Mobo - HOT transistor - 100 deg. C

[larrymoencurly]

Any electronics experts here?

I have an Elitegroup P5-TX mobo with a CPU voltage regulator that exceeds 100 deg. C, and that's with a much larger heatsink than was originally on it (a ridiculously large heatsink with a fan blowing on it won't drops it below 80 C). The CPU is a 200 MHz K6 @ 2.9V (not K6-2). I've replaced the electrolytic capacitors and the transistor, and the waveforms don't show slow rise/fall times or excess ringing (the signals look a lot like those from my PA-2007, whose regulator runs cool). I even tried a couple of different inductors, in case saturation was the problem. The schottky diode runs cool. The transistor is a FRP45N03L (45 amps, 30 volts, logic level).

I realize that this board isn't worth spending any money on, but I'm just trying to learn something here.

 

[jamarno]

I'm no expert, but sometimes when the current limiting kicks in, it makes the regulator efficiency go bad. The limiting resistor may be a metal bar or even a bare jumper wire.

 

[adolfmuhammed]

Why is there too much current going through the regulator?
Maybe a high load, a short circuit?
Regulators adjust the voltage by burning up the excess as heat. Maybe adjusting the regulators input voltage downward would help.
Does/should the cpu+motherboard work?

 

[BUTCH1]

Have you checked the voltage going in to the regulator??

 

[larrymoencurly]

5V goes in, and the CPU voltage, 2.2-3.5V, comes out. I can't find any shorts, and I isolated not only the transistor from the heatsink but also the heatsink from the mobo, just in case. I don't think the transistor should run nearly this hot because its original heatsink was teeny, and now I'm using one with 5 times the surface area and 2-3 times thickness. The transistor is rated for 0.028 ohms when turned on, so it should run cool even with a K6-3.

I changed the inductor and got the transistor down to about 85 deg. C, but somehow the inductor core shattered, and the transistor runs a lot hotter with every other inductor I've tried.

 

[BUTCH1]

Does it regulate for the CPU only or the memory, bridge ect.?
it might be just a leaky transistor that is on the way out. You
might find a replacment at R shack or go to partsexpress.com, they sell 'em...

 

[larrymoencurly]

This IC has separate outputs for the 3.3V and the CPU voltage, and it was the transistor for the latter that blew. Both transistors are 30V, 45A logic level MOSFETs.