Increasing capacitance

[Cohibaman]

When recapping a switching power supply, space permitting, what are the pros & cons of increasing the capacitance of the output filtering caps?

[budm]

Re: Increasing capacitance

Some info here.

[Cohibaman]

Re: Increasing capacitance

Some info here.

Excellent reads! Thanks!

[Behemot]

Re: Increasing capacitance

The first one is more for DC-DC VRM than SMPS.

But usually the upper limit of capacitance is very far for ATX PSU, especially with all that other capacitive load (almost everything these days has its own VRM with input capacities) increasing it does not make much difference. You just cannot lower ESR too much, compensation may not like it if it's not designed for it (new PSUs using polymers from factory are).

Is there any particular PSU you ask about?

[Cohibaman]

Re: Increasing capacitance

It was a general question on switching power supplies vs linear mainly, but also came up when I recently recapped my McIntosh MS300 power supply with Panasonic FR's and a few Nichicons. It's a TOP249Y based 66 watt switcher powering an IBM PowerPC motherboard, a HD and a CD ROM. I resisted increasing the capacitance since ESR would be already be lower with the same size Pannys, and I know ESR can influence feedback/regulation. I figured I could always add caps to the 4-pin molex connectors going to the drives and to the 20-pin ATC connector later on if the pros outweighed the cons.

(the second pic with the switcher is before the recap)

[Behemot]

Re: Increasing capacitance

Yeah FR have too low ESR, Chemi-Con KYA are much better suited for these old PSUs as they have high capacitance while reasonable ESR. That's why I have went for them. But as I do not work on such old stuff anymore, I am taking KYB next with slightly lower ESR.

Adding caps to cables (where you do not drain power from) is not a good idea, than the power will flow forwards and backwards, basically making oscillator. Filtration caps should always be as close to rectifier as possible with minimum ESR (but it must be of level which the circuitry accepts).

[momaka]

Re: Increasing capacitance

I resisted increasing the capacitance since ESR would be already be lower with the same size Pannys, and I know ESR can influence feedback/regulation.

It can, but what you have up there is a discontinuous flyback PSU. Those will usually take very low ESR caps fine. You may get some high-pitched whining noises, though. Won't damage anything, but could be an annoyance for some people.

In regular PC PSUs and those that don't use a flyback topology, it's best to not go much lower in ESR. But choosing between staying with the same ESR or same capacitance, always choose same capacitance (or higher). In other words, never lower the capacitance just to match the ESR. PSUs are like the exact opposite of motherboards, where the ESR is more important.