Filtering the ambient noise caused by PSU

By Dr. Min Zhang, the EMC Consultant

One of the questions people often asked is “how do I filter the noise coming from the PSU in my pre-compliance test set-up?”

This is a typical issue in our pre-compliance set-up, especially the power supply unit (PSU) we use is often designed for EMC testing. They will provide a stable output voltage, but the output voltage contain a high level of conduced RF current, both in differential and common mode.

You can always use an EMC tent with filter on the tent wall. But even with a tent, you might still need to add some filter on your PSU. Here we give you some practical tips in dealing with the noise generated by PSU.

Most of the PSU in the market has isolated topology (such as flyback), which means the conducted noise is shown on both input and output sides. For this reason, we propose the following filter configuration in a pre-compliance set-up.

As it can be seen, on the mains cable, since the Live, Neutral and PE wires are inside one cable bundle, we can only apply a common mode filter by using a two-turn ferrite. Depending on the PSU, sometimes, you might need a two-turn 75 ferrite and a two-turn 31 material to give you the optimal filtering on the mains. Note, this also reduces the noise on the DC output significantly.

On the DC output, follow the same principle. However, since we have the output lines separate, we can apply a differential mode filter by using a two-turn or three-turn 75 material ferrite. This essentially forms an inductor and should work with the output capacitor of the PSU and the input capacitor of the LISN as part of a C-L-C pi filter.

The input capacitors on the LISN should be configured according to

By having this configuration, you should expect a relatively clean ambient noise on your LISN output.