Film Capacitors Not for High Frequencies?

[laingalion]

So from I read from the wiki and people talking on forums, film capacitors are great for high frequency stuff.
I need a reliable capacitor that can handle high voltages at high frequencies (50V peak at 60 kHz, square-ish waves from full bridge amp).

I went looking at datasheets and I noticed that for high frequencies the "permissible AC voltage" drops FAST.
Example, page 15:
http://www.farnell.com/datasheets/1468249.pdf
and page 35 (3rd pdf page):
http://www.farnell.com/datasheets/815363.pdf

Are there film capacitors that work in high voltage and high frequencies? Do they exist? Or am I misunderstanding the datasheet and/or how film capacitors work?

[The Electrician]

Look up the specs on metallized polypropylene capacitors, MKP style.

[Rufus]

Are there film capacitors that work in high voltage and high frequencies? Do they exist? Or am I misunderstanding the datasheet and/or how film capacitors work?

High frequency and high voltage across a capacitor means high current which means resistive and dielectric losses cause the capacitor to heat up. The Wima data sheet states the plots are for a 10 degree C internal temperature rise.

The plots are more maximum dissipation rating than maximum voltage rating.

[laingalion]

Are there film capacitors that work in high voltage and high frequencies? Do they exist? Or am I misunderstanding the datasheet and/or how film capacitors work?

High frequency and high voltage across a capacitor means high current which means resistive and dielectric losses cause the capacitor to heat up. The Wima data sheet states the plots are for a 10 degree C internal temperature rise.

The plots are more maximum dissipation rating than maximum voltage rating.

That makes sense.

Doing a bit more thinking about my application I realized these capacitors will not be seeing the full wave, but rather ripples. I will be using these as snubber capacitors connected between the full bridge source voltage and ground, and will be placed right next to the MOSFETs. If the rating graphed is due to the dissipation rating then I can likely get away with a much larger voltage.

Thanks a lot, saved me a ton of trouble redesigning.

[Conrad Hoffman]

You need the lowest dissipation factor you can find at the frequency of interest, and that's usually polypropylene. Just as a reference point, I built a small induction heater running at a similar frequency. The resonating cap was a very low loss polypropylene, yet it still got hot and I was duty cycle limited! Several caps in parallel can solve the problem.