Several common switching power supply structures and principles

1. Forward circuit The working process of the circuit:

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; after S is turned off, the inductor L continues to flow through VD2, and VD1 is turned off. After S is turned off, the excitation current of the transformer flows back to the power supply through the N3 winding and VD3 , so the voltage it bears after S is turned off is. Ø Transformer core reset: After the switch S is turned on, the transformer excitation current starts from zero and increases linearly with the increase of 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 next time it is turned on. 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*Ton/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 the N1 winding increases linearly, and the inductor energy storage increases; after S is turned off, the current of the N1 winding is cut off, and the magnetic field energy in the transformer is released to the output end through the N2 winding and VD. The voltage after S is turned off is: us=Ui+N1*Uo/N2 Working mode of the flyback circuit: Current continuous mode: When S is turned on, the current in the N2 winding has not 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, , so the flyback circuit should not work in the load open state. Idealized waveform of the flyback circuit

3. Half-bridge circuit

Half-bridge circuit schematic

Working process: S1 and S2 are turned on alternately, so that an AC voltage with an amplitude of Ui/2 is formed on the primary side of the transformer. By changing the duty cycle of the switch, the average value of the secondary rectified voltage ud can be changed, and the output voltage Uo is changed. When S1 is turned on, the diode VD1 is in the on state, and when S2 is turned on, the diode VD2 is in the on state. When both switches are turned off, the current in the transformer winding N1 is zero, and VD1 and VD2 are both in the on state, each sharing half of the current.

When S1 or S2 is turned on, the current of the inductor L gradually increases, and when both switches are turned off, the current of the inductor L gradually decreases. The peak voltages of S1 and S2 when they are off are both Ui. Due to the DC isolation effect of the capacitor , the half-bridge circuit has an automatic balancing effect on the DC component of the primary voltage of the transformer caused by the asymmetric conduction time of the two switches, so it is not easy for the transformer to be magnetized and DC magnetic saturated.

Idealized waveform of half-bridge circuit:

Full bridge circuit

Full bridge circuit schematic

Working process: In the full-bridge inverter circuit, the two diagonal switches are turned on at the same time, and the upper and lower switches of the same half-bridge are turned on alternately, so that the primary side of the transformer forms an AC voltage with an amplitude of Ui. The output voltage can be changed by changing the duty cycle.

Idealized waveform of full-bridge circuit

Push-pull circuit:

Working process: In the push-pull circuit, the two switches S1 and S2 are turned on alternately, and opposite-phase AC voltages are formed at both ends of the windings N1 and N'1 . The output voltage can be changed by changing the duty cycle. When S1 is turned on, the diode VD1 is in the on state, and the current of the inductor L gradually increases. When S2 is turned on, the diode VD2 is in the on state, and the current of the inductor L also gradually increases. When both switches are turned off, VD1 and VD2 are both in the on state, each sharing half of the current. The peak voltage that S1 and S2 bear when they are off is 2 times Ui. When S1 and S2 are turned on at the same time, it is equivalent to a short circuit in the primary winding of the transformer, so the two switches should be avoided from being turned on at the same time.