[Brians256]

I think I mentioned this before, but I wonder if it is possible to simply replace the MOSFETs with higher quality ones. I need some expert EE advice from some of you gurus out there.

Now, I've some some research to figure out what causes the heat in the DC-DC converter power MOSFETs. If I recall correctly, the main culprit depends upon the frequency at which the MOSFETs are switched on/off. I think Intersil or National has a good appnote on this.

At lower frequencies, the main factor in how much heat is generated is RDSon, which stands for the resistance from drain to source when the MOSFET is switched fully on. When the switching frequency is low, the transistor spends most of its time either fully on or fully off. So, the generated heat is mostly caused when current is flowing through it.

With higher power MOSFETs, you need to work harder to turn the MOSFET on and then off. Think of industrial strength valves. You don't ask grandma Hazel to turn off the water supply valve for Hoover dam, right? Well, it turns out that you need beefier MOSFET control circuitry to make sure that the MOSFETs switch on/off quickly so that the MOSFET spends most of the time fully on or fully off. If you don't, then the MOSFET starts acting like a high power heater element as it spends time time in the "almost on" state. Since I am sorta stuck with the MOSFET controller on the mobo, I need to pick a MOSFET that is as easy to turn on/off as possible while still being able to easily handle the power our CPU will need. This is one of the reasons that mobo mfgrs use multiple MOSFETs. It is easier to get three or four MOSFETs with lower power specs turning on/off quickly. Much cheaper.

At higher frequencies, other factors come into prominence. A MOSFET is turned on by current flowing in/out (depending upon whether it is a N-channel or P-channel MOSFET) of the gate. This gate acts like a capacitor storing up charge and then releasing it. However, and here is an important part, nothing comes for free. Moving those electrons in and out of the gate is lossy, because the gate has a charging "resistance". Every time you move the electrons in/out, you generate waste heat. Turning the gate on and off very rapidly means that you generate little bits of heat VERY often. It adds up.

The best thing to do is to lower the amount of electrons you need to shove in and then pull out, right? Well, that doesn't come for free either. A MOSFET with lower gate capacitance cannot handle as much power flowing through the Drain to Source (that is the main power path leading (in our case) to the CPU).

If I'm not mistaken, there are other effects that I (to be honest) can't remember. There is the Miller something or other and I think a couple of other effects.

Now, I'm sure that the transistors used by most motherboard manufacturers are not the best available, because that simply is not needed. But, can you get a transistor with a lower RDSon or lower gate capacitance without messing adversely with some other important parameter?

Also, the inductors (the little wire wrapped doughnuts) can get hot too, because you are dumping lots of current in and out of them. If you have higher value inductors, they tend not to get as hot. But, you can't do that without changing the filter capacitors, I think. The MOSFETs might get driven harder with higher value inductors too (I don't remember on this one).

Any thoughts on this one?