I'm using this FET as the pass element in a linear heater.
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(IRLIZ44GPBF) Power supply is 48 V @1 A. Max dissipation in the FET ~12W. (it's in series with ~50 ohm resistor) I want to spec the maximum temperature. Something greater than 100 C... 400 K (~130C) is a nice round number. The junction to case (thermal) resistance is 3.1 C/W So worst case junction temp is something like 130 +37 = 167 C. Does that seem reasonable? Figures 8 and 9 kinda hint that this may work... I guess I just want someone to hold my hand and help read the spec sheet.
TIA George H.
Oh It's been running on my bench for a day, with no problem. But it's a little tricky getting the right amount of insulation such that the Fet's dissipating 12W @130 C... 11W at 135C at the moment. I guess I could just turn it up and see where it breaks.
Right it's got an insulated tab. I'll go search for something else.
I ran it at 150 C for an hour (I'm off the calibration curve for my diode temp sensor).. and then turning it up more, I melted the solder on the fet (air wired) at ~155 C (+36C) ~=190 C...
Yeah that's what I've got... well actually there are three 15 ohm resistors in series with the FET. I may have to try two 22 ohm, 'cause at the moment there are only three mounting holes on the hot plate. Re-reading the spec sheet it says I can run the case temperature at 150 C and still put 12 Amps through it. Ron (@150 C) = 0.06 ohm, I^2 R = ~8 Watts. I think it will be fine at 130 C. (I'll cook it for a few days at 150 C and if that works.....)
Fet in series with the resistor, both making heat, is neat because, above half power, the dissipation shifts from the fet to the resistors. At full power, it's all in the resistors. If you control fet current, it's still linear.
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John Larkin Highland Technology, Inc
picosecond timing precision measurement
Semiconductors age much faster at elevated junction temperatures. Running it at 167 C for a few days isn't going to bring that failure mechanism to light, nor will it make any issues with differential expansion & contraction come to light (if the latter is a problem...)
I'd try to find a better part, if there's something out there.
Back in the old days, the metal-case power transistors often were rated to 200C. There's one FET with that high a rating at DigiKey, SCT10N120 ( a SiC NMOS power device). Kinda pricey, since it replaces with an old 2N3055...
Well I went looking at insulated FET's and unless I go with the bigger to-247 pac I seem to be stuck with ~3C/W junc to case. (there are some space issues that have been designed into the hot plate... I'll have to see if things can get moved a bit.) For the to-247 thermal resistance is like
1 C/W. Or I can always reduce the max temp spec.
Another heater story. (As long as no one reports me to the SPCIC*) I use a TIP120 (Tc max = 150 C) as part of a heater for a Rb lamp. Typically the lamp runs at ~120C @~ 2W heater power. But I've run it at 150 C for hours/ days at a time. With out any visible signs of problems.
I think the issue is the glass-transition point of the epoxy. As has been pointed out, the transistor itself is okay at 200C. Now where's Syd Rumpo when we need him?
It should...its melting point is about twice that of pure silicon.
Even with metal-case power transistors the derating means you can't actually dissipate much power at those high temperatures. Reliability suffers, too, at high temperatures.
George should really move over to a PWM circuit, then all the heat will be in the power resistors. There are plenty of easy-to-use PWM chips. One by Linear Technology, part of their TimerBlox series, the LTC6992, is really easy to use. See AoE III, pages 432 to 434.
I think he may have noise concerns. I've done PWM with deliberately slow edges, basically a trapezoid, which is good unless you have magnetic field issues.
Slowing the edges is as simple as adding a series gate resistor.
Generating the PWM is easy.
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John Larkin Highland Technology, Inc
picosecond timing precision measurement
PWM is great in low-to-moderate-performance applications. I use it that way sometimes too. All the advantages you cite are real. However, there are a fair number of applications in EO systems where higher order effects are su per-important, so switching junk is death and destruction.
An approximate analogue square-rooter is a wonderful thing inside heater FB loops.
Right. My first choice was a PWM with rolled off edges. (I was thinking of an RC roll-off, being easy) But that caused worry, heater current next to signal lines. I hadn't thought about magnetic fields from the heater. (I think I've told this heater noise story before, but in our noise measurement apparatus, I found that the noise on the heater leads capacitivly coupled to the signal lines... (looking at noise from 10 k resistor in a LN2 probe) and screwed up the measurement. I had to make my own low noise voltage source... Cap. multiplier my noise "hammer".)
Oh thanks! (I was trying to slow down a comparator output, and the National Semi, support engineer told me to hang a cap on the output... good advice.)
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