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What is a MOSFET? And is it important to cool it?

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January 5, 2008 7:18:44 PM

I'd like to see some pictures of a mosfet and i was reading a video card cooling review and they said that it is necessary to cool the MOSFET's on a 8800GT, and well since the motherboard has 3 (well at least mine does) is it important to add RAM sink's to it?

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January 5, 2008 7:30:21 PM

im sure. they said, "the vf900 cu came with enough ramsinks for the ram, but was missing some for the 8800gt's MOSFET...."

that was in the hardocp 8800gt mega heatsink roundup review.

anyone have any pictures of a mosfet?
January 5, 2008 8:07:23 PM

The MOSFETs are used as the switching devices in the DC-DC voltage converter that creates the CPU VRM voltage (~0.8 - 1.6V) from the +12V rail. Like most electronic components, they have real-world operational limitations, and as such, generate heat during operation. The current required by modern CPUs can reach almost 200A (!!!) If the devices are too hot to touch, it may be desirable to add a heatsink. Care must be taken, as the metal tabs on the MOSFETs may not be at ground potential, and a short-circuit would not be a good thing.

EDIT: There is a similar DC-DC converter on the video card for the power-hungry GPU, as well.
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January 5, 2008 8:28:56 PM

mosfets' engineering handles heat pretty well. I doubt you would neet a heatsink one on your mobo if you had a hot 8800. The fun ones you can cook eggs on.
January 5, 2008 9:07:50 PM

I think my amplifier, subwoofer and car audio system has a MOSFET power supply, whatever it is, im not sure, but it probably regulates power or something.
January 5, 2008 9:16:27 PM

Hi

Since nobody has answered your question (what does a MOSFet transistor look like)!!

MOSFets are 3-connection transistors and a surface mount MOSFet looks like this:


They typically dissipate little heat when completely on or off... It's the high speed switching that generates the heat (like most transistors)!! They are used for DC-DC conversion as previously stated. I guess they use PWM to generate the lower core voltages (from the 12V rail) at the necessary high currents drawn by todays CPUs. That's right - isn't it?

To test if MOSFets needs active cooling (e.g. motherboard regulators supplying the core voltage for a QUAD-CPU, 2x dual-core Opteron, etc.) just use the ol' finger test when the system is under load!!

Bob
January 5, 2008 11:00:56 PM

bobwya said:
Hi

Since nobody has answered your question (what does a MOSFet transistor look like)!!

MOSFets are 3-connection transistors and a surface mount MOSFet looks like this:
http://content.answers.com/main/content/wp/en-commons/thumb/8/8b/250px-D2PAK.JPG



Actually, this package you are illustrating could contain many different types of components; it is important not to generalize. When in doubt, check the part number on the package. You could be looking at a Schottky dual rectifier. Likewise, MOSFETs are available in a wide variety of surface-mount power packages. Just because you see a device that looks like this, it is not necessarily a MOSFET. If you see devices that look like this, and they are near the CPU socket, they are likely to be MOSFETs, though.
January 6, 2008 12:04:06 AM

Hi,

Well hey I think you take my point that Mr Shadow was showing the guy a bunch of heatsinks on a graphics card isn't much use!!

Sure if you want to get pedantic I am just showing a typical/common MOSFet package and not an actual MOSFet (e.g. electron microscope image of the actual device!!)

Hell I building a circuit just now and using a TO220 packaged MOSFet so I am aware that they come in different packages... I am just trying not to confuse the guy!!

Bob
January 6, 2008 6:34:35 AM

http://www.madboxpc.com/reviews/MSI/p35_platinum/fotos/...

in the link there are some mosfets on an 8800gts they are the lined up, small, black things and theres 9 in a row

as people have said, mosfets come in a range of shapes & sizes but will generally look something like the link above when were talking about graphics cards.... not allways though!

they are generally near the power connector of the card and will be in close proximity to some capacitors and coils (not allways).

the only thing adding heatsinks would do is extend the life of the mmosfets (maybe). theres no point in that because other components such as capacitors are far more likely to put the card out of use while its still usefull for gaming (2-4 years).

some mobos come with heatsinks on the mosfets... these are generally for the overclocking boards which are designed to push quad core 200w cpus to their limit and then a bit more. most boards dont have heatsinks on the mosfets unless their designed to drive the most power hungry cpus... + it looks cool

the only time it may be worth it is if your looking for an extreme overclock with a voltage mod. just increasing the frequency wont make a considerable difference to the power consumption of the gpu or ram. only when you start to bump up the voltage will it start to put pressure on the mosfets because mosfets provide the power for the card. power consumption increases exponentialy with increase in voltage but linearly with just frequency increase (i think)

so in conclusion, if the card your talking about didnt have heatsinks on the mosfets when you bought it then you wont need to put any on... unless you have increased the gpu/ram voltage on your card ....................simple!
January 6, 2008 8:53:05 AM

psymanproductions said:



the only thing adding heatsinks would do is extend the life of the mmosfets (maybe). theres no point in that because other components such as capacitors are far more likely to put the card out of use while its still usefull for gaming (2-4 years).


Actually the capacitors in that picture are ALL solid-state capacitors and should have a very long life span!!

Without extra active cooling (i.e. a fan!!) on my GPU (X1950 Pro 512Mb) it starts to flake out when OC and it has a waterblock covering the core AND ram chips... So something else is overheating on the card!! MOSFets DO BENEFIT FROM ACTIVE COOLING!! With the sort of AMPages they have to conduct this is common sense. If you look at the datasheets for a MOSFet it will specify a maximum current WITH A heatsink. When overclocking you typically will draw more current through the MOSFets (higher frequency chips = more switching transistors, especially if you bump up the core/ram voltages).

Bob
January 7, 2008 1:19:49 PM

bobwya said:
Actually the capacitors in that picture are ALL solid-state capacitors and should have a very long life span!!

Without extra active cooling (i.e. a fan!!) on my GPU (X1950 Pro 512Mb) it starts to flake out when OC and it has a waterblock covering the core AND ram chips... So something else is overheating on the card!! MOSFets DO BENEFIT FROM ACTIVE COOLING!! With the sort of AMPages they have to conduct this is common sense. If you look at the datasheets for a MOSFet it will specify a maximum current WITH A heatsink. When overclocking you typically will draw more current through the MOSFets (higher frequency chips = more switching transistors, especially if you bump up the core/ram voltages).

Bob


yes the caps in the pic are solid state... so what? i was using that pic purely to give an example of what mosfets look like.

generaly a graphics card will have a few electrolytic caps which have a shorter life span than most other components generaly found on a graphics card.

like i sed in my last message, AND I QUOTE "the only time it may be worth it is if your looking for an extreme overclock with a voltage mod."

i can olny speak from personal experience... same for you. i personaly have never reached the point where the mosfets are the limiting factor when oc'ing so yes, as far as i am concerned the quote in the above paragraph is correct... if youve had a different experience, dont take cheap shots at my common sense, just put your info up politely so the person who started the thread can have that piece of information with out having to read through all this crap!!! your info is valid... as is mine.

i never said he wont see any advantage from cooling his mosfets... so please read my posts properly before getting clever.
January 7, 2008 1:54:27 PM

Hi psymanproductions,

I didn't mean to bad mouth what you have been saying... I guess what I was trying so say is that MOSFets appear (in my experience) to benefit from active cooling/good airflow when overclocking (even on stock voltages). I wasn't really thinking life-span vs. short-term overclocking stability... Sorry didn't mean to get sucked into that one (though I must say that it is good that manufactures are moving over to using solid-state capacitors for longevity reasons).

Like I said in my previous post I am speaking from experience as the only chips on my X1950 Pro card without watercooling are the MOSFets. The card has more OC headroom if I cool it down with a fan (despite having a full coverage waterblock on it). I have not applied any voltage mods./new BIOS (yet!!)

Bob
January 7, 2008 10:55:26 PM

Thanks guys now not only do i understand what a mosfet is i know what its function is and best of all i know what it looks like!
January 12, 2008 5:10:44 PM

metal oxide semiconductor) field effect transistor = mosfet

high current switch it gets hot

so you cool it

why mosfet

since the new chips suck up 150w they need alot current at 1.5v thats 100 amps

they why u cool it!
!