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Old 3rd January 2005, 12:04 PM   #1
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Default bypassing electrolytic capacitors

Hi everyone, Iīve got a question:

Should electrolytic caps be bypassed at the load or at the capīs pins?
In my case itīs an output-decoupling 10000ĩF cap between FET and output; elect. capīs pins or between FETīs leg and output-pin?

And if I have several electrolytic caps (3*10000ĩF80V) in parallel, where do I bypass then? There where I take the supply voltage from and ground? Well, canīt bypass individually, these 1,5ĩF film caps arenīt that cheap...

Thank y'all, David
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Old 5th January 2005, 12:48 PM   #2
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bypassing an electrolytic, at least to my way of thinking, compensates for the ESL of the capacitor, in that an electrolytic has a higher impedance at high frequencies due to the series inductance. so you could bypass one time near the electrolytic with a 100nF film capacitor at the power supply.

it's not really "bypassing", rather "decoupling" but place, as close as possible to the chip being driven -- with 100nF or 10nF ceramics on each of the power pins. these "bypass" caps decouple the power supply lines, removing whatever RFI, EMI nasties may have been picked up along the way.

fwiw, National shows the power pins of the LM4780 decoupled with 10u and 100n capacitors.
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Old 5th January 2005, 12:50 PM   #3
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Watch the type of film that you use too - some are quite inductively wound
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