Class D question

[portreathbeach]

I am learning about class D amps and have read a couple of pfd files and a few threads on here about there operation. I have a question that I expect has a simple answer, so here it is.....

The output of the comparator will always be a a PWM square wave. It switches N and P MOSFETS when either positive or negative right? If the output of the comparator has to be positive or negative, that means 1 of the MOSFETS (in half bridge) or 2 (full bridge) would be on all the time?

Please can someone explain

 

[Ouroboros]

Well, not ALL the time. In a simple half-bridge, either the top FET or the bottom FET will be on (but never both on at the same time). The output before the filter will simply be the PWM switching waveform but at a higher power level. The output filter reconstructs the audio signal by simple low-pass filtering.

 

[portreathbeach]

Thanks Ouroboros, got it now. Didn't even think about the low pass filter. So when you have no input into the amp, you have a 50/50 duty cycle, and at the high switching frequency (say 150KHz), the low pass filter will not pass this, so the speaker sees nothing, and is silent.