With the development of science and technology and the advancement of semiconductor materials, PD power supplies are moving towards miniaturization, high frequency and high power. EMC design is becoming more and more challenging. EMC includes EMI and EMS. EMI includes RE, CE, Harmonic and Flicker. EMS includes ESD, EFT, Surge, RS, CS, DIP and PFM. Due to the large uncertainty and many variables of RE and CE, it is more difficult and time-consuming. This article mainly introduces some practical debugging methods for RE and CE.

CE is divided into differential mode noise and common mode noise. The source of differential mode noise is the switching current after filtering by the BUS capacitor. A small amount of residual current is transmitted to the input port and LISN through the line. The transmission diagram is shown below:

Differential mode noise is mainly filtered out by filters, which include LC filtering andπType filtering, LC filter is placed on the AC line, L usually uses the differential mode component of the common mode inductor, and cooperates with the X capacitor to attenuate the noise. This method has a larger filter size and higher cost, but the surge current borne by the post-stage BUS capacitor is small and the capacitor temperature is low.πThis type of filter is usually composed of adding a differential mode inductor in the middle of the BUS capacitor. The filter is small in size and low in cost, but the BUS capacitor after L has a large current stress and a high temperature.

The source of common mode noise is mainly the transient voltage changes generated when the power device works at high frequency. The transmission diagram is shown below:

One path is to flow to the secondary side and the ground through Cps and Cse, and the other path is to flow directly to the ground through Cpe.

There are several ways to suppress common mode noise:

• Adding Y capacitors between the primary and secondary sides of the power circuit provides a low impedance path for common mode noise
• Add shielding windings to the transformer or adjust the positions of the moving and static points of the windings
• Add a small common-mode inductor to the input or output to suppress high-frequency noise
• The transformer winding and the switching point are far away from the input line.

RE is noise propagated through electromagnetic waves in space. The rapid changes in current and voltage caused by high-frequency turn-on and turn-off operations of power devices are the main cause of RE. There are mainly the following ways to suppress RE noise:

• Increase the driving resistance, reduce the turn-on speed of the switch tube, reduce dv/dt and di/dt, and reduce the noise amplitude.
• Reduce the area of ​​the power circuit and the drive circuit.
• Add small common-mode inductors to the input and output to reduce noise emission through the line.
• The Y capacitor bypasses the common-mode current flowing through the transformer into a differential-mode current. This loop is a source of radiation interference, and the loop area should be as small as possible.
• The drain of the main switch tube is a strong interference source. RCD absorption can reduce this interference. The closer the RCD is to the drain, the smaller the radiation.
• Add shielding, and note that the shielding should be connected to a noise-free point, such as the GND of the BUS capacitor or the output capacitor.