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IamSmooth

Sat Oct 05 2019, 04:23PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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Posts: 1567

I'm working on a project that occasionally causes me to exceed the current limitations of a mosfet's current (40A). Are there any simple techniques to protect the mosfet from this happening?

I've considered using a variable DC power supply with a fixed maximum current of say 20A. I've thought of using some fastblow fuses, but I don't think they would work fast enough.

Any suggestions?

2Spoons

Sun Oct 06 2019, 09:33AM

Registered Member #2939 Joined: Fri Jun 25 2010, 04:25AM
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Posts: 615

IamSmooth wrote ...

I've considered using a variable DC power supply with a fixed maximum current of say 20A.

This is your best bet. So long as to don't have a lot of C in there too.

Fuses are useless. Fuses are for massive over-current faults, by which time your silicon is already toast.

Adrenaline

Mon Oct 07 2019, 03:14PM

Registered Member #235 Joined: Wed Feb 22 2006, 04:59PM
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2Spoons wrote ...

Fuses are useless. Fuses are for massive over-current faults, by which time your silicon is already toast.

That isn't true. You just have to make sure your I2T of your fuse is less than that of the device you want to protect.

2Spoons

Mon Oct 07 2019, 09:21PM

Registered Member #2939 Joined: Fri Jun 25 2010, 04:25AM
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Posts: 615

Adrenaline wrote ...

You just have to make sure your I2T of your fuse is less than that of the device you want to protect.

Try " a lot less" - fuses are not precision devices. The trip current also varies with temperature. Which means using grossly over-rated silicon if you don't want your fuse to pop in normal operation.
An electronic current limiter is a much better option.

IamSmooth

Mon Oct 07 2019, 11:11PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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I've looked at the I2t of fuses. If I read the chart correctly, a 20A fast blow has a trip time of about 0.1s if the current is 40A. This is not a lot of time for a semiconductor.

I've decided to get a variable power supply to limit the current during my testing phase.

Adrenaline

Tue Oct 08 2019, 01:09PM

Registered Member #235 Joined: Wed Feb 22 2006, 04:59PM
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Yes, for this use case using current limiting is the way to do it.

My point was fuses can and do protect silicon, in sort circuit conditions.

Patrick

Tue Oct 08 2019, 05:40PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

I would use pulse by pulse current limiting if possible. With modern ICs its much easier than in past years, and you can use a loop homemade current transformer or un-ravel the nichrome on a high watt cement resistor. Low power loss through heating either way.

The fuses' slow thermal inertia may cause problems with the ultra low mass semiconductor's high speed inertia.

I have a thread somewhere with about 8 pages of discussion on your very issue.

Current limiting unlike fusing can be used as a diagnostic tool, which surprised me. It can indicate short, open, shoot through under varying loads and saturation with a oscilloscope.

IamSmooth

Tue Oct 08 2019, 10:42PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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Posts: 1567

Patrick, any way you can find that thread and post a link to it here?

Patrick

Wed Oct 09 2019, 06:45AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Unfortunately the pics dont work. But the UC 280XX or similar is the key. you dont need to setup the whole circuit. you can spoof everything except transistor gate drive, current limit and timing cycle. in this there was another IC i used that linearly rolled back the duty cycle. cant remember.

Im wishing there were a large cache of pics we could sift through. Im sure there are threads ill never see again. I have 5,400 pics going back 12+ years, theyre on 4 different computers and only 3% or so are cropped and fit for posting on the interwebs.

klugesmith

Wed Oct 09 2019, 09:20PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

IamSmooth wrote ...

I've looked at the I2t of fuses. If I read the chart correctly, a 20A fast blow has a trip time of about 0.1s if the current is 40A. This is not a lot of time for a semiconductor.

Agreed, and it can be similarly revealing to look at I2t numbers for the semiconductors. Or infer it from half-cycle-at-power-frequency specs.

I think 20A circuit breakers in household panels are supposed to conduct 40A for about a minute.
Of course they trip as fast as they can in event of a short circuit (>> 1000 A).
The logarithmic delay-vs-overcurrent charts span many decades, and are not constant I2T.

Can anyone tell us about the time delay control mechanisms? Maybe the answers are on the Internet.
Thermal methods (bimetal parts that move)?
Hydraulic mechanisms (solenoid plungers with viscous damping)?
Decades ago, one brand became notorious for failing to trip under a condition where they needed to trip.

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