Recommend primary capacitor ratings

[japlytic]

Here are some values I recommend for the two 200V/250V primary capacitors used in most PC power supplies, board space permitting:

<200W: 220uF
200-250W: 330uF
251W-350W: 470uF
351-400W: 680uF
401W-450W: 820uF
451-550W: 1000uF
551-650W: 1200uF
651-750W: 1500uF
751-850W: 1800uF
851W-1000W: 2200uF

Some of these have been overrated, especially at the higher wattage end.

[larrymoencurly]

Re: Recommend primary capacitor ratings

Some values I've seen:

220uF: 90W Gateway/Newton, 250W cheapo (PC P&C's prime example of a bad PSU)
470uF: 200W Dell/Delta, 550W Q-tec
680uF: 300W Antec & Fortron/Sparkle, 350W Enermax
820uF: 300W Delta (oddly, rated for down to 100VAC, not the usual 90VAC)
1000uF: 350W Fortron/Sparkle

[Per Hansson]

Re: Recommend primary capacitor ratings

Well, here my question comes again then ;-)

Can someone more knowledgeable than me please comment on the very small rating of the Primary bulk cap, only 270uF in the S12 430w PSU, and 220uF in the 350w Seasonic?

Is it due to the active PFC or something? I mean I've seen Primary caps in L&C powersupplies with almost twice the farad rating!!!

[Oklahoma Wolf]

Re: Recommend primary capacitor ratings

It's the active PFC I'm sure. My FSP 600W with active PFC has a single 470uf 400v Capxon on the primary side, while the older 530W with no PFC has two Teapo 1200uf 200v capacitors.

[Chris1992]

Re: Recommend primary capacitor ratings

Which would explain the Q-tec's abnormally low primary cap.

[PeteS in CA]

Re: Recommend primary capacitor ratings

Per, there are two factors involved.

First, in P/Ss without active PFC, the two I/P lytics are connected in series (necessary for the I/P voltage range switch). This means that the net capacitance is cut in half. IOW, if you have two 470uF lytics, the net capacitance is 235uF.

Second, when you have active PFC, the voltage on the lytic is typically in the 325V-350V range (and regulated). Without PFC, the voltage across the I/P lytics is typically in the 260V-270V range (unregulated). The formula to calculate the energy stored in the lytics is .5(C)(V^2). Thus, for the same net capacitance, if the PFC bus voltage is only 325V and the non-PFC bus voltage is 265V, the lytic in the PFC is storing ~50% more energy.

[Per Hansson]

Re: Recommend primary capacitor ratings

Thanks for the explanation Pete, I really appreciate it!

[Oklahoma Wolf]

Re: Recommend primary capacitor ratings

Me too

Just remembered something about my 530W - next to the two 1200uf Teapos, there is a place for a PFC board... I wonder how much that would have affected Fortron's choice of input capacitors had the PFC circuitry been added. Then again, now that I think about it, that may just have been a soldering point for a passive PFC coil...

[jonnyGURU]

Re: Recommend primary capacitor ratings

An SMPS I know told me pretty much the same thing OP is posting. He said that 1000uF in a 500W and 1500uF in a 550W is widely accepted, but said that it's more "accepted" to use a 1500uF in a 600W.

Of course, that doesn't take into account the 470uF~820uF caps found in 500W and up active PFC PSU's.

[dood]

Re: Recommend primary capacitor ratings

I have a Topower server PSU that's supposed to be 420w... has two CapXon 1200uf 200v primary caps...