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Old 1st November 2007, 05:44 PM   #1
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Default Lateral MOSFET torture...

I did a fun little experiment with a Hitachi 2SJ83 lateral mosfet a couple of weeks ago. It is probably the same die as used in the 2SJ50, the parameters in the datasheet look the same.

Mounted on the small "heatsink" (~4degK/W) shown in the picture I gave it 50V drain to source and then applied 10V to the gate with a 330 ohm gate resistor to suppress oscillation. The current after a short short while was 6A. (300W dissipation in the 100W rated part) The fan in the picture was NOT running.

Can anyone guess:
a) How long it survived?
b) How hot the "heatsink" got?

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Joakim
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Old 1st November 2007, 05:50 PM   #2
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It survived 3.750 seconds.
The heatsink peaked 23C over ambient.
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Old 1st November 2007, 05:53 PM   #3
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Nope, wrong answer
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Old 1st November 2007, 05:55 PM   #4
AndrewT is offline AndrewT  Scotland
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10Vgs and only 6A!
I would expect around 9A or 10A.

My estimate is around 20seconds.
and the sink probably didn't have time to change too much due to the relative thermal inertias of the junction and the sink.
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Old 1st November 2007, 06:22 PM   #5
djQUAN is offline djQUAN  Philippines
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Quote:
Originally posted by AndrewT
10Vgs and only 6A!
I would expect around 9A or 10A.

My estimate is around 20seconds.
and the sink probably didn't have time to change too much due to the relative thermal inertias of the junction and the sink.

probably because he was using rather thin wires to connect the drain and source to the supply.
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Old 1st November 2007, 06:54 PM   #6
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I didn't have any fast responding instruments for measuring the current, I just used the ones on the power supply. So it's 6A after maybe 0.2 seconds or something. The initial current might have been greater, but the laterals conduct less current at the same Vgs the hotter they get...

The resistance of that "thin wire" is 6 milliohms so it is probably not a problem.
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Old 1st November 2007, 09:48 PM   #7
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So? How long it survived?
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Old 2nd November 2007, 03:18 PM   #8
wg_ski is offline wg_ski  United States
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My guess is maybe 5 minutes, with a heat sink temp of 150C.
Mosfets will take junction temps of around 300C before they die instantly. I've tortured a few hexfets in my day - with the epoxy package split open, glowing orange inside, but still beahving as an amplifier. Didn't work after it was turned off, cooled down, and turned back on, however.
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Old 4th November 2007, 04:13 PM   #9
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Glowing orange and still behaving as an amplifier? I'm not surprised it didn't work afterwards though! The lateral mosfet in my experiment does something interesting though:



The device under test is a lateral mosfet, a Hitachi 2SJ83, a 2SJ50 equivalent in a weird plastic package. It is mounted on the small "heatsink" (~4degK/W) shown in the picture in the first post. I gave it 50V drain to source and then applied 10V to the gate with a 330 ohm gate resistor to suppress oscillation. The current after about 0.1 seconds was 6A. (300W dissipation in the 100W rated part) The fan in the picture was NOT running.

The current starts decreasing very rapidly, maybe to 2 amps after a second. (the current has a negative temperature coefficient above 100mA) The heatsink continues to heat up. This negative temperature coefficient is not enough to limit the temperature much though due to de woefully inadequate heatsink.

Next, the FET starts to draw current into the gate, thereby lowering the current through it and the dissipation. When the current going into the gate exceeded 8mA a small pop was heard, the gate voltage dropped to 1.2V (drawing 25mA then), the drain current dropped to 0.2A and the heatsink started cooling off. After turning off the drain-source power supply the gate was still drawing current.

By turning off the gate supply the transistor stops drawing gate current of course, but when turning on the gate supply it doesn't draw any current. Maybe it wasn't destroyed? I did the same experiment a couple of times, and it does the same thing each time

If I drop the gate voltage so the gate current never exceeds 8mA I can get the heatsink to stabilize at 165 degrees celsius with 40W of power dissipation. The transistor is mounted without insulating washer and the temperature is measured right below it so the die temperature seems to never want to exceed 200 degrees by much.

It is interesting that it has this built-in overtemperature protection, no wonder they got the reputation of being rugged and outlasting fuses! But I wonder, what is it that causes this? Is it something with the built-in gate protection zeners maybe? The thyristor effect when exceeding 8mA of gate current is very interesting
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Old 4th November 2007, 05:05 PM   #10
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I had always doubted that external zeners plus fuses would be enough to protect these FETs but I happened to run across these usenet posts by Phil Allison (same guy that wrote the VI-limiting article on the ESP pages):
http://groups.google.com/group/rec.a...b54f59a162286e
http://groups.google.com/group/rec.a...f891609d862346
http://groups.google.com/group/rec.a...74bc5db48967ed

I had a hard time believing that there really is a semiconductor component that will outlast fuses with such simple protection so I had to try it. Especially the part about mounting bolts working loose without meltdown...

http://groups.google.com/group/rec.a...eaa3a11d8efada

He thinks the zeners must be chosen wisely. But, just think of what happens if the mosfet is cool and then gets hit by 14V limited by the gate protection zeners. I'm pretty sure the thermal time constant of the leadout wire is shorter than the one of the die, so I guess the drain leadout wire fuses in short order at the current which ensues...
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