Are oil capacitors suitable for snubber circuits

Yeah, pretty much. Excuse my brevity.. polypropylene has the lowest loss, so they don't really make low ESR, ESL caps out of anything else. Generic film (mylar, etc.) comes close but it's not really what you're looking for.

Tim

-- Deep Fryer: a very philosophical monk. Website:

Are they low ESR, etc?

i

Their internal construction (as with most types) can vary quite a bit, though generally they have a pretty high current rating for their capacitance, compared to something like metalized mylar. But the good ones are sold specifically as snubber or commutation capacitors.

There are also high current polypropylene foil capacitors that are made specifically for high peak current applications with heavy connections. Though there are also metalized film versions that emphasize self healing instead of high current ratings. Don't assume any particular type meets your peak and RMS current requirements unless you read that it does in a data sheet.

So, I only should look for polypropylene caps? Is that right? I do not really mind that, I just want to understand.

i

You start with the RMS current and peak current the cap must handle. the instantaneous waste heat will be peak current squared times ESR and the average heat will be RMS current squared times ESR. Many capacitors do not dissipate heat at all well. The peak current rating (often listed as I^2 * time) is based on something melting inside the cap and the RMS rating is based on overall internal temperature rise.

You will be ready to shop for a capacitor when you have the design well enough defined to at least estimate the peak current and duration and RMS current. Then you dive into data sheets. Or you just spend what ever it takes to get the best caps you can find and hope they are good enough.

I think an industrial distributor like Newark may have a few more high current choices than Digikey does.

Here are some to study:

Thanks. Another person also mentioned polypropylene.

What is low enough ESR for my purposes, how would I arrive at a number?

i

Yes. I've used the CDE oil-filled caps in 500 kVA inverters. Look at the snubber capacitors at:

They don't *have* to be oil-filled. Their chemistry depends on the application.

Cheers, John

Polypropylene caps have the largest dV/dt capability of any dielectric I know.

Graham

BTW, on the opening page at

in the "Site Features" box, click on "Technical Papers". That will produce a list of documents one of which is titled "Design of Snubbers for Power Circuits".

Just thought you'd like to know.

John

I see. Thanks.

Thank you, I will read these datasheets.

Thanks. I found a digikey part number 338-1142-ND, that I think fits what I need 100%.

I got it. Thanks a lot.

i

Thank you, that's a great paper.

i