Low pass filter on PSU and target amplifier

Question

We are designing an LLC resonator PSU to feed a class D amplifier.

The switching frequency of the LLC PSU is 80-140 kHz. At the end of the PSU we are thinking to add a low pass filter (PI filter) with a cutoff frequency of 50 kHz.

This AMP board will be fed by 2 PSUs...one as discussed above, second we want any laptop brick to be able to feed it (usually the switching frequency of those laptop bricks are 500 kHz to 1.2 MHz) so we are thinking to add a second low pass filter (LC) with cutoff frequency of 400 kHz.

1. Can we have a second low pass filter (LC) on the AMP? In case of using the LLC resonator there will be 2 filters..one on the PSU PCB (cutoff frequency at 50 kHz) and second on the AMP PCB, cutoff frequency at 400 kHz.

2. In case that we power the AMP PCB using a laptop brick, there will only be 1 filter (400 kHz cutoff). Should we use an LC or PI filter?

Answer 1

Can we have a second LPF (LC) on the AMP ?

Yes, most class D amplifiers have both a low-pass filter between the power supply and the amplifier where the power goes into the amplifier, and also a low-pass output filter where the amplified output signal goes out.

should we use a LC or PI filter ?

I recommend using exactly the filter recommended in the datasheets for the parts you are using, getting that working first, before experimenting with anything else. For example, if you are using a MC56F series chip, first tray to get it to work using exactly the filters recommended in "LLC Resonant AC/DC Switched-Mode Power Supply using the MC56F8013 and MC56F8257". For example, if you are using the ADAU1592, first try to get it to work using exactly the filters recommended in the "Applications Information" schematic shown on p. 21 of the ADAU1592 datasheet. For example, if you are using the SSM2529, first try to get it to work using exactly the filters recommended in the "Applications Information" schematic shown on p. 50 of the SSM2529 datasheet.

cutoff frequency at 400Khz

That doesn't sound right. Chi Ho Li, "Design and analysis of a basic class D amplifier"; Würth Elektronik, "Class D Amplifiers: Dimensioning and Calculating the Filter"; Richard Palmer, "Design Considerations for Class-D Audio Power Amplifiers"; and Bruno Putzeys, "Switching Amplifier (Class D) Basics" all imply that a class D audio amplifier typically has an output filter cutoff frequency around 40 kHz to 80 kHz. (Is your system perhaps not typical in some way?) Often the filter between the power supply and the class D amplifier has a cutoff frequency well below 50 Hz.