Impact of driver grounding point location on conducted emissions

1. Background

When doing the conducted emission experiment, it was found that the PE grounding position of the driver's motor line would have a great impact on the test results, causing the test to exceed the standard. I wonder if my colleagues have encountered this. Now I will share with you the interesting scene I encountered.

2. Driver structure and conducted emission test layout

2.1 Driver structure grounding instructions

The heat sink of the drive inverter module is connected through an L-shaped metal bridge, which serves as the user-side PE interface;

2.2 LISN Schematic

2.3 Conducted Emission Test Grounding Instructions

Conducted emission test layout and driver structure diagram

3. Noise path analysis

3.1 Overall noise flow

LISN test noise path

3.2 Simplified analysis of noise path

LISN test path simplification

3.3 Analysis of grounding point optimization path

Noise source near PE of motor line:

Motor grounding position changes

4. Analysis and summary

4.1 Design weaknesses

• (1) The motor PE line terminal is designed without a grounding point near the I drive noise circuit;

• (2) The Y capacitor grounding point of the port capacitor is not designed close to the port, resulting in serious externalization of the filter capacitor and failure to achieve the desired filtering effect;

4.2 Path Analysis Summary

(1) The ground bar between the user interface and the heat sink is too narrow, and the resulting inductance level is sufficient to affect the shunt of motor noise, resulting in excessive conducted emissions;

(2) By changing the location of the grounding point, the magnitude of the noise source flowing through the LISN is changed, thereby changing the test results, which can be explained by the path or by common impedance coupling;

5. Thoughts and inspirations

• The port filter capacitor should be placed close to the port, especially the grounding point, to prevent the capacitor from being seriously plugged in;

• The PE grounding point of the motor should be placed close to the noise source for return current;

• The width of the ground bridge between the user interface and the heat sink should be as large as possible to reduce impedance, and focus on the path impedance of the multiple frequency band of the carrier frequency;