Electronics > Projects, Designs, and Technical Stuff

ATX Flyback Transformer

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engrguy42:
LTSpice sure doesn't like nano-henries does it? Freakin' slow as molasses on a cold day in January at the South Pole...

Anyway, here's what I have so far. I used mH for the transformer inductances to speed things up. At 100mSec I slam on the 2.4 amp load to the 12V output, and it's just a tiny blip. I'm gonna let it run for a while (like 0.5 seconds) to see if it holds or it drops off..

HackedFridgeMagnet:

--- Quote from: engrguy42 on June 21, 2020, 09:35:55 pm ---LTSpice sure doesn't like nano-henries does it? Freakin' slow as molasses on a cold day in January at the South Pole...

--- End quote ---

u got your polarity wrong.

engrguy42:
WTF??? This isn't fair.

So I fixed the ability to handle switching on load and maintain output voltage, but now if I run the sim longer 8mSec after it picks up load the freakin' controller shuts down. WTF?? I never told it to shut down !!! Apparently when the "Ith" (threshold) pin voltage gets up to 2 v it shuts down. And it stays off for like 50mSec, then turns on again. And keeps repeating the cycle (BTW, load is still applied the entire time).

WTF?? Now it's just trying to piss me off.

So now I have a freakin' science project to determine what happens with the "Run/SS" and "Ith" pins and what that crap all means. I have no clue how it works. And of course the datasheet for the LTC3873 doesn't tell you directly. I suppose you have to search for clues and figure it all out.

I hate electronics.   

Buriedcode:
Look at your transformer.  This is a flyback converter, remember what I said about current not flowing in both the primary and secondary at the same time?

I realize you're getting quite into this, but others have suggested you start at the basics, but you don't seem to like links to articles.  Do you want someone to provide a full explanation?

engrguy42:

--- Quote from: Buriedcode on June 22, 2020, 12:04:15 am ---Look at your transformer.  This is a flyback converter, remember what I said about current not flowing in both the primary and secondary at the same time?

I realize you're getting quite into this, but others have suggested you start at the basics, but you don't seem to like links to articles.  Do you want someone to provide a full explanation?

--- End quote ---

A while back (my post from yesterday) I changed to a forward converter in order to get a higher output and showed the updated schmatic. I'm applying a 30 watt, 2.4 amp load and therefore need (I believe) a forward converter. And it seems to work okay other than my latest problem with shutting off.

And yes, I'm reading a lot of info on this, most recently the rather hilarious App Note 19 from Linear Technologies, "LT1070 Design Manual".

EDIT: BTW, my favorite troubleshooting hint from that app note:

12. Didn’t Read the Data Sheet
Then you shall have no pie.   

:-DD

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