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ATX Flyback Transformer

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engrguy42:
Okay, well here's what I have so far.

Dedicated 9VDC connected to the Vcc of the controller. Full 160VDC feeding the switching circuit. No ground on the 120VAC, only on the secondary of the associated isolating 1:1 transformer.

And the 12VDC output is flat at 12VDC for about 20mSec after I switch on the 30watt, 2.4 amp load to the 12VDC. Then it decays, and settles at just under 11 volts with +/- 0.5 volt ripple, with a ripple frequency of 60Hz (not 120Hz as I'd expect for a full wave). BTW, the 160VDC input voltage at Vin looks identical in waveform to the Vout shown in the trace. 

T3sl4co1l:
Oh, you did have a ground on the AC side. Yeah, that.  Poor bridge, it's shorted out...

Other than that, I'd guess it's hitting current limit, i.e., for the values shown, it's running at the design maximum current, whatever that is.  This is best illustrated by Ith saturating to its maximum 2V, i.e. it's commanding all the current it can.

Tim

engrguy42:

--- Quote from: T3sl4co1l on June 22, 2020, 07:12:10 pm ---Oh, you did have a ground on the AC side. Yeah, that.  Poor bridge, it's shorted out...

Other than that, I'd guess it's hitting current limit, i.e., for the values shown, it's running at the design maximum current, whatever that is.  This is best illustrated by Ith saturating to its maximum 2V, i.e. it's commanding all the current it can.

Tim

--- End quote ---

So if I removed the 120VAC ground, why is it still showing 60Hz ripple on the DC??

And what exactly is limiting this? Is it running out of duty cycle? Is it a limitation of the controller?

engrguy42:
Oh wait, I see. If there's a ground on BOTH the AC and DC a diode gets shorted. So no grounds at all on the AC side? I thought I tried that before and LTSpice freaked.

EDIT: Ok, thanks...no AC grounds at all is working great. Maybe that will give me more "oomph" to "oomph" my 12VDC output so it doesn't languish at 11V.  :D

engrguy42:
Wow, well I guess I answered my own question again...Looks like this circuit isn't being limited, at least for the load I'm applying. In fact all I did was tweak the primary of the switching transformer and now the 12VDC output under load is flat as a pancake. And the Ith volts are hanging between 1.3 and 1.4 volts, not the 2 volt ceiling.

Previously I had the primary set to 400mH, and secondary at 10mH, giving a turns ratio of a little over 6:1. I then brought that down to 300mH, for a ratio of about 5.5, and as you can see in the attached plot it's doing fine.

And that's one area of this DC switching and the associated transformers that I haven't grasped yet. With AC it's pretty easy and straightforward. But with this DC crap it's about inductance magnitudes and relative magnitudes and stuff. And it's a whole freakin' science project to figure out what transformer parameters to use.

Anyway, that's for another day. At least at this point I have a working forward converter, so now I can go back and tweak things per the actual power supply design.   

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