ATX mosfet replacement - Page 1
Forum section: electronics / beginners

author: alxnik
author_rank: Regular Contributor
timestamp: August 03, 2013, 06:31:22 pm
content: Hello Everyone,I recently blown an ATX power supply. After opening it, I found out that one of the 2 mosfets that drive the psu has blown. The mosfets used are toshiba 2SK2749, which as I found out are pretty obsolete nowadays and I cannot seem to find a pair anyway.So I decided to start searching for a replacement mosfet altogether (I will replace the still functioning one too, to have a pair of the same type). After looking at the specs of this specific mosfet, I found out that it has a huge Vds of 900V. Although I found similar ones like the STW7N95K3, I can't help but wonder...why 900V? I mean for a standard 220-230V AC input psu whouldn't 400V be enough?While we are at this subject, given that this is not a cheap power supply so supposedly resonably engineered, if I change the mosfets to ones with lower Rds or different input capacitance, could this affect the overall stability/efficiency of the psu for the worse?Thanks!

author: tom66
author_rank: Super Contributor
timestamp: August 03, 2013, 07:33:31 pm
content: The MOSFETs may be rated as such because some SMPS topologies expose them to double the input voltage.When they failed other parts may have been damaged so check resistors, diodes and capacitors on or around their gates for damage as this is the driving network.

author: Tita
author_rank: Contributor
timestamp: August 03, 2013, 07:50:49 pm
content: Look at this video and you will know what ports need to be replaced!View the entered video !

author: wraper
author_rank: Unknown
timestamp: August 03, 2013, 08:07:51 pm
content: You don't need to replace another mosfet generally. You may want to do so only if it's flyback type psu with transistors connected in parallel. If they are not in parallel then matching them won't give you anything.

author: alxnik
author_rank: Regular Contributor
timestamp: August 03, 2013, 08:33:30 pm
content: Thanks for the replies!@Tita: Very cool video, I think if I had looked at it before, it would have troubleshooted the psu much faster. FYI, I had a blown fuse and according to Dave's videos, I traced the place where the short had the lowest resistance possible (the MOSFETS) and after checking them, I found the one which was shorted.@Tom66: That kinda makes sense. Can you name such topologies off the top of your head? It would be interesting to identify the specific topology. Although I haven't checked the driving network, after removing the bad MOSFET, I reconnected the psu and it works pefrectly on all output rails. I am sure that right now it's completely out of the manufacturers specs but at least the components work. Good thinking to check the driving network before soldering the new transistor in though.@Wraper: Agreed. But given that it is a high efficiency psu and has PFC, wouldn't a transistor mismatch screw with these specs, whatever the topology?

author: wraper
author_rank: Unknown
timestamp: August 03, 2013, 09:42:14 pm
content: No, it wouldn't impact anything. Only thing you don't want to do is to connect different MOSFETS in parallel because of the different RDS(on) so the power dissipated by them will be uneven. Only thing need to check is that this psu isn't flyback what is very possible accounting that it has PFC and 900V MOSFETS. Aren't those MOSFETS connected in parallel? And i highly suggest you that on the first powering you put some light bulb in series between the mains and PSU. It can save you some firework. Bulb should be high enough wattage 100-200W. If bulb lights up something is wrong (it will light up instead of psu). BTW, check PFC, I assume it's active? First of all check ESR or at least capacitance of filtering capacitor. If it's dead PFC can give huge transients on it's output which can damage other parts. When PSU is under the load there must be ~350-400V on the filtering capacitor, without load and on standby PFC may be not working and voltage will be lower (mains voltage x1.4). If pfc doesn't work PSU can seem to be working fine but power MOSFETS will be highly stressed because of increased duty cycle to keep output voltage. Just thought that you also should check small value electrolytic capacitors on primary side, they are faulty quiet often (especially if they are obscure brand) and can cause other parts to fail .

author: alxnik
author_rank: Regular Contributor
timestamp: August 03, 2013, 11:56:09 pm
content: Too tired to open it up again right now, but from what I found from the net, it's a forward design with 2 parallel transistors - this answers my question about the Vds actually. Since I am looking for MOSFETs with lower Rds than the 1.6ohm that the current ones have I will look for a matching pair (hope this will increase its efficiency). The PFC is passive, which should have been obvious before because it has a big transformer before the PWM stage.Right now the psu is nice and stable albeit with only one transistor so I hope that any fireworks should have happened by now. Since the transistors are in parallel, I am assuming that it is actually usable for half the specified output power, although I'm not really going to try it out. I guess I will check the input capacitors anyway while I am at it although given that it's a coolermaster psu, I'm hoping that they haven't used dodgy components.Well, that answers all the questions I had, thanks guys. That sure was fast.

author: wraper
author_rank: Unknown
timestamp: August 04, 2013, 01:17:35 am
content: You shouldn't worry too much about efficiency. Passive PFC is crap and heavy piece of iron generally. I assume your PSU is either quiet old or really low end because almost nobody uses passive PFC nowadays. I doubt that it is really high efficiency one because of that kind of PFC. I wouldn't bother to search lowest RDS(on) MOSFETS, you won't gain any sensible efficiency increase (>1%). Actually you should also look at gate capacitance. If it is significantly higher (and it's usually higher on lower RDS(on) MOSFETS) PSU can loose efficiency because existing circuit may not be able to provide enough current to charge gate fast enough to switch MOSFET rapidly.

author: alxnik
author_rank: Regular Contributor
timestamp: August 04, 2013, 11:58:58 am
content: Yes, It is actually old by now. I'm too much of a cheapskate to buy a new one.Nevertheless, thank you for the very cool info.