Case of electromagnetic compatibility shielding absorbing material CE exceeding standard caused by switch control line of aircraft power supply product

[Phenomenon Description]: The testing system for a certain aircraft power supply product is shown in Figure 1. According to GJB 151A Air Force internal standard CE102, the 28V DC port CE exceeds the standard. The test results are shown in Figure 2.

▲ Figure 1 Schematic diagram of the testing system

▲ Figure 2 Test data before optimization

Root cause analysis:

1) The interference from the 28V power supply is coupled to the LISN through the switch control line, causing the CE test to exceed the standard. The interference return path is shown in Figure 3;

2) The positive and negative currents of the 28V two-stage filtering common mode inductor are unbalanced (5A current, 4.5A for the positive line, 0.5A for the switch control line, and 5A for the negative line reflux). The inductor current is shown in Figure 4, and the positive and negative line currents are not equal. There is magnetic saturation in the magnetic core, and the common mode inductor has not played its due filtering role.

▲ Figure 3 Schematic diagram of interference reflux path

▲ Figure 4 Schematic diagram of common mode inductor current flow

[Solution]: Wrap the 28V switch control line and the 28V positive line together into the input common mode inductor. The new common mode inductor is shown in Figure 5.

▲ Figure 5 Schematic diagram of new common mode inductor

▲ Figure 6 Optimized Test Data

【 Experience Sharing 】:

1) The 28V switch control line itself is a positive part of 28V. When winding a common mode inductor, this control line should be included, otherwise there will be an imbalance in the positive and negative current of the common mode inductor, leading to saturation problems.

2) The switch control line adopts capacitive magnetic bead capacitors for filtering, which causes significant power interference and limited magnetic bead insertion loss, resulting in insufficient filtering. In the early stage of design, this signal line should be wound together with the power supply positive line into a two-stage filtering common mode inductor.