Project: C694 – A 48V-12V Bi-directional DC-DC converter (Automotive Application)
Timescale: 1 working week
Scope of work: The DUT failed bulk current injection test. The scope of the work therefore is to have the issue solved.
Challenges: There was no solid, continuous ground plane, the chassis reference is long loop, which is a high impedance path to the test ground plane, making fixes difficult.
The client is a well-established Tier-1 supplier, their customer is a UK based OEM.
During the BCI test, the CAN bus lines were disturbed, the CAN function related chip stopped working, causing the BCI failure.
Because there’s no solid ground plane, nor good chassis reference connection. All the measures, such as filtering capacitors, common mode choke failed to work effectively. The chassis reference is shown in Figure 1.
What we mean by high impedance connection causing problem is demonstrated in Figure 2.
There are two fixes here, one is to increase the 4 layer board to 6, so we could have solid ground plane or chassis plane. Second option is a little bit risky, which is to remove the ESD protection diodes. The reason is that when BCI noise got injected into the system, the ESD protection diodes divert the noise to the chassis reference, this causes voltage increase due to the high impedance. So there’s a big voltage difference between two different points in the ground plane. This causes the chip to malfunction. By removing the ESD diode, this problem went away.
The rationale here is that the CAN chip has its internal ESD protection up to 15kV. So removing the ESD protection at the connector might still pass the ESD test.
But of course, if the design was done right in the first place, we would not have to risk it by removing the ESD protection.
Lesson learnt: Grounding design is the most important thing in system design. If the ground is not defined properly, then most of the EMC fixes will fail to show effect.