SEPIC Converter oscillation

I assume this is a high frequency thing on the drain of the MOSFET. It is likely the capacitance of the MOSFET and the inductor interacting.

Is it really a problem? It may just be the normal sort of ugly wave form you see in switchers. If this is the case, don't try to get rid of it. Anything you do to get rid of it makes a new and real problem.

If it's not a ringout with a fast decay but a stable oscillation this might fly into the OP's face at the EMC lab. When they give the thumbs down for EMC cert.

I don't think this is what the OP was talking about. It is fairly hard to make a switcher have a sustained oscillation above the switching rate. It it was below the switching rate, he would likely have said something about the pulses being modulated or the like.

If it's way below the switching rate some of it usually show up in the output. Until a couple years ago I thought the same, that it is hard to sustain oscillation on a switcher. Until one fine day where a client asked me to see what can be done to improve the EMI of a system so it would pass at the EMC lab. Found a nice stand of "trees" around 260MHz. Before disassembling much I asked their engineers about clock rates and such. The consensus was that there wasn't anything that could generate this. Turns out it was the switch mode supply. Singing like a bird. Forgot which manufacturer but it was one of the big famous ones.

The "stand of trees" effect can be a burst oscillation on the switching edge. It can also be just the selection from the existing spectrum by something resonant. I don't think the OP was talking of either of these. He was looking with a scope. This sort of thing is hard to see.

Power10?

I also chased it with a scope, an EMCO near field kit and a wideband amp in front of the scope. It was oscillation. Didn't bother the functionality of the loop though but I guess they had designed a bit "on the edge" there.

I really don't remember, this was just one of the layers to peel in that case. Like usual it ends up with a laundry list of changes. That power supply was outta there afterwards. We didn't want to fix the problems of an OEM supplier, just informed them about it. Of course, I had offered to redesign it for them later but haven't heard back ;-)

Yes that is a scope, in the same sort of way a formula 1 car is still a car. I have never seen a problem like what you had but can imagine how it could happen if there was enough wrong with the design and layout.

I once had a DC-DC converter of my own design act very strange. It turned out that when the layout was done a large plane area was connected to the drain of the MOSFET instead of ground. You could see harmonics of the 150KHz switching frequency on a scope probe held more than a foot away from the PCB. It made so much EMI it messed with other supply circuits on the same PCB.

Yep, but the fix was easy: Switched manufacturers, problem gone.

The most embarrassing scenario is when you turn on the new super-duper design and the lab radio goes weeeeeooouuu ... phshhhhhhht....

On May 5, 11:39 am, Joerg wrote: [.... about switchers ....]

You must have lived a charmed life.

KABLAM FIZZZKKKKKKA-POOF and throwing the casings of the capacitors clear across the lab is by far more embarrassing.

Oh, I had my share of those. The first one was as a teenager when I was still blissfully unaware that even large electrolytics have their limits on the ripple current you can burden them with. Transformer-less tripler from 230VAC since a transformer just wasn't in the budget. Cranked out a stiff 910VDC, sweet. Just don't ever plug in the power cord reversed. In Europe those weren't polarised so a Sharpie dot had to suffice. Turned it on, fired up the amp. 500W out, great, 700W, 900W, 1200W, yeehaw! Then the fluorescents in my room dimmed in a weird fashion. Even more weird because I knew for sure there was no dimmer. KAFOOMP!

One of the caps went straight into the ceiling, taking out a big chunk of plaster, then returned from orbit still hissing. Or rather, its wrinkled can returned because the rest had come out of it and was raining down as fluffy snow. I guess it had celebrated its last seconds by mutating into rocket fuel. Luckily I wasn't with my head over the amp, else I'd be disfigured now. Had to mix plaster, get up on a ladder, then re-paint the whole ceiling. Oh, and my shirt had lots of burn holes in it and so did my bed and the carpet and a chair and... Mom was not enthused about my hobbyist activities that day.

"I'm pretty sure we shouldn't be breathing the yellow smoke."

Been there :-)

The real stench came from tantalums. They let of a greenish cloud. Cured me pretty much from ever using them.

I've used kabillions of tantalums. The only time I ever had a magic stinky (and violent) smoke release was when the dummy populating the cards plugged them in backwards. We had some pretty strict quality standards for 'em though.

If you want stink, try frying a selenium rectifier. ...eewwwww!