I am working on a project for my heat transfer class, and I have decided to analyze the heat dissipation of a power amplifier. The only trouble is that I need a couple of bits of information that haven't proved easy to find:

What is the efficency of a power transistor? In other words, how much heat will have to be dissipated from a 200 Watt amplifier?

What is the maximum operating temperature of a power transistor?

With such a wide variety of people here on AR15.com, someone has to know this. Right?

It depends on what configuration the amplifier is, i.e., class A, class B, Class AB, class C, etc.  Class A is least efficient (hottest) but is the most linear.  Class C is most efficient (coolest) but is non-linear in many applications.

Audio applications usually use a class AB type amp.  If my memory serves me, they're about 60-70% efficient.  In this case, they'll pass along 60-70% of their power supply energy consumption to the load, while dissipating 30-40% of their energy consumption as heat.

Maximum transistor heat values can be found in their specifications.  Note that with increased heat, the transistor's overall specifications may be derated.

This data is for a Crown Studio Reference 2 power amp. Its rated at 360 / channel into 8 ohms. This amp, driven to rated output with compressed rock music will disipate 840 BTUs per hour, or about 250 watts, which is a fair amount of heat.

You will notice that power amps have some decent sized 'heat sinks' (fins) on them and/or built in cooling fans. Power transistors can get too hot to touch.

If you are going to "analyze" the heat dissipation you really need a lot of information, like voltages, currents, and load impedances. These are all different, depending on the design of the amplifier.

There are numerous technical papers on the heat dissipation of Crown's amplifiers on their web site.
[url]www.crownaudio.com[/url]

Thanks, this sounds like some pretty good information, esp. the thermal sheets on the crown amplifier page.

As for the analysis, what I am analyzing is how to dissipate the heat, not the amount of heat generated. I am essentially saying: I have X watts of heat to dissipate. This is the best fin design for doing so (fin length, thickness, material, orientation, positioning, etc.)

However, I'm still not sure where to find good information for the maximum temperature of the transistors.

Pick up the phone and call Madrigal (Mark Levinson) or Krell. They're such fanatics, am sure they'd be delighted, to chat the numbers you seek.
[url]www.madrigal.com[/url]
[url]www.krellonline.com[/url]

In the 80's, Audio magazine used a Levinson 23 amp, to arc weld, with a square wave input.

 Go to barnes and Noble they usually have an Engineering Handbook on the shelf. Failing that go to your University Library. They definately have an engineering handbook on the shelf probably many different fields as well.

Heat transfer would definately be a product of area and some sort of constant based on material type. Aluminum for instance will have a higher constant of heat transfer then iron or whatever.    

200 watt amps may have 4-6 cascaded transistors.

Sub-books will give you specs on individual power transistors.

To get this info you will need to pick an amp, find out the type of transistors used in the output stages, and then get a transitor data sheet for that type transistor from the semiconductor manufacturer.....ECS