Capacitors and the Computer PSU Mon, Mar-05-2007
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Right here on this very site, several versions of the FSP Epsilon design developed an early reputation of having a bit of a ripple and noise issue that saw ripple exceeding ATX specs at full load. I wanted to see just how much capacitor quality had a bearing on this problem. And I use the word "problem" loosely, for it has yet to be proven that up to 200mV of ripple is really that much worse than the usual spec of 120mV. So, I put the unit on the bench and grabbed the same brand and model caps used from the Antec recap, and set to work on one of FSP's latest and greatest designs.

The insides look a bit anemic for a 600 watt power supply, but this isn't your father's 600 watt power supply. It's far more efficient with a higher switching frequency than the designs of yesteryear. You say your father doesn't have any 600 watt power supplies? Hey, my mind's made up, don't confuse me with facts!

Moving on, we need to remove the main PCB just like we did on the Antec. See the brown and blue wires in the below picture? Those are our targets on this unit. Make a cap placement diagram like with the Antec.

Some of the caps we're going to replace. These are made by Capxon, a lesser known but still respected company. You can see one of the screws to remove to get the PCB out - the FSP is a little screwy in how it's mounted in there, bad pun intended.

A close-up of the two wires keeping us from pulling out the PCB. And they're glued in on this one. Gently pry or cut the wires away from the glue so you can desolder them.

The PCB out of the enclosure. Again, we'll ignore the primary side with the single huge cap. The secondary side on the right is where the action is.

That black thing between the big coils and the heatsink is another cap with a sock pulled over it. Don't worry, it's not planning to rob a liquor store, it's just covered up to keep it from heating up too much around those hot little coils and output diode packs. Carefully cut or pry the glue away from all caps.

You may be wondering what's the deal with only two coils in this one. Well, this is a group regulated design, meaning that two rails share a coil. In this case, it's the 5v and 12v rails. While this means loads on one will always affect the other, it is a common practice when space and cost savings are important. Most older designs that didn't cost a mint (and even some that did and still do) use this type of design.

This time, we aren't going to pay too much attention to anything other than the 12v section, for 'tis that area where the ripple is at its worst. But, I ripped out every cap on the secondary and replaced them with Chemi-Con anyway. 12v output is handled by two OST 2200uF caps in parallel, where they are attached to protection circuitry that splits the unit into four 12v rails. Use a good powerful iron again, because this unit has some big traces to solder.

Above we see one of a couple places where soldering the FSP becomes a game of how steady we can keep our irons. One small slip could easily wipe out that little SMD resistor. Soldering on this PSU is not for the newbie I'm afraid.

Old caps gone, never to return. Keep the sock for that one near the heatsink - you'll need it for the new one.

As with the Antec, we fit our new caps in place, holding them in with masking tape if necessary, and then solder them up. Above is a picture of one cap soldered in, ready for lead trimming. Just to the right, you can see the holes of another cap location, and the second place you need to be reeeeeally careful with that iron.

Nice brand new Chemi-Cons all soldered in. KZE and KY used.

Everything back together. Time for testing. Power it on outside a computer as with the Antec. Again, verify all active rails.

3.3v works fine.

12v rail - an oddball measurement like this is typical of a group regulated PSU being run without a load. It's in the ballpark, which is good enough to tell it works.

Here we have the other half of the odd effects of group regulation, a high 5v rail. This is normal. They will even out when the PSU is loaded properly.

Anybody curious to see how they did now? Next page, please. Things are about to get... interesting.

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