How does this simple class d amp for car work?

[wonderfulaudio]

Hi,

Attached is a class D amplifier for car use I found on the net. The circuit looks very compact and I would like to build it just for fun and get to learn class D. The amp directly converts the battery voltage into amplified audio using an inverter kind of mechanism, I know its not a perfect design as PSRR will be compromised. Probably, the fact that it uses feedback makes it work for car use just fine.

I would like to still understand certain things like:
1) What components decide the oscillation frequency
2) The output should have LC filter but there is only C, L is missing, how does it work?
3) How is dead time controlled by BD139/BD140?

Thanks and Warm Regards,
WonderfulAudio

 

[voodooless]

I don’t think this is any good for a car audio system.

The transformer at the end is 3:1 step down, so output voltage swing will be compromised heavily. At best it will do .5W or so into 4R. Maybe the schematic is wrong. It’s also missing the inductance value.

As for the LC: there is an L, the transformer. And the speaker will also have an inductance.

The frequency is determined by the square wave generator and ramp converter.

Dead time: no idea, way above my pay grade

The thing seems to be post filter feedback, though. Pretty cool true.

 

[wonderfulaudio]

The transformer drawn is misleading, it shows more windings on primary than secondary but it's the other way round, it's 1:3 so it boosts 12 volts to 36 volts theoretically.
Also as far as I understand the transformer L does not filter the oscillation frequency, it just passes that high frequency, there is supposed to be extra LC network to low pass and filter out this oscillation frequency. If the subwoofer coil is used as L then it varies a lot from driver to driver and EMI on the long wires to the driver coil is not good. Nevertheless I do find this circuit quite interesting for the low parts it uses along with post filter feedback after the transformer
I hope somebody more knowledgeable could explain more