[Power Data Summary] Summary of the characteristics and differences of the five classic structures of the full bridge

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In the power electronics design of the circuit, there are many topological circuit designs, each of which has its own special functions and functions. We can select different topological circuits to complete the design project according to the needs of the design, and make the product more perfect. But for novices, it is difficult to identify various topological circuits from the beginning and be familiar with their advantages and disadvantages. Therefore, this article specifically designs half-bridge, full-bridge, flyback, forward and other topological circuits for everyone. The differences and characteristics have been summed up, I hope everyone can gain something from it.

Single-ended forward

Single-ended - one-way drive of the pulse transformer through a switching device.

Forward--the phase relationship of the original/sub-side of the pulse transformer ensures that when the switch is turned on and the primary side of the pulse transformer is driven, the transformer supplies power to the load at the same time.

The biggest problem of this circuit is that the switch tube T operates alternately in the on/off state. When the switch tube is turned off, the pulse transformer is in the "idle" state, in which the stored magnetic energy will be accumulated to the next cycle until the inductor The device is saturated, causing the switching device to burn out. The flux reset circuit formed by D3 and N3 in the figure provides a channel for venting excess magnetic energy.

Single-ended flyback

The flyback circuit is opposite to the forward circuit. The original/paying phase relationship of the pulse transformer ensures that when the switch is turned on and the primary side of the pulse transformer is driven, the transformer does not supply power to the load, that is, the original/paid side is switched on and off. . The problem that the magnetic energy of the pulse transformer is accumulated is easy to solve. However, due to the leakage inductance of the transformer, a voltage spike will be formed on the primary side, which may break through the switching device. A voltage clamping circuit is needed to protect the circuit formed by D3 and N3. From the circuit schematic diagram, the flyback type is very similar to the forward type. On the surface, it is only the difference between the same name of the transformer, but the circuit works differently, and the functions of D3 and N3 are different.

Push-pull (transformer center tap)

The characteristic of this circuit structure is: symmetrical structure, the primary side of the pulse transformer is two symmetrical coils, the two switch tubes are connected in a symmetrical relationship, and the wheel is broken, and the working process is similar to the class B push-pull power amplifier in the linear amplifying circuit.

Main advantages: high frequency transformer core utilization (compared to single-ended circuit), high power supply voltage utilization (compared to the half-bridge circuit to be described later), large output power, low voltage of both bases Flat, the drive circuit is simple.

The main disadvantages are: low utilization of the transformer windings and high requirements on the withstand voltage of the switching tubes (at least twice the supply voltage).

Full bridge

The characteristic of the circuit structure is that four identical switching tubes are connected into a bridge structure to drive the primary side of the pulse transformer.

In the figure, T1 and T4 are a pair, driven by the same group of signals, and simultaneously turned on/off; T2 and T3 are another pair, driven by another group of signals, and simultaneously turned on/off. Two pairs of switch tube wheels are turned on/off, and a positive/negative alternating pulse current is formed in the primary coil of the transformer.

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

The main disadvantages are: the number of switching tubes used is large, and the parameters are required to be consistent, the driving circuit is complicated, and it is difficult to achieve synchronization. This circuit structure is usually used in ultra-high power switching power supply circuits above 1KW.

Half bridge

The structure of the circuit is similar to the full bridge type, except that two of the switch tubes (T3, T4) are replaced by two equal-value capacitors C1 and C2.

Main advantages: It has a certain anti-unbalance ability, and the circuit symmetry requirements are not very strict; the adaptable power range is large, from tens of watts to kilowatts; the switch tube withstand voltage requirements are lower; the circuit cost is higher than the full bridge circuit Inferior. Such circuits are often used in various unregulated output DC converters, such as electronic fluorescent lamp drive circuits.

After a series of examples, I believe that you have a better understanding of the common topologies. By understanding the advantages and disadvantages of these different circuit topologies, we can apply it more flexibly to the design, and perhaps we can sum up our own theory to design, thus adding a solid foundation for our design career.