“The main circuit of the switching power supply is composed of an input electromagnetic interference filter (EMI), a rectifier filter circuit, a power conversion circuit, a PWM controller circuit, and an output rectifier filter circuit. The auxiliary circuits include input over-voltage and under-voltage protection circuits, output over-voltage and under-voltage protection circuits, output over-current protection circuits, and output short-circuit protection circuits.
First, the circuit composition of the switching power supply
The main circuit of the switching power supply is composed of an input electromagnetic interference filter (EMI), a rectifier filter circuit, a power conversion circuit, a PWM controller circuit, and an output rectifier filter circuit. The auxiliary circuits include input over-voltage and under-voltage protection circuits, output over-voltage and under-voltage protection circuits, output over-current protection circuits, and output short-circuit protection circuits.
The circuit composition block diagram of the switching power supply is as follows:
Second, the principle and common circuit of the input circuit
1. Principle of AC input rectifier filter circuit:
①Lightning protection circuit: When there is a lightning strike and high voltage is generated and imported into the power supply through the power grid, the circuit composed of MOV1, MOV2, MOV3: F1, F2, F3, and FDG1 is used for protection. When the voltage applied to both ends of the varistor exceeds its working voltage, its resistance value decreases, so that high-voltage energy is consumed on the varistor. If the current is too large, F1, F2, and F3 will burn and protect the subsequent circuit.
②Input filter circuit: The double π-type filter network composed of C1, L1, C2, and C3 mainly suppresses the electromagnetic noise and clutter signal of the input power supply, prevents interference to the power supply, and also prevents high-frequency clutter generated by the power supply itself. Interference with the grid. When the power is turned on, it is necessary to charge C5. Due to the large instantaneous current, adding RT1 (thermistor) can effectively prevent inrush current. Because the instantaneous energy is all consumed on the RT1 resistance, the resistance value of RT1 decreases after the temperature rises after a certain period of time (RT1 is a negative temperature coefficient element), at this time, it consumes very little energy, and the subsequent circuit can work normally.
③ Rectifier and filter circuit: After the AC voltage is rectified by BRG1 and filtered by C5, a relatively pure DC voltage is obtained. If the capacity of C5 becomes smaller, the output AC ripple will increase.
2. Principle of DC input filter circuit:
①Input filter circuit: The double π-type filter network composed of C1, L1, and C2 is mainly to suppress the electromagnetic noise and clutter signal of the input power supply, prevent interference to the power supply, and also prevent the high-frequency clutter generated by the power supply itself from affecting the power grid. interference. 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 conducting due to the existence of C6, and the current forms a loop through RT1. When the voltage on C6 is charged to the regulated value of Z1, Q2 is turned on. If C8 is leaking or the circuit of the latter stage is short-circuited, the voltage drop generated by the current on RT1 increases at the moment of starting, and Q1 is turned on so that Q2 does not conduct without the gate voltage, RT1 will be burned in a very short time, so that the Protect the back-end circuit.
3. Power conversion circuit
1. The working principle of MOS tube:
At present, the most widely used insulated gate field effect transistor is the MOSFET (MOS tube), which works by using the electroacoustic effect on the surface of the semiconductor. Also known as surface field effect devices. Since its gate is in a non-conductive state, the input resistance can be greatly improved, up to 105 ohms. The MOS tube uses the gate-source voltage to change the amount of charge induced on the surface of the semiconductor, thereby controlling the drain current. .
2. Common schematic diagram:
3. Working principle:
R4, C3, R5, R6, C4, D1, D2 form a buffer, which is connected in parallel with the switch MOS tube, so that the voltage stress of the switch tube is reduced, EMI is reduced, and no secondary breakdown occurs. When the switch tube Q1 is turned off, the primary coil of the transformer is prone to generate peak voltage and peak current. These components are combined to absorb the peak 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 turned off immediately. The junction capacitors CGS and CGD in R1 and Q1 form an RC network together, and the charging and discharging of the capacitors directly affect the switching speed of the switch. If R1 is too small, it is easy to cause oscillation, and the electromagnetic interference will be very large; if R1 is too large, the switching speed of the switch tube will be reduced. Z1 usually limits the GS voltage of the MOS tube to below 18V, thus protecting the MOS tube.
The gate controlled voltage of Q1 is a saw-shaped wave. When its duty cycle is larger, the conduction time of Q1 is longer, and the energy stored by the transformer is more; when Q1 is turned off, the transformer passes through D1, D2, R5 , R4, C3 release energy, and at the same time achieve the purpose of magnetic field reset, and prepare for the next energy storage and transfer of the transformer. According to the output voltage and current, the IC adjusts the duty cycle of the saw wave of the ⑥ pin, thus stabilizing the output current and voltage of the whole machine. C4 and R6 are the peak voltage absorption circuits.
4. Push-pull power conversion circuit:
Q1 and Q2 will be turned 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.
Fourth, the output rectifier filter circuit:
1. Forward rectifier circuit:
T1 is a switching transformer whose primary and secondary phases are in phase. D1 is a rectifier diode, D2 is a freewheeling diode, and R1, C1, R2, and C2 are peak clipping circuits. L1 is a freewheeling Inductor, and C4, L2, and C5 form a π-type filter.
2. Flyback rectifier circuit:
T1 is a switching transformer whose primary and secondary phases are opposite. D1 is a rectifier diode, and 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 rectifier circuit:
Working principle: When the upper end of the transformer secondary is positive, the current passes through C2, R5, R6, R7 to make Q2 conduct, the circuit forms a loop, and Q2 is a rectifier tube. The gate of Q1 is turned off due to being reverse biased. When the lower end of the transformer secondary is positive, the current passes through C3, R4, and R2 to make Q1 conduct, and Q1 is a freewheeling tube. The gate of Q2 is turned off due to being reverse biased. L2 is a freewheeling inductor, and C6, L1, and C7 form a π-type filter. R1, C1, R9, and C4 are peak clipping circuits.
5. Principle of voltage regulation loop
1. Schematic diagram of the feedback circuit:
2. Working principle:
When the output U0 rises, after the sampling resistors R7, R8, R10 and VR1 divide the voltage, the voltage of pin U1 ③ rises. When it exceeds the reference voltage of pin U1 ②, pin U1 ① outputs a high level, which makes Q1 turn on, and the optocoupler OT1 emits light. The diode emits light, the phototransistor is turned on, and the potential of the UC3842 pin ① becomes lower accordingly, thereby changing the output duty ratio of the U1 ⑥ pin and reducing the U0. When the output U0 decreases, the voltage of pin U13 decreases, when it is lower than the reference voltage of pin U12, pin U11 outputs a low level, Q1 does not conduct, the light-emitting diode of the optocoupler OT1 does not emit light, the phototransistor does not conduct, and the potential of pin UC38421 rises High, thus changing the output duty ratio of U1⑥ pin to increase, and U0 to decrease. cycle, 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. Such as feedback resistor capacitance error, leakage, virtual welding, etc., will produce self-excited oscillation, the fault phenomenon is: abnormal waveform, empty, full load oscillation, unstable output voltage and so on.
6. Short circuit protection circuit
1. In the case of short-circuit at the output end, the PWM control circuit can limit the output current within a safe range, and it can implement the current-limiting circuit in a variety of ways. Add some circuits.
2. There are usually two types of short-circuit protection circuits. The following figure is a small-power short-circuit protection circuit. The principle is briefly described as follows:
When the output circuit is short-circuited, the output voltage disappears, the optocoupler OT1 does not conduct, the voltage at pin 1 of UC3842 rises to about 5V, the voltage divider between R1 and R2 exceeds the reference of TL431, making it conductive, the potential of pin 7 of UC3842 is pulled down, and the IC stops working . After the UC3842 stops working, the potential of the ① pin disappears, the TL431 is not conducting, the potential of the UC3842 ⑦ pin rises, the UC3842 restarts, and the cycle begins again and again. When the short-circuit phenomenon disappears, the circuit can automatically return to the normal working state.
3. The following figure is the medium power short circuit protection circuit, and its principle is briefly described as follows:
When the output is short-circuited, the voltage of pin ① of UC3842 rises, and the potential of pin ③ of U1 is higher than that of pin ②, the comparator flips pin ① to output a high potential to charge C1, when the voltage across C1 exceeds the reference voltage of pin ⑤, pin U1 ⑦ outputs a low potential, UC3842① When the pin is lower than 1V, the UCC3842 stops working, the output voltage is 0V, and the cycle starts again and again. When the short circuit disappears, the circuit works normally. R2 and C1 are charge and discharge time constants, short-circuit protection does not work when the resistance value is incorrect.
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, the primary current of the transformer increases, the voltage drop across R3 increases, the voltage at pin ③ increases, and the output duty ratio of pin 6 of UC3842 increases gradually. When the voltage at pin ③ exceeds 1V, the UC3842 turns off and has no output. .
5. The following figure is a protection circuit that uses a current transformer to sample current. It has low power consumption, but high cost and complicated circuit. Its working principle is briefly described as follows:
When 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, the UC3842 will stop working, and the cycle will resume. When the short-circuit or overload disappears, the circuit will recover by itself.
7. Output current limiting protection
The above picture is a common output current limiting protection circuit, and 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 of U1 ③ pin is higher than the reference voltage of ② pin, U1 ① pin Output high voltage, Q1 is turned on, the optocoupler produces photoelectric effect, the voltage of pin ① of UC3842 decreases, and the output voltage decreases, so as to achieve the purpose of output overload current limiting.
Eight, the principle of output overvoltage protection circuit
The function of the output overvoltage protection circuit is to limit the output voltage within a safe value range when the output voltage exceeds the design value. When the internal voltage regulation loop of the switching power supply fails or the output overvoltage occurs due to improper operation by the user, the overvoltage protection circuit protects to prevent damage to the electrical equipment at the later stage. 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 rises, the voltage regulator tube (Z3) breaks down and conducts, and the control terminal of the thyristor (SCR1) gets the trigger voltage, so the thyristor is turned on. If the Uo2 voltage is shorted to ground, the overcurrent protection circuit or the short circuit protection circuit will work and stop the operation of the entire power supply circuit. When the output overvoltage phenomenon is eliminated, the trigger voltage of the control terminal of the thyristor is discharged to the ground through R, and the thyristor returns to the off state.
2. Photoelectric coupling protection circuit:
As shown in the figure above, when there is an overvoltage phenomenon in Uo, 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 optocoupler emits light, so that the phototransistor of the optocoupler conducts Pass. The base of Q1 is turned on, and the power of the 3-pin of 3842 is reduced, so that the IC is turned off and the whole power supply is stopped. Uo is zero, and the cycle starts again and again.
3. Output voltage limiting protection circuit:
The output voltage limiting protection circuit is shown in the figure below. When the output voltage rises, the Zener tube is turned on and the optocoupler is turned on. The base of Q1 has a driving voltage and the channel is turned on. The voltage of UC3842③ rises, the output decreases, and the Zener tube is not turned on. UC3842③ The voltage decreases and the output voltage increases. Over and over again, the output voltage will stabilize within a range (depending on the voltage regulator value of the Zener tube).
4. Output overvoltage lockout circuit:
The working principle of Figure A is that when the output voltage Uo increases, the voltage regulator tube 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 also turned on, and the Vcc voltage is turned on through R1. , Q1, R2 make Q2 always on, UC3842 ③ pin is always high and stops working. In Figure B, when UO rises, the voltage of pin ③ of U1 increases, and pin 1 outputs a high level. Due to the existence of D1 and R1, pin U1 1 always outputs a high level, Q1 is always on, and pin 1 of UC3842 is always low and stops working . Positive feedback?
Nine, power factor correction circuit (PFC)
1. Schematic diagram of the principle:
2. Working principle:
The input voltage is EMI filter composed of L1, L2, L3, etc. BRG1 rectifies one way to send the PFC inductor, the other way is divided by R1, R2 and sent to the PFC controller as the sampling of the input voltage to adjust the duty of the control signal ratio, that is, changing the turn-on and turn-off time of Q1 to stabilize the PFC output voltage. L4 is the PFC inductor that stores energy when Q1 is on and releases energy when Q1 is off. D1 is the startup diode. D2 is PFC rectifier diode, C6, C7 filter. One PFC voltage is sent to the post-stage circuit, and the other is divided by R3 and R4 and sent to the PFC controller as a sampling of the PFC output voltage to adjust the duty cycle of the control signal and stabilize the PFC output voltage.
10. Input overvoltage and undervoltage protection
1. Schematic diagram:
2. Working principle:
The input overvoltage and undervoltage protection principles of AC input and DC input switching power supply are roughly the same. The sampling voltage of the protection circuit comes from the input filtered voltage. The sampling voltage is divided into two channels, one of which is divided by R1, R2, R3, R4 and then input to the comparator pin 3. If the sampling voltage is higher than the reference voltage of pin 2, the comparator pin 1 outputs a high level to control the main controller to make it Off, the power supply has no output. The other path is divided by R7, R8, R9 and R10 and then input to the comparator pin 6. If the sampling voltage is lower than the reference voltage of pin 5, the comparator pin 7 outputs a high level to control the main controller to turn it off, and the power supply has no output. .