Bizzare behaviour from SG/UC3525

Pooh Bear · 2005-06-22T08:30:45+00:00

Hi,you may have noted my posts about problems recently with my attempts tomake a high power smps.I'll give a quick overview.The application is a high power pro-audio amplifier.Traditional psus use nice toroids that we know well. You can optimisethem for low copper losses that helps keeps weight down.We want to make our amps lighter though. Like some competitors that arealso using SMPSs.Various mfrs have used switch-mode techonology. The fixed duty cycleapproach appears to be universal. I expect this is since a classicforward converter doesn't have the bandwidth to respond to audiotransients. Not to mention cross-regulation to other rails issues.Unlike most 'forward converters' - see argument about this terminologyanother day - PLS ! they run at a fixed duty cycle. I.e. max. Kinda 90%.Ok - so that's the rough overview.So - I have this pcb with my design that's 'plain vanilla' - no frills.The 'control circuitry' i.e. SG 3525 and some other shit is located on aplug-in pcb via a right-angle pcb header.I work-up the prototype - the waveforms look right etc etc....Then I get trouble as I've mentioned here before. The UVLO I can dealwith. If the parts can't 'agree' when to stop working I'll tell themmyself !So - that one's fixed. Ok - finito. ( or so you might think )Next problem - apparent 'jitter' on the trailing edge wavefom at theload ( high side ) with high input volts on the highside drive. Iinvestigate this. I add a zener clamp on the highside IGBT for examplein case of gate resonance. No good. I modify the signal path to reduceinductance. No good.I then find by accident that if I try to 'provoke' the highside jitterit's affected by my hand capacitance on the switch that I have includedto 'stop switching' to the shutdown pin on the 3525. Uh ?I check the supervisory supply. I discover that if the supervisorysupply ( provides drive to the 3525 ) is higher than before - changedtransforner ratio - then the jitter occurs earlier ( I'm ramping theinput volts on a variac...

Jun 22, 2005 43 Replies

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Pooh Bear 21 years ago

That is indeed what I've done. :-)

Graham

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Pooh Bear 21 years ago

Me too ! The 3525 was chosen simply since almost *every* similar design I've seen uses it.

LT are a bit pricey normally though. I'm wondering about On-Semi.

Incidentally, another company ( Italian FWIW ) makes their 'audio smps' with one of IR's 'electronic ballast driver' chips. Basically any old cheap way to make the necessary waveforms !

Graham

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Pooh Bear 21 years ago

Cor blimey !

Can you still get a *genuine* SG data sheet for this part ?

Yup.

That accords with my results.

Graham

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Frithiof Andreas Jensen 21 years ago

polyester type

hot.

I seem to remember that I had *three* different values on there, probably:

100 nF, 10 nF and 500 pF and a funny layout too, there was an application note on the proper placing of the capacitors so they were put in first, then all the rest had to fit wherever it could.

I think there were shottky's on the outputs too so that they could not pull the substrate negative.

Those fast drivers are always a pain in the bum!

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