MOBO FET's

[Steve48]

Hello,

This question will take somebody with some real technical experence. I recently had to replace a MOBO due to the fact that one of the FETs had shorted out. Thes FETs also have a coil and a capacitor as filters. After a bit of reasearch I found out 5hat thes appear to be 5 volt regulators. Well I replaced the shorted out FET with a FET with a higher current rating and the board worked..but the FEY, capacitor and coli got way too hot! I eneded up replacing the MOBO and am using the same Power supply unit and the same FET's (or facsimile) stay relarivly cool. Any Ideas as to why the FET would overheat on the other MOBO? Even replacing the Cap. had no effect. Could it be that the other FET tat it is paired with was open? This problem occured only a few days after I "fixed" a 64 meg chip.
I will never do that again.....RAM is too cheap to mess with.
Any Ideas?

Thanks in advance,
Steve

[Hardtek1976]

Hello,

As far as I know FET's (field effect transistor) are rarely if ever used in a regulation circuit as they are a high gain, "low" power device. You have probably mistaken this as a regulator IC, a device that provides a constant voltage at it's output no matter what the input is. The output voltage must match exactly, a higher current rating is OK. Thus if the voltage rating is correct the reason the IC is getting hot and the coil too is that there is a short on it's output side , probably caused by the capacitor being shorted. Also as a note that capacitors only get hot if that the voltage across it is higher than it's rating or it is shorted.
Hope this helps.
Just re-read your post, since you have replaced the cap, the voltage must be too high across it. Double check the ratings of the regulator IC.
Ron

[This message has been edited by Hardtek1976 (edited 03-21-2001).]

[Hardtek1976]

Me again,

It's been a long day and the grey matter is in neutral. Should also note if the cap is on the primary (input) side than you may have another regulator shorted also causing an excessive voltage higher than the cap's rating.

Ron

[Steve48]

Hello

Thanks for the answer Ron. They are indeed FET's. More specificily they are power MOSFETS (CEB6030L). Apparently they are used to keep the CPU stable. All I know is that when one shorts out, it wont give the right CPU speed as well as other problems. Like I said before, the "SAME" FETS in a new MOBO with the exact same CPU (AMD k6 233)there is NO heat problems. At first I thought maybe the CPU was faulty, but since no FETS overheat in the new MOBO I ruled that out. My only other guess is that since there are 2 of them (MOSFETS) in a type of series configuration, maybe the other one is also shorted, partially shorted or open. The coil and cap are for RF filtering, and as such are output components, and in fact the Case power on LED is connected there.

Steve