Transistors

Hi, You don't mention the type of power supply, it could be linear, shunt or series regulated or, of course, a switched mode supply...

Let's assume its a switcher... most switched mode supplies are pretty efficient, the figures quoted on the spec sheet will be the most optimistic, if you say you're only using them at 20% of load the efficiency could be much worse, making the heat needing to be dissipated almost as much as if you were running them at 100% output.

Most transistors will have a thermal coefficient for the style of case used with the die.... the heatsink will have a thermal coefficient to ambient with air flow and without...

So you have to work on these parameters starting with the maximum die temperature that is useable and calculating the temperature decrease to case and then to ambient...

Only then can you run them knowing you are within safe limits...

John.

Let's say the die to case thermal coeff is 0.5 *F the power disspated is 100 watts and the max die temperature is 250 *F...

Then the case temperature at maximum power and die temperature will be 0.5 x 100 = 50 *F

So the case maximum temperature will be 200*F

If the heatsink has a thermal coeff of 0.5 and it still needs to dissipate 100 Watts. then the heatsink temperature at the location of the transistor will be 150 *F (this assumes a perfect case to heatsink connection if not as in the real world the temperature should be derated).

The calculations are quite easy its knowing the actual values which is difficult!!

John.

To calculate the actual temperature of the transistor junctions, you need to know some things:

What is the transistor p/n? If you have this you can look up the part and get the maximum temperature, and the junction to case thermal resistance. The next best thing is the transistor package type (TO-3, etc) if you can't get the actual part number.

What kind of heatsink is it? If you know this, you cann lookup the thermal resistance for it. You don't need to know a part number, if you know the dimensions and shape, google "heatsinks" and you should be able to find the same one on a manufacturers website.

Assuming you find the junction to case thermal resistance (θ_JC) and the θ for the heatsink, you can calculate the internal temperature of the junction, here:

http://www.daycounter.com/Calculators/Heat-Sink-Temperature-Calculator.phtml

There is a good tutorial, here:

http://focus.ti.com/lit/an/sboa021/sboa021.pdf#search=%22to-3%20thermal%20resistance%22

Remember to do everything in degrees Celsius. (140F = 60C)

As far as your original question, most power transistors are rated to run at up to 125C, at the junction, although you would want to operate them much cooler, say 85C.

Cool, thanks for all of the information. Sorry, I forgot to say, it is a switching power supply. The only way I can describe the case is that they look like very large TO220 transistors (They are kind of buried, so it's hard to tell). The heat sinks are made out of 3mm thick stamped aluminum and has "fingers" extending out and bent to allow for better heat dissipation. These are quite large, made out of a 12x5 cm plate, with 4 transistors mounted on each one. So it looks like this is still a safe temperature for this, I just wasn't sure, because with the origional fans, the heat sinks barely got warmer than 40C, and are now at 60C.

Thanks again!

If you're interested in keeping the unit(s) quieter, check into Peltier junction devices.

They are solid state, absorb heat on one side and release it to the other (presumably to a heatsink, water-cooling, etc)

If it's a switcher, those are probably MOSFETs. You're probably OK with temperature, but the hotter you run them, the quicker they will fail, so ultimately you might want to find a fan that's quiet, but gives you the airflow you need.

At least, make sure you keep the fan, heatsinks, and vents clean and clear of dust so that you maximize the cooling that you have with your smaller fan.