mosfet dissipation
what`s the safe % of mosfet dissipation?
If max dissipation of a mosfet in datasheet is 150W, could i use it on 120W rms (20V rms x 6A rms). I`m asking this becouse I know that datasheet numbers of dissipation are bit of optimistic... 
You need to work back from the wattag rating using the temperature derating curves. Also keep in mind SOA. As a rule of thumb with good thermally efficient insulators (i.e. kapton) and nice heatsink I work to 3/4 rated power.

Mosfets are no different from BJTs when it comes to dissipation.
At 50% of the SOA a 150 watt device is only good for 75 watts. The output devices of an output stage capable of 150 watts in 4 Ohm should therefore have a total power figure of 300 watts minimum. 20V/6A= 3.33 Ohms. Continuous power in 8 Ohms should be close to 75 watts, two 150 watt devices would be sufficient. But in the words of Nelson Pass: heatsinks are the most important parts of amplifiers. 
It's not as simple as giving some percentage value. You need to do a proper SOA analysis to make an informed judgement.
As an example, here's the quick and dirty thermal analysis I did for my amplifier. The power supply is +/56V, output maximum power is 100W, and it uses two Hitachi 2SK1058 and 2SJ162 pairs, with thermal transfer pads to a Conrad 300mmx75mmx48mm heatsink: Rth (transistor) = 1.25 deg/watt mounting pad = 0.2 deg/watt Rth (heatsink) = 0.37 deg/watt Max Temp (transistor) = 150 deg Dissipation at 50W out = 76.8W (from simulation  1/2 power point is generally worst case, but it's a good idea to do a few simulations to find the worst case) Ambient = 25 deg Temp (transistor) = Ambient + dissipation / thermal resistance Temp (transistor) = 25 + 76.8 / (((1.25+0.2)/4)+0.37) Temp (transistor) = 130 degrees. My design works reasonably comfortably with two pairs of output transistors and a fairly hefty heatsink. Of course, you should also check that you won't exceed current and voltage ratings for your devices, as well. Hope that's been of some help. Cheers, Suzy 
Miss Jackson,
your quick calculation is a bit off. At 76.8 watts dissipation divided over 2 pairs of your Hitachis, the die temperature is 81 C. (with the assumed data you posted) At a transistor temperature of 105 C above ambient dissipation is 143 watts. Max dissipation will be a lot higher if you hook your 100 watt in 8 to a 4 Ohm loudspeaker. 
No, the calculation's good, it's just my description that's lacking. The 76W is across all the MOSFETs.
:) Cheers, Suzy 
No, Jacco's right. Using the correct equation,
76.8 * (((1.25+0.2)/4)+0.37) + Ambient and not 76.8 / (((1.25+0.2)/4)+0.37) + Ambient There is a 56.256 degree rise, add 25 degrees ambient, and you get Jacco's 81 degrees. 
Re: mosfet dissipation
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or less, and they don't break. Run them at 50 watts under the same conditions, and you will lose a small percentage. 
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Cheers, Suzy 
SJ,
i thought you Bike B. feed the fish and hit the hay at 21.15 ? 
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