When using electrolytic caps in a power supply is it any advantage in using a cap that is twice or three times the output voltage, like a 35 volt cap on a 13 volt supply. Also any ideas on electrolytic caps behavior when the caps are tested at -55 C, any changes?
They'll tend to last longer. I don't believe there's much advantage over about double, though.
Assuming you're talking about aluminum electrolytics, capacitance drops and ESR increases by a fairly large factor. Limits may not be specified as low as -55°C (maybe at -40° or -25°C), even if they are rated for operation that low. Check the data sheets carefully if that's a real consideration for you.
Best regards, Spehro Pefhany
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I have heard that capacitors may become leaky if not used reasonably close to their rated voltage, because the voltage helps maintain the aluminum oxide dielectric. I have used 35 VDC capacitors for 5 VDC regulator outputs without problem, but usually I specify 10 or 16 volt. I don't think it is as much a problem with tantalum.
R0 = resistance of foil, tabs & terminals R1 = resistance of electrolyte R2 = dielectric loss resistance C1 = terminal capacitance C2 = dielectric loss capacitance
R2 and C2 give a large variation in ESR with frequency. typically the effect of R2,C2 peters out above 10kHz, so you can take the ESR at
100kHz as the combined value of R0 and R1. this can be seen from the ripple current multiplier tables a decent cap data sheet has.
R0+R1 = ESR @ 100kHz
R2 = ESR @ 100Hz - (Ro + R1)
and pick C2 to get the right values for ESRs in the 100H - 10kHz range
the reason the ripple current varies with temperature is the loss in the cap is kept constant (for a given lifetime) so lower ESR means more current. you can thus translate a ripple-current multiplier table (eg see LXZ cap datasheet) into an ESR multiplier.
ESR_multiplier = 1/(ripple_multiplier)2
the LXZ table for 220uF - 560uF caps is:
120Hz 1kHz 10kHz 100kHz
0.5 0.85 0.94 1.00
so the ESR multiplier is:
4 1.4 1.13 1
the ESR of these caps have at 100Hz is 4x th 100kHz value....
ESR variation with temperature is due to the increased conductivity of the electrolyte.
R1(T) = R1o*exp[(To-Tcore)/E]
R1o = ESR at temperature To
E = temperature sensitivity factor
R1 is usually 5x Ro, and you can calculate E from the two ESR measurements (-10C, 20C) for a given cap family.
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