EMI and Signal Integrity Investigation of a High-Power EV DC Charger

This case study describes an on-site EMC and signal-integrity investigation of a high-power DC charger used for electric trucks. The system exhibited intermittent charging dropouts, particularly when multiple charger posts were operating simultaneously or when a new vehicle was connected. Initial analysis showed that the issue could not be attributed to a single EMC mechanism; instead, it was caused by a combination of grounding-related EMI and weak PLC signal integrity over long cable lengths, requiring system-level architectural considerations.

Client Context

The client is a small-to-medium-sized company specialising in the design of DC fast chargers for electric trucks. The system under investigation is a high-power charger rated above 1 MW, supplying multiple charging posts simultaneously.

Scope of Work

The charger system was experiencing intermittent charging dropouts, where charging would stop unexpectedly without an obvious fault condition. In addition, when a new vehicle was connected to one charging post, charging at other posts could be interrupted.

The scope of this engagement was to investigate the root cause of these issues and implement an effective solution to restore stable operation across the system.

Technical Approach

This was a genuine real-world EMI problem, affecting system operation rather than compliance test results. From the outset, two possible root causes were considered:

• EMI-related issues, where electromagnetic noise interferes with charger operation and causes unintended trips. The interference could be a transient event such as relay/contact switching, or it could be high frequency noise coupled from the DC output

• Signal and power integrity issues, where communication signals become too weak or distorted to function reliably

If the issue were purely EMI-related, mitigation through shielding, filtering and grounding would likely be sufficient. However, if signal or power integrity were the dominant factors, a more fundamental electrical architecture change would be required.

Actions Taken

• On-site measurements were carried out using a dedicated test set-up

• A grounding-related weakness was identified within the system

• The client was advised on modifications to the grounding design

• Return currents on the communication lines were measured, allowing rapid identification of unintended current paths and coupling mechanisms

The investigation confirmed that the problem was a combination of both EMI and signal integrity effects.

Key Findings

CP 1 kHz PWM Signal Quality

The CP 1 kHz PWM signal quality was good. A clean waveform was observed, with no evidence that transient events were introducing glitches or noise capable of disrupting communication. At this stage, transient effects were not considered a risk to CP PWM integrity.

PLC Signal Level

The higher-frequency PLC signal level was observed to be relatively low. Notably, the signal was stronger closer to the charging post and weaker at the converter side, which is located tens of metres away from the charging post. This suggests that the PLC signal originates at the vehicle side and propagates toward the VSECC, suffering attenuation along the cable length.

Effect of Added Capacitance and Cable Length

Introducing a small capacitance between CP and PE was effective at filtering high-frequency noise without degrading the CP PWM signal. However, the same capacitance significantly attenuated the already weak PLC signal. Even very small capacitance values were sufficient to cause noticeable degradation.

In another test, we added an inductor to the PE wire, this also degraded the CP PLC signal, indicating any addtional L and C component in the signal transmission line will cause issues.

This highlights an important system-level limitation: long cables are inherently unsuitable for carrying low-level PLC signals. From a physics standpoint, long cables behave as distributed L–C networks, and increasing length inevitably leads to signal attenuation.

If this interpretation holds, adopting an EE architecture, in which the VSECC is relocated closer to the charging post, may be necessary.

Noise Source Mitigation

If signal attenuation due to cable length is the dominant mechanism, relocating noise sources such as DC power cables further away from the communication lines is likely to offer only marginal improvement.

Outcome

This project demonstrates how real-world charger failures can stem from a complex interaction between EMI, grounding, and signal integrity, rather than a single isolated issue. By combining targeted measurements with system-level analysis, the client was able to understand the true root causes and identify the most effective path forward.