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What is the working principle of "synchronous rectification"?

Publisher: Administrator Date:2021-12-07

Synchronous rectification technology uses power MOSFETs with extremely low on-state resistance to replace rectifier diodes, which can greatly reduce the loss of the rectifier circuit, improve the efficiency of the DC/DC converter, and meet the needs of low-voltage, high-current rectifiers. This article starts from the analysis of the principle diagram of the synchronous rectification circuit in the textbook "Power Electronics Technology", introduces the reverse resistance working area of the power MOSFET and the basic principle of the synchronous rectification technology, and discusses the driving circuit and the gate voltage waveform in the synchronous rectification circuit. Analyzed.

The basic circuit structure of synchronous rectification:

Power MOSFET is a voltage-controlled device, and its volt-ampere characteristic is linear when it is turned on. When using a power MOSFET as a rectifier, the gate voltage must be synchronized with the phase of the rectified voltage to complete the rectification function, so it is called synchronous rectification. PS7516 and PS7616 are lithium battery boost output 5V1A, 2A synchronous rectification boost classic IC, FP6717, FP6716 are also the leader of lithium battery boost output 5V3A, 5V2A.

Why apply synchronous rectification technology:

With the development of electronic technology, the working voltage of the circuit is getting lower and lower, and the current is getting larger and larger. Low-voltage operation is conducive to reducing the overall power consumption of the circuit, but it also poses new problems for power supply design.

The loss of the switching power supply is mainly composed of three parts: the loss of the power switch tube, the loss of the high-frequency transformer, and the loss of the output rectifier tube. In the case of low voltage and large current output, the conduction voltage drop of the rectifier diode is relatively high, and the loss of the output rectifier tube is particularly prominent. Fast recovery diodes (FRD) or ultra-fast recovery diodes (SRD) can reach 1.0~1.2V. Even if low-drop Schottky diodes (SBD) are used, a voltage drop of about 0.6V will be generated, which leads to increased rectification losses. Large, the power efficiency is reduced.
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