What are the advantages and disadvantages of 5 classic switching power supply structures?

1. Single end exciting type

Single end: Drive the pulse transformer through a single switching device.

Forward excitation: the phase relationship between the original and the paying side of the pulse transformer ensures that when the switch tube is on and the original side of the pulse transformer is driven, the paying side of the transformer supplies power to the load at the same time.

The biggest problem of this circuit is that the switching tube T works alternately in the on/off state. When the switching tube is off, the pulse transformer is in the "no-load" state, in which the stored magnetic energy will be accumulated to the next cycle until the inductor is saturated, so that the switching device is burned. The magnetic flux reset circuit constituted by D3 and N3 in the figure provides a channel for releasing excess magnetic energy.

2, single end flyback

Flyback circuit and forward circuit in contrast, the pulse transformer original/pay side phase relationship, to ensure that when the switch tube is on, driving the pulse transformer original side, the transformer pay side does not supply power to the load, that is, the original/pay side staggered off. It is easy to solve the problem of magnetic energy accumulation in the pulse transformer. However, due to leakage of the transformer, voltage spikes will be formed on the primary side, which may breakdown the switching device. Therefore, voltage clamp circuit needs to be set to protect the circuit composed of D3 and N3. From the schematic diagram of the circuit, flyback and forward are very similar, the surface is only the transformer with the same name of the difference, but the circuit is different, D3, N3 function is different.

3, push-pull (transformer center tap) type

The characteristics of this circuit structure are: symmetrical structure, pulse transformer original side is two symmetric coil, two switch tube connected into a symmetric relationship, wheel flow is broken, the working process is similar to the linear amplifier circuit in class B push-pull power amplifier.

Main advantages: high utilization rate of magnetic core of high-frequency transformer (compared with single-ended circuit), high utilization rate of power supply voltage (compared with the half-bridge circuit described later), large output power, low level of both tube bases, simple driving circuit.

Main disadvantages: low utilization rate of transformer winding, high voltage requirement of switching tube (at least twice of the supply voltage).

4. Full bridge type

This circuit structure is characterized by four identical switching tubes connected into a bridge structure to drive the original side of the pulse transformer.

In the figure, T1 and T4 are a pair, driven by the same set of signals, and at the same time switch on/off terminal; T2 and T3 are another pair, driven by another set of signals, and turn on/off terminals at the same time. Two pairs of switching tubewheel flow/break, forming a positive/negative alternating pulse current in the primary side coil of the transformer.

Main advantages: Compared with push-pull structure, the primary side winding is reduced by half, and the switching tube voltage is reduced by half.

Main disadvantages: the number of switch tubes used is large, and the required parameter consistency is good, the drive circuit is complex, and it is difficult to achieve synchronization. This kind of circuit structure is usually used in the super large power switching power supply circuit above 1KW.

5. Half bridge type

The structure of the circuit is similar to the full bridge type, but the two switch tubes (T3, T4) are replaced by two equivalent large capacitors C1 and C2.

Main advantages: It has certain anti-unbalance ability, and the requirement of symmetry of the circuit is not very strict; Adaptable power range is large, from tens of watts to kilowatts can be; Low pressure requirement of switching tube; Circuit cost is lower than full bridge circuit. This circuit is often used in a variety of unregulated output DC converter, such as electronic fluorescent lamp drive circuit.