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Topic: [SOLVED} MOSFET continuously conducts (Read 8084 times) previous topic - next topic

Jiggy-Ninja

How's your heat dissipation? You're going to have 2.5A max going though that MOSFET and it's going to be taking all that heat. If you don't have good heat sinking you're going to toast it.

Do you have a large enough heatsink with a good thermal connection to the MOSFET?
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outsider

Can LM324 pull all the way down to 0V?

pwillard

Only with a +/- gnd referenced supply such as a +12 -12 GND supply.  It cannot go "rail to rail".

allanhurst

An IRF520 is an enhancement mode device, and should be pretty well off with 0v g-s

Would need 8ish volts to turn it on hard.   

Since it's being used in an open loop linear mode, it'll probably get pretty hot - but unlike a bipolar that should try and shut it down..

OP - there's something else going on - more details of the load  please - or is the 2ohm the load and you're strapping the drain directly to +ve?

The LM324 will pull down to a few mv, but won't sink much current - a 10k from g-s may help here

regards

Allan

dwightthinker

#19
Sep 28, 2016, 11:49 pm Last Edit: Sep 28, 2016, 11:59 pm by dwightthinker
It is possible that that the MOSFET is being damaged during
soldering.
You really should be using a grounded iron.
It is not real difficult to add a ground wire.
The FET has a reasonably large input capacitance but
they can be damaged.
Even with a grounded tip, install the MOSFET last.
If the voltage of the opamp output is zero than I'd say
your looking at a leaking gate.
Your inputs for the 324 are not well balanced on impedance.
As has been noted, the 324 does not do well near the negative
terminals level.
Have you measured across the 1K resistor?
There should be zero volts. Any voltage at all is an
indication of a blown FET.
Lastly, you should have a method of clamping the voltage
into the analog input. This is especially true with your
lack of filtering. Pulses could easily exceed the input spec
for analog in.
Dwight

raschemmel

The OP hasn't posted any test results that are conclusive regarding damage. At this point I am treating it as speculation until the OP posts the resistance comparison of a suspect device compared to a brand new unused device. That would be conclusive if the difference is noticable.
Simply saying it won't turn off with out having any further information is not conclusive. There may be some other explanation.  There has been no mention of overheating or smoke or exploding devices so it's too soon to tell.
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Jiggy-Ninja

The OP hasn't posted any test results that are conclusive regarding damage.
Really?

First post:
Quote
Then at some point (not always immediately) my MOSFETs, rather than switching off with a gate voltage of 0, seem to continue to conduct current (just as if its a 50hm resistor).
Seems pretty clear he's noticing a degradation over time compared to initial operation.

Second post:
Quote
Also disconnected the gate and connected it to ground - same result.
Has tried directly grounding the gate. Still leaks like an open faucet. Assuming it's wired properly, that seems pretty definitive.

Right now, I'm leaning towards the fault being caused by a good toasting, courtesy of Mr Watt.
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outsider

#22
Sep 29, 2016, 11:17 am Last Edit: Sep 29, 2016, 11:23 am by outsider
I still think it's the 324 op-amp not pulling all the way to GND, look at the OPs drawing and imagine 50 mV at the output of the Vfollower, applied to the + input of the second amp would cause the output to rise, forcing the FET to conduct until the voltage at the top of the 2 R resistor and the - input  was at 50 mV, causing 25 mA to flow through the sense resistor.
The 324 datasheet states the Vlo voltage at output may be as high as 20 mV @ 5V VCC, what could it be at 12V?

A quick stab with a DMM probe on the gate would tell the tale.

SteveShaw

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.

Using an earthed socket for the iron, have replaced the failed MOSFET and everything is now functioning correctly

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 - which would perhaps explain why the circuit was seeming to work but not turning off - no idea how damaging the device changed it but out of interest personal i'll test the failed device further to see if it really does behave like a D rather than E device and if it an indeed be turned off with a -ve input.

Big thanks again to all those helping me out here

raschemmel

#24
Sep 29, 2016, 02:59 pm Last Edit: Sep 29, 2016, 03:28 pm by raschemmel
You can edit the post title in your original post to read:

"[SOLVED]  MOSFET continuously conducts"


SEE REPLY # 21

Quote
It is possible that that the MOSFET is being damaged during
soldering.
You really should be using a grounded iron.

It is not real difficult to add a ground wire.
The FET has a reasonably large input capacitance but
they can be damaged.
Even with a grounded tip, install the MOSFET last.  
Quote
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 - which would perhaps explain why the circuit was seeming to work but not turning off - no idea how damaging the device changed it but out of interest personal i'll test the failed device further to see if it really does behave like a D rather than E device and if it an indeed be turned off with a -ve input.  
Depletion_and_enhancement_modes

Quote
For N-type depletion-load devices, the threshold voltage might be about -3 V, so it could be turned off by pulling the gate 3 V negative (the drain, by comparison, is more positive than the source in NMOS). In PMOS, the polarities are reversed.
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TomGeorge

Hi,
Do you have the gnd of the arduino connected to gnd of the load circuit, that is the gnd end of the 2R resistor?
Without the gnd to the arduino your current sense from 2R resistor is not valid.

What is the maximum current you put through the load and does the MOSFET have a heatsink?

As you are not PWM controlling the MOSFET then it will be dissipating all your load energy.

Do you have a fuse in the load circuit?

Tom... :)
Everything runs on smoke, let the smoke out, it stops running....

dwightthinker

I am not surprised that the ungrounded iron did the
damage.
At a previous place I worked, we damaged about $1500s worth
of MOSFETs using ungrounded soldering irons. These were nice
Wellers that the assemblers took home and cut the ground pin
off.
MOSFETs come in two types. Some have an input protection
zener diode and some don't.
We had both types in our design. None of the zener input ones
failed. Almost all of the unprotected ones where damaged.
We fixed the plugs and had no more issues.
The other part that I've found to be sensitive to static is
the parts like 4051, 4052 and 4053.
 Just handling these part while ungrounded would damage them.
Also, the more sensitive parts should always be the last parts
to be installed. This keeps the unconnected leads from being
static antennas.
Dwight

Jiggy-Ninja

#27
Sep 29, 2016, 04:18 pm Last Edit: Sep 29, 2016, 04:21 pm by Jiggy-Ninja
Quote
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.
Quote
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.
Quote
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.

Quote
Device is rated for 9.2 A @25 degree
Your point? You might as well have said that it has a 100V VDS 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.
Quote
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.
Quote
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.

Quote
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.
Quote
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.
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Jiggy-Ninja

These were nice
Wellers that the assemblers took home and cut the ground pin
off.
Did you ever find out why these people vandalized company property?
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raschemmel

#29
Sep 29, 2016, 04:27 pm Last Edit: Sep 29, 2016, 04:34 pm by raschemmel
Quote
These were nice
Wellers that the assemblers took home and cut the ground pin
off.  
That's messed up.

That's just wrong on so many levels...


Quote
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.
The load current is a function of the two parameters we don't have, the Load voltage and the load current. It has nothing to do with the 5V drive signal.

In conclusion, the damage was NOT heat related so that rules out "toasting".



Quote
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. 
You forget we don't know the load voltage (or current) but heatsinking is not an issue because the OP never reported overheating.
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