Circuit composition and principle of switching power supply

Switching power supplies have become an important part of our lives, touching every aspect of life. Switching power supplies are very familiar to engineers. In this article, the editor will give you some technical benefits to help novice friends understand the circuit and principle of switching power supplies.

Circuit composition of switching power supply

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 overcurrent protection circuit, output short-circuit protection circuit, etc.

The circuit block diagram of the switching power supply is shown in Figure 1:

figure 1

Principles of input circuits and common circuits

AC input rectification and filtering circuit principle (Figure 2):

(2) Input filter circuit: The double π 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 to 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 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.

DC input filter circuit principle (Figure 3):

image 3

(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. It also prevents 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, and C7 form an anti-surge circuit. At the moment of starting, Q2 does not conduct due to the presence 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 conducts. If C8 leaks or the subsequent circuit is short-circuited, the voltage drop on RT1 caused by the current at the moment of starting increases, Q1 conducts, and Q2 does not conduct because of the lack of gate voltage. RT1 will burn out in a very short time to protect the subsequent circuit.

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 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:

Figure 4

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 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 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 the peak voltage absorption circuit.

Push-pull power conversion circuit:

Q1 and Q2 will turn on in turn.

Figure 5

Power conversion circuit with drive transformer:

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