Reply 20 of 36, by gdjacobs
PC-Engineer wrote on 2020-08-01, 20:38:
In nearly all cases in Switching Power Supply Units/Circuits the function of the Electrolytic Capacitors are for filtering (mini […]
In nearly all cases in Switching Power Supply Units/Circuits the function of the Electrolytic Capacitors are for filtering (minimalizing the AC ripples on the DC voltages). As higher the current, as lower the needed amplitude of the AC Ripple, as highe r the needed capacity. The power dissipation on the ESR correlates with the frequency and the current. The main effect of a higher ESR is a higher temperature of the capacitor. The good message is, that the modern low ESR electrolytic capacitors accept higher temperatures.
For Recaping you should follow this rules:
- rated Voltage must be equal or higher (the higher voltage rating of the capacitor doesn’t lead to higher voltage in the circuit!)
- rated Capacity must be equal or higher (would not take more than double)
- Pitch of the legs must be equal
- Diameter should be equal or larger (better heat dissipation), depends on place around
- Hight should be equal or larger (better heat dissipation), depends on place above
As higher the capacity and/or as higher the voltage, as higher the volume/surface of the capacitor (hight and/or diamater). So for temperature critical capacitors for the CPU voltage regulators it can be smart to use capacitors with a notch higher voltage rating and so a higher hight/diameter for better heat dissipation.
You can test the thermal result in situations with high CPU load (Prime95) with your fingers on the capacitors. I use for recapping low ESR capacitors from Panasonic, Epcos and Yageo. And I never had capacitors which are not long-touchable.
The first criteria is correct.
Sufficient capacity (capacitance?) is more important on a 50/60hz line rectifier than smoothing the output of high frequency switching transistors. There you want an appropriate ESR as higher ESR directly translates into higher ripple voltage.
Matching the leg pitch is desirable but not essential. Sometimes close enough is good. Sometimes you bodge your way through (usually when you don't have SMD parts on hand).
There are rules of thumb for heat dissipation and case size, but the best solution is to use a capacitor with appropriately high ripple current rating.
All hail the Great Capacitor Brand Finder