I did additional testing, with the Single-MOSFET Test Rig. Here are the results:
Beginning Ambient Temperature = 24*C
Highest Volts/Amps = 13.82V @ 2A
Soak Time = 10 Minutes, Temp was settled in only two minutes.
Final MOSFET face temperature = 52.3*C
Heat Sink Considerations
The MOSFET was mounted in grease directly to an Aluminum Plate, being 125mm x 50mm. No insulator was used. This is in keeping with the OEM test method. Direct aeration was provided by a 60mm x 60mm, 12vdc fan. (I originally reported thatIthe fan is an 80mm unit; I was wrong.)
Mathematical Extrapolations
Voltage Extrapolation Factor (The Design Goal is 30V/2A)
30.0V/13.82V = 2.17
MOSFET Rise In Temperature
52.3*C – 24.0*C = 28.3*C Rise
Estimated MOSFET Rise at 30V Design Goal
28.3*C x 2.17VEF = 61.4*C
Estimated MOSFET Temperature at 30V/2A Design Goal
61.4*C + 24.0*C = 85.4*C
Conclusion
I estimate the Single-MOSFET is capable, thermally, of the design goal, and possibly more, with a proper heat sink and cooling. It would be good to do operational testing, at full power, but I will have to think on how to configure that much wattage.
Merry Christmas!
I wanted to push the test rig to the MOSFET heat limit, to see what the wattage might be. That is to say that the design goal (30v @ 2a,) expressed as wattage is 60w. But, I only have a ~14vdc/55a DUT supply. So, I could load the test rig at 14v @ 4a.
I upgraded the on-board fuse to 4a and ran the test. The rig performed as before, to the 14v @ 2a point, with some variations... Mainly, that for this similar supplied wattage (~14v @ 2a,) the temperature reading rose to 62.0*C. I attribute this to several things:
- Much larger DUT supply cables were used, which significantly reduced the resistance/heat loss in the cables.
- The heat sink had to be re-seated, in order to change the fuse.
- The thermocoupler contacted the MOSFET's face in a different location.
The most curious result followed, when I attempted to advance the test above the prior range. There was only a small amount of throw left in the pot turn... maybe 1/8-1/4 turn... This put the amperage just over 2a. The temperature only rose to 67.1*C.
I am at a bit of a loss. The DUT supply can supply 55a. So, shouldn't the throw of the pot span this whole amount? I will put the MOSFET onto the oscilloscope, to see what is happening, but, presently, I do not understand. Your thoughts?