I obtained two NPN BJT power transistors from DigiKey. They are by ST Micro and the part number is BUT30V. These are 100 amp, 250 watt parts in the "ISOTOP" (SOT-227) package.
They are intended to be used as pass transistors for a 13.8 volt, 30 ampere linear power supply regulator. I attached each one to it's own heatsink and used Indium foil as the thermal interface material, along with two heavy springs to maintain a constant pressure on the heatsink during thermal expansion and contraction. Before wiring them in, I intended to run around 20 volts DC at up to 10 amperes through them to insure the TIM was properly conducting the heat to the heatsink. To do this, I setup a power supply set to 20 volts with the current limit set (initially) to 3 amperes, intending to go up to 10 amperes eventually. The base was driven via a 10 ohm, 1 watt resistor connected to a 1.5 to 20 volt adjustable power supply, starting out at 1.5 volts. (See the test setup image below).
Anyway, when I turned on the 20 volt "big" supply, I had a collector current of around 3 amperes (caused by the 1.5 volts through the 10 ohm resistor). I slowly turned up the base drive and the collector current increased as I expected. At around 7 amperes collector current, the transistor collector voltage went down to around 7 volts and the collector current at around 7 amperes (it "jumped" to that value). I turned off the "big" supply and removed the base drive (the base was now open - free floating). Turned the big supply back on and got the same 7 volts, 7 ampere load from the transistor! It was conducting with no base drive. Then I tried connecting the base directly to the emitter and no change - still 7 amps collector current.
Thinking that somehow I blew this monster of a transistor, I tried the second one. Same thing, it started out with full collector voltage of 20V and zero amperes. Upon connecting the base drive, I got the same low collector current as the first one. Upon ramping up, this transistor suddenly snapped into the same "conducting with no base drive" mode as the first one. Connected the base to the emitter - same thing - no change. And the second transistor had virtually the same collector voltage and current as the first one!
Then I tried applying a NEGATIVE bias relative to the emitter of -5 volts through the 10 ohm resistor. No effect, no change.
To see if I was maybe doing something wrong, I stuck an old 2N3055 I had lying around on a heatsink and connected it the same way. IT worked. I could control the collector current, the collector current dropped to zero when removing the base drive, and it got hot as I expected it to. NOTE that the monster BUT30V transistors didn't even have time to get hot (or warm). However, connecting them to the "big" power supply and just letting them draw the 7 amperes with a 7 volt collector voltage makes them heat up, along with the heatsink (at least the indium foil works!).
Thinking possibly they were oscillating, I hooked the silly-scope to the collector. Nothing. Pure DC.
Here's a link to the PDF datasheet: BUT30V Datasheet
These transistors are supposedly "high current bipolar modules" intended for "motor control, smps, ups and welding equipment", yet both seem to have died under conditions that a little TO-220 part could handle. What the heck? Did I do something wrong?
Thank goodness I did this test, because if these went into their "semi-shorted" mode in the power supply, it would have nuked my Kenwood TS-590SG HF radio (a $1700 to $1800 radio!).
I would appreciate it if someone could take a look at the data sheet and tell me if I did something wrong, because I'm not seeing it.
(click thumb for full res)