EMC Rectification Case Sharing - CE Current Method

I. Introduction

With the continuous development of electromagnetic compatibility technology in China, a large number of electronic devices are integrated in the confined space of vehicles, such as navigation systems, car audio, reversing radars, and airbag electronic integrated systems. These electronic devices may emit different frequency bands of interference signals to the surroundings. For example, when driving under high-voltage power lines, the interference to the vehicle's electronic control circuits can lead to ineffective operations. This can lightly affect the driving experience and, in severe cases, cause fatal accidents. Therefore, the related testing of automotive electronics plays an increasingly important role in ensuring the safety of vehicle travel.

II. Testing Standards

Conducted Emission, abbreviated as CE, also known as conducted disturbance, refers to the electromagnetic phenomenon where voltages or currents within electronic or electrical equipment or systems are transmitted through signal lines, power lines, or ground lines, becoming a source of interference for other electronic or electrical equipment or systems. Conducted emissions are divided into two testing modes, which we call the current method and the voltage method. Today, we will mainly understand the current method, which primarily measures frequencies from 0.15MHz to 245MHz.

III. Case Sharing

1. Data Analysis:

This is a vehicular product, and the current test item that is not passing is the current method, with the test standard being CISPR 25-2016. According to the standard for test setup, we obtained the following data:

From the data, the high-frequency part is relatively poor. The test frequency band does not reach GHz, and it is more of a power envelope rather than a data envelope.

2. Verification Plan

We already know that the problem is power supply noise. At this point, we can take out our verification tool, the ferrite ring. In determining the problem, we can use the ferrite ring to verify the noise type and radiation path. After adding the ferrite ring, the test results are as follows:

Normally, a ferrite ring wrapped around a wire would have a better effect. Considering that the current method test has a requirement for the length of the wire bundle, wrapping a wire bundle would increase the line length, which may lead to test result deviations. Therefore, we directly clamp it to see the effect. Currently, it appears that the noise of the exceeding frequency bands belongs to common-mode noise.

3. Verification Measures

We have previously determined that the noise is transmitted through the wire bundle and captured by the current probe, and a common-mode filter can effectively reduce the common-mode noise exceeding the standard. Up to this point, we simply add a common-mode filter, but the test data after adding it is as follows:

From the data, the common-mode filter has an effect, but it is far from the effect of the ferrite ring. This situation indicates that the noise is still transmitted through the wire bundle. However, we have added a common-mode filter on the positive and negative poles of the power supply. Theoretically, there should be no noise left. Why is that the case? In addition to the positive pole, the vehicular power supply also has an ACC signal. The previous measures only addressed the positive and negative poles without adding anything to the ACC. So, what can be added to the ACC to reduce the data?

The above measures are the final solution: add common-mode filters on the positive and negative poles of the power supply, and add ferrite beads to the ACC signal line to prevent noise from being radiated through low-impedance lines.

IV. Conclusion

Generally speaking, the ACC signal does not have such a large amount of noise. The situation can only be caused by coupling to the power supply noise. In addition to some peripheral rectification measures, there is another solution, which is to pay attention to the wiring position during the layout to avoid areas with strong radiation capabilities, such as around crystal oscillators, inductors, switching chips, etc.