A summary of power supply knowledge: understanding the EMI circuit of power supply

Why do we need EMI circuits?

When the power supply is working, the switching transistor will generate a lot of EMI and RFI, which will seriously affect the normal operation of other electronic equipment in the house. Therefore, in order to ensure that the power supply is not affected by the input noise and outgoing voltage surge peaks in the power grid, a two-way protection measure is required at this stage of the power supply.

The mains current entering the power supply must first be filtered by the EMI filter

Noise can be divided into two types according to the conduction mode: common mode noise (CMN) and differential mode noise (DMN).

　Common mode and differential mode filtering 1. Common mode noise (CMN) is the noise that exists at the input end (transmission line and neutral line) of electrical equipment using AC power, and the phases of the two lines to the ground are kept in phase. It can be suppressed by placing an inductor in series with each transmission line in the electromagnetic interference filter and connecting the two transmission lines and the ground with a Y capacitor.

2. Differential mode noise (DMN) is the noise that comes from the live wire of the power supply and returns through the neutral wire. It exists in the AC line and neutral conductor.

The EMI filter circuit is located before the rectifier bridge, because this design can filter the noise before the current passes through the rectifier bridge diode. The components of the EMI filter current must include two Y capacitors and two X capacitors, two electromagnetic coils, an MOV (varistor) and a fuse. These components are indispensable. In addition, it is necessary to briefly introduce here that the full name of MOV is "varistor", which mainly protects the power supply when the grid surge voltage peaks.

However, in some low-end power products, some manufacturers will omit this MOV to save costs. If your power supply has an EMI circuit and there is no MOV, it is best to connect a power outlet with a surge protector or a UPS power supply, otherwise it will cause great damage to your power supply and hardware system.

Generally, there is a thermistor NTC (full name: negative temperature coefficient thermistor, the higher the temperature, the lower the resistance value) after the EMI filter circuit, which is usually used to protect the internal components of the power supply when a large current enters. As the name suggests, the thermistor is a resistor that controls the resistance value by the temperature. When the temperature of the thermistor is low, the resistance value is usually around 6-12 ohms. When the power supply is started, the temperature of the resistor rises and the resistance value is about 0.5-1 ohm.

For high-performance power supplies, they will be equipped with a relay, which will bypass the thermistor after the power supply is started, reducing the heat loss of electricity and helping to improve the efficiency of the power supply .