Bootstrap power supply-based flyback power supply circuit and control method thereof

Abstract

The invention discloses a flyback power supply circuit based on bootstrap power supply and a control method thereof. The flyback power supply circuit comprises a first MOS tube; a grid electrode of the first MOS tube is electrically connected with a collector electrode of the triode, a drain electrode is electrically connected with one end of the pre-charging resistor, and a source electrode is electrically connected with a negative electrode of the second voltage stabilizing diode, one end of the supporting capacitor, a power supply output end of the bootstrap power supply circuit and a power supply positive electrode of the modulation circuit; the other end of the pre-charging resistor is electrically connected with the negative electrode of the first diode and the dotted terminal of the main side inductor of the transformer; the positive electrode of the first diode is electrically connected with one end of a voltage input end Vin; the synonym end of the main side inductor is electrically connected with the power supply input end of the bootstrap power supply circuit and the control output end of the modulation circuit. According to the invention, the purposes of reducing the input loss, improving the starting speed and reducing the design complexity and the size of the transformer are achieved with relatively low cost.

Description

Technical Field

[0001] This invention belongs to the field of electrical engineering, specifically relating to a flyback power supply circuit based on bootstrap power supply and its control method. Background Technology

[0002] Currently, flyback auxiliary power supplies on the market generally use the following two methods to power their flyback modulation circuits: 1. The primary side high voltage is powered by LDO or TVS voltage limiting modulation; 2. The primary side is current-limited excitation, and the auxiliary power supply voltage generated by the secondary side is reversed to power the flyback modulation circuit.

[0003] For method 1: using LDO or TVS voltage-limiting modulation for power supply, it is generally applied in circuits where the primary and secondary sides need to be isolated. When the input voltage is high, the power supply circuit suffers significant losses and heat generation. Furthermore, if the adjustment circuit is adapted to the higher voltage, the low-voltage input cannot provide the current required for modulation, resulting in a narrow input voltage range adaptability and significant losses in the auxiliary power supply circuit.

[0004] Method 2, which uses conventional primary-side current-limiting excitation and reverse-biased auxiliary power supply to power the flyback modulation circuit, has the following disadvantages: ① When the primary-side excitation current-limiting resistor is small, its normal operating losses are large, resulting in significant heat generation; ② When the primary-side excitation current-limiting resistor is large, the excitation current it can provide is small. When the secondary-side load is large, multiple excitation cycles may be required to achieve stable output, resulting in a long start-up time and consequently, a higher required input voltage; ③ This method requires an additional output winding for isolated output applications, increasing the complexity of transformer design and transformer size. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a flyback power supply circuit and its control method based on bootstrap power supply, which addresses the shortcomings of the prior art and achieves the goals of reducing input loss, improving startup speed, reducing transformer design complexity and transformer size at a lower cost.

[0006] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows: A flyback power supply circuit based on bootstrap power supply includes a first MOSFET Q1; the gate of the first MOSFET Q1 is electrically connected to the collector of transistor Q2, the drain is electrically connected to one end of pre-charge resistor R1, and the source is electrically connected to the negative terminal of second Zener diode D2, one end of support capacitor C1, the power supply output terminal of bootstrap power supply circuit and the power supply positive terminal of modulation circuit. The other end of the pre-charge resistor R1 is electrically connected to the negative terminal of the first diode D1 and the same-name terminal of the main-side inductor L1 of the transformer T1; the positive terminal of the first diode D1 is electrically connected to one end of the voltage input terminal Vin; the opposite-name terminal of the main-side inductor L1 is electrically connected to the power supply input terminal of the bootstrap power supply circuit and the control output terminal of the modulation circuit. The positive terminal of the second Zener diode D2 is electrically connected to one end of the second resistor R2, and the other end of the second resistor R2 is electrically connected to the base of the transistor Q2; the emitter of the transistor Q2 is electrically connected to the other end of the supporting capacitor C1, the other end of the voltage input terminal Vin, the negative power supply terminal of the bootstrap power supply circuit, and the negative power supply terminal of the modulation circuit. The same-name terminal of the secondary inductor L2 of the transformer T1 is electrically connected to one end of the second capacitor C2 and one end of the load R3; the opposite-name terminal of the secondary inductor L2 is electrically connected to the positive terminal of the third diode D3, and the negative terminal of the third diode D3 is electrically connected to the feedback input terminal of the modulation circuit and the other end of the load R3.

[0007] In a further improvement, the bootstrap power supply circuit includes a fourth diode D4, the negative terminal of which is electrically connected to the negative terminal of the fifth Zener diode D5 and the power supply output terminal of the bootstrap power supply circuit; the positive terminal of the fourth diode D4 is electrically connected to one end of the bootstrap capacitor C3 and the drain of the third MOSFET Q3, and the other end of the bootstrap capacitor C3 is electrically connected to the power supply input terminal of the bootstrap power supply circuit; the gate of the third MOSFET Q3 is electrically connected to the positive terminal of the fifth Zener diode D5, one end of the fourth resistor R4, and one end of the fourth capacitor C4; the other ends of the fourth resistor R4 are electrically connected to the negative power supply terminal of the bootstrap power supply circuit.

[0008] In a further improvement, the modulation circuit includes a modulation chip U1. The VCC port of the modulation chip U1 is electrically connected to the positive power supply of the modulation circuit. The GND port and Vfeed port are electrically connected to the negative power supply of the modulation circuit, one end of the fifth resistor R5, the GND terminal of the first optocoupler OP1, the positive terminal of the sixth diode D6, and one end of the eighth resistor R8. The negative terminal of the sixth diode D6 is electrically connected to one end of the fifth capacitor C5 and the negative input terminal of the first optocoupler OP1. The other end of the eighth resistor R8 is electrically connected to the other end of the fifth capacitor C5 and one end of the seventh resistor R7. The positive input terminal of the first optocoupler OP1 is electrically connected to one end of the sixth resistor R6. The other ends of the seventh resistor R7 and the other end of the sixth resistor R6 are electrically connected to the feedback input terminal of the modulation circuit. The collector of the first optocoupler OP1 is electrically connected to the COPM terminal of the modulation chip U1. The other end of the fifth resistor R5 is electrically connected to the Is terminal of the modulation chip U1 and the source of the fourth MOSFET Q4. The base of the fourth MOSFET Q4 is electrically connected to the OUT terminal of the modulation chip U1, and the drain is electrically connected to the control output terminal of the modulation circuit.

[0009] In a further improvement, the first MOSFET Q1 is a depletion-type MOSFET.

[0010] A further improvement is made, wherein the resistance value R1' of the pre-charge resistor R1 satisfies: R1'>Vin_max / Iq1_max Vin_max represents the maximum allowable value of the input voltage Vin in the design; Iq1_max: The maximum current allowed for the first MOSFET.

[0011] A further improvement is that the capacitance of the bootstrap capacitor C3 is between C3min1 and C3min2, and the withstand voltage of the bootstrap capacitor C3 should be greater than 1.2 times the peak voltage of the fourth MOSFET Q4, wherein: C3min1=Pu1 / (Freq*(Vin_min-Vcc) 2 ) C3min2=Pu1 / (Freq*(Vin_min+Vout*N-Vcc) 2 ) Pu1 represents the power consumption of the modulation chip U1; Freq is the output frequency of modulation chip U1; Vin_min is the minimum allowable value for the input voltage design; Vcc is the operating voltage of modulation chip U1; Vout is the rated output voltage of the power supply; N is the turns ratio of transformer T1.

[0012] A further improvement is made, wherein the resistance value R4' of the fourth resistor R4 satisfies: Iz*R4'≥Vgs Iz is the maximum current of the fifth Zener diode D5 in the nonlinear region before Zener breakdown; Vgs is the Miller plateau voltage of the third MOSFET Q3.

[0013] A control method for the above-mentioned flyback power supply circuit based on bootstrap power supply includes the following steps: State 1: Before Vin is powered on, the first MOSFET Q1 is turned on. When Vin is powered on, Vin supplies power to the support capacitor C1 and the modulation circuit through the pre-charge resistor R1. State 2: When the voltage of the supporting capacitor C1 reaches the operating threshold of the modulation circuit, the modulation circuit starts to work, and the flyback power supply starts to output; the control output terminal of the modulation circuit is in PWM state, and the bootstrap power supply circuit starts to work; at this time, the fifth Zener diode D5 of the bootstrap power supply circuit is not turned on, the third MOSFET Q3 is in diode working mode, and the fourth diode D4, the third MOSFET Q3 and the bootstrap capacitor C3 form a conventional bootstrap circuit. Under the PWM input of the control output terminal of the modulation circuit, current is provided to the power supply output terminal of the bootstrap power supply circuit and the positive power supply terminal of the modulation circuit. At this time, the precharge resistor R1 and the bootstrap power supply circuit simultaneously provide current to the supporting capacitor C1 and the modulation circuit. State 3: When the voltage of the supporting capacitor C1 reaches the conduction threshold of the second Zener diode D2, the second Zener diode D2 conducts, the first MOSFET Q1 is turned off, the pre-charge resistor R1 no longer provides current to the supporting capacitor C1 and the modulation circuit, and the current required by the modulation circuit is only provided by the bootstrap power supply circuit. State 4: When the voltage of the supporting capacitor C1 reaches the conduction threshold of the fifth Zener diode D5, the fifth Zener diode D5 conducts and the third MOSFET Q3 conducts. At this time, the fourth diode D4, the third MOSFET Q3 and the bootstrap capacitor C3 are no longer in bootstrap mode. The bootstrap capacitor C3 is connected between the control output terminal of the modulation circuit and the negative power supply terminal and is used as an absorption capacitor. State 5: When the modulation circuit absorbs current and the voltage of the support capacitor C1 drops below the turn-off threshold of the fifth Zener diode D5, the fifth Zener diode D5 turns off, the third MOSFET Q3 turns off, and the third MOSFET Q3 is in diode operating mode. The fourth diode D4, the third MOSFET Q3 and the bootstrap capacitor C3 form a conventional bootstrap circuit, which continues to provide current to the support capacitor C3 and the modulation circuit. State Six: Adjust the parameters of the fifth Zener diode D5, the fourth resistor R4, and the third MOSFET Q3 to satisfy Iz*R4'≥Vgs, so that the flyback power supply circuit approaches a stable State Six and no longer switches between State Four and State Five; at this time, the fifth Zener diode D5 is not fully turned on and operates in current-limiting mode, and the third MOSFET Q3 is not fully turned on and operates in the linear region; the fourth diode D4, the third MOSFET Q3, and the bootstrap capacitor C3 form a conventional bootstrap circuit to provide current to the modulation circuit, while excess energy is absorbed by the third MOSFET Q3, which is in the linear region.

[0014] 1. The bootstrap power supply circuit of the present invention can reduce one set of coils in the transformer of the isolated flyback power supply, reduce pin occupation, reduce transformer design complexity, and reduce transformer size and occupancy area on PCB.

[0015] 2. The bootstrap power supply circuit of the present invention has the function of absorbing and utilizing leakage inductance spike energy and reducing system voltage stress.

[0016] 3. The bootstrap power supply circuit of the present invention can reduce static power consumption. Attached Figure Description

[0017] The accompanying drawings, which form part of this application, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings: Figure 1 This is a schematic diagram of the overall circuit of the present invention; Figure 2 A circuit diagram of the bootstrap power supply circuit; Figure 3 This is a circuit diagram of the modulation circuit. Detailed Implementation

[0018] The embodiments of the present invention will be described in detail below with reference to the accompanying drawings, but the present invention can be implemented in many different ways as defined and covered by the claims.

[0019] like Figure 1 The diagram shows a flyback power supply circuit based on bootstrap power supply, including a first MOSFET Q1 and a fifth Zener diode D5. The gate of the first MOSFET Q1 is electrically connected to the collector of the transistor Q2, the drain is electrically connected to one end of the pre-charge resistor R1, and the source is electrically connected to the negative terminal of the second Zener diode D2, one end of the support capacitor C1, the power supply output terminal of the bootstrap power supply circuit, and the power supply positive terminal of the modulation circuit.

[0020] The other end of the pre-charge resistor R1 of the fifth Zener diode D5 is electrically connected to the negative terminal of the first diode D1 and the same-name terminal of the main inductor L1 of the transformer T1; the positive terminal of the first diode D1 is electrically connected to one end of the voltage input terminal Vin; the opposite-name terminal of the main inductor L1 of the fifth Zener diode D5 is electrically connected to the power input terminal of the bootstrap power supply circuit and the control output terminal of the modulation circuit. The positive terminal of the fifth Zener diode D5 and the second Zener diode D2 are electrically connected to one end of the second resistor R2, and the other end of the second resistor R2 is electrically connected to the base of the transistor Q2; the emitter of the transistor Q2 is electrically connected to the other end of the supporting capacitor C1, the other end of the voltage input terminal Vin, the negative power supply terminal of the bootstrap power supply circuit, and the negative power supply terminal of the modulation circuit. The same-name terminal of the secondary inductance L2 of the fifth Zener diode D5 transformer T1 is electrically connected to one end of the second capacitor C2 and one end of the load R3; the opposite-name terminal of the secondary inductance L2 of the fifth Zener diode D5 is electrically connected to the positive terminal of the third diode D3, and the negative terminal of the third diode D3 is electrically connected to the feedback input terminal of the modulation circuit and the other end of the load R3.

[0021] like Figure 2As shown, the bootstrap power supply circuit includes a fourth diode D4. The cathode of the fourth diode D4 is electrically connected to the cathode of the fifth Zener diode D5 and the power supply output terminal of the bootstrap power supply circuit. The anode of the fourth diode D4 is electrically connected to one end of the bootstrap capacitor C3 and the drain of the third MOSFET Q3. The other end of the bootstrap capacitor C3 is electrically connected to the power supply input terminal of the bootstrap power supply circuit. The gate of the third MOSFET Q3 is electrically connected to the anode of the fifth Zener diode D5, one end of the fourth resistor R4, and one end of the fourth capacitor C4. The other ends of the fourth resistor R4 are electrically connected to the negative power supply terminal of the bootstrap power supply circuit.

[0022] like Figure 3 As shown, the modulation circuit includes a modulation chip U1. The VCC port of the modulation chip U1 is electrically connected to the positive power supply of the modulation circuit. The GND port and Vfeed port are electrically connected to the negative power supply of the modulation circuit, one end of the fifth resistor R5, the GND terminal of the first optocoupler OP1, the positive terminal of the sixth diode D6, and one end of the eighth resistor R8. The negative terminal of the sixth diode D6 is electrically connected to one end of the fifth capacitor C5 and the negative input terminal of the first optocoupler OP1. The other end of the eighth resistor R8 is electrically connected to the other end of the fifth capacitor C5 and one end of the seventh resistor R7. The positive input terminal of the first optocoupler OP1 is electrically connected to one end of the sixth resistor R6. The other ends of the seventh resistor R7 and the other end of the sixth resistor R6 are electrically connected to the feedback input terminal of the modulation circuit. The collector of the first optocoupler OP1 is electrically connected to the COPM terminal of the modulation chip U1. The other end of the fifth resistor R5 is electrically connected to the Is terminal of the modulation chip U1 and the source of the fourth MOSFET Q4. The base of the fourth MOSFET Q4 is electrically connected to the OUT terminal of the modulation chip U1, and the drain is electrically connected to the control output terminal of the modulation circuit.

[0023] The control method for the above-mentioned flyback power supply circuit based on bootstrap power supply is as follows: State 1: Before Vin is powered on, the first MOSFET Q1 is turned on. When Vin is powered on, Vin supplies power to the support capacitor C1 and the modulation circuit through the pre-charge resistor R1. State 2: When the voltage of the supporting capacitor C1 reaches the operating threshold of the modulation circuit, the modulation circuit starts to work, and the flyback power supply starts to output; the control output terminal of the modulation circuit is in PWM state, and the bootstrap power supply circuit starts to work; at this time, the fifth Zener diode D5 of the bootstrap power supply circuit is not turned on, the third MOSFET Q3 is in diode working mode, and the fourth diode D4, the third MOSFET Q3 and the bootstrap capacitor C3 form a conventional bootstrap circuit. Under the PWM input of the control output terminal of the modulation circuit, it provides current to the positive terminal -VCC of the power supply. At this time, the precharge resistor R1 and the bootstrap power supply circuit simultaneously provide current to the supporting capacitor C1 and the modulation circuit. State 3: When the voltage of the supporting capacitor C1 reaches the conduction threshold of the second Zener diode D2, the second Zener diode D2 conducts, the first MOSFET Q1 is turned off, the pre-charge resistor R1 no longer provides current to the supporting capacitor C1 and the modulation circuit, and the current required by the modulation circuit is only provided by the bootstrap power supply circuit. State 4: When the voltage of the supporting capacitor C1 reaches the conduction threshold of the fifth Zener diode D5, the fifth Zener diode D5 conducts and the third MOSFET Q3 conducts. At this time, the fourth diode D4, the third MOSFET Q3 and the bootstrap capacitor C3 are no longer in bootstrap mode. The bootstrap capacitor C3 is connected between the control output terminal of the modulation circuit and the negative power supply terminal and is used as an absorption capacitor. State 5: When the modulation circuit absorbs current and the voltage of the support capacitor C1 drops below the turn-off threshold of the fifth Zener diode D5, the fifth Zener diode D5 turns off, the third MOSFET Q3 turns off, and the third MOSFET Q3 is in diode operating mode. The fourth diode D4, the third MOSFET Q3 and the bootstrap capacitor C3 form a conventional bootstrap circuit, which continues to provide current to the support capacitor C3 and the modulation circuit. State Six: Adjust the parameters of the fifth Zener diode D5, the fourth resistor R4, and the third MOSFET Q3 to satisfy Iz*R4'≥Vgs, so that the flyback power supply circuit approaches a stable State Six and no longer switches between State Four and State Five; at this time, the fifth Zener diode D5 is not fully turned on and operates in current-limiting mode, and the third MOSFET Q3 is not fully turned on and operates in the linear region; the fourth diode D4, the third MOSFET Q3, and the bootstrap capacitor C3 form a conventional bootstrap circuit to provide current to the modulation circuit, while excess energy is absorbed by the third MOSFET Q3, which is in the linear region.

[0024] Parameter selection: To achieve the fastest power-on speed, resistor R1 should be as small as possible, but its minimum value should ensure the safety of Q1. The selection of the resistance value of R1 should follow the following formula: The resistance value R1' of the pre-charge resistor R1 satisfies: R1'>Vin_max / Iq1_max Vin_max represents the maximum allowable value of the input voltage Vin in the design; Iq1_max: The maximum allowable current for the first MOSFET; C3min2≥Pu1 / (Freq*(Vin_min+Vout*N-Vcc) 2 ).

[0025] Selection of the second Zener diode D2: When the second Zener diode D2 is turned on, Q1 is turned off, and R1 no longer supplies power to the modulation circuit. Therefore, the turn-on threshold of D2 should be greater than the minimum start-up operating voltage of the modulation circuit, with a certain margin (usually 20%).

[0026] Selection of the fifth Zener diode D5 The conduction threshold of the fifth Zener diode D5 is usually set to the rated operating voltage of the modulation circuit.

[0027] Selection of bootstrap capacitor C3 The bootstrap capacitor C3 should be chosen with a relatively small value, while ensuring the bootstrap current meets the requirements of the modulation circuit, in order to reduce the power consumption of Q3. The extreme operating condition is when the input voltage Vin is at its minimum and the power supply output voltage is 0V. In this case, without considering the leakage inductance of transformer T1, the input of the bootstrap power supply circuit is at its theoretical minimum. The minimum value of C3, C3min1, under this extreme operating condition is calculated using the following formula: C3min1≥Pu1 / (Freq*(Vin_min-Vcc) 2 ) Pu1: Power consumption of the U1 modulation chip, including the power consumption of driving MOSFET Q4; Freq: The output frequency of the U1 modulation chip; Vin_min: The minimum allowable value for the input voltage design; Vcc: Operating voltage of the U1 modulation chip; Since the actual driving current of transformer T1 is large when the output voltage is less than the rated output voltage, and the energy brought by leakage inductance (voltage spike energy) is large, the capacitance value of C3 can be further reduced based on C3min1.

[0028] Under normal operating conditions, the power supply output voltage is the rated output voltage. The minimum value of capacitor C3 is calculated using the following formula: C3min2≥Pu1 / (Freq*(Vin_min+Vout*N-Vcc) 2 ) Vout: Rated output voltage of the power supply; N: Transformer T1 turns ratio; Considering both operating conditions, the C3 capacitance value should be selected from a suitable value between C3min1 and C3min2.

[0029] To ensure the stability of capacitor C3, the voltage rating of capacitor C3 should be greater than 1.2 times the peak voltage of Q4.

[0030] Choice of D5, R4, Q3 The selection of parameters for D5, R4, and Q3 is a key factor determining whether the system can operate stably in state six. To ensure the system remains stable in state six, rather than switching between states four and five, the voltage across the fourth resistor R4 needs to be slightly greater than the Miller plateau voltage of Q3 in the nonlinear region of D5. The resistance value must satisfy the following: Iz*R4'≥Vgs Iz: The maximum current of D5 in the nonlinear region before Zener breakdown; Vgs: Miller plateau voltage of Q3.

[0031] The examples described above are only used to illustrate the technical solutions of the present invention, but the present invention is not limited to the above embodiments. Any obvious improvements, substitutions or modifications that can be made by those skilled in the art without departing from the essence of the present invention shall fall within the protection scope of the present invention.

Claims

1. A flyback power supply circuit based on bootstrap power supply, characterized in that, It includes a first MOSFET (Q1); the gate of the first MOSFET (Q1) is electrically connected to the collector of the transistor (Q2), the drain is electrically connected to one end of the pre-charge resistor (R1), and the source is electrically connected to the negative terminal of the second Zener diode (D2), one end of the support capacitor (C1), the power supply output terminal of the bootstrap power supply circuit, and the power supply positive terminal of the modulation circuit; The other end of the pre-charge resistor (R1) is electrically connected to the negative terminal of the first diode (D1) and the same-name terminal of the main-side inductor (L1) of the transformer (T1); the positive terminal of the first diode (D1) is electrically connected to one end of the voltage input terminal Vin; the opposite-name terminal of the main-side inductor (L1) is electrically connected to the power supply input terminal of the bootstrap power supply circuit and the control output terminal of the modulation circuit. The positive terminal of the second Zener diode (D2) is electrically connected to one end of the second resistor (R2), and the other end of the second resistor (R2) is electrically connected to the base of the transistor (Q2); the emitter of the transistor (Q2) is electrically connected to the other end of the supporting capacitor (C1), the other end of the voltage input terminal Vin, the negative power supply terminal of the bootstrap power supply circuit, and the negative power supply terminal of the modulation circuit. The same-name terminal of the secondary inductor (L2) of the transformer (T1) is electrically connected to one end of the second capacitor (C2) and one end of the load (R3); the opposite-name terminal of the secondary inductor (L2) is electrically connected to the positive terminal of the third diode (D3), and the negative terminal of the third diode (D3) is electrically connected to the feedback input terminal of the modulation circuit and the other end of the load (R3).

2. The flyback power supply circuit based on bootstrap power supply as described in claim 1, characterized in that, The bootstrap power supply circuit includes a fourth diode (D4), the negative terminal of which is electrically connected to the negative terminal of the fifth Zener diode (D5) and the power supply output terminal of the bootstrap power supply circuit; the positive terminal of the fourth diode (D4) is electrically connected to one end of the bootstrap capacitor (C3) and the drain of the third MOSFET (Q3), and the other end of the bootstrap capacitor (C3) is electrically connected to the power supply input terminal of the bootstrap power supply circuit; the gate of the third MOSFET (Q3) is electrically connected to the positive terminal of the fifth Zener diode (D5), one end of the fourth resistor (R4), and one end of the fourth capacitor (C4); the other end of the fourth resistor (R4) is electrically connected to the negative power supply terminal of the bootstrap power supply circuit.

3. The flyback power supply circuit based on bootstrap power supply as described in claim 1, characterized in that, The modulation circuit includes a modulation chip (U1). The VCC port of the modulation chip (U1) is electrically connected to the positive power supply of the modulation circuit. The GND port and Vfeed port are electrically connected to the negative power supply of the modulation circuit, one end of the fifth resistor (R5), the GND terminal of the first optocoupler (OP1), the positive terminal of the sixth diode (D6), and one end of the eighth resistor (R8). The negative terminal of the sixth diode (D6) is electrically connected to one end of the fifth capacitor (C5) and the negative input terminal of the first optocoupler (OP1). The other end of the eighth resistor (R8) is electrically connected to the other end of the fifth capacitor (C5) and the first optocoupler (OP1). One end of the seventh resistor (R7); the positive input terminal of the first optocoupler (OP1) is electrically connected to one end of the sixth resistor (R6), and the other ends of the seventh resistor (R7) and the sixth resistor (R6) are electrically connected to the feedback input terminal of the modulation circuit; the collector of the first optocoupler (OP1) is electrically connected to the COPM terminal of the modulation chip (U1); the other end of the fifth resistor (R5) is electrically connected to the Is terminal of the modulation chip (U1) and the source of the fourth MOS transistor (Q4); the base of the fourth MOS transistor (Q4) is electrically connected to the OUT terminal of the modulation chip (U1), and the drain is electrically connected to the control output terminal of the modulation circuit.

4. The flyback power supply circuit based on bootstrap power supply according to claim 1, characterized in that, The first MOSFET (Q1) is a depletion-type MOSFET.

5. The flyback power supply circuit based on bootstrap power supply according to claim 1, characterized in that, The resistance value R1' of the pre-charge resistor (R1) satisfies: R1'>Vin_max / Iq1_max Vin_max represents the maximum allowable value of the input voltage Vin in the design; Iq1_max: The maximum current allowed for the first MOSFET.

6. The flyback power supply circuit based on bootstrap power supply according to claim 1, characterized in that, The capacitance value of the bootstrap capacitor (C3) is between C3min1 and C3min2, and the withstand voltage of the bootstrap capacitor (C3) should be greater than 1.2 times the peak voltage of the fourth MOSFET (Q4), wherein: C3min1=Pu1 / (Freq*(Vin_min-Vcc) 2 ) C3min2=Pu1 / (Freq*(Vin_min+Vout*N-Vcc) 2 ) Pu1 represents the power consumption of the modulation chip (U1); Freq is the output frequency of the modulation chip (U1); Vin_min is the minimum allowable value for the input voltage design; Vcc is the operating voltage of the modulation chip (U1); Vout is the rated output voltage of the power supply; N is the turns ratio of the transformer (T1).

7. The flyback power supply circuit based on bootstrap power supply according to claim 1, characterized in that, The resistance value R4' of the fourth resistor (R4) satisfies: Iz*R4'≥Vgs Iz is the maximum current of the fifth Zener diode (D5) in the nonlinear region before Zener breakdown; Vgs is the Miller plateau voltage of the third MOSFET (Q3).

8. A control method for a flyback power supply circuit based on bootstrap power supply as described in any one of claims 1-7, characterized in that, Includes the following steps: State 1: Before Vin is powered on, the first MOSFET (Q1) is turned on, Vin is powered on, and Vin supplies power to the support capacitor (C1) and the modulation circuit through the pre-charge resistor (R1); State 2: When the voltage of the supporting capacitor (C1) reaches the operating threshold of the modulation circuit, the modulation circuit starts to work, and the flyback power supply starts to output; the control output terminal of the modulation circuit is in PWM state, and the bootstrap power supply circuit starts to work; at this time, the fifth Zener diode (D5) of the bootstrap power supply circuit is not turned on, the third MOSFET (Q3) is in diode working mode, and the fourth diode (D4), the third MOSFET (Q3) and the bootstrap capacitor (C3) form a conventional bootstrap circuit. Under the PWM input of the control output terminal of the modulation circuit, current is provided to the power supply output terminal of the bootstrap power supply circuit and the positive power supply terminal of the modulation circuit. At this time, the precharge resistor (R1) and the bootstrap power supply circuit simultaneously provide current to the supporting capacitor (C1) and the modulation circuit. State 3: When the voltage of the supporting capacitor (C1) reaches the conduction threshold of the second Zener diode (D2), the second Zener diode (D2) conducts, the first MOSFET (Q1) is turned off, the pre-charge resistor (R1) no longer provides current to the supporting capacitor (C1) and the modulation circuit, and the current required by the modulation circuit is only provided by the bootstrap power supply circuit. State 4: When the voltage of the supporting capacitor (C1) reaches the conduction threshold of the fifth Zener diode (D5), the fifth Zener diode (D5) conducts, and the third MOSFET (Q3) conducts. At this time, the fourth diode (D4), the third MOSFET (Q3), and the bootstrap capacitor (C3) are no longer in bootstrap mode. The bootstrap capacitor (C3) is connected between the control output terminal of the modulation circuit and the negative power supply terminal, and is used as an absorption capacitor. State 5: When the modulation circuit absorbs current and the voltage of the supporting capacitor (C1) drops below the turn-off threshold of the fifth Zener diode (D5), the fifth Zener diode (D5) turns off, the third MOSFET (Q3) turns off, and the third MOSFET (Q3) is in diode mode. The fourth diode (D4), the third MOSFET (Q3), and the bootstrap capacitor (C3) form a conventional bootstrap circuit, which continues to provide current to the supporting capacitor (C3) and the modulation circuit. State Six: Adjust the parameters of the fifth Zener diode (D5), the fourth resistor (R4), and the third MOSFET (Q3) to satisfy Iz*R4'≥Vgs, so that the flyback power supply circuit approaches a stable State Six and no longer switches between State Four and State Five; at this time, the fifth Zener diode (D5) is not fully turned on and operates in current limiting mode, and the third MOSFET (Q3) is not fully turned on and operates in the linear region; the fourth diode (D4), the third MOSFET (Q3), and the bootstrap capacitor (C3) form a conventional bootstrap circuit to provide current to the modulation circuit, while excess energy is absorbed by the third MOSFET (Q3) in the linear region.

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