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Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Oops! My bad!
Sorry
Anyway, I will consider what Klugesmith said, and i was hoping to get 2-3 times the (air dissipation=1) oil heat dissipation as Pinky's Brain said/link indicates.
Oooo, pinky's Pdf describes everything I need know.
Registered Member #1792
Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527
You can directly measure the junction temperature of a diode by characterizing its forward voltage under different temperatures:
The way to accurately characterize a diode's temperature dependence (it should still work for a string of diodes I think), is to fix their temperature at ambient, and a few points above ambient. So for example you can put them in a container with a fixed heat source and thermometer, then wait for the whole thing to come to equilibrium. Then quickly run a fixed reference current through them and measure the forward voltage. For a fixed current, the forward voltage should be a linear function of temperature. Take a few data points to make sure of this. Doing the measurement quickly ensures you won't significantly affect the diode temperature by additional heating.
Then using this technique, you could run your diode string in air and oil at fixed power dissipation, find the junction temp rise above ambient, and thus find how far you can push the power dissipation in each medium.
It's kind of involved, but in this way you'll know exactly what temperature you're operating at.
Registered Member #2123
Joined: Sat May 16 2009, 03:10AM
Location: Bend, Oregon
Posts: 312
Somebody's going to learn why it's not a good idea to design an electronic circuit that runs its components at, or slightly above, maximum specification.
I use the word "rectifier" with pride, I went from 5U4's to Seleniums. Selenium rectifiers stink when they burn out (not to mention releasing all those poisonous fumes).
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
Patrick wrote ... ... assuming air has thermal conductivity of 0.024 K (Wm/K) and FR 3 oil, from this datasheet...here.
I should add a word about thermal conductivity. For solid materials it's important in calculations about die attachment, lead wires, and heat sink attachments. Thermal conductivity of air or oil would be directly relevant if the heated part were packed in something fluffy, like goose down or Thinsulate, to immobilise the fluid. But in most electronic equipment cooling applications, convection is critical. A microscopic thickness of fluid is heated by conduction, then replaced by unheated fluid (moved by gravity, fans, or pumps). When comparing oil with air, the densities and viscosities and Reynolds numbers are as important as the bulk thermal conductivities.
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Diodes dissipating heat
