Do N channel Mosfets have a current requirement?

[rjenkinsgb]

Circuit runs good on 25vdc 40a & 2uh choke but if amps goes up to 45a the amp meter suddenly pegs out above 60a loud hum sound it must be pulling 150a when both mosfets explode.

It may be that the oscillator stops when there is too high a load on the coil, it's resonance is being damped too much?

In that case, both FETS are trying to be biassed on but limited to a low gate voltage due to the cross connected diodes, in to what's effectively a short circuit load.
If that is happening, they are both conducting but in a relatively high resistance state, so their power dissipation does crazy and they rapidly cook.

That is why I so strongly dislike that self-oscillating design; there is nothing to guarantee proper gate drive and it has conduction overlap, which is why it needs the extra series choke; for commutation.

 

[gary350]

Scope shows the same wave on, 15, 20, 25 vdc, only difference is amplitude. I'm not sure what I should be seeing? There is a continuous wave form with no load, but load makes the wave change. Camera is slow wave flashes 3 times in each photo that is why each pictures looks like 3 waves. 2nd photo show the circuit under load. Scope says 3.225KHz but my meter says 75KHz loaded.

I have a video but not sure how to upload it here?

 

[ChrisP58]

What I want to see is both signals of the two Drain terminals on the scope at the same time. You'll need a two channel scope to do that.

What is important is to see the timing of the two signal in relation to each other.

 

[gary350]

What I want to see is both signals of the two Drain terminals on the scope at the same time. You'll need a two channel scope to do that.

What is important is to see the timing of the two signal in relation to each other.

I don't have a scope that does that.

If I take signal from center tap then scope will show both signals, left, right, left, right, left, right, left, right, left, right, left, right, left, right, left, right, left, right every other pulse will be left and the other pulse will be right.

 

[ChrisP58]

I don't have a scope that does that.

If I take signal from center tap then scope will show both signals, left, right, left, right, left, right, left, right, left, right, left, right, left, right, left, right, left, right every other pulse will be left and the other pulse will be right.

Which tells you nothing about the relative timing of the two signals, nor the true (source referenced) waveforms of the drain voltages.

You need a decent 2 (or more) channel scope to do this kind of work.

 

[shortbus=]

You need a decent 2 (or more) channel scope to do this kind of work.

I just checked online, the Dollar Store doesn't carry them.

 

[morrissel311]

It sounds to me like you're reaching a point where one mosfet is not fully of when the other turns on. At that point you've got a dead short through the mosfets.
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