Detailed Interpretation of Switching Power Supply Circuit

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Detailed Interpretation of Switching Power Supply Circuit [Copy link]

The main circuit of the switching power supply is composed of input electromagnetic interference filter (EMI), rectifier filter circuit, power conversion circuit, PWM controller circuit, and output rectifier filter circuit. The auxiliary circuits include input over-voltage and under-voltage protection circuit, output over-voltage and under-voltage protection circuit, output over-current protection circuit, output short-circuit protection circuit, etc.

The circuit block diagram of the switching power supply is as follows:

Principles of input circuits and common circuits

1. Principle of AC input rectification and filtering circuit:

①, Lightning protection circuit: When there is a lightning strike and high voltage is generated and introduced into the power supply through the power grid, the circuit composed of MOV1, MOV2, MOV3: F1, F2, F3, FDG1 provides protection. When the voltage applied to the two ends of the varistor exceeds its working voltage, its resistance value decreases, causing the high voltage energy to be consumed in the varistor. If the current is too large, F1, F2, F3 will burn out to protect the subsequent circuit.

②. Input filter circuit: The double π-type filter network composed of C1, L1, C2, and C3 is mainly used to suppress the electromagnetic noise and clutter signals of the input power supply to prevent interference with the power supply, and also prevent the high-frequency clutter generated by the power supply itself from interfering with the power grid. When the power is turned on, C5 needs to be charged. Due to the large instantaneous current, adding RT1 (thermistor) can effectively prevent surge current. Because the instantaneous energy is completely consumed in the RT1 resistor, the temperature rises after a certain period of time and the resistance of RT1 decreases (RT1 is a negative temperature coefficient element). At this time, the energy it consumes is very small, and the subsequent circuit can work normally.

③. Rectification and filtering circuit: After the AC voltage is rectified by BRG1, it is filtered by C5 to obtain a relatively pure DC voltage. If the capacity of C5 becomes smaller, the output AC ripple will increase.

2. Principle of DC input filter circuit:

① Input filter circuit: The double π filter network composed of C1, L1, and C2 is mainly used to suppress the electromagnetic noise and clutter signals of the input power supply to prevent interference with the power supply, and also to prevent the high-frequency clutter generated by the power supply itself from interfering with the power grid. C3 and C4 are safety capacitors, and L2 and L3 are differential mode inductors.

②, R1, R2, R3, Z1, C6, Q1, Z2, R4, R5, Q2, RT1, C7 form an anti-surge circuit. At the moment of starting, Q2 is not turned on due to the existence of C6, and the current forms a loop through RT1. When the voltage on C6 is charged to the regulated voltage value of Z1, Q2 is turned on. If C8 leaks or the subsequent circuit is short-circuited, the voltage drop on RT1 caused by the instantaneous current at the start-up increases, Q1 is turned on, and Q2 is not turned on due to the lack of gate voltage. RT1 will burn out in a very short time to protect the subsequent circuit.

Power conversion circuit

1. The working principle of MOS tube:

The most widely used insulated gate field effect transistor is MOSFET (MOS tube), which works by using the electroacoustic effect on the surface of the semiconductor. It is also called surface field effect device. Since its gate is in a non-conductive state, the input resistance can be greatly increased, up to 105 ohms. MOS tube uses the size of the gate-source voltage to change the amount of induced charge on the semiconductor surface, thereby controlling the size of the drain current.

2. Common schematic diagrams:

3. Working principle:

R4, C3, R5, R6, C4, D1, and D2 form a buffer, which is connected in parallel with the switch MOS tube to reduce the voltage stress of the switch tube, reduce EMI, and prevent secondary breakdown. When the switch tube Q1 is turned off, the primary coil of the transformer is prone to generate spike voltage and spike current. These components combined together can absorb the spike voltage and current well. The current peak signal measured from R3 participates in the duty cycle control of the current working cycle, so it is the current limit of the current working cycle. When the voltage on R5 reaches 1V, UC3842 stops working and the switch tube Q1 is immediately turned off. R1 and the junction capacitors CGS and CGD in Q1 together form an RC network. The charging and discharging of the capacitor directly affects the switching speed of the switch tube. If R1 is too small, it is easy to cause oscillation and electromagnetic interference will also be large; if R1 is too large, it will reduce the switching speed of the switch tube. Z1 usually limits the GS voltage of the MOS tube to below 18V, thereby protecting the MOS tube. The gate controlled voltage of Q1 is a saw-shaped wave. When its duty cycle is larger, the longer the Q1 conduction time is, the more energy the transformer stores. When Q1 is turned off, the transformer releases energy through D1, D2, R5, R4, and C3, and also achieves the purpose of magnetic field reset, preparing for the transformer to store and transfer energy next time. The IC adjusts the duty cycle of the saw-shaped wave of pin ⑥ according to the output voltage and current, thereby stabilizing the output current and voltage of the whole machine. C4 and R6 are peak voltage absorption circuits.

4. Push-pull power conversion circuit:

Q1 and Q2 will turn on in turn.

5. Power conversion circuit with drive transformer:

T2 is the driving transformer, T1 is the switching transformer, and TR1 is the current loop.

Output rectifier filter circuit

1. Forward rectifier circuit:

T1 is a switching transformer, the phases of its primary and secondary are in phase. D1 is a rectifier diode, D2 is a freewheeling diode, R1, C1, R2, C2 are a peak clipping circuit. L1 is a freewheeling inductor, and C4, L2, C5 form a π-type filter.

2. Flyback rectifier circuit:

T1 is a switching transformer, the phases of the primary and secondary poles are opposite. D1 is a rectifier diode, R1 and C1 are peak clipping circuits. L1 is a freewheeling inductor, R2 is a dummy load, and C4, L2, and C5 form a π-type filter.

3. Synchronous rectification circuit:

Working principle: When the upper end of the transformer secondary is positive, the current passes through C2, R5, R6, and R7 to turn on Q2, and the circuit forms a loop. Q2 is a rectifier. The gate of Q1 is cut off because it is in reverse bias. When the lower end of the transformer secondary is positive, the current passes through C3, R4, and R2 to turn on Q1, and Q1 is a freewheeling tube. The gate of Q2 is cut off because it is in reverse bias. L2 is a freewheeling inductor, and C6, L1, and C7 form a π-type filter. R1, C1, R9, and C4 are a peak clipping circuit.

Voltage stabilization loop principle

1. Feedback circuit schematic diagram:

2. Working principle:

When the output U0 increases, after the sampling resistors R7, R8, R10, and VR1 divide the voltage, the voltage of U1 pin ③ increases. When it exceeds the reference voltage of U1 pin ②, U1 pin ① outputs a high level, Q1 is turned on, the optocoupler OT1 light-emitting diode is turned on, the phototransistor is turned on, and the potential of UC3842 pin ① becomes lower accordingly, thereby changing the output duty cycle of U1 pin ⑥ to decrease and U0 to decrease. When the output U0 decreases, the voltage of U1 pin ③ decreases. When it is lower than the reference voltage of U1 pin ②, U1 pin ① outputs a low level, Q1 is not turned on, the optocoupler OT1 light-emitting diode is not turned on, the phototransistor is not turned on, and the potential of UC3842 pin ① increases, thereby changing the output duty cycle of U1 pin ⑥ to increase and U0 to decrease. Repeating the cycle, the output voltage remains stable. Adjusting VR1 can change the output voltage value.

The feedback loop is an important circuit that affects the stability of the switching power supply. If the feedback resistor and capacitor are wrong, missing, or poorly soldered, self-oscillation will occur. The fault phenomena are: abnormal waveform, oscillation under no load or full load, unstable output voltage, etc.

Short circuit protection circuit

1. In the case of a short circuit at the output end, the PWM control circuit can limit the output current to a safe range. It can use a variety of methods to implement the current limiting circuit. When the power current limiting does not work in the case of a short circuit, only another part of the circuit can be added.

2. There are usually two types of short-circuit protection circuits. The following figure is a low-power short-circuit protection circuit. Its principle is briefly described as follows:

When the output circuit is short-circuited, the output voltage disappears, the optocoupler OT1 is not conducting, the voltage at pin 1 of UC3842 rises to about 5V, the voltage divided by R1 and R2 exceeds the reference of TL431, making it conducting, the VCC potential at pin 7 of UC3842 is pulled down, and the IC stops working. After UC3842 stops working, the potential at pin 1 disappears, TL431 is not conducting, the potential at pin 7 of UC3842 rises, and UC3842 restarts, and the cycle repeats. When the short circuit disappears, the circuit can automatically return to normal working state.

3. The figure below is a medium power short circuit protection circuit, and its principle is briefly described as follows:

When the output is short-circuited, the voltage on pin 1 of UC3842 rises, and the potential on pin 3 of U1 is higher than that on pin 2, the comparator flips pin 1 to output a high potential, charging C1. When the voltage across C1 exceeds the reference voltage on pin 5, pin 7 of U1 outputs a low potential, pin 1 of UC3842 is lower than 1V, UCC3842 stops working, and the output voltage is 0V, and the cycle repeats. When the short circuit disappears, the circuit works normally. R2 and C1 are the charging and discharging time constants. If the resistance value is incorrect, the short circuit protection will not work.

4. The following figure is a common current limiting and short circuit protection circuit. Its working principle is briefly described as follows:

When the output circuit is short-circuited or overcurrent occurs, the primary current of the transformer increases, the voltage drop across R3 increases, the voltage at pin ③ increases, and the output duty cycle of pin ⑥ of UC3842 gradually increases. When the voltage at pin ③ exceeds 1V, UC3842 is turned off and has no output.

5. The figure below shows a protection circuit that uses a current transformer to sample current. It has low power consumption, but high cost and a relatively complex circuit. Its working principle is briefly described as follows:

If the output circuit is short-circuited or the current is too large, the voltage induced by the TR1 secondary coil will be higher. When the UC3842 pin ③ exceeds 1 volt, UC3842 stops working, and the cycle repeats. When the short circuit or overload disappears, the circuit recovers automatically.

7. Output current limiting protection

The figure above is a common output current limiting protection circuit. Its working principle is briefly described as above: when the output current is too large, the voltage across RS (manganese copper wire) rises, the voltage at pin ③ of U1 is higher than the reference voltage at pin ②, pin ① of U1 outputs a high voltage, Q1 is turned on, the optocoupler produces a photoelectric effect, the voltage at pin ① of UC3842 decreases, and the output voltage decreases, thereby achieving the purpose of output overload current limiting.

Principle of output overvoltage protection circuit

The function of the output overvoltage protection circuit is to limit the output voltage to a safe value when the output voltage exceeds the design value. When the internal voltage stabilization loop of the switching power supply fails or the output overvoltage phenomenon is caused by improper operation of the user, the overvoltage protection circuit will protect to prevent damage to the subsequent electrical equipment. The most commonly used overvoltage protection circuits are as follows:

1. SCR trigger protection circuit:

As shown in the figure above, when the output of Uo1 increases, the voltage regulator (Z3) breaks down and turns on, and the control end of the thyristor (SCR1) receives the trigger voltage, so the thyristor turns on. The voltage of Uo2 is short-circuited to the ground, and the overcurrent protection circuit or short-circuit protection circuit will work, stopping the operation of the entire power supply circuit. When the output overvoltage phenomenon is eliminated, the control end trigger voltage of the thyristor is discharged to the ground through R, and the thyristor returns to the disconnected state.

2. Photoelectric coupling protection circuit:

As shown in the figure above, when Uo has an overvoltage phenomenon, the voltage regulator tube breaks down and conducts, and current flows through the optocoupler (OT2) R6 to the ground, and the light-emitting diode of the photocoupler emits light, thereby turning on the photosensitive transistor of the photocoupler. The base of Q1 is turned on, and the voltage of pin 3 of 3842 is reduced, so that the IC is turned off, and the entire power supply stops working. Uo is zero, and it repeats over and over again.

3. Output voltage limiting protection circuit:

The output voltage limiting protection circuit is shown in the figure below. When the output voltage increases, the voltage regulator tube is turned on, the optocoupler is turned on, the base of Q1 is driven by voltage and the channel is turned on, the voltage of UC3842③ increases, the output decreases, the voltage regulator tube is not turned on, the voltage of UC3842③ decreases, and the output voltage increases. Repeating this cycle, the output voltage will be stable within a range (depending on the voltage regulation value of the voltage regulator tube).

4. Output overvoltage lock circuit:

The working principle of Figure A is that when the output voltage Uo increases, the voltage regulator is turned on, the optocoupler is turned on, and the base of Q2 is turned on. Due to the conduction of Q2, the base voltage of Q1 is reduced and it is also turned on. The Vcc voltage makes Q2 always turned on through R1, Q1, and R2. The 3rd pin of UC3842 is always high and stops working. In Figure B, UO increases, the voltage of the 3rd pin of U1 increases, and the 1st pin outputs a high level. Due to the existence of D1 and R1, the 1st pin of U1 always outputs a high level, Q1 is always turned on, and the 1st pin of UC3842 is always low and stops working. Positive feedback?

Power Factor Correction Circuit (PFC)

1. Schematic diagram:

2. Working principle:

The input voltage is rectified by L1, L2, L3 and other components of the EMI filter. One path of the BRG1 rectifier is sent to the PFC inductor, and the other path is sent to the PFC controller as a sample of the input voltage after being divided by R1 and R2, which is used to adjust the duty cycle of the control signal, that is, to change the on and off time of Q1 and stabilize the PFC output voltage. L4 is the PFC inductor, which stores energy when Q1 is turned on and releases energy when Q1 is turned off. D1 is the start diode. D2 is the PFC rectifier diode, and C6 and C7 are used for filtering. One path of the PFC voltage is sent to the post-stage circuit, and the other path is sent to the PFC controller as a sample of the PFC output voltage after being divided by R3 and R4, which is used to adjust the duty cycle of the control signal and stabilize the PFC output voltage.

Input over-voltage and under-voltage protection

1. Schematic diagram:

2. Working principle:

The input over-voltage and under-voltage protection principles of switching power supplies with AC input and DC input are roughly the same. The sampling voltage of the protection circuit comes from the input filtered voltage. The sampling voltage is divided into two paths. One path is input to the comparator pin 3 after voltage division by R1, R2, R3, and R4. If the sampling voltage is higher than the reference voltage at pin 2, the comparator pin 1 outputs a high level to control the main controller to shut down, and the power supply has no output. The other path is input to the comparator pin 6 after voltage division by R7, R8, R9, and R10. If the sampling voltage is lower than the reference voltage at pin 5, the comparator pin 7 outputs a high level to control the main controller to shut down, and the power supply has no output.

END

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