Let's see! Detailed explanation of the components of a switching power supply[Copy link]
1. Circuit composition of switching power supplyThe 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:
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. It also prevents 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, after a certain period of time, the temperature rises and the RT1 resistance 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 startup will increase, Q1 will turn on and Q2 will not turn on due to the lack of gate voltage. RT1 will burn out in a very short time to protect the subsequent circuit.3. Power conversion circuit
1. Working principle of MOS tube: The most widely used insulated gate field effect tube is MOSFET (MOS tube), which works by using the electroacoustic effect on the surface of 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. The 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 diagram:
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 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. The larger its duty cycle is, the longer the Q1 conduction time is, and the more energy the transformer stores. When Q1 is cut off, the transformer releases energy through D1, D2, R5, R4, and C3, and at the same time achieves the purpose of resetting the magnetic field, preparing for the transformer to store and transfer energy next time.IC adjusts the duty cycle of the saw wave at pin ⑥ according to the output voltage and current, thus stabilizing the output current and voltage of the whole machine. C4 and R6 are the peak voltage absorption circuit.
4. Push-pull power conversion circuit: Q1 and Q2 will be turned on in turn.
T1 is a switching transformer, and the phases of its primary and secondary are in phase. D1 is a rectifier diode, D2 is a freewheeling diode, and R1, C1, R2, and C2 are a peak-cutting circuit. L1 is a freewheeling inductor, and C4, L2, and C5 form a π-type filter.
T1 is a switching transformer, and the phases of its primary and secondary are opposite. D1 is a rectifier diode, and R1 and C1 are a peak-cutting circuit. L1 is a freewheeling inductor, R2 is a dummy load, and C4, L2, and C5 form a π-type filter.
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 tube. 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.5. 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 at pin ③ of U1 increases. When it exceeds the reference voltage at pin ② of U1, pin ① of U1 outputs a high level, turning on Q1, the light-emitting diode of the optocoupler OT1, and the phototransistor, and the potential at pin ① of UC3842 becomes lower accordingly, thereby changing the output duty cycle of pin ⑥ of U1 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 light-emitting diode of the optocoupler OT1 does not emit light, the phototransistor is not turned on, and the potential of UC3842 pin 1 increases, thereby changing the output duty cycle of U1 pin 6 and increasing U0. This cycle repeats, so that 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, leaking, or poorly soldered, self-oscillation will occur. The fault phenomena are: abnormal waveform, oscillation under no load or full load, unstable output voltage, etc.6. 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 during 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 the 1st pin of UC3842 rises to about 5V, the voltage divided by R1 and R2 exceeds the TL431 reference, making it conducting, the VCC potential at the 7th pin of UC3842 is pulled down, and the IC stops working. After UC3842 stops working, the potential at the 1st pin disappears, the TL431 is not conducting, the potential at the 7th pin 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. Its principle is briefly described as follows:
When the output is short-circuited, the voltage of UC3842 pin ① rises, and the potential of U1 pin ③ is higher than that of pin ②, the comparator flips pin ① to output high potential, charging C1. When the voltage across C1 exceeds the reference voltage of pin ⑤, U1 pin ⑦ outputs low potential, UC3842 pin ① 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 not correct, 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 is 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 repeats this cycle. 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.8. 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 SCR (SCR1) receives the trigger voltage, so the SCR 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 SCR is discharged to the ground through R, and the SCR 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 the cycle repeats..
3. Output voltage limiting protection circuit:
The output voltage limiting protection circuit is as shown in the figure below. When the output voltage increases, the Zener diode is turned on, the optocoupler is turned on, the base of Q1 is driven by the voltage and is turned on, the voltage of UC3842③ increases, the output decreases, the Zener diode is not turned on, the voltage of UC3842③ decreases, and the output voltage increases. Repeating this process, the output voltage will be stable within a range (depending on the voltage regulation value of the Zener diode).
The working principle of Figure A is that when the output voltage Uo increases, the Zener diode 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 turned on. The Vcc voltage makes Q2 always turned on through R1, Q1, and R2, and the UC3842 pin ③ is always high and stops working.In Figure B, UO increases, the voltage at U1's pin 3 increases, and pin 1 outputs a high level. Due to the presence of D1 and R1, U1's pin 1 always outputs a high level, Q1 is always turned on, and UC3842's pin 1 is always at a low level and stops working. Positive feedback?9. 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 is sent to the PFC inductor through BRG1 rectification, and the other path is sent to the PFC controller as a sampling of the input voltage after being divided by R1 and R2 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 on and releases energy when Q1 is off. D1 is the start-up diode.D2 is the PFC rectifier diode, C6 and C7 are filters. One path of the PFC voltage is sent to the subsequent 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 to adjust the duty cycle of the control signal and stabilize the PFC output voltage.10. Input over-voltage and under-voltage protection
1. Schematic diagram:
2. Working principle:
The input over-voltage and under-voltage protection principles of AC input and DC input switching power supplies 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 being divided 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 divided by R7, R8, R9, and R10 and input to the comparator pin 6. If the sampled 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.
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