My fully discrete ClassD
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:
The 0.05 ohms represent the fuses...
And the prototype: (not the latest)
Any ideas, suggestions ?
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.
Have you tried increasing your final driver 33R emitter resistors? Maybe try just increasing the N-channel ones to something like 47 or 56R.
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 ?
About that P-channel heating, have you matched your mosfets? I suppose you did, but maybe they have drifted...
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 !
Re: My fully discrete ClassD
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"?
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.
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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