Abit IP35-E with a really hot mosfet

[yyonline]

So I pulled apart one of my test PCs to clean out the dust this weekend.

When pulling it apart, I noticed some heat discoloration on the back of the PCB. I saw one of the mosfets was running hot, so I figured I would put a heatsink on it and be done with it.

That was until I realized just how hot it was running. My infrared thermometer puts the temperature of this mosfet at 97°C. This seems really hot, as all the other mosfets on the board are much lower - around 50-60°C. I also don't think it can be good for the Sanyo WG cap right next to it. The placement of the mosfet also makes it difficult to put a heatsink on it, as it can't be too tall due to the video card placement.

Now, the board works just fine. It's stable, it has good caps, and it's hooked up to a good Seasonic power supply. It's my most used test box, which has a bunch of PCI disk controllers in it. I looked at upgrading it awhile ago, but it's really hard to find things with enough PCI slots these days...or a BIOS that still boots with 3 add-in storage controllers.

So I'm seeking some advice from the technical gurus. Is the mosfet bad perhaps, if so, would I be seeing stability issues? Any way to test it? Is it supposed to run that hot? Given that I already put it back in the case, I had to steal a photo from Google images. I've highlighted the mosfet location on the board, if that sheds any light on anything.

[Wizard]

Re: Abit IP35-E with a really hot mosfet

Read the marking on it and look up on 'net and chances this is not a MOSFET. Rather, it is a linear regulator that reduces initial voltage to lower voltage for point of use supply.

MOSFET *always* have a inductor coil next to this transistor with capacitor or two. Linear regulator run hot and is a problem if the pad is not on sufficient copper pad as outlined in the datasheet for SMD regulator. Hence the overheating regulator.

Cheers, Wizard

[yyonline]

Re: Abit IP35-E with a really hot mosfet

Thanks for the info. A quick search found that it is a Fairchild FDD8778, which is described as a 25V N-Channel PowerTrench MOSFET.

From looking at the board, it definitely looks like it's on a copper plane, and by the looks of it, it's not any smaller than those that other similar components are on.

I have a total of 6 IP35 based systems, and this part is in all of them, but only runs hot on one of them.

It heats up to 90°+ after only 2-3 minutes of the system being on. The datasheet says its rated up to 175°C, but I can't imagine that would be good for nearby components that aren't rated that high.

Any ideas for how I could cool this thing down? Could it be defective? Or not soldered on very well, thus poor heat transfer to the board? (Though wouldn't the dark spot on the back of the board suggest otherwise?) Or something else altogether?

Thanks again for any advice.

[shovenose]

Re: Abit IP35-E with a really hot mosfet

stick a heatsinks on all your mosfets...its just a good thing to do

[Th3_uN1Qu3]

Re: Abit IP35-E with a really hot mosfet

When you factor in the junction to case resistance (read the datasheet), the device may actually be operating close to its limits. If you have an oscilloscope check the gate drive waveform, it could be a bad gate resistor giving too little drive to the mosfet and as a result it operates in a high resistance state and heats up a lot more.

[Wizard]

Re: Abit IP35-E with a really hot mosfet

I do not see a coil or inductor next to the MOSFET, so I wondered how they use this as linear regulator or something? Cannot do chopping into capacitors without inductor.

Cheers, Wizard

[yyonline]

Re: Abit IP35-E with a really hot mosfet

I don't have a scope. Is there anything I can check for without one? Or is this a case of definitely needing the right tool to get the job done?

[Th3_uN1Qu3]

Re: Abit IP35-E with a really hot mosfet

I do not see a coil or inductor next to the MOSFET, so I wondered how they use this as linear regulator or something?

It might be a reg. If that's the case then it doesn't have a reason to get hot so it's still suspicious.

I don't have a scope. Is there anything I can check for without one? Or is this a case of definitely needing the right tool to get the job done?

I've worked a lot of time without one, and when i finally got one it opened new horizons. Pick up a 20MHz or something, you don't need anything too fancy.

[davidutz]

Re: Abit IP35-E with a really hot mosfet

Hello,

I have an Abit IP35 MOBO that has discoloration on the pcb near that mosfet you were talking about. The motherboard doesn't displays any image at all, the fans are running and also I get no beep. I changed CPU, RAM and video card but no succes. I also re-wrote the bios chip with a Galep programmer but still no succes.

I also bought a pci tester card that isn't showing any error and I checked the voltages from the RAM slots they are 1.8v.
I looked for bad caps but it seems they are ok.

Could you give me a hint on what to do next...

Thank you !

[stj]

Re: Abit IP35-E with a really hot mosfet

check for cpu voltages.

[davidutz]

Re: Abit IP35-E with a really hot mosfet

The CPU voltages are ok !

[RJARRRPCGP]

Re: Abit IP35-E with a really hot mosfet

The motherboard doesn't displays any image at all, the fans are running and also I get no beep.

That can be a bad socket, especially LGA 775. Do any of the socket pins look funky?

[momaka]

Re: Abit IP35-E with a really hot mosfet

I suspect your motherboard may have a different issue, but nonetheless...
First, check resistance between Drain and Source on that hot MOSFET, just to make sure it is not shorted. If not, then change the cap next to it. Even though it may look good, it could be cooked by now, judging by how hot that MOSFET runs.

While on the topic, I will mention that I have seen plenty of motherboards with hot-running MOSFETs. For older ASUS and ASUS-built AsRock, it's usually a MOSFET or two around the NB, SB, or RAM area and normally used as linear regulators. The same applies for Foxconn, though it is not as frequent. One example is the Dell OptiPlex GX270 motherboard with its hot-running MOSFET near the AGP slot.
And with Intel, it's usually the CPU VRM - either because the copper pads for the MOSFETs are too small and can't dissipate the heat properly or because they have the MOSFETs free-standing on some models (which IMO is just plain stupid).

[brethin]

Re: Abit IP35-E with a really hot mosfet

Hello,

I have an Abit IP35 MOBO that has discoloration on the pcb near that mosfet you were talking about. The motherboard doesn't displays any image at all, the fans are running and also I get no beep. I changed CPU, RAM and video card but no succes. I also re-wrote the bios chip with a Galep programmer but still no succes.

I also bought a pci tester card that isn't showing any error and I checked the voltages from the RAM slots they are 1.8v.
I looked for bad caps but it seems they are ok.

Could you give me a hint on what to do next...

Thank you !

When you say PCI tester card I assume you mean PCI Post Card. If so when you turn it on do the codes changes as it tries to boot? What code is displayed after say 3 mins?