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| Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
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29th Aug 2011, 9:19 pm
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Dekatron
Join Date: Jan 2004
Location: Newcastle upon Tyne, Tyne & Wear, UK.
Posts: 8,195
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Film capacitors at high frequencies
Hi Gents, following on from an earlier thread about failures in X class caps, I have just seen an article in a professional magazine (Bodo's PowerSystems) relating to component lifetimes.
There is the usual stuff on e-caps and ripple current, but there is also a section on film caps and their rated voltages, where a "popular" cap is rated at 400vDC and 250v AC (50Hz), but only a few volts at 100KHz. If this can be assumed to be an X cap, then in use it will see some of the switching frequency from downstream; might this be the cause of some of the earlier failures? This may be due to the higher frequencies but could also be due to dV/dt, a known problem with this type of cap if used as a snubber in SMPS. The article implies that this effect is mostly noticed in metalised polyester types, not polypropylene or foil/ film types. Ed |
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29th Aug 2011, 10:51 pm
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Dekatron
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 16,536
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Re: Film capacitors at high frequencies
Isn't this because of dielectric losses at high ac currents? For the same RMS applied voltage you get 1000 times the current at 100kHz than at 100Hz.
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