Snubber in LiteOn PS-5281-7VR

[everell]

I have recently acquired a LiteOn PS-5281-7VR 250 watt atx power supply. This is the second LiteOn power supply I have seen with an electrolytic capacitor in the snubber circuit. Capacitor value is 2.2 uF/450 volts. Measured capacitance is 1.98 uF and esr is 8.0.

So what's the beef? Every other power supply I have looked at has a ceramic capacitor in this circuit. Seems like an electrolytic in this location is just asking for trouble. The first one I saw had a bulging capacitor. Replacing the capacitor fixed the power supply. This one is not bulging, and the power supply is operational.

I thought that the proper value would be more like .1 uF, which is one tenth of the value. Here are some pictures and a drawing of the snubber. What do you think?

[Wester547]

Re: Snubber in LiteOn PS-5281-7VR

Was the bulging capacitor an LTEC by any chance? Or a Chemi-con, Nichicon, Rubycon, or something? Or OST or Taicon? Since most of those brands aren't esteemed and don't deal well with stress.

[everell]

Re: Snubber in LiteOn PS-5281-7VR

The bulging cap incident was a few years ago, so I don't remember the brand. I only remember that it was unusual to see an aluminum electrolytic capacitor in this circuit. And I still think it is unusual......and maybe a time bomb.

[momaka]

Re: Snubber in LiteOn PS-5281-7VR

What were the symptoms of the power supply that had this cap failed? Just refused to power up or unstable power? Or ticking? I'm just curious because I have a few LiteON PSUs as well. I think one or two of them are still in service.

And yes, an electrolytic in that spot almost seems like planned obsolescence. Or just poor design.

[stj]

Re: Snubber in LiteOn PS-5281-7VR

is the polarity marked on the pcb?
maybe it was designed for a film cap and they decided the electrolytic was cheaper.

[goodpsusearch]

Re: Snubber in LiteOn PS-5281-7VR

everell is right. This is not random at all. Lite on does that.

See this thread for example:

https://www.badcaps.net/forum/showth...331#post377331

There is a 2.2uF 400V capacitor among the big pink resistor and the mosfet. That cap has gone high esr and I replaced it back then with 2.2uF 450V Nichicon PW if I remember correct.

[ben7]

Re: Snubber in LiteOn PS-5281-7VR

is the polarity marked on the pcb?
maybe it was designed for a film cap and they decided the electrolytic was cheaper.

+1
Looks like they might have not had enough room to put a film cap in.
I do agree, a film or ceramic cap would be better.

I thought that the proper value would be more like .1 uF, which is one tenth of the value. Here are some pictures and a drawing of the snubber. What do you think?

I'm not a PSU design expert (I want to be one some day though :P), but I think that is normal. I've seen some with 10nF caps, up to 1 or 2uF!!! And I've seen some with varying snubber resistor values, from 33K up to 100K, or maybe more. Hm, I don't recall them well enough to see if there is a relationship between either the frequency, topology, DC bus voltage, or wattage, and the capacitance/resistance.

-Ben

[momaka]

Re: Snubber in LiteOn PS-5281-7VR

I guess I will be looking at my LiteON PSUs when I get back home. Might even try a little experiment. I'm thinking, just open up one of my STF HiPro or Bestec PSUs and just copy the snubber circuit from that.

Now let me go out a bit with my bike today. I saw a few busted TVs two days ago on the side of the road here. Could definitely come in handy for the parts .

[goodpsusearch]

Re: Snubber in LiteOn PS-5281-7VR

good luck momaka!