I'm testing a 5V out buck converter designed around the MC34063 (cut down 78S40). Rsc is 0.22R, the inductor 330uH and it runs at about
100kHz.According to the data sheet I should have a peak current of under 1.5A which is the switch transistor rating. TI say minimum Rsc is 0.2R.
At 15V in I had a short circuit current of 1.93A (on 2 samples). I wound the supply up to 24V while measuring Isc and instant failure.
What am I missing?
TIA
Are you sure the inductor isn't saturating?
Sylvia.
Let's see...with 15v in, switch on, the inductor has ~9v across it, current rising at 9v/330uH = 27mA/uS. Ripple current @ 1.5A load is roughly 27mA/uS x (10uS x 5.5v/(5.5v+9v)) = 100mA.
Looks roughly okay.
100Khz looks a little fast--is there significant switch-off delay? That would make i(L) coast past the desired limit, worsening with higher applied voltages.Otherwise, inductor saturation, per Sylvia.
Cheers, James Arthur
Doh!
Thanks. Chose an inductor I knew was OK and then increased the value and didn't recheck Isat. But I knew the inductor was OK so didn't check for that. Forgot rule 1, if you can't find the fault it's where you're not looking.
I know that feeling well enough, from many occasions. Each time I vow never to be so blind again, and each time....
;)
Sylvia.
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