Replacing Caps in Power supplies

[davmax]

Re: Replacing Caps in Power supplies

Many thanks 999999999 for the Elna document link. Much appreciated. The document has been used to create a contribution to convert formulae and cap data into practical understanding. I have posted it under Recapping Your Motherboard to assist with understand of caps on motherboards where caps are usually the hottest.

Again thanks for your contribution.

[Logistics]

Re: Replacing Caps in Power supplies

Okay, after reading a little bit about PSU caps and ripple current, it seems that for any given rail, the capacitor's ripple current would have to be rated at the regulators maximum output current.

I'm looking at this old AT PSU I have and it says that the load current at the +12V rail will be a minimum of 1.6A and a maximum of 9A. So does this mean ideally it would need a capacitor which can handle a maximum ripple current equal to or greater than the maximum output current of the regulator?

[davmax]

Re: Replacing Caps in Power supplies

Logistics. The PS output current value is a DC value and is seperate from the AC ripple current that passes through the cap. Only AC can pass through a cap. Supply output ripple voltage can be about 50mV. Taking this 50mV value and an output cap with a ESR of 9 milli ohm the ripple current through the cap will be 50/9 = 5.56 amps, rather high for most caps. This means the output design must share ripple across several caps or the ripple voltage must be reduced either by inductor design or an increase in capacitance. Does this help?

[Logistics]

Re: Replacing Caps in Power supplies

So aside from upgrading to nice caps (this one has f-you's) would it be good to run two capacitors of half the capacitance in parrallel rather than a single of the full capacitance? I seem to remember discussions earlier which led me to believe that adding another cap of the same capacitance, (doubling the capacitance) was a bad idea because it would stress components.

Edit: Oops, jumbled up my first post.

[davmax]

Re: Replacing Caps in Power supplies

I was just trying to clarify the difference between DC load current and AC ripple current. When replacing caps it is best to keep matters simple.
1. Go for known quality.
2. Replace like for like ie same capacitance and same voltage rating, although 10V caps can be replaced by 6.3V.
In other words do not stray from the original design. Capacitance can be increased by 20% without much problem. Doubling capacitance will reduce ripple but will also slow the power supply response time to a changing load ie a CPU. A slowed response can cause voltage overshoot, it depends on the actual design at what point this occurs.