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bogdan_borko 8th June 2006 08:32 AM

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...

richie00boy 8th June 2006 08:58 AM

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.

jacco vermeulen 8th June 2006 09:05 AM

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.

suzyj 8th June 2006 12:55 PM

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

jacco vermeulen 8th June 2006 01:20 PM

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.

suzyj 8th June 2006 01:25 PM

No, the calculation's good, it's just my description that's lacking. The 76W is across all the MOSFETs.

:)

Cheers,

Suzy

Tim__x 8th June 2006 04:22 PM

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.

Nelson Pass 8th June 2006 06:51 PM

Re: mosfet dissipation
 
Quote:

Originally posted by bogdan_borko
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...

With a heat sink at 55 deg C, I run them continuously at 25 watts
or less, and they don't break. Run them at 50 watts under the
same conditions, and you will lose a small percentage.

suzyj 8th June 2006 09:28 PM

Quote:

Originally posted by Tim__x
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.

Eeek! Thanks for that. My only excuse was it was really late at night.

Cheers,

Suzy

jacco vermeulen 8th June 2006 09:47 PM

SJ,

i thought you Bike B. feed the fish and hit the hay at 21.15 ?


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