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Old 26th March 2005, 12:29 PM   #1
MikeB is offline MikeB  Germany
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Default My fully discrete ClassD

Hi !

I've nearly sucessfully built my first ClassD, fully discrete using standard devices.
It plays like hell, but, i have some problems left..
First problem, without the 1nF in the VAS deadtime seems to be too short,
heating up the P-Channel...
The second problem, driver for opening the mosfets heats up, so i have to turn off after ~5 minutes.

The schematic i've constructed:

Click the image to open in full size.

The 0.05 ohms represent the fuses...

And the prototype: (not the latest)

Click the image to open in full size.

Click the image to open in full size.

Any ideas, suggestions ?

MikeB
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Old 26th March 2005, 02:07 PM   #2
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Default Go ahead, discrete class D is great!...good luck!

Even with those little problems sounding good!...this is a promiss that will be very good when those details turned fixed.

regards,

Carlos
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Old 26th March 2005, 02:12 PM   #3
subwo1 is offline subwo1  United States
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Hi MikeB,
Have you tried increasing your final driver 33R emitter resistors? Maybe try just increasing the N-channel ones to something like 47 or 56R.
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Old 26th March 2005, 02:58 PM   #4
MikeB is offline MikeB  Germany
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Hi subwo1 !

No, have not tried yet, but you're right, the n-channel is faster !
The 1st version used 10ohms, then i increased to 33ohms, but did
not show any change... I will try !

What i do not understand, only the P-channel heats up, the N-channel
keeps cold, should crossconduction not heat up both ?

Mike
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Old 26th March 2005, 03:15 PM   #5
Giaime is offline Giaime  Italy
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Beautiful prototype!

About that P-channel heating, have you matched your mosfets? I suppose you did, but maybe they have drifted...
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Old 26th March 2005, 03:39 PM   #6
MikeB is offline MikeB  Germany
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Hi Giaime !

I did not really match the mosfets, i only matched them for identical
Vgs-threshold... I can't avoid having different Rds-ON, also the
p-channel has higher recoverytimes, but i have no idea how to
measure that !

Mike
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Old 26th March 2005, 04:05 PM   #7
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Default Re: My fully discrete ClassD

Quote:
Originally posted by MikeB [some snippage]
I've nearly successfully built my first ClassD, fully discrete using standard devices. It plays like hell, but, i have some problems left.. First problem, without the 1nF in the VAS deadtime seems to be too short, heating up the P-Channel... The second problem, driver for opening the mosfets heats up, so i have to turn off after ~5 minutes.

And the prototype: (not the latest)

Click the image to open in full size.

Any ideas, suggestions?
Since your output inductor is a coreless solenoid, it may be injecting enough stray flux back into your amp to cause problems. As a test, space it back on the end of a twisted pair of wires, 20 or 30 cm away from the rest of the circuitry.

I don't suggest it, but as a test to demonstrate how much trouble this inadvertent coupling can cause, you could then bend it back down very close over your circuit board, moving it and twisting it around until your circuit starts misbehaving (and probably breaks).

Regards -- analogspiceman

PS: What did you mean by "VAS"?
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Old 26th March 2005, 04:37 PM   #8
subwo1 is offline subwo1  United States
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Quote:
Originally posted by MikeB
What i do not understand, only the P-channel heats up, the N-channel
keeps cold, should crossconduction not heat up both ?
One thing that could heat the P-channel more is its greater rds-on. But like you were thinking, it would also seem to me that both should heat up, still. It may have something to do with the P-ch's body diode recovery being forced by the N-ch. Maybe something will occur to me later.
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Old 26th March 2005, 04:41 PM   #9
MikeB is offline MikeB  Germany
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Hi analogspiceman !

Hmm, i hoped the heatsinks would give some "shielding"...
A bit dumb, as the heatsinks are made of alloy...
Would it be enough for testing to put some steelplate between
coil and rest of amp ?

With "VAS" i meant the 2nd diffamp with currentmirror, loaned from
another ClassAB-design. With the 150ohm the voltageswing is
reduced to +/- 1.2volt, also giving low zout. The 1nF slows down this
signal, and with the 0.6v vbe-threshold of the following bjts i "cut"
this signal at it's half, giving ~100ns deadtime.

Mike
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Old 26th March 2005, 07:52 PM   #10
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I don't know if this is your problem, but 36 volts of gate drive is probably not good for those output transistors... At least not in the long run. But i guess puncturing the gate oxide would be more terminal than just heating up. Like Short and Kaboom!
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