With the development of technology and the advancement of semiconductor materials, PD power supplies are moving towards miniaturization, high frequency, and high power. EMC design is becoming increasingly challenging. EMC encompasses EMI and EMS, and EMI includes RE, CE, Harmonic, and Flicker. EMS includes ESD, EFT, Surge, RS, CS, DIP, and PFM. Due to the high uncertainty and numerous variables of RE and CE, it is more difficult and takes longer. 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 that after the switching current passes through the BUS capacitor filter, a small amount of residual current is transmitted through the line to the input port and LISN. The transmission schematic diagram is shown as follows:

Differential mode noise is mainly filtered out by filters. The filtering methods include LC filtering and π -type filtering. The LC filter is placed on the AC line. L usually adopts the differential mode component of the common mode inductor and is combined with the X capacitor to attenuate the noise. This type of filter is relatively large in size and high in cost, but the surge current that the subsequent BUS capacitor bears is small and the capacitor temperature is low. π -type filters are usually composed of a differential mode inductor added in the middle of the BUS capacitor. The filter size is small and the cost is low, but the current stress of the BUS capacitor after L is relatively large and the temperature is relatively high.

The main source of common-mode noise is the voltage transient changes generated when power devices operate at high frequencies. The transmission schematic diagram is shown as follows:

One of the paths flows to the secondary side and the earth through Cps and Cse, and the other path goes directly to the earth through Cpe.

The suppression measures for common-mode noise mainly include the following several ways:

1. A Y capacitor is added between the primary and secondary sides of the power circuit to provide a low-impedance path for common-mode noise

2. Add shielding windings inside the transformer or adjust the positions of the moving and static points of the windings

3. A common-mode inductor with a small inductance is added to the input or output to suppress noise in the high-frequency band

4. The moving points of the transformer windings and switch tubes are far away from the input lines

RE is the noise propagating through spatial electromagnetic waves. The rapid changes in current and voltage caused by the high-frequency turn-on and turn-off operations of power devices are the main reasons for the generation of RE. The suppression measures for RE noise mainly include the following several ways:

1. Increase the driving resistance, reduce the turn-on speed of the switching tube, decrease dv/dt and di/dt, and reduce the noise amplitude.

2. Reduce the area of the power circuit and the drive circuit.

3. Small inductance common mode inductors are added to the input and output to reduce noise transmission through the circuit.

4. The Y capacitor bypasses the common-mode current flowing through the transformer to a differential mode current. This circuit is a source of radiation interference, and the circuit area should be as small as possible.

5. The drain of the main switch tube is a strong interference source. RCD absorption can weaken this interference amount. The closer the RCD is to the drain, the smaller the radiation amount will be.

6. Add shielding. Note that the shielding should be connected to the noise static point, such as GND of the BUS capacitor or output capacitor