MC34063/78S40 buck

[Raveninghorde]

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

 

[Sylvia Else]

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.

 

[James Arthur]

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

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

 

[Raveninghorde]

Are you sure the inductor isn't saturating?

Sylvia.

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.

 

[Sylvia Else]

Doh!

I know that feeling well enough, from many occasions. Each time I vow
never to be so blind again, and each time....



Sylvia.