Quick charging adapter circuit

Abstract

The utility model relates to a quick charge adapter circuit, including transformation circuit and synchronous rectification circuit, transformation circuit includes transformer T1, transformer T1 includes first secondary side coil and second secondary side coil, the first end of second secondary side coil is used as the positive pole of direct current power supply VOUT2, synchronous rectification circuit includes NMOS transistor Q1 and rectification controller U4, the second end of second secondary side coil is used as the positive pole of direct current power supply VOUT2. The drain electrode of the NMOS tube Q1 is connected with the second end of the second secondary side coil, and the grid electrode of the NMOS tube Q1 is connected with the control end of the rectification controller U4; the power output end of the rectification controller U4 is connected with the first end of the first secondary side coil, the speed setting end of the rectification controller U4 is grounded through a resistor R75, and the monitoring end of the rectification controller U4 is connected with the drain electrode of the NMOS tube Q1 through a resistor R74. Compared with traditional diode rectification, the synchronous rectification circuit reduces conduction loss and improves rectification efficiency.

Description

Technical Field

[0001] The utility model relates to the technical field of power supply, in particular to a fast charging adapter circuit. Background Art

[0002] As an indispensable device in modern life, fast chargers are widely used in various electronic products, such as smartphones, laptops, tablets, wearable devices, power tools, etc.

[0003] Modern chargers usually use switching mode power supply technology (SMPS), whose main advantages are high efficiency and small size. The switching power supply converts AC power into stable DC power by high-frequency switching. Compared with traditional linear power supplies, the switching power supply has obvious advantages in energy conversion efficiency, size and weight.

[0004] Its basic telephone includes a rectifier and filter circuit that rectifies and filters the AC power, then forms high-voltage direct current, which is then stepped down by a transformer. The direct current needs to be modulated before stepping down, and then it can be coupled to the secondary side of the transformer for output. High-frequency switching signals are used here, and the size of the transformer can be made very small.

[0005] For example, a Chinese invention patent application document with publication number CN117767513A discloses a flyback power adapter circuit. Figure 1 , the design is based on the above working principle, but in the rectifier output part of the secondary coil, two diodes D1 and D3 are used for rectification. The traditional diode rectification has large conduction loss, low rectification efficiency and large heat loss. Utility Model Content

[0006] The utility model aims to provide a fast charging adapter circuit to solve the technical problems of conduction loss and low efficiency in the existing fast charging adapter rectifier output circuit.

[0007] To achieve the above purpose, the specific technical solution of a fast charging adapter circuit of the utility model is as follows:

[0008] A fast charging adapter circuit includes an EMC circuit for input filtering and surge current protection of input AC power, a rectifying and filtering circuit for rectifying and filtering AC power, a transformer circuit for stepping down a DC voltage, and a synchronous rectifying circuit for rectifying the transformed power supply, wherein the transformer circuit includes a transformer T1, the transformer T1 includes a primary coil, a first secondary coil, and a second secondary coil, wherein a first end of the second secondary coil serves as a positive electrode of a DC power supply VOUT2, and the synchronous rectifying circuit is arranged at a second end of the second secondary coil;

[0009] The synchronous rectification circuit includes an NMOS tube Q1 and a rectification controller U4, wherein the drain of the NMOS tube Q1 is connected to the second end of the second secondary coil, the source of the NMOS tube Q1 is grounded, and the gate of the NMOS tube Q1 is connected to the control end of the rectification controller U4; the power output end of the rectification controller U4 is connected to the first end of the first secondary coil, the ground end of the rectification controller U4 is grounded, the speed setting end of the rectification controller U4 is grounded through a resistor R75, the monitoring end of the rectification controller U4 is connected to the drain of the NMOS tube Q1 through a resistor R74, and the power input end of the rectification controller U4 is grounded through a capacitor C39.

[0010] Furthermore, the voltage transformation circuit includes an NMOS tube Q3 and a PWM controller U1 for driving the NMOS tube Q3, the second end of the primary coil is connected to the drain of the NMOS tube Q3, the source of the NMOS tube Q3 is grounded through a current sensing resistor, the gate of the NMOS tube Q3 is connected to the driving end of the PWM controller U1 through a gate protection circuit, and the source of the NMOS tube Q3 is connected to the current sampling port of the PWM controller U1.

[0011] Furthermore, the gate protection circuit includes a diode D10, a resistor R21, a capacitor C10, a resistor R20, a resistor R19, a voltage regulator Z4 and a resistor R18. The resistor R18 and the voltage regulator Z4 are arranged between the gate and the source of the NMOS tube Q3. The anode of the voltage regulator Z4 is connected to the source of the NMOS tube Q3, and the cathode of the voltage regulator Z4 is connected to the gate of the NMOS tube Q3; the gate of the NMOS tube is connected to the driving end DRV of the PWM controller U1 through the resistor R19 and the resistor R20 in sequence, the cathode of the diode D10 is connected to the driving end DRV of the PWM controller U1, and the anode of the diode D10 is connected to the gate of the NMOS tube Q3 through the resistor R21 and the capacitor C10 in sequence.

[0012] Furthermore, the feedback pin of the PWM controller U1 is grounded through the capacitor C5 and the phototransistor Q8 respectively; a feedback circuit for feeding back to the PWM controller U1 is provided, including a light-emitting diode D18 and a three-terminal voltage regulator U5, the cathode of the light-emitting diode D18 is connected to the cathode of the three-terminal voltage regulator U5, the anode of the light-emitting diode D18 is connected to one end of the first secondary coil through the resistor R7, the anode of the three-terminal voltage regulator U5 is grounded, and a series voltage divider circuit composed of a resistor R5 and a resistor R6 is provided, one end of the resistor R5 is connected to one end of the first secondary coil, the other end of the resistor R5 is connected to one end of the resistor R6, the other end of the resistor R6 is grounded, the reference electrode of the three-terminal voltage regulator U5 is connected to the voltage dividing node of the resistor R5 and the resistor R6, the light-emitting diode D18 and the phototransistor Q8 constitute a photoelectric coupler, which is used to feed back the voltage state of the second secondary coil to the PWM controller U1.

[0013] The utility model provides a fast charging adapter circuit with the following advantages:

[0014] Compared with traditional diode rectification, synchronous rectification uses NMOS tube Q1 as the rectification element, which can significantly reduce conduction loss, improve rectification efficiency and the overall efficiency of the system. Since the on-resistance of the NMOS tube is much lower than the forward voltage drop of the diode, the conduction loss and heat generation are greatly reduced, the heat dissipation performance is improved, and the demand for heat sinks and fans is reduced, thereby increasing the life and reliability of electronic components. The rectifier controller U4 optimizes the rectification process by precisely controlling the switching state of the NMOS tube Q1, maintains efficient and stable rectification performance, and improves the dynamic response capability and output stability of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 The schematic diagram of the EMC circuit and the rectifier and filter circuit provided by the utility model;

[0016] Figure 2 The schematic diagram of the voltage conversion circuit and synchronous rectification circuit provided by the utility model;

[0017] Figure 3 The schematic diagram of the PWM control circuit in the voltage conversion circuit provided by the utility model;

[0018] Figure 4 This is a schematic diagram of a gate protection circuit in a voltage conversion circuit provided by the utility model. DETAILED DESCRIPTION

[0019] In order to make the purpose, technical solution and advantages of the utility model more clear, the utility model is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described here are only used to explain the utility model and are not used to limit the utility model.

[0020] See also Figure 1 and Figure 2 The utility model provides a fast charging adapter circuit, including an EMC circuit for input filtering and surge current protection of input alternating current, a rectifying and filtering circuit for rectifying and filtering alternating current, a transformer circuit for stepping down a direct current voltage, and a synchronous rectifying circuit for rectifying the transformed power supply. The transformer circuit includes a transformer T1, and the transformer T1 includes a primary coil, a first secondary coil, and a second secondary coil. The first end of the second secondary coil serves as the positive electrode of the direct current power supply VOUT2, and the synchronous rectifying circuit is arranged at the second end of the second secondary coil.

[0021] See also Figure 1The EMC circuit includes a fuse F1 in the live line L and a varistor in the neutral line N in series. The fuse F1 and the varistor are connected to a common-mode inductor L3 at the subsequent stage. The two coils of the common-mode inductor L3 are respectively connected in series in the live line L and the neutral line N. The subsequent stage of the common-mode inductor L3 is connected in parallel with an X capacitor C31. The subsequent stage of the common-mode inductor L3 is connected to a common-mode inductor L5.

[0022] The AC power is filtered and protected from surge current through fuse F1, common mode inductor L3, common mode inductor L5 and varistor.

[0023] When the AC power is connected, the current first passes through the fuse F1. The function of the fuse F1 is to melt in the case of overcurrent to protect the subsequent circuits.

[0024] When the power is just turned on, the resistance value of the varistor is higher, which can limit the surge current; under normal working conditions, the resistance value is reduced, reducing the impact on the circuit.

[0025] The common-mode inductor consists of two windings, which can filter the common-mode noise on the live wire and the neutral wire at the same time, thereby improving the circuit's anti-interference ability.

[0026] The current passes through the X capacitor C31, which is connected in parallel between the live wire and the neutral wire to provide differential mode filtering, mainly filtering out high-frequency noise.

[0027] Then the current passes through the common-mode inductor L5 and cooperates with the common-mode inductor L3 to form a two-stage common-mode filter circuit to further suppress common-mode noise.

[0028] Through multi-stage filtering and surge protection, it ensures that the subsequent circuits work in a clean and stable power supply environment, improving the reliability and anti-interference ability of the entire power supply system.

[0029] The rectifier and filter circuit includes a rectifier part and a filter part. The rectifier part includes two parallel rectifier bridges U2 and U3, which convert AC power into DC power and provide the DC voltage required by the subsequent circuit. The two rectifier bridges improve the load capacity.

[0030] The filtering part includes an inductor L7, an inductor L8 and a resistor R51, two parallel capacitors C32 and C33, and two parallel capacitors C34 and C35. The two ends of capacitor C32 and capacitor C33, capacitor C34 and capacitor C35 are respectively connected to the positive and negative poles of the direct current, and inductor L7 and inductor L8 are respectively connected in series to the negative and positive poles of the direct current line.

[0031] One end of capacitor C33 is connected to the first end of inductor L8, the other end of capacitor C33 is connected to the first end of inductor L7, one end of capacitor C34 is connected to the second end of inductor L8, the other end of capacitor C34 is connected to the second end of inductor L7.

[0032] Inductors L7 and L8 and capacitors C32, C33, C34, and C35 together form a π-type filter circuit, which helps to reduce ripple and noise in the DC output and make the output current smoother and more stable.

[0033] By connecting multiple capacitors in parallel (C32, C33 and C34, C35), the total capacitance value is increased, further improving the filtering effect. This configuration can effectively filter out high-frequency interference and low-frequency noise.

[0034] The presence of inductors L7 and L8 creates impedance to the DC current, thereby reducing current spikes and rapid changes during the filtering process and maintaining current continuity and stability.

[0035] The parallel capacitors (C32, C33 and C34, C35) can share the voltage and current, reducing the burden on a single capacitor, thereby extending the service life of the capacitor and improving the reliability of the circuit.

[0036] The π-type filter circuit composed of multiple inductors and capacitors effectively improves the stability of the power supply, reduces the fluctuation of the power supply output, and ensures that the subsequent circuits work in a stable power supply environment.

[0037] Through the above inductor and capacitor configuration, the filter circuit can effectively smooth the output current, improve the filtering efficiency, reduce the current peak and rapid changes, share the capacitor load and increase the stability and reliability of the circuit. This design makes the DC power supply output more stable and clean, and is suitable for electronic devices with high requirements for power quality.

[0038] See also Figure 4 The voltage conversion circuit is provided with an NMOS tube Q3, the second end of the primary coil is connected to the drain of the NMOS tube Q3, the source of the NMOS tube Q3 is grounded through a current detection resistor, a resistor R18 and a voltage regulator tube Z4 are respectively provided between the gate and the source of the NMOS tube Q3, the anode of the voltage regulator tube Z4 is connected to the source of the NMOS tube Q3, and the cathode of the voltage regulator tube Z4 is connected to the gate of the NMOS tube Q3;

[0039] The gate of the NMOS tube is connected to the driving end DRV of the PWM controller U1 through resistors R19 and R20 in sequence. A diode D10 is also provided. The cathode of the diode D10 is connected to the driving end DRV of the PWM controller U1. The anode of the diode D10 is connected to the gate of the NMOS tube Q3 through resistors R21 and capacitors C10 in sequence.

[0040] The circuit composed of diode D10, resistor R21, capacitor C10, resistor R20, resistor R19, voltage regulator Z4 and resistor R18 mainly protects the gate of NMOS tube Q3, adjusts the driving signal and limits the voltage, forms a gate protection circuit, and ensures the stable operation of NMOS tube Q3.

[0041] Gate drive signal conditioning:

[0042] The RC network composed of the diode D10, the resistor R21 and the capacitor C10 is used to adjust the rise and fall time of the gate drive signal of the NMOS tube Q3, to ensure the smooth switching of the NMOS tube Q3, and to avoid noise and oscillation generated during high-frequency switching.

[0043] D10 is turned on when the PWM control signal changes from low to high, quickly charging C10, causing the gate voltage to rise rapidly through R21, causing Q3 to turn on quickly.

[0044] When the PWM control signal changes from high to low, D10 is blocked, and the RC time constant of R21 and C10 controls the speed at which the gate voltage drops, preventing oscillation or spike interference from occurring during the shutdown process of Q3.

[0045] Gate voltage protection and current limiting:

[0046] The anode of the voltage regulator Z4 is grounded, and the cathode is connected to the gate of the NMOS tube Q3. This connection method can clamp the gate voltage to prevent the gate voltage from exceeding its rated value. The voltage regulator Z4 protects the gate from excessive voltage, ensuring that the NMOS tube operates within a safe voltage range to avoid breakdown damage.

[0047] When the gate voltage exceeds the regulated value, the voltage regulator Z4 is turned on to limit the gate voltage to a safe range.

[0048] Resistor R18 plays a current limiting role to prevent large current from directly impacting the gate, and at the same time cooperates with Zener diode Z4 for clamping protection.

[0049] The gate protection circuit protects and optimizes the gate drive of the NMOS tube Q3 through reasonable resistance voltage division, current limiting and voltage stabilization protection, as well as signal adjustment of the RC network, ensuring its stable and efficient operation in high-frequency switching power supply control.

[0050] See also Figure 3The first end of the PWM controller U1 is the fault feedback pin, which is connected to the anode of the voltage regulator tube Z3, and the cathode of the voltage regulator tube Z3 is connected to the power supply VCC2, and the first end is also grounded through a capacitor; the second end of the PWM controller U1 is the maximum operating frequency setting pin, which is grounded through a resistor R9; the third end of the PWM controller U1 is the feedback pin, which is grounded through a capacitor C5 and a phototransistor Q8 respectively; the fifth end of the PWM controller U1 is the current sampling end, which is grounded through a capacitor C3; the sixth end of the PWM controller U1 is the ground end, which is grounded; the seventh end of the PWM controller U1 is the driving end, which outputs a PWM wave; the eighth end of the PWM controller U1 is the power supply end, which is connected to the VCC2 power supply and grounded through a capacitor C7, and the tenth end of the PWM controller U1 is a high voltage pin; the fourth end of the PWM controller U1 is a zero crossing detection and over-power protection pin, which is grounded through a capacitor C4 and a resistor R8 respectively;

[0051] In order to power the PWM controller U1, a second secondary coil is introduced through the transformer T1 to provide an auxiliary power supply circuit, including an NPN-type transistor Q9 and a voltage regulator Z2. The emitter of the transistor Q9 is used to output the VCC2 power supply, the base of the transistor Q9 is connected to the cathode of the voltage regulator Z2, the anode of the voltage regulator Z2 is grounded, a resistor R10 is connected between the collector and the emitter of the transistor Q9, a capacitor C8 is connected between the collector of the transistor Q9 and the anode of the voltage regulator Z2, a capacitor C9 is connected between the emitter of the transistor Q9 and the anode of the voltage regulator Z2, a first end of the second secondary coil of the transformer T1 is connected to the collector of the transistor Q9 through a resistor R11 and a diode D3 in sequence, the anode of the diode D3 is connected to the resistor R11, the cathode of the diode D3 is connected to the collector of the transistor Q9, and the second end of the second secondary coil of the transformer T1 is grounded.

[0052] The first end of the second secondary coil of the transformer T1 is connected to the fourth terminal of the PWM controller U1 through the resistor R13 and the resistor R12 in sequence. A diode D4 is connected in parallel to both ends of the resistor R12, the cathode of the diode D4 is connected to the fourth terminal of the PWM controller U1, and the anode of the diode D4 is connected to the connection point of the resistor R13 and the resistor R12.

[0053] The transistor Q9, voltage regulator Z2, capacitor C8, resistor R10 and capacitor C9 circuit form a voltage regulator circuit, which is used to provide a stable VCC2 power supply. The transistor Q9 is mainly used for voltage regulation and serves as a control element for the output voltage; the voltage regulator Z2 provides a reference voltage, which is used to set the base voltage of the transistor Q9, thereby controlling the output voltage; capacitors C8 and C9 are used for filtering, smoothing the voltage regulation output, and reducing power supply noise; and resistor R10 provides a bias voltage.

[0054] The diode D3 and resistor R11 circuit is used to obtain power from the secondary coil of the transformer and provide voltage for the collector of Q9; the diode D3 converts the AC power of the secondary coil of the transformer into DC power; the resistor R11 ensures that the circuit and components are not affected by excessive current.

[0055] The circuit of resistor R13, resistor R12 and diode D4 is connected to the 4th terminal of PWM controller U1 to realize zero-crossing detection and over-power protection. Resistors R13, R12 and diode D4 are used to realize zero-crossing detection of the secondary coil voltage of the transformer, ensuring that the next switching cycle is driven when the secondary voltage drops to zero, and realizing over-power protection to prevent power supply overload damage.

[0056] The circuit of voltage regulator Z3 and phototransistor Q8 is connected to the first terminal of PWM controller U1 for fault detection and protection. Voltage regulator Z3 sets the fault detection voltage threshold. Phototransistor Q8 is used for feedback control and realizes isolated feedback through an optical coupler. Through voltage regulator Z3 and phototransistor Q8, the fault conditions in the system are monitored. When the voltage exceeds the set value, the protection mechanism is triggered to avoid circuit damage.

[0057] See also Figure 2 In the first secondary coil of the transformer T1, one end of the first secondary coil serves as the positive electrode of the DC power supply VOUT2, and the other end of the first secondary coil is provided with a synchronous rectification circuit, including an NMOS tube Q1 and a rectification controller U4, the drain of the NMOS tube Q1 is connected to the other end of the second secondary coil, the source of the NMOS tube Q1 serves as the negative electrode of the DC power supply VOUT2, and the gate of the NMOS tube Q1 is connected to the control end of the rectification controller U4.

[0058] The first end of the rectifier controller U4 is a high-voltage starting end, which is connected to one end of the first secondary coil; the second end of the rectifier controller U4 is a ground end, which is grounded; the third end of the rectifier controller U4 is a speed setting end, which is grounded through a resistor R75; the fourth end of the rectifier controller U4 is grounded through a capacitor C39; the fifth end of the rectifier controller U4 is a control end, which controls the switching state of the external NMOS tube Q1; the sixth end of the rectifier controller U4 is connected to the drain of the NMOS tube Q1 through a resistor R74.

[0059] Compared with traditional diode rectification, synchronous rectification uses NMOS tube Q1 as the rectification element. The on-resistance of NMOS tube is much lower than the forward voltage drop of diode, so it can significantly reduce the conduction loss and improve the rectification efficiency. Synchronous rectification can reduce power loss and improve the overall efficiency of the system. Since the on-resistance of NMOS tube is low, the conduction loss is reduced and the heat generated is also reduced accordingly. This not only improves the power conversion efficiency, but also improves the heat dissipation performance and reduces the need for heat sinks and fans. By reducing heat loss, the temperature rise of the circuit is reduced, and the life and reliability of electronic components are improved. The rectifier controller U4 optimizes the rectification process by precisely controlling the switching state of NMOS tube Q1, and can maintain efficient and stable rectification performance under different load conditions. The rectifier controller can respond quickly to voltage and current changes, improve the dynamic response capability of the system, and ensure the output stability and quality of the power supply.

[0060] A feedback circuit for feeding back to the PWM controller U1 is also provided, including a light emitting diode D18 and a three-terminal voltage regulator U5, wherein the cathode of the light emitting diode D18 is connected to the cathode of the three-terminal voltage regulator U5, the anode of the light emitting diode D18 is connected to one end of the first secondary coil through a resistor R7, the anode of the three-terminal voltage regulator U5 is grounded, a series voltage divider circuit consisting of a resistor R5 and a resistor R6 is provided, one end of the resistor R5 is connected to one end of the first secondary coil, the other end of the resistor R5 is connected to one end of the resistor R6, the other end of the resistor R6 is grounded, and the reference electrode of the three-terminal voltage regulator U5 is connected to the voltage dividing node (common node) of the resistor R5 and the resistor R6. The light emitting diode D18 and the phototransistor Q8 form a photoelectric coupler, which is used to feed back the voltage state of the second secondary coil to the PWM controller U1, and is composed of a photoelectric coupler and a three-terminal voltage regulator U5, which is responsible for detecting the output voltage and feeding it back to the PWM controller to adjust the switching frequency and duty cycle.

[0061] The photoelectric coupler composed of light-emitting diode D18 and phototransistor Q8 feeds back the output voltage change to PWM controller U1 in real time. The PWM controller adjusts the switching frequency and duty cycle according to the feedback signal to ensure that the output voltage is stable at the set value.

[0062] The three-terminal voltage regulator U5 ensures the stability of the feedback signal, reduces the impact of voltage fluctuations on the system, and improves the stability of the entire power supply system.

[0063] The feedback circuit can dynamically monitor the output voltage and adjust the working state of the PWM controller in time, thereby improving the power conversion efficiency. Especially when the load changes, it can respond and adjust the output quickly to maintain efficient operation.

[0064] The feedback circuit can monitor and feed back the output voltage. When an overvoltage condition is detected, the output voltage is reduced by adjusting the PWM controller U1 to avoid damage to the load equipment and improve system reliability.

[0065] Electrical isolation is achieved through the photocoupler to reduce noise interference, improve the purity of the feedback signal, and ensure that the PWM controller receives accurate feedback signals, thereby achieving stable output.

[0066] In summary, the utility model provides a fast charging adapter circuit. Compared with the traditional diode rectification, the synchronous rectification uses the NMOS tube Q1 as the rectification element, which can significantly reduce the conduction loss and improve the rectification efficiency and the overall efficiency of the system. Since the on-resistance of the NMOS tube is much lower than the forward voltage drop of the diode, the conduction loss and heat generation are greatly reduced, the heat dissipation performance is improved, the demand for heat sinks and fans is reduced, and the life and reliability of electronic components are improved. The rectifier controller U4 optimizes the rectification process by accurately controlling the switching state of the NMOS tube Q1, maintains efficient and stable rectification performance, and improves the dynamic response capability and output stability of the system.

[0067] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A fast charging adapter circuit, comprising an EMC circuit for input filtering and surge current protection of input AC power, a rectifying and filtering circuit for rectifying and filtering AC power, and a transformer circuit for stepping down DC voltage, characterized in that: It also includes a synchronous rectification circuit for rectifying the transformed power supply, the transformation circuit includes a transformer T1, the transformer T1 includes a primary coil, a first secondary coil and a second secondary coil, the first end of the second secondary coil serves as the positive electrode of the DC power supply VOUT2, and the synchronous rectification circuit is arranged at the second end of the second secondary coil; The synchronous rectification circuit includes an NMOS tube Q1 and a rectification controller U4, wherein the drain of the NMOS tube Q1 is connected to the second end of the second secondary coil, the source of the NMOS tube Q1 is grounded, and the gate of the NMOS tube Q1 is connected to the control end of the rectification controller U4; the power output end of the rectification controller U4 is connected to the first end of the first secondary coil, the ground end of the rectification controller U4 is grounded, the speed setting end of the rectification controller U4 is grounded through a resistor R75, the monitoring end of the rectification controller U4 is connected to the drain of the NMOS tube Q1 through a resistor R74, and the power input end of the rectification controller U4 is grounded through a capacitor C39.

2. A fast charging adapter circuit according to claim 1, characterized in that: The voltage conversion circuit includes an NMOS tube Q3 and a PWM controller U1 for driving the NMOS tube Q3. The second end of the primary coil is connected to the drain of the NMOS tube Q3. The source of the NMOS tube Q3 is grounded through a current-sense resistor. The gate of the NMOS tube Q3 is connected to the driving end of the PWM controller U1 through a gate protection circuit. The source of the NMOS tube Q3 is connected to the current sampling port of the PWM controller U1.

3. A fast charging adapter circuit according to claim 2, characterized in that: The gate protection circuit includes a diode D10, a resistor R21, a capacitor C10, a resistor R20, a resistor R19, a voltage regulator Z4 and a resistor R18. The resistor R18 and the voltage regulator Z4 are arranged between the gate and the source of the NMOS tube Q3. The anode of the voltage regulator Z4 is connected to the source of the NMOS tube Q3, and the cathode of the voltage regulator Z4 is connected to the gate of the NMOS tube Q3. The gate of the NMOS tube is connected to the driving end DRV of the PWM controller U1 through the resistor R19 and the resistor R20 in sequence, the cathode of the diode D10 is connected to the driving end DRV of the PWM controller U1, and the anode of the diode D10 is connected to the gate of the NMOS tube Q3 through the resistor R21 and the capacitor C10 in sequence.

4. A fast charging adapter circuit according to claim 3, characterized in that: The feedback pin of the PWM controller U1 is grounded through the capacitor C5 and the phototransistor Q8 respectively; a feedback circuit for feeding back to the PWM controller U1 is provided, including a light emitting diode D18 and a three-terminal voltage regulator U5, the cathode of the light emitting diode D18 is connected to the cathode of the three-terminal voltage regulator U5, the anode of the light emitting diode D18 is connected to one end of the first secondary coil through the resistor R7, the anode of the three-terminal voltage regulator U5 is grounded, and a series voltage divider circuit composed of a resistor R5 and a resistor R6 is provided, one end of the resistor R5 is connected to one end of the first secondary coil, the other end of the resistor R5 is connected to one end of the resistor R6, the other end of the resistor R6 is grounded, the reference electrode of the three-terminal voltage regulator U5 is connected to the voltage dividing node of the resistor R5 and the resistor R6, the light emitting diode D18 and the phototransistor Q8 constitute a photoelectric coupler, which is used to feed back the voltage state of the second secondary coil to the PWM controller U1.

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