> With an improved sink & heat dissipation ability it may do much more than that?
No. On the Infinite heatsink at 25 deg C, 50 Watts in the transistor will raise the internal parts (silicon and seals) to bad temperature.
You can cheat by pulling the heatsink down to freezing... the specs don't actually cover this because it would be "stupidly expensive" and the extreme temperature difference (150 inside, 0 less than a millimeter away) creates as much problem as it solves. CPU-overclockers do things like this, but they have temperature meters and in any case are willing to take risks for mild speed increase.
> 50W is the max for this mosfet.
On an INFINITE heat-sink.
A power device has an electronic part and a thermal (heat) part. Unlike glass tubes, which are both in one, power transistors give you just the electronic bit and you must supply the thermal section.
If you do nothing, a TO220 is good for 1W-2W dissipation.
If you do infinitely well you get (for this part) 50W max dissipation; that will never happen.
(At the 50W level, according to Pease a fair approximation to "infinite heat sink" is an old VW air-cooled engine head; you H-D riders can use your old shovelheads.)
The actual safe dissipation declines with temperature. 50W at 25 deg C (room temp), _zero_ Watts at 150 deg C, so 40W at 125deg C, 30W at 100 C (spit-sizzle), 20W at 75 C, and 10W at 50 deg C.
_IF_ your heatsink were out in the open and vertical, 7.5 C/W, and you put 10W in it, that's 75 deg C rise above 25 deg C air, 100 deg C, the MOSFET is rated 30W at 100 deg C, it is "OK".
However mounted sideways inside a hot chassis, it is probably 100 deg rise above 40 deg C ambient, 140 deg C at the MOSFET, the MOSFET is safe for 4 Watts but dissipating 10 Watts, it is "not OK".
In fact modern silicon will "work" HOT. I've seen a power transistor melt the solder on its legs yet still hold a heavy load in regulation. It will leak a lot so you need a beefy driver.
But the real limit is the package. Steady 150 deg C or thousands of 50 deg C hot/cool cycles will crack the seals, let moisture in, which rots the silicon. And this is treacherous because it will work for hours or months or years, then fail "for no reason".
Here's another real-world benchmark. A transistor/chip "20 Watt" audio amplifier has a heatsink to dissipate 10 Watts. Look around your scrap-heap, see what is used for small practice-amps and hi-fis. That's a bit generous because audio amps run hot/warm/hot with the beat and your VVR will run more steady; OTOH a tranny-amp has no other heat in the box while yours is under hot glass.
Another path is CPU heatsinks. These are usually fan-blown, which is noise and power-hassle and eventual failure. But they work good. (I put that solder-melting regulator on a CPU sink and it ran just-hot for years.) There are thermostat fans which run silent, in this case at full-power and at very-low power, then run for the medium-power condition where the noise may be no-probem. Of course a thermstat is another part to fail.
> silver jubilee
i.e., another two-EL34 max-power monster? 50W max output means about 80W of total power demand. When VVR is turned to half voltage the current goes to half, power goes to 1/4, an equal amount is dropped in the VVR. 80W/4= 20 Watts possible dissipation in the VVR MOSFET.
You need to stay under 75 deg C at the MOSFET. Inside a 40 deg Chassis you have only 35 deg C allowable rise. 35C/20W= 1.75 degrees C per watt heat rating. This is a BIG heatsink.
You want the sink from a "40 Watt" tranny amp, a P-100 sink with fan, or a recent (big) Pentium sink with no fan but ample air-space.
> I am using the NTE2973
Good. The NTE2973 is in the new-fangled fully-insulated case. The tab won't shock you (do shrink-tube any leg you might lay a finger on). You could glom that right to the side of the chassis, though it appears you have no side on that chassis?
Since you have insulated case, top-mounting may be an option? Better air-space.
BTW: a higher-power MOSFET or more MOSFETs isn't a great answer. You can get a marginal increase of thermal safety by putting less power in each HOT device, but it is still HOT and close to unreliability.
Topic: Hot VVR Mosfet (Read 7147 times)