematic diagram according to Embodiment 1 of the present invention;

[0026] Figure 2 is a voltage waveform diagram according to Embodiment 1 of the present invention;

[0027] Figure 3 is a partial schematic diagram according to Embodiment 1 of the present invention;

[0028] Figure 4 is a schematic diagram according to Embodiment 2 of the present invention;

[0029] Figure 5 is a voltage waveform diagram according to Embodiment 2 of the present invention;

[0030] Figure 6 is Figure 4 a schematic diagram of the power factor correction control circuit in DETAILED DESCRIPTION OF THE EMBODIMENTS

[0031] To further illustrate the embodiments, the present invention provides drawings, which are part of the disclosure of the present invention. They are mainly used to illustrate the embodiments and can be combined with the relevant descriptions in the specification to explain the operating principles of the embodiments. With reference to these contents, those of ordinary skill in the art should be able to understand other possible implementation manners and the advantages of the present invention. The components in the drawings are not drawn to scale, and similar component symbols are usually used to represent similar components.

[0032] According to an embodiment of the present invention, a single-stage power factor correction control circuit and a switching power supply are provided.

[0033] Embodiment 1

[0034] The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments. As Figure 1 shown, the single-stage power factor correction control circuit according to an embodiment of the present invention includes a grid input Vin, a rectifier bridge DB1, a capacitor C1, a capacitor C2, a switching transistor S1, a switching transistor S2, a switching transistor S3, a diode D1, an inductor L1, a transformer T1, a first output rectifier circuit, an output capacitor Co, a power factor correction control circuit, and a resonant control drive circuit;

[0035] Among them, the grid input Vin is connected to the first end and the third end of the rectifier bridge DB1. The second end of the rectifier bridge DB1 is sequentially connected to the positive electrode of the capacitor C1 and the first end of the switching transistor S1. The second end of the switching transistor S1 is sequentially connected to one end of the inductor L1 and the first end of the switching transistor S2. The other end of the inductor L1 is connected to the first input end of the transformer T1. The second input end of the transformer T1 is connected to one end of the capacitor C2. The output end of the transformer T1 is connected in parallel with the first output rectifier circuit. The first output rectifier circuit is connected in parallel with the output capacitor Co. The other end of the capacitor C2 is sequentially connected to the first end of the switching transistor S3, the negative electrode of the diode D1, and the fourth end of the rectifier bridge DB1. The positive electrode of the diode D1 is sequentially connected to the negative electrode of the capacitor C1, the second end of the switching transistor S3, and the second end of the switching transistor S2 and grounded. The third end of the switching transistor S3 is connected to the power factor correction control circuit. The third end of the switching transistor S2 is connected to the second end of the resonant control drive circuit. The third end of the switching transistor S1 is connected to the first end of the resonant control drive circuit.

[0036] In one embodiment, for the above capacitor C1, the capacitor C1 is a polarized capacitor.

[0037] In one embodiment, for the above output capacitor Co, the output capacitor Co is a polarized capacitor.

[0038] In one embodiment, when the current in the inductor L1 flows from right to left during the negative half cycle, the diode D1 conducts and the rectifier bridge DB1 is cut off, and the current does not pass through the input grid Vin.

[0039] In one embodiment, when the current in the inductor L1 flows from left to right during the positive half cycle, if the switching transistor S3 conducts, the rectifier bridge DB1 is cut off, and the current does not pass through the input grid Vin.

[0040] In one embodiment, when the current of inductor L1 flows from left to right during the positive half cycle, if switch S3 is turned off and rectifier bridge DB1 is turned on, the current passes through the input power grid Vin.

[0041] In one embodiment, the resonant control drive circuit supplies energy to output capacitor Co and the load through transformer T1 and the first output rectifier circuit.

[0042] In one embodiment, the resonant control drive circuit adopts frequency feedback control of the output voltage or output current. Its control method is similar to that of a general series resonance or series-parallel resonance circuit, symmetrically and complementarily driving switch S1 and switch S2, and obtaining the set output voltage Vo or output current Io through frequency control.

[0043] The present invention also provides a switching power supply, which is composed of the above single-stage power factor correction control circuit.

[0044] In one embodiment, when switch S3 is always on, the input power grid Vin, rectifier bridge DB1 and capacitor C1 form an uncontrolled rectifier circuit without power factor correction function. At this time, the operating voltage of capacitor C1 is the lowest and the grid current harmonic is also very large; when switch S3 is always off, the positive half cycle resonant current of inductor L1 all passes through rectifier bridge DB1 and input power grid Vin to charge capacitor C1. The current flowing through switch S1 must be less than the positive half cycle current of inductor L1. Therefore, the charging current is always greater than the discharging current, and the voltage of capacitor C1 will keep rising until the device is damaged. As Figure 2 and Figure 3 shown, by controlling the conduction time of switch S3, the voltage of capacitor C1 can be limited and stabilized. When the positive half cycle current of inductor L1 flows, by controlling the conduction time of switch S3, the magnitude of the current passing through rectifier bridge DB1 and input power grid Vin to charge capacitor C1 can be controlled. The longer the conduction time of switch S3, the smaller the current charging capacitor C1. On the contrary, the shorter the conduction time of switch S3, the larger the current charging capacitor C1. When the conduction time of switch S3 is at a certain value, the charging and discharging currents of capacitor C1 are equal and the voltage of capacitor C1 is in a steady state. Therefore, by controlling the conduction time of switch S3, the voltage of capacitor C1 can be limited and stabilized, avoiding overstress of circuit devices.

[0045] In one embodiment, according to the input power grid voltage, the conduction time or conduction duty ratio of switch S3 is controlled and modulated, so that the average value of the current flowing through rectifier bridge DB1 and the power grid tracks the input power grid voltage in the power frequency cycle, then the power factor correction function can be realized, the input power factor is improved, and the current harmonic is reduced.

[0046] In one embodiment, the power factor correction control circuit feedback-controls to stabilize the voltage of the bus capacitor C1, implementing the power factor correction function.

[0047] Embodiment 2

[0048] As Figure 4 shown, the single-stage power factor correction control circuit according to an embodiment of the present invention includes a grid input Vin, a rectifier bridge DB1, a capacitor C1, a capacitor C2, a switching transistor S1, a switching transistor S2, a switching transistor S3, a diode D1, an inductor L1, a transformer T1, a first output rectifier circuit, an output capacitor Co, a power factor correction control circuit, a resonance control drive circuit, a second output rectifier circuit, a current transformer CT1, a capacitor C3, and a switching transistor S4;

[0049] Wherein, the grid input Vin is connected to the first end and the third end of the rectifier bridge DB1, the second end of the rectifier bridge DB1 is sequentially connected to the positive electrode of the capacitor C1 and the first end of the switching transistor S1, the second end of the switching transistor S1 is sequentially connected to one end of the inductor L1 and the first end of the switching transistor S2, the other end of the inductor L1 is connected to the first input end of the transformer T1, the second input end of the transformer T1 is connected to one end of the capacitor C2, the output end of the transformer T1 is connected in parallel with the first output rectifier circuit, the first output rectifier circuit is connected in parallel with the output capacitor Co, the other end of the capacitor C2 is sequentially connected to the Vds_s3 terminal of the power factor correction control circuit, the first end of the switching transistor S3, the negative electrode of the diode D1, and the second input end of the current transformer CT1, the positive electrode of the diode D1 is sequentially connected to the negative electrode of the capacitor C1, the second end of the switching transistor S3, and the second end of the switching transistor S2 and grounded, the third end of the switching transistor S3 is connected to the Vg_s3 terminal of the power factor correction control circuit, the third end of the switching transistor S2 is connected to the second end of the resonance control drive circuit, the third end of the switching transistor S1 is sequentially connected to the first end of the resonance control drive circuit and the Vg_s1 terminal of the power factor correction control circuit, the first input end of the current transformer CT1 is connected to the fourth end of the rectifier bridge DB1, the output end of the current transformer CT1 is connected in parallel with the capacitor C3, the negative electrode of the capacitor C3 is connected to the first end of the switching transistor S4 and grounded, the positive electrode of the capacitor C3 is sequentially connected to the Vc terminal of the power factor correction control circuit and the second end of the switching transistor S4, the third end of the switching transistor S4 is connected to the Vg_s4 terminal of the power factor correction control circuit, and the Vin_rec terminal of the power factor correction control circuit is connected to the second output rectifier circuit;

[0050] Among them, the capacitor C3 is a polarized capacitor.

[0051] In one embodiment, as Figure 4 shown, the input grid current is sampled by the current transformer CT1, and charge integration is performed through the capacitor C3. Each time the switch tube S3 is turned on, the conducting switch tube S4 is turned on to release and reset the voltage across the capacitor C3 to zero. When the capacitance of the capacitor C1 is large enough, it can be approximately considered that the switching operating frequency of the resonant circuit is basically the same throughout the power frequency cycle. From I*t = C*V, it can be known that the peak value of the capacitor C3 is proportional to the average current flowing through the input grid during the switching period, and its magnitude characterizes the average value of the input current during the switching period. By using peak comparison control to modulate the conduction duty cycle of the switch tube S3, the average value of the input current can be modulated and controlled.

[0052] In one embodiment, as Figure 6 shown, an operational amplifier negative feedback circuit is used to achieve the voltage control of the bus capacitor C1, and PI integral control is used as loop compensation;

[0053] In one embodiment, as Figure 5 and Figure 6 shown, the operational amplifier negative feedback output Vcomp and the input voltage Vin's bread wave voltage pass through a multiplier and serve as the peak comparison reference for the voltage Vc of the capacitor C3. When Vc rises to the reference, the switch tube S3 is turned on; through charge integration to follow the peak control of the input grid, the input current follows the input voltage indirectly, achieving power factor correction.

[0054] In one embodiment, when the current of the inductor L1 changes from the positive half-cycle to the negative half-cycle, the switch tube S3 is still in the conducting state and replaces D1 to conduct, reducing the conduction loss; when the negative half-cycle current flowing through the switch tube S3 gradually decreases to the set comparison value, the switch tube S3 is turned off; it is ensured that when the current conducts in the positive half-cycle, the current will flow into the input grid through the rectifier bridge DB1.

[0055] In one embodiment, when the switch tube S3 is a Mosfet, the magnitude of the voltage Vds_s3 across the switch tube S3 can be detected to judge the conduction current of the switch tube S3. When it increases from a lower negative value to the set threshold Vth, a rising edge action is detected to trigger the turn-off of the switch tube S3.

[0056] To facilitate the understanding of the above technical solutions of the present invention, the working principle or operation mode of the present invention in the actual process will be described in detail below.

[0057] In actual application,

[0058] When the switching transistor S1 is turned on and the switching transistor S2 is turned off, and when the switching transistor S3 or the diode D1 is turned on, the current passes through the switching transistor S1, the inductor L1, the transformer T1, the resonant capacitor C2, the switching transistor S3 or the diode D1, and the capacitor C1;

[0059] When the switching transistor S1 is turned off and the switching transistor S2 is turned on, and when the switching transistor S3 or the diode D1 is turned on, the current passes through the switching transistor S2, the inductor L1, the transformer T1, the resonant capacitor C2, the switching transistor S3 or the diode D1;

[0060] When the switching transistor S1 is turned on and the switching transistor S2 is turned off, and when the switching transistor S3 and the diode D1 are turned off, the current passes through the switching transistor S1, the inductor L1, the transformer T1, the resonant capacitor C2, the rectifier bridge DB1, and the input power grid Vin;

[0061] When the switching transistor S1 is turned off and the switching transistor S2 is turned on, and when the switching transistor S3 and the diode D1 are turned off, the current passes through the switching transistor S1, the inductor L1, the transformer T1, the resonant capacitor C2, the rectifier bridge DB1, and the input power grid Vin.

[0062] In summary, the present invention provides a single-stage power factor correction control circuit, which realizes power factor correction within a wide range of input power grid voltages, reduces current harmonic distortion, can also limit and stabilize the bus capacitor voltage, and avoids overstress of devices. The single-stage power factor correction control circuit of the present invention limits and stabilizes the voltage of the capacitor C1 by controlling the on-time of the switch S3, preventing overstress of circuit devices. At the same time, the input power grid current is sampled, and through capacitor integration, the peak voltage of the capacitor characterizes the average current. By adopting the peak control method and controlling the on-time of the switch S3, the input current tracks the input power grid voltage, realizing the power factor correction function. The single-stage power factor correction circuit of the present invention has a simple circuit and convenient control. Compared with the two-stage circuit, the circuit cost is lower. Compared with the passive charge pump PFC, it can well balance the bus capacitance voltage and the power factor, and can be applied to a wide range of input and output load ranges.

[0063] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A single-stage power factor correction control circuit, characterized in that, It includes grid input Vin, rectifier bridge DB1, capacitor C1, capacitor C2, switching transistor S1, switching transistor S2, switching transistor S3, diode D1, inductor L1, transformer T1, first output rectifier circuit, output capacitor Co, power factor correction control circuit, resonant control drive circuit, second output rectifier circuit, current transformer CT1, capacitor C3 and switching transistor S4; Among them, the grid input Vin is connected to the first end and the third end of the rectifier bridge DB1. The second end of the rectifier bridge DB1 is sequentially connected to the positive electrode of the capacitor C1 and the first end of the switching transistor S1. The second end of the switching transistor S1 is sequentially connected to one end of the inductor L1 and the first end of the switching transistor S2. The other end of the inductor L1 is connected to the first input end of the transformer T1. The second input end of the transformer T1 is connected to one end of the capacitor C2. The output end of the transformer T1 is connected in parallel with the first output rectifier circuit. The first output rectifier circuit is connected in parallel with the output capacitor Co. The other end of the capacitor C2 is sequentially connected to the Vds_s3 terminal of the power factor correction control circuit, the first end of the switching transistor S3, the negative electrode of the diode D1 and the second input end of the current transformer CT1. The positive electrode of the diode D1 is sequentially connected to the negative electrode of the capacitor C1, the second end of the switching transistor S3 and the second end of the switching transistor S2 and grounded. The third end of the switching transistor S3 is connected to the Vg_s3 terminal of the power factor correction control circuit. The third end of the switching transistor S2 is connected to the second end of the resonant control drive circuit. The third end of the switching transistor S1 is sequentially connected to the first end of the resonant control drive circuit and the Vg_s1 terminal of the power factor correction control circuit. The first input end of the current transformer CT1 is connected to the fourth end of the rectifier bridge DB1. The output end of the current transformer CT1 is connected in parallel with the capacitor C3. The negative electrode of the capacitor C3 is connected to the first end of the switching transistor S4 and grounded. The positive electrode of the capacitor C3 is sequentially connected to the Vc terminal of the power factor correction control circuit and the second end of the switching transistor S4. The third end of the switching transistor S4 is connected to the Vg_s4 terminal of the power factor correction control circuit. The Vin_rec terminal of the power factor correction control circuit is connected to the second output rectifier circuit; Among them, the capacitor C3 is a polarized capacitor; The input grid current is sampled through the current transformer CT1, and charge integration is performed through the capacitor C3. Each time the switching transistor S3 conducts, the switching transistor S4 is turned on to release and reset the voltage across the capacitor C3 to zero. According to I*t = C*V, the peak value of the capacitor C3 is proportional to the average current flowing through the input grid during the switching period, and its magnitude characterizes the average value of the input current within the switching period. Peak comparison control is adopted to modulate the conduction duty ratio of the switching transistor S3, and the average value of the input current can be modulated and controlled; An operational amplifier negative feedback circuit is used to realize the voltage control of the bus capacitor C1, and PI integral control is used as loop compensation; The negative feedback output Vcomp of the operational amplifier and the input voltage Vin form a "mantou wave" voltage. After passing through the multiplier, it serves as the peak comparison reference for the voltage Vc of the capacitor C3. When Vc rises to the reference, the switching transistor S3 is turned on; by means of charge integration to follow the peak control of the input power grid, the input current follows the input voltage indirectly, achieving power factor correction. When the current of the inductor L1 changes from the positive half-cycle to the negative half-cycle, the switching transistor S3 is still in the on state, replacing D1 to conduct, reducing the conduction loss; when the negative half-cycle current flowing through the switching transistor S3 gradually decreases to the set comparison value, the switching transistor S3 is turned off; ensuring that when the current conducts in the positive half-cycle, the current will flow into the input power grid through the rectifier bridge DB1. When the switching transistor S3 is a Mosfet, the magnitude of the voltage Vds_s3 across the switching transistor S3 can be detected to judge the conduction current of the switching transistor S3. When it increases from a lower negative value to the set threshold Vth, a rising edge is detected and an action is triggered to turn off the switching transistor S3.

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