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ATX Flyback Transformer

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engrguy42:
So Tim, it sounds like you're saying that the gate voltage will, due to the gate capacitance, decay as the gate voltage charges the gate capacitance? And that seems to match with the trace below where I plotted gate voltage vs. Vout, and the gate voltage peaks decay until they get around 4V, which I assume is the Vgs threshold?

But I don't follow what you're saying after that, and how to resolve this.

Anyone? 

T3sl4co1l:
Ohhh you're so close!  Gate charge is drawn from the supply.  Repeated charging means current draw.  Gate drive voltage is essentially supply voltage.  What's supply voltage doing then?  And what is the shutdown and startup threshold for supply? :D

Tim

engrguy42:
Ahh, so we're gonna play games...  :D

Anyway, if I can ever get the server to respond, here's a plot of Vcc vs Vout. After startup it decays to 4V, which is the point of shutdown. So is this a threshold setting issue or a "don't let the Vcc get so low" issue?

engrguy42:
Okay, well, (fingers crossed) I think I've resolved the minor shutdown/cycling annoyance. Instead of powering Vcc from the incoming bridge rectifier, I added a fixed 9VDC supply to power Vcc. At this point I'm not really interested in the idiosyncracies of a particular controller, but rather getting the basic design principles down. Since I'm trying to model what's actually a CM6800 I'm not gonna spend a lot of time on a science project with a different controller.

In any case, it seems to be working fine. Slammed the 2.4A, 30 watt load and it's fine. I'll let it run for the full 0.5 seconds and see if anything else pops up.

engrguy42:
Dumb question...

On the DC side of a full wave bridge rectifier, if you ground the negative lead (which apparently LTSpice must have or its brain explodes...) that no longer gives you a full wave rectified output, right? Only half wave?

Because after I apply my 2.4 amp load, it maintains perfect 12VDC output for about 25mSec, then decides to start decaying until it drops to around 10.8 volts, and from then on it has ripple between 10.5 and 11 VDC. And the period of the ripple is about 16.667 mSec.

I go to all that trouble of making a full wave bridge, and it ignores 50% of my input waveform. That sucks.

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