Small, high voltage capacitor

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Hi guys,

Do you know of any HV (preferably SMD) caps out there, with a reasonable capacity ( 68nF and more) rated at 1500V or more?

So far I've found:
http://sk.farnell.com/jsp/search...

They are great in everything, aside from the price. We'll most likely use them, since they are far smaller than our current solution ( http://www.wima.com/EN/WIMA_MKS_... ). The main problem with that is that we'll end up at 6x the price.

The questions:
1. With size in mind, do I have any other option than ceramic caps? If so, what is it?
2. Is that the price I'm going to get from all manufacturers for such a device?

Thanks,

David

There are pointy haired bald people.
Time flies when you have a bad prescaler selected.

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Look at the MKP film capacitors from Wima, etc. They are metalized polypropylene, and tend to be more compact than polyester for the same voltage rating.

Tom Pappano
Tulsa, Oklahoma

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Your challenge is "small" and "high voltage" and "68nf".

Some basic principles:

1) for a given dielectric, the capacitor volume increases more or less linearly with capacitance at a given voltage rating.

2) for a given dielectric, the capacitor volume increases more or less linearly with voltage rating for a given capacitance.

Thus, you would expect a 68nf cap to be about 68x the volume, all else equal, compared to a 1nf cap, both rated at the same voltage. Likewise, a 1500V cap would be about 15x the volume compared to a 100V cap with the same capacitance.

Film dielectric tends to give more capacitance per unit volume at higher voltages because of the breakdown mechanisms in ceramic materials. Also, you may find that the high voltage and (relatively) high C in a ceramic cap may force you into the realm of "high-K ceramics" where temperature coefficients can eat you alive (imagine a cap with less than 50% the value at 0C compared to 27C).

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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Thanks guys!

I'll take a look at those MKP caps, but at a quick glance they seem to be pretty much the same stuff we're using now.

Quote:
Also, you may find that the high voltage and (relatively) high C in a ceramic cap may force you into the realm of "high-K ceramics" where temperature coefficients can eat you alive (imagine a cap with less than 50% the value at 0C compared to 27C).
Many thanks for this warning, I didn't realize this. I looked it up, and the caps from farnell seem to have a reasonable characteristic (worst case -15% capacitance vs. temperature). Anyway, they are only voltage filtration caps, so +/-20% shouldn't be a big problem.

There are pointy haired bald people.
Time flies when you have a bad prescaler selected.

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ESR might change too over temperature. Some LDOs require the ESR of the output cap to be within a certain range or they won't regulate properly. Probably not an issue in your case I guess.

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Quote:
ESR might change too over temperature. Some LDOs require the ESR of the output cap to be within a certain range or they won't regulate properly. Probably not an issue in your case I guess.
Fortunately not in this part of the circuit :-)

There are pointy haired bald people.
Time flies when you have a bad prescaler selected.

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Daqq,

There may be other high voltage SMD caps typically used in higher power RF transmitters.