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

source:Industry News     Popular:adapter     release time:2021-09-11 09:26:49     Article author:sznbone    

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

  1. Single-ended forward

  Single-ended: The pulse transformer is unidirectionally driven by a switching device.

  Forward: The primary/secondary phase relationship of the pulse transformer ensures that when the switch tube is turned on and the primary side of the pulse transformer is driven, the secondary side of the transformer simultaneously supplies power to the load.

  The biggest problem of this circuit is that the switch tube T works alternately in on/off states. When the switch tube is turned off, the pulse transformer is in a "no-load" state, and the stored magnetic energy will be accumulated in the next cycle until the inductance The device saturates, causing the switching device to burn out. The magnetic flux reset circuit composed of D3 and N3 in the figure provides a channel for venting excess magnetic energy.

What are the advantages and disadvantages of the 5 classic switching power supply structures?(图1)

  2, single-ended flyback

  The flyback circuit is opposite to the forward circuit. The primary/secondary phase relationship of the pulse transformer ensures that when the switch tube is turned on and the primary side of the pulse transformer is driven, the secondary side of the transformer does not supply power to the load, that is, the primary/secondary side is switched on and off alternately . The problem of the accumulation of magnetic energy of the pulse transformer 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 down the switching device. A voltage clamp circuit is required to protect the circuit composed of D3 and N3. From the circuit schematic diagram, the flyback and the forward are very similar. On the surface, it is only the difference between the same name end of the transformer, but the working mode of the circuit is different, and the functions of D3 and N3 are also different.

  3, push-pull (transformer center tap) type

  The characteristics of this circuit structure are: 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 circulation is broken. The working process is similar to the class B push-pull power amplifier in the linear amplifier circuit.

  Main advantages: high utilization rate of high-frequency transformer core (compared with single-ended circuit), high utilization rate of power supply voltage (compared with half-bridge circuit to be described later), high output power, low power for both tube bases Flat and simple driving circuit.

  The main disadvantages: low utilization of transformer windings and relatively high requirements for the withstand voltage of the switching tube (at least twice the power supply voltage).

  4, full bridge

  The characteristic of this circuit structure is that four identical switch 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, which are driven by the same set of signals and turn on/off at the same time; T2 and T3 are another pair, which are driven by another set of signals and turn on and off at the same time. The two pairs of switching tube wheels circulate/break, forming positive/negative alternating pulse currents in the primary coil of the transformer.

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

  The main disadvantages: the number of switch tubes used is large, and the parameter consistency is required to be good, the drive 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.

  5, half bridge

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

  Main advantages: Has a certain degree of anti-unbalance ability, and the circuit symmetry requirements are not very strict; It can adapt to a large power range, from tens of watts to kilowatts; the switch tube has a lower withstand voltage requirement; the circuit cost is higher than that of a full-bridge circuit Inferior. This kind of circuit is often used in various non-regulated output DC converters, such as electronic fluorescent lamp driving circuits.


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