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Old 12th July 2010, 05:47 PM   #1
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Default Fast capacitors for SMPS

Which electrolytic capacitors are considered "fast" and suitable for SMPS?
A different question: What details in capacitors datasheets indicate their "speed"?
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Old 12th July 2010, 06:25 PM   #2
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I've never seen the phrase "fast capacitor". However, to answer your question, you want caps with a low equivalent resistance (ESR) and a low equivalent inductance (ESL). Some cheap electrolytics may have very poor ESR and ESL.
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Old 12th July 2010, 06:31 PM   #3
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Snap-ins have impressed me.
http://myweb.msoe.edu/williamstm/Ima...Discharge2.jpg
This was a 220uF 400V snap-in discharged into a xenon flash tube. I forget if the Y axis is light intensity or current. Whatever it is, it discharges quite quickly, still ESR limited, but nearly critically damped.

For smaller values, can-with-leads style caps are okay, but make sure they are low ESR type. "General purpose" caps make excellent AC resistors!

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Old 12th July 2010, 06:35 PM   #4
stoc005 is offline stoc005  United States
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Default Caps for SMPS

Look for ones that have a ripple current rated at 100KHz or more. If the rating is at 120 Hz, maybe that cap is not suitable for SMPS duty. You want the ESR to stay low at 100KHz --> over 500KHz. Look at the ESR vs frequency chart for that cap. Of course, the higher the ripple current rating the better.
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Old 12th July 2010, 06:39 PM   #5
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I remember some raving in the past about photo-flash caps
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Old 12th July 2010, 06:52 PM   #6
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Quote:
Originally Posted by tinitus View Post
I remember some raving in the past about photo-flash caps
Photoflash caps- yes!

I remember that as the closest I came to dying while sitting down ....
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Old 14th July 2010, 08:06 PM   #7
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Photoflash caps are wonderfully resistive. My camera has a 330V, 220uF cap in it the size of an average 2200uF 35V cap. If they weren't so resistive, there would be no way the GT8G132 would be able to turn off the flash, which enables multiple flashes. This incredibly small transistor is rated for 150A peak, suggesting a total ESR around 2 ohms. (Part of this will be due to wiring, of course. They use really flimsy wire.)

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