Working principle of switching power supply and schematic diagram of switching power supply

　　The switching power supply uses a circuit to control the switch tube to turn it on and off at high speed.

　　Convert direct current into high-frequency alternating current and provide it to the transformer for transformation, thereby generating the required set or sets of voltages! The reason for converting to high-frequency alternating current is that the efficiency of high-frequency alternating current in the transformer transformer circuit is much higher than that of 50HZ. Therefore, the switching transformer can be made very small, and it is not very hot when working!! The cost is very low. If 50HZ is not converted into high frequency, the switching power supply will be meaningless.

　　The working process of the switching power supply is:

　　Power supply → input filter → full-bridge rectifier → DC filter → switching tube (oscillation inverter) → switching transformer → output rectification and filtering.

　　The AC power input is rectified and filtered to DC

　　The switch is controlled by a high-frequency PWM (pulse width modulation) signal, and the DC is added to the primary of the switching transformer.

　　The secondary of the switching transformer induces a high-frequency voltage, which is supplied to the load after rectification and filtering.

　　The output part is fed back to the control circuit through a certain circuit to control the PWM duty cycle to achieve the purpose of stable output.

　　When AC power is input, it usually passes through something like an oscillator to filter out interference on the power grid, and also to filter out interference from the power supply to the power grid;

　　When the power is the same, the higher the switching frequency, the smaller the size of the switching transformer, but the higher the requirements for the switching tube;

　　The secondary of the switching transformer can have multiple windings or one winding can have multiple taps to obtain the required output;

　　Generally, some protection circuits should be added, such as no-load, short-circuit and other protections, otherwise the switching power supply may be burned.

　　Mainly used in industry and some household appliances, such as televisions, computers, etc.

　　Switching Power Supply Schematic Analysis

　　1. Forward circuit

　　Working process of the circuit:

　　a> After the switch S is turned on, the voltage across the transformer winding N1 is positive at the top and negative at the bottom, and the voltage across the coupled N2 winding is also positive at the top and negative at the bottom. Therefore, VD1 is in the on state, VD2 is in the off state, and the current of the inductor L gradually increases;

　　b> After S is turned off, the inductor L continues to flow through VD2, and VD1 is turned off. After S is turned off, the transformer's exciting current flows back to the power supply through the N3 winding and VD3, so the voltage that S withstands after it is turned off is .

　　c> Transformer core reset: After the switch S is turned on, the transformer's excitation current starts from zero and increases linearly with time until S is turned off. In order to prevent the transformer's excitation inductance from saturation, it is necessary to try to make the excitation current drop back to zero within a period of time from the time S is turned off to the time it is turned on again. This process is called the transformer's core reset.

　　Idealized waveform of the forward circuit:

　　The core reset time of the transformer is:

　　Tist=N3*Tone/N1

　　Output voltage: When the output filter inductor current is continuous:

　　Uo/Ui=N2*Ton/N1*T

　　Core reset process:

　　2. Flyback circuit

　　Flyback circuit schematic

　　The transformer in the flyback circuit acts as an energy storage element and can be regarded as a pair of mutually coupled inductors.

　　Working process:

　　After S is turned on, VD is in the off state, the current of N1 winding increases linearly, and the inductor energy storage increases;

　　After S is turned off, the current of N1 winding is cut off, and the magnetic field energy in the transformer is released to the output end through N2 winding and VD. The voltage after S is turned off is: us=Ui+N1*Uo/N2

　　Working mode of flyback circuit:

　　Current continuous mode: When S is turned on, the current in the N2 winding has not yet dropped to zero.

　　Output voltage relationship: Uo/Ui=N2*ton/N1*toff

　　Current discontinuous mode: Before S is turned on, the current in the N2 winding has dropped to zero.

　　The output voltage is higher than the calculated value of the above formula, and increases as the load decreases. In the extreme case where the load is zero, the flyback circuit should not work in the open-load state.

　　Idealized waveform of flyback circuit