Electronics > Projects, Designs, and Technical Stuff
ATX Flyback Transformer
engrguy42:
And here's the traces for the gate voltage and the voltage on the diode side of the output inductor during the heavy overload. This controller supplies a fixed 200kHz, and the gate voltage is showing that 200kHz at a 30% duty cycle. And I recall for this config you need a duty cycle below 50%, so it looks reasonable. And it also looks like it's not running out of duty cycle.
Oh, and you can also see the Vout trace at 5V, when it's supposed to be at 12V...
engrguy42:
First of all, LT data sheets SUCK!!! :--
They tell you how to size the voltage divider resistors to give 1.2v into the FB pin at the desired output volts, but otherwise they tell you freakin' NOTHING!! Nothing on how to size the C for threshold (Ith) pin, or the R for the Vcc pin, or the size of the sense resistor, or what to do with the Run/SS pin. |O
Anyway, I plotted the feedback voltage as I start the power supply, then apply the big load at around 20mSec, then remove it at around 40mSec and everything returns to normal.
Attached is the FB voltage (this controller wants 1.2V here at the desired output) and the Ith threshold pin voltage. When output volts is good the Ith volts go to 0.9, and when the output is heavily loaded it slams to the ceiling of 2.0V, while the FB voltage drops down to around 0.7 volts (which means 7V output), then then decays down to around 0.3v.
I dunno, something is limiting the ability of the controller to boost the very low voltage. :-//
T3sl4co1l:
Can't see the schematic; did you edit the attachment by any chance? I see the thumbnail but not the full size, something got out of sync.
Tim
engrguy42:
--- Quote from: T3sl4co1l on June 21, 2020, 02:13:26 pm ---Can't see the schematic; did you edit the attachment by any chance? I see the thumbnail but not the full size, something got out of sync.
Tim
--- End quote ---
Yeah, for some reason it's not uploading to the server. I got a message it's loading too slow or the file's too big. Which it isn't.
EDIT: Okay, there it is...some sort of server hiccup I suppose
engrguy42:
BAM !!!! I think I got it...
On a hunch, I assumed that with a higher turns ratio (N1:N2) I'd get more secondary amps and therefore more stored energy to feed the load. So I cranked the turns ratio up to about 4.5:1 (ie, a L1:L2 ratio of 20:1). And the result (it's still plotting) is just a tiny blip in output voltage as I slam a 2.4 amp, 30 watt load on the 12 output. And it instantly recovers.
BTW, I also added a reset winding with diode to discharge the transformer rather than the RC/diode circuit on the existing primary, and it seems to work great.
I'll post an image of the circuit and traces when it finishes. Sometime next month probably. One thing I hate about LTSpice is it's so freakin' slow on complex circuits.
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