Circuit composition principle of switching power supply

1、 Circuit composition of switching power supply

The main circuit of 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. Auxiliary circuits include input overvoltage and undervoltage protection circuit, output overvoltage and undervoltage protection circuit, output overcurrent protection circuit, output short-circuit protection circuit, etc.

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

2、 Principle and common circuit of input circuit

1. AC input rectifier filter circuit principle:

① Lightning protection circuit: when there is a lightning strike and the high voltage is introduced into the power supply through the grid, the circuit composed of MOV1, MOV2, MOV3: F1, F2, F3 and FDG1 is protected. When the voltage applied at both ends of the varistor exceeds its working voltage, its resistance value will be reduced, so that the high voltage energy will be consumed on the varistor. If the current is too large, F1, F2, F3 will burn the protection circuit of the later stage.

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 signal of the input power supply, prevent the interference to the power supply, and also prevent the interference of the high-frequency clutter generated by the power supply itself to the power grid. When the power is turned on, C5 should be charged. Due to the large instantaneous current, adding RT1 (thermistor) can effectively prevent the surge current. Because the instantaneous energy is consumed on the RT1 resistance, the RT1 resistance value decreases after the temperature rises after a certain period of time (RT1 is a negative temperature coefficient element). At this time, the energy consumed by it is very small, and the subsequent circuit can work normally.

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

2. DC input filter circuit principle:

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 signal 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 capacitance, L2 and L3 are differential mode inductance.

② R1, R2, R3, Z1, C6, Q1, Z2, R4, R5, Q2, RT1, C7 constitute anti-surge circuit. At the moment of starting, Q2 is not connected due to the existence of C6, and the current flows through RT1 to form a circuit. When the voltage on C6 is charged to the regulated value of Z1, Q2 is on. If C8 leakage or post-stage circuit short circuit occurs, the voltage drop on RT1 caused by the current at the instant of starting will increase. Q1 conduction will make Q2 non-conductive without grid voltage, and RT1 will be burned in a short time to protect the post-stage circuit.

3、 Power conversion circuit

1. Working principle of MOSFET: At present, the widely used insulated gate field effect transistor is MOSFET (MOSFET), which works by using the electro-acoustic effect of the semiconductor surface. Also known as surface field effect devices. Because its gate is in non-conductive state, the input resistance can be greatly increased, up to 105 ohms. MOS transistor uses the gate source voltage to change the amount of induced charge on the semiconductor surface, thus controlling the drain current.

2. Common schematic diagrams:

3. Working principle:

R4, C3, R5, R6, C4, D1 and D2 form buffers and are connected in parallel with the switch MOS tubes to reduce the voltage stress and EMI of the switch tubes and avoid secondary breakdown. When the switch Q1 is turned off, the primary coil of the transformer is prone to generate peak voltage and peak current. These components can well absorb the peak voltage and current when combined. 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 immediately turns off. The junction capacitors CGS and CGD in R1 and Q1 form an RC network. The charge and discharge of the capacitors directly affect the switching speed of the switch. If R1 is too small, it is easy to cause oscillation and electromagnetic interference is also large; If R1 is too large, the switching speed of the switch tube will be reduced. Z1 usually limits the GS voltage of MOS tubes to below 18V, thus protecting MOS tubes. The grid controlled voltage of Q1 is a saw-shaped wave. When its duty cycle is larger, the longer Q1 conduction time is, the more energy the transformer stores; When Q1 is cut off, the transformer releases energy through D1, D2, R5, R4 and C3, and also achieves the purpose of magnetic field reset, which is ready for the lower storage and transmission of energy of the transformer. The IC adjusts the duty cycle of the ⑥ pin saw wave according to the output voltage and current, thus stabilizing the output current and voltage of the whole machine. C4 and R6 are peak voltage absorption circuits.