The composition of switching power supply circuit and a detailed explanation of general circuits

1. The composition of switching power supply circuit

The main circuit of switching power supply consists of an input EMI filter, a rectifier filter circuit, a power conversion circuit, a PWM controller circuit, and an output rectifier filter circuit. The auxiliary circuit includes input overvoltage and undervoltage protection circuit, output overvoltage and undervoltage protection circuit, output overcurrent protection circuit, output short circuit protection circuit, etc. Block- switching power supply circuit diagram is as follows:

Second, principle of input circuit and common circuits

1. AC input rectifier filter circuit principle:

①Lightning protection circuit: When lightning strikes and high voltage generated and imported into power supply through grid, circuit consisting of MOV1, MOV2, MOV3: F1, F2, F3 and FDG1 will protect it. When voltage applied to both ends of varistor exceeds its operating voltage, its resistance decreases, resulting in high voltage consumption of varistor. If current is too large, F1, F2 and F3 will burn out and protect varistor. subsequent circuit.

②Input filter circuit: double π-type filter network, consisting of C1, L1, C2 and C3, mainly suppresses electromagnetic interference and interference signals of input power supply, prevents power supply interference, and also prevents high noise. generated by power supply itself. Frequency interference interferes with operation of power system. When power is on, C5 needs to be charged. Due to large instantaneous current, adding RT1 (thermistor) can effectively prevent current surge. Since all instantaneous energy is spent on resistor RT1, value of resistance RT1 decreases after a certain period of time when temperature rises (RT1 is an element with a negative temperature coefficient), during which time it consumes very little power, and subsequent circuit can work normally. ③ Rectification and filtering circuit: after AC voltage is rectified by BRG1, it is filtered by capacitor C5 to obtain a relatively pure DC voltage. If capacitance C5 becomes smaller, AC output ripple will increase. 2. Working principle of DC input filter circuit:

①Input filter circuit: double π-type filter network, consisting of C1, L1 and C2, mainly suppresses electromagnetic interference and interference signals of input power supply, prevents power supply interference, and also prevents high-frequency interference. waves generated by power supply itself interfere with power system. C3 and C4 are protective capacitors, and L2 and L3 are differential inductors.

②R1, R2, R3, Z1, C6, Q1, Z2, R4, R5, Q2, RT1, C7 form an overvoltage protection circuit. At moment of starting, due to presence of C6, Q2 is not turned on, and current forms a loop through RT1. Q2 turns on when voltage at C6 is charged to regulated value of Z1. If C8 electricity leaks or circuit of subsequent stage is short-circuited, voltage drop generated by current on RT1 increases at moment of starting, and Q1 turns on so that Q2 does not conduct without gate voltage, and RT1 will burn out in a short time, so protect subsequent circuit.

3. Energy Conversion Scheme

1. Working principle of MOS tube:

Currently, most widely used insulated gate FET is MOSFET (MOSFET), which uses an electro-acoustic effect on surface of a semiconductor. Also known as surface field effect devices. Because its gate is in a non-conducting state, input resistance can be greatly increased, up to 105 ohms. The MOS lamp uses gate-to-source voltage to change amount of charge induced on surface of semiconductor, thereby controlling drain current..

2. General schematic diagram:

3. How it works:

R4, C3, R5, R6, C4, D1 and D2 form a buffer and are connected in parallel to switch MOS tube, so that switch tube voltage is reduced, electromagnetic interference is reduced, and secondary failure does not occur. When switching tube Q1 is turned off, primary winding of transformer tends to generate peak voltage and peak current. The combination of these components can absorb peak voltage and current well. The current peak signal measured from R3 participates in duty cycle control of current duty cycle, so it is current limit of current duty cycle. When voltage across R5 reaches 1V, UC3842 stops working and Q1 switch tube turns off immediately. The junction capacitances CGS and CGD in R1 and Q1 together form an RC circuit, and charging and discharging of capacitance directly affects switching speed of switching tube. If R1 is too small, it will easily cause fluctuations, and electromagnetic interference will be large; if R1 is too large, switching speed of switching tube will be reduced. Z1 normally limits GS voltage of MOS tube below 18V, thereby protecting MOS tube. The voltage controlled by gate of Q1 is a sawtooth wave. The larger duty cycle, longer conduction time of Q1, more energy will be stored in transformer, when Q1 is disconnected, transformer will pass through D1, D2, R5, R4 and C3 release energy, and at same time achieve purpose of resetting magnetic field, preparing for next accumulation and transfer of energy of transformer. Depending on output voltage and current, IC continuously adjusts duty cycle of sawtooth wave at terminal ⑥, thereby stabilizing output current and voltage of entire machine. C4 and R6 are 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 control transformer, T1 is switching transformer, and TR1 is current loop.

Fourth, output rectifier filter circuit

1. Direct rectification scheme:

T1 is a switching transformer, its primary and secondary poles are in phase. D1 is a rectifier diode, D2 is a flyback diode, and R1, C1, R2, and C2 are peak limiting circuits. L1 is a free inductance and C4, L2 and C5 form a π-type filter.

2. Flyback rectifier circuit:

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

3. Synchronous rectification scheme:

Working principle: when upper end of secondary of transformer is positive, current flows through C2, R5, R6 and R7 to make Q2 conductive, circuit forms a loop, and Q2 is a rectifier tube. The gate of Q1 is off because it is reverse biased. When lower end of transformer secondary is positive, current flows through C3, R4 and R2 to make Q1 conductive and Q1 is a freewheel. Gate Q2 is off because it is reverse biased. L2 is a free inductance and C6, L1 and C7 form a π-type filter. R1, C1, R9 and C4 are peak limit circuits.

Five. Voltage regulation loop principle

1. Schematic diagram of the feedback circuit:

2. How it works:

When output signal U0 rises, after sampling resistors R7, R8, R10 and VR1 divide voltage, voltage at U1 terminal ③ rises, and when it exceeds reference voltage at U1 terminal ②, U1 terminal ① outputs a high level, creating conductivity Q1, and light. The coupling light emitting diode OT1 emits light, phototransistor turns on, and potential at UC3842 pin ① decreases accordingly, thereby changing output duty cycle of U1 terminal ⑥ to decrease, and U0 decreases. When output U0 decreases, voltage at U1③ decreases, and when it is lower than reference voltage at U1②, U1① outputs low, Q1 does not conduct, light emitting diode of optocoupler OT1 does not emit light, phototransistor does not conduct, and terminal potential UC3842 ① rises to a high level, resulting in a change in duty cycle of output loop at U1 pin ⑥ increases, U0 decreases. Several times to keep output voltage stable. Adjusting VR1 can change value of output voltage. The feedback loop is an important circuit that affects stability of a switching power supply. If feedback resistor and capacitor are incorrect, leaking or welding, etc., self-excited oscillations will occur, and malfunction phenomena: abnormal waveform, oscillations at empty and full load, unstable output voltage, etc.

6. Short circuit protection circuit

1. In event of a short circuit at output terminal, PWM control circuit can limit output current to a safe range. It can use various methods to implement current limiting circuit. When current limit does not work with a short circuit, only another part of circuit is added. 2. There are usually two types of short circuit protection circuits. The following figure shows a low power short circuit protection circuit. The principle is briefly described as follows:

When output circuit is shorted, output voltage disappears, optocoupler OT1 does not conduct, voltage at UC3842 pin ① rises to about 5V, divided voltage of R1 and R2 exceeds reference value of TL431, making it conductive. , UC3842 ⑦ VCC contact potential decreases, IC stops working. After UC3842 stops working, ① pin potential disappears, TL431 does not conduct UC3842 ⑦ pin rises, UC3842 restarts, and cycle repeats. When short circuit phenomenon disappears, circuit can automatically return to normal working state.

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

When output is shorted, voltage at pin ① of UC3842 rises, and when potential at pin ③ of U1 is higher than that at pin ②, comparator reverses pin ① and outputs a high potential to charge C1. C1 exceeds reference voltage of pin ⑤, U1 pin ⑦ is low potential, pin ① of UC3842 is lower than 1V, UCC3842 stops working, output voltage is 0V, and cycle repeats When short circuit disappears, circuit works normally. R2 and C1 are charge and discharge time constants, and short circuit protection will not work if resistance value is wrong.

4. The figure below shows general circuit of current limiting and short circuit protection. Its working principle is briefly described as follows:

When output circuit has a short circuit or overcurrent, primary current of transformer increases, voltage drop across resistor R3 increases, voltage at terminal ③ increases, and output duty cycle of terminal ⑥ of UC3842 gradually increases. voltage on pin ③ exceeds 1V, UC3842 is closed without output.

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

If output circuit is short-circuited or current is too large, voltage induced by secondary of TR1 will be higher. When voltage at pin ③ of UC3842 exceeds 1V, UC3842 will stop working and cycle will repeat. When short circuit or overload disappears, circuit will recover by itself by oneself.

7. Output current limit protection

The above figure shows general current limit protection circuit of output terminal, and its working principle is briefly described in above figure: when output current is too large, voltage at both ends of RS (manganese-copper wire) rises, and voltage at The ③ pin of U1 is higher than reference voltage of ② pin, U1 ① pin outputs high voltage, Q1 is on, optocoupler has a photoelectric effect, UC3842 pin ① voltage decreases, and output voltage decreases to achieve purpose of limiting output overload current.

Eight, principle of output surge protection circuit

The function of output overvoltage protection circuit is to limit output voltage within a safe range when output voltage exceeds rated value. When internal voltage regulation circuit of switching power supply fails or an output overvoltage occurs due to user misuse, overvoltage protection circuit will protect it to prevent damage to downstream electrical equipment. The most commonly used surge protection circuits:1. Thyristor protection circuit:

As shown in figure above, when output signal Uo1 increases, regulator tube (Z3) fails and becomes conductive, and trigger voltage appears on thyristor control terminal (SCR1), so thyristor turns on. If Uo2 is shorted to ground, overcurrent protection circuit or short circuit protection circuit will operate, stopping entire power circuit. When overvoltage phenomenon at output is eliminated, operation voltage of thyristor control terminal is reset to ground through R, and thyristor returns to off state.

2. Photoelectric clutch protection scheme:

As shown in figure above, when an overvoltage phenomenon occurs in Uo, voltage regulator tube breaks down and conducts, and current through optocoupler (OT2) R6 flows to ground, and light emitting diode of optocoupler emits light, so that optocoupler phototransistor is turned on. The Q1 base is powered on, pin ③ 3842 goes down, chip turns off, entire power supply stops, Uo is zero, and cycle repeats.

3.Output voltage limit protection circuit:The output voltage limit protection circuit is shown in figure below. When output voltage rises, voltage regulator tube turns on and optocoupler turns on. , output voltage is reduced, voltage regulator tube is not included, voltage of UC3842③ is reduced, and output voltage is increased. Re-output voltage will be stable within a certain range (depending on Zener tube voltage regulation value).

4. Output overvoltage blocking circuit:

The working principle shown in figure A is that when output voltage Uo rises, Zener tube turns on, optocoupler turns on, and Q2 base turns on electrically. Due to inclusion of Q2, base voltage of Q1 is also turned on, and Vcc. A voltage is passed through R1, Q1, and R2 to make Q2 always on, and pin 3 of UC3842 is always high and stops working. In Figure B, UO is rising. The voltage at U1③ pin rises, ① pin outputs a high level due to presence of D1 and R1, ① terminal of U1 always outputs a high level. Q1 is always on, pin ① of UC3842 is always low and stops working. Positive feedback?

9. Power Factor Correction (PFC)

1. Schematic diagram:

2. How it works:

The input voltage passes through an EMI filter composed of L1, L2, L3, etc., BRG1 rectifies one path and sends it to PFC, and other path is divided into R1 and R2 and then sent to PFC. controller as an input voltage sample to adjust control signal The duty cycle of Q1 changes on and off times of Q1 to stabilize output voltage of PFC. L4 is a PFC inductor that stores energy when Q1 is on and releases energy when Q1 is off. D1 - starting diode. D2 is PFC rectifier diode and C6 and C7 are filter. One PFC voltage path is sent to post-stage circuit, and other path is divided into R3 and R4, and then sent to PFC controller as PFC output voltage sampling to adjust control signal duty cycle and stabilize. output voltage of KKM.

10. Input overvoltage and undervoltage protection

1. Schematic diagram:

2. How it works:

The principles of AC input overvoltage and undervoltage protection and DC switching power supply are roughly same. The protection circuit's sampling voltage comes from filtered input voltage. The sampling voltage is divided into two channels, one of which is divided into R1, R2, R3 and R4, and then applied to pin 3 of comparator. If sampling voltage is higher than reference voltage at pin 2, pin 1 of comparator outputs a high level to control main controller to make it turn off, power supply has no output. The other way is divided into R7, R8, R9 and R10, and then applied to comparator pin 6. If sampling voltage is lower than reference voltage at pin 5, comparator pin 7 outputs a high level to control main controller to turn it off, and power supply has no way out.