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Old 6th September 2010, 05:29 PM   #1
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Default P channel class d anyone ?

Today in lack of anything to do i decided to revisit a cheapo class d topology that i've seen in manuals for flat screen tv's and such.

This is the circuit:
Click the image to open in full size.

I call it the "Cheapo s**t class d"
Since i dident intend this as a project, i dident put down much time in layouting, so heres a video of what i ended up with:
YouTube - Simple quick and dirty class d amplifier

Its running at around 600kHz, but started out at over 1MHz so i had to slow it down a bit. The fets are IRF9540 and IRF540.

Sound ? Pretty darn good for such a lashup, the low frequency response is wicked, it goes right down to DC!!!

In which thread was the similar circuits posted ? I can't remember.
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Old 7th September 2010, 04:01 AM   #2
savu is offline savu  Romania
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fredos did something similar a couple of years ago
1'st rule of chess:
The only way to get smarter is by playing a smarter opponent.
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Old 7th September 2010, 04:24 AM   #3
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I have been thinking of a P channel high side to replace the
high side driver and N channel fet.
I'll bet the specs are better then average due to your high switching rate.
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Old 7th September 2010, 05:22 AM   #4
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Not quite, with a LM311 you cant get below around 0.1% THD due to its internal construction.

Above 400kHz is really just a waste of switching instants, since 300-400kHz is fully enough to produce a 0.01% THD or better full range class d amplifier.

Also, the P channel fet class d is not a good design, it was simply used because high side gate drivers dident exist back then and noone had yet figured out how to drive a high side N channel fet.

Why this kinda circuit is still in use in commercial products is that it dramatically reduces production costs.
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Old 7th September 2010, 06:41 AM   #5
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Just wanted to remind you that the IR2110 already existed more than 20 years ago ! The current versions have better specs though.

Because I didn't like how it behaved back then I did an amp with complementary output stage as well. It had a DC path parallel to the capaciator coupling. And it wasn't self osciallating. The schematic can be found somewhere on this forum.


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Old 7th September 2010, 09:38 AM   #6
Eva is offline Eva  Spain
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This circuit demands clean, damped and very low impedance supply rails. Otherwise signal-dependent and load-dependent parasitic turn-on of the wrong MOSFET can arise.

In fact, the power supply is in series with gate drive.
I use to feel like the small child in The Emperor's New Clothes tale
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Old 7th September 2010, 10:55 AM   #7
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Yeh its not a good design. I'm not gonna waste time on making any boards for this one, just play with when nothing else to do.
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Old 7th September 2010, 02:29 PM   #8
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It works better from the same rail, low voltage only. That way you don't have to worry about Vgs(max) or DC restoration hacks (as shown).

For most MOSFETs, you need shorting-mode commutation, because Vgs(th) is small and shoot-through occurs when both transistors are switching.

My block-model class D amp seems to be working fairly well, by the way. "Complementary" H bridge output.

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Old 7th September 2010, 05:30 PM   #9
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I have been messing around with it some more and eventually damaged something which caused the amp to temporarily stick to one of the rails above a certain volume, so i decided to short the speaker terminals and see what would happen, volume on max and heavy clipping and rail sticking, whilest still on a heatsink it just kept on going, got hot but nothing else.

I've never before had an amplifier survive full volume into a dead short!
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Old 7th September 2010, 06:43 PM   #10
m-tech is offline m-tech  Serbia
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A year ago I made similar amp ( on single sided PCB ). Sound is quite nice ( for me ), but I never done any measurements.
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Last edited by m-tech; 7th September 2010 at 06:51 PM.
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