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Old 26th November 2005, 02:48 PM   #1
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Question Snubber capacitors choice

I am looking for snubber caps to put on my chip amp PSU.

People say that MKP caps are the best among all, but I also found some others that look very good on paper too.

- ceramic caps
- multicoated ceramic ( KDPU )
- tantalum

Ceramic are commonly used in HF applications, so why wouldn't they suit PSU filtering up to MHz ?

Tantalum, for larger values have a good reputation too.

What characteristics should be analyzed to compare snubber caps ?
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Old 26th November 2005, 03:00 PM   #2
pooge is offline pooge  United States
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You should use an X or Y rated cap. These are safety rated for full line voltage and surges. They are safety rated by their failure modes.
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Old 26th November 2005, 03:14 PM   #3
sam9 is offline sam9  United States
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You might look up "Quencharc" in the MOuser catalog. These are too pricy for me but, I've read sometime back of at least one forum member utilizing them.
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Old 26th November 2005, 03:21 PM   #4
Bare is offline Bare  Canada
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I've used Solen Film 'n foil caps (600v types) with no problems whatsoever. Easy to find and buy.
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Old 26th November 2005, 03:24 PM   #5
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Quote:
Originally posted by pooge
You should use an X or Y rated cap. These are safety rated for full line voltage and surges. They are safety rated by their failure modes.

My problem is not especially reliability but rather quality.

I have seen some datasheets that show the impedance vs. frequency graph.

I assume that the cap must have the highest possible resonnance frequency to damp correctly the MHz + rubbish.

Is there something else in favour of one of those caps ?

PS : I'm not willing to spend much on small caps, that's why I'm asking the question
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Old 26th November 2005, 03:34 PM   #6
Tweeker is offline Tweeker  United States
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Something to look at when comparing caps in general is thier dielectric constant (k). A lower K dielectric will absorb less RF than a higher one. This is important in some applications, as among other things this absorbed RF becomes heat.

Polypropylene has the second lowest K (~2.2) of commonly used solid dielectrics. Teflon is lower, but suffers from high wallet leakage. If your going to abuse a cap, poly is probably your choice. Snubber capacitors for SCRs and IGBTs are almost invariably polypropylene.

Ceramic has a much higher K, allowing a higher capacitance for a given amount of plate area, so less inductance and ESR is possible. Ceramics tend to be used for local HF coupling and decoupling as the inductance is critical here. Having the worlds lowest ESR isnt necessarily desirable in a snubber, they can lead to high Q circuits that may resonate and ring.

Tantalums have thier place with some much desired properties, but are not famous for reliability and really dont like being reversed(pop!).

Heres some gratuitous pictures of cap abuse.
Click the image to open in full size.Click the image to open in full size.
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Old 26th November 2005, 05:01 PM   #7
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Great information !

Thanks a lot

I'll stick with the MKP caps then, I found some not too expensive ones that look very good.

Greets
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Old 26th November 2005, 10:47 PM   #8
sam9 is offline sam9  United States
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Quote:
My problem is not especially reliability but rather quality.
the X and Y cap comment was not about reliability but about safety. More specificly about caps catching fire or shorting in a way that injures the user.


With rasgasrd to snubbers, however, the following article is often cited:

http://www.hagtech.com/pdf/snubber.pdf

even if you find the calculations a bit difficult to apply exactly, note that without the inclusion of a series resistor the benefits of snubber caps (regardless of type) is rather limited.
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Old 27th November 2005, 01:03 AM   #9
Tweeker is offline Tweeker  United States
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I dont really see the point of 600 volt caps next to 63 volt or whatever electrolytics. For caps on the primary side of the transformer x and y ratings are a must.
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Old 27th November 2005, 01:14 AM   #10
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Quote:
Originally posted by Tweeker
Polypropylene has the second lowest K (~2.2) of commonly used solid dielectrics. Teflon is lower, but suffers from high wallet leakage.
I'll have to remember that.


Cheers,
Francois.
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