Protected MOSFETs

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JL posted a couple of circuit suggestions in the "slow fade-in circuit"  
thread that used protected MOSFETs.  (*)

We've had a few discussions about those gizmos in this very boutique.  
I've actually never used one, but IIRC the wisdom has been that they're  
not nearly as robust as their makers claim.

It's been awhile, though--anybody have recent experience with them,  
favourite parts and so on?

Cheers

Phil Hobbs

(*) I'm trying to break myself of the habit of starting tangential  
discussions far down existing threads, which is why this new one.  We  
need more engineering round here.

--  
Dr Philip C D Hobbs
Principal Consultant
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Re: Protected MOSFETs
Phil Hobbs wrote:

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I was using the VND14NV04 to switch rectified AC 12V RMS (just the  
bridge, no filter cap). In a 100 channel lab device, there were 4  
failures over 5 years. Interestingly, the failure mode has always been  
half-open. No data from purely DC applications, it was enough for me to  
send them to the trashcan.

I have good experience with the IPS161, although it is not exactly what  
you are asking, but IMHO close enough to mention it.

    Best regards, Piotr

Re: Protected MOSFETs
On Sat, 26 Sep 2020 02:48:10 -0400, Phil Hobbs

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I was thinking about a protected-gate mosfet, which is pretty common
these days. They basically have a roughly 40 volt symmetric zener from
gate to source. That's nice.

I tried three of the claimed really protected mosfets, the ones with
current and thermal limits. The ones that I tried were terrible. One
didn't really protect itself. One oscillated horribly. Something else
was nasty about the third.

They seem to power their protection circuits from the gate drive,
which means that you wind up doing half the work.

If you find a good one, let us know.

Lately I've been putting some power parts on a PCB copper pour with a
shutdown thermistor incorporated into the MAX809 powerup reset
circuit. Seems to work.

You can do fun stuff around a MAX809.

We need a MAX809 sort of thing that has a temperature shutdown built
in. May exist. Isolated tab would be good.

Zetex makes a SOT23 nickel RTD with isolation.









--  

John Larkin         Highland Technology, Inc

Science teaches us to doubt.

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Re: Protected MOSFETs
On 2020-09-26 11:13, snipped-for-privacy@highlandsniptechnology.com wrote:
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Those are the ones I was talking about.

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Not a bad idea.  I usually use LM393s for that, but in situations where  
you need a POR anyway, dorking a MAX809 might be a win.

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MIC280 looks nice--settable local plus remote temperature measurement  
and thermal alarm, SMBus.  I've never used it.  Seems to be the only  
game in town, but doesn't have an externally-accessible POR function.

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Cheers

Phil Hobbs


--  
Dr Philip C D Hobbs
Principal Consultant
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Re: Protected MOSFETs

sniptechnology.com:
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which ones and what did you try? other than one type from Onsemi some 10 ye
ars ago I've not heard of any issues

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usually ~1mA gate drive and a series resistor how is that half the work?



Re: Protected MOSFETs
On Saturday, September 26, 2020 at 11:14:13 AM UTC-4, snipped-for-privacy@highlandsniptechnology.com wrote:
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Right, I tested a few of these for a simple heater (I  
was mostly interested in the thermal limit) and none of them  
were as robust as I wanted.  Nothing like thermally protected  
bjt parts (lm317, lm395)

George H.  
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Re: Protected MOSFETs
On 9/26/2020 1:48 AM, Phil Hobbs wrote:
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This is disturbing.

We have a few thousand of the NCV8402STT1G protected FET out and in use  
over the last couple of years. They are used as a uC interface to drive  
20-40 feet or so of wires in customers' construction warning signs to  
operate running time meters (100mA) and some logic lines <10MA. We have  
had no failures so far, but I have never bench tested them and cannot  
testify as to their robustness. No failures does not mean there will be  
none in the future.



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