I'll be convinced by the resistance measurements. (preferably out of circuit)
Still no load in schematic .
schematic still "incomplete"
replying "liad is constant current" does not satisfy schematic requirements
Read Reply# 14
I didn't say it answered all the questions about the circuit, but it answered
your question about why the PWM is being filtered. It's a constant current driver. I know that because Dave Jones (EEVBlog) made a video about a circuit using the exact same topology, but with a pot in place of the LPF to adjust the output current.
Plus, you know, the fact that the picture's named "Constant Current Load.jpg" is a pretty big hint.
I agree that the load is important, but the circuit is not working the way I think you think it is.
post complete schematic that shows load and it's connections
We don't know any mire now than we did at Reply # 14.
How can you think the mosfet is "toasted" in absence of ANY mention of device temperature ?
Read OP, Reply # 6, & Reply # 14.
See ANY temp info ? (answer: NO !)
You're drawing conclusions without the most imposrtant info (temp ,resistance & load wiring)
Show me the evidence.
Where did I say that I knew it was toasted? I said I suspected it, and was leaning towards it. I very specifically asked in reply #15 about the heat dissipation setup. I asked for the important info.
The MOSFET isn't being switched between saturation and cutoff like when it's used as a digital switch, but is operating in the linear region due to the op amp feedback circuit. This means that, depending on the load, it can dissipate very significant amounts of power.
How can mosfet "work" if it is damaged ?
I am not stupid, and would appreciate not being treated like I am. You could at least give me the courtesy of
thinking about my words before ranting.
Semiconductor devices have multiple failure modes, not just "open". They can also fail short, sometimes all at once, sometimes progressively.
Device is rated for 9.2 A @25 degree
Your point? You might as well have said that it has a 100V V
DS maximum. That's not the limit I was talking about.
The maximum junction temperature is 175C, and with 62C/W Junction-to-Ambient resistance it only needs about 3W of power to hit that limit and start damaging the junction if there's no heat sink.
Would YOU choose a 9 A mosfet for a 50 mA load ? (answer: NO !)
For a professional production circuit? I wouldn't expect to ever see that.
For a hobbyist project or a development prototype? Wouldn't surprise me.
Also, 50 mA is not the load, it's the fault current when the device is supposed to be off.
So then we can assume load current is 5 A +/- 4A.
You rant at me earlier in this same post about drawing conclusions without evidence, and then you pull
this out?
The MOSFET is part of an adjustable dummy current load. It is the driving circuitry that defines what current will go through it. And if you do that math, with a 5V PWM signal the maximum current that will go through is 2.5A.
If the heat sinking is inadequate, this kind of amperage could very easily cause the MOSFET to burn out.
I mentioned thermal damage as one possibility. The others mentioned (gate damage from an ungrounded iron, inductive voltage spikes from the load) are good possibilities too. We don't know yet.
I've solved the issue
Thanks for the good feedback and thoughts, I've done some more testing and come to the conclusion that the cause of the damaged MOSFETS is indeed the soldering, and that my iron is not earthed. I measured the voltage on the tip relative to ground at 70V AC ! This seems to be the culprit not the circuit.
I don't think that's enough evidence to conclude it's the iron. I assume that when you're soldering, the board itself is not earthed but left floating. If the 70 VAC mains frequency you measured was capable of damaging the MOSEFT in this case, it would damage
everything that couldn't handle at least 100V.
An earthed iron is necessary because the fragile MOSFET gates are very easily broken by static electricity. You are not going to be able to measure static electricity with a typical multimeter.
I've checking the failed and a new MOSFET in a component checker, which seemed to show the failed one as a Depletion Mode device and a new on as an Enhanced Mode device
An unsurprising result, given the symptoms.
I think it's too early to say that the problem has been fixed. You said in your OP that the problem didn't always show up immediately. Wait for a time and give it an opportunity to get damaged. If it goes for a long enough time to give you confidence that it isn't getting damaged, perfect. If not, we'll need to revisit the inductive spikes of overheating possibilities.