Exceeding Vgs rating

On Saturday, April 7, 2018 at 3:43:57 PM UTC-4, Pimpom wrote:
> On 4/7/2018 11:17 PM, bloggs.fredbloggs.fred@gmail.com wrote:
> > On Friday, April 6, 2018 at 6:01:50 AM UTC-4, Pimpom wrote:
> >> I'm designing a small simple circuit in which a MOSFET drives a
> >> low-power load. The very low frequency gate drive may, on rare
> >> occasions, exceed the max Vgs rating of 12V by about 1V, possibly 2V.
> >>
> >> There are a number of ways to limit the gate voltage but I want
> >> to avoid them and keep the circuit as simple as possible, and it
> >> won't cause a disaster if the transistor fails. What do you think?
> > 
> > Does this source have an impedance?
> 
> Not enough to protect the gate if it does break down.
> 
> > And you do have quiescent current loading limitations?
> 
> Not sure what you mean by that.
> 
> > What is the range, both minimum and maximum, of your input signal?
> 
> Normally not much above the gate threshold. The circuit is about 
> proportional control, not hard switching. Exceeding Vgs(max) is a 
> remote possibility under abnormal conditions.
> 
> > What is this low power load and how mow much voltage variation across it can it take?
> 
> It can take the full supply voltage continuously.
>   > There's not enough context to offer a fix.
> 
> I'm not really looking for a fix. I can think of more than one 
> that will cost no more than 1 US cent. The aim is for elegance 
> with the least possible parts count.

You should use one of your one penny solutions.

On Saturday, 7 April 2018 17:19:21 UTC+1, George Herold  wrote:
> On Saturday, April 7, 2018 at 1:10:59 AM UTC-4, mike wrote:

> > Thought experiment:
> > Do two designs.
> > Start one with a quality mindset.
> > Start the other with a minimum cost mindset.
> I'm always doing both at once... biggest bang
> for smallest buck.  That is the value I deliver to 
> my customers.  (well, part of it.) 

And that is primarily what consumer product engineering is about, bang for buck. Consumer and industrial versions of the same product have a lot of differences, and they're mainly for that reason.


NT

I can think of more than one 
> > that will cost no more than 1 US cent. The aim is for elegance 
> > with the least possible parts count.
> 


an elegant design would remove all doubt of a long term reliability issue for 1 cent

mark

On 06/04/2018 20:18, mike wrote:

<snip>

> You guys talk like ignoring specifications is acceptable practice.
> IT IS NOT!
> Testing a few samples under controlled conditions is NOT reason to 
> exceed specifications.

Oh dear.  My latest design has microprocessors, FPGAs and lots of analog 
stuff, almost all of which will be used outside the manufacturer's 
specifications, in some cases way outside.  Luckily, the customers are 
aware of this.

The connectors and the PCB are within spec though.  Phew.

Cheers
-- 
Clive

On Tue, 10 Apr 2018 17:01:13 +0100, Clive Arthur
<cliveta@nowaytoday.co.uk> wrote:

>On 06/04/2018 20:18, mike wrote:
>
><snip>
>
>> You guys talk like ignoring specifications is acceptable practice.
>> IT IS NOT!
>> Testing a few samples under controlled conditions is NOT reason to 
>> exceed specifications.
>
>Oh dear.  My latest design has microprocessors, FPGAs and lots of analog 
>stuff, almost all of which will be used outside the manufacturer's 
>specifications, in some cases way outside.  Luckily, the customers are 
>aware of this.
>
>The connectors and the PCB are within spec though.  Phew.
>
>Cheers

That is NOT ACCEPTABLE and you are a BAD PERSON.



-- 

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement 

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

On Tuesday, 10 April 2018 20:39:41 UTC+1, John Larkin  wrote:
> On Tue, 10 Apr 2018 17:01:13 +0100, Clive Arthur
> <cliveta@nowaytoday.co.uk> wrote:
> >On 06/04/2018 20:18, mike wrote:
> >
> ><snip>
> >
> >> You guys talk like ignoring specifications is acceptable practice.
> >> IT IS NOT!
> >> Testing a few samples under controlled conditions is NOT reason to 
> >> exceed specifications.
> >
> >Oh dear.  My latest design has microprocessors, FPGAs and lots of analog 
> >stuff, almost all of which will be used outside the manufacturer's 
> >specifications, in some cases way outside.  Luckily, the customers are 
> >aware of this.
> >
> >The connectors and the PCB are within spec though.  Phew.
> >
> >Cheers
> 
> That is NOT ACCEPTABLE and you are a BAD PERSON.

Lol. I won't say a thing about unknown & thus effectively unspecified transistors then. Someone's in-house part numbers maybe.

If there are transistors that can't do a few milliamps at a handful of volts I've not met them.

Hi-rel electronics did teach me one thing: how inefficient it was & how to do it all cheaply.


NT

On Tuesday, April 10, 2018 at 7:25:12 PM UTC-4, tabb...@gmail.com wrote:
> On Tuesday, 10 April 2018 20:39:41 UTC+1, John Larkin  wrote:
> > On Tue, 10 Apr 2018 17:01:13 +0100, Clive Arthur
> > <cliveta@nowaytoday.co.uk> wrote:
> > >On 06/04/2018 20:18, mike wrote:
> > >
> > ><snip>
> > >
> > >> You guys talk like ignoring specifications is acceptable practice.
> > >> IT IS NOT!
> > >> Testing a few samples under controlled conditions is NOT reason to 
> > >> exceed specifications.
> > >
> > >Oh dear.  My latest design has microprocessors, FPGAs and lots of analog 
> > >stuff, almost all of which will be used outside the manufacturer's 
> > >specifications, in some cases way outside.  Luckily, the customers are 
> > >aware of this.
> > >
> > >The connectors and the PCB are within spec though.  Phew.
> > >
> > >Cheers
> > 
> > That is NOT ACCEPTABLE and you are a BAD PERSON.
> 
> Lol. I won't say a thing about unknown & thus effectively unspecified transistors then. Someone's in-house part numbers maybe.
> 
> If there are transistors that can't do a few milliamps at a handful of volts I've not met them.
> 
> Hi-rel electronics did teach me one thing: how inefficient it was & how to do it all cheaply.
> 
> 
> NT

Can you say more about inefficient?  Do you mean there was testing and 
lots of rejects?  

George H. 

On Tue, 10 Apr 2018 16:25:07 -0700 (PDT), tabbypurr@gmail.com wrote:

>On Tuesday, 10 April 2018 20:39:41 UTC+1, John Larkin  wrote:
>> On Tue, 10 Apr 2018 17:01:13 +0100, Clive Arthur
>> <cliveta@nowaytoday.co.uk> wrote:
>> >On 06/04/2018 20:18, mike wrote:
>> >
>> ><snip>
>> >
>> >> You guys talk like ignoring specifications is acceptable practice.
>> >> IT IS NOT!
>> >> Testing a few samples under controlled conditions is NOT reason to 
>> >> exceed specifications.
>> >
>> >Oh dear.  My latest design has microprocessors, FPGAs and lots of analog 
>> >stuff, almost all of which will be used outside the manufacturer's 
>> >specifications, in some cases way outside.  Luckily, the customers are 
>> >aware of this.
>> >
>> >The connectors and the PCB are within spec though.  Phew.
>> >
>> >Cheers
>> 
>> That is NOT ACCEPTABLE and you are a BAD PERSON.
>
>Lol. I won't say a thing about unknown & thus effectively unspecified transistors then. Someone's in-house part numbers maybe.
>
>If there are transistors that can't do a few milliamps at a handful of volts I've not met them.
>
>Hi-rel electronics did teach me one thing: how inefficient it was & how to do it all cheaply.
>
>
>NT

I recall a study that found JAN-TX transistors to be less reliable
than regular ones.


-- 

John Larkin         Highland Technology, Inc

lunatic fringe electronics 

On 4/10/2018 4:25 PM, tabbypurr@gmail.com wrote:
> On Tuesday, 10 April 2018 20:39:41 UTC+1, John Larkin  wrote:
>> On Tue, 10 Apr 2018 17:01:13 +0100, Clive Arthur
>> <cliveta@nowaytoday.co.uk> wrote:
>>> On 06/04/2018 20:18, mike wrote:
>>>
>>> <snip>
>>>
>>>> You guys talk like ignoring specifications is acceptable practice.
>>>> IT IS NOT!
>>>> Testing a few samples under controlled conditions is NOT reason to
>>>> exceed specifications.
>>>
>>> Oh dear.  My latest design has microprocessors, FPGAs and lots of analog
>>> stuff, almost all of which will be used outside the manufacturer's
>>> specifications, in some cases way outside.  Luckily, the customers are
>>> aware of this.

If you're exceeding the breakdown voltage specs of your FPGAs and 
microprocessors
by a large margin (way outside), you must be a lucky designer.
Even the worst of the clueless designers I've known knew better.

There once was a design for a lithium ion battery charger inside
an expensive instrument.  When everything was working right, there
was no problem with voltages.  But, if the battery voltage was allowed
to drop below a certain point, the FET that managed the
charge disconnect broke down and failed shorted.
The fire department was not amused.  They didn't learn about
that problem until there were many in the field.  Big recall.  SAD.

It's very hard to teach engineers to do bad things to their designs
(at least in thought experiments)
and see what happens.  I used to sit down with engineers and say things 
like,
"What happens if that cap shorts?"
"What happens if those two un-keyed ribbon connectors get swapped in 
production?"  "Put on backwards?"
"How much smoke comes out if this device if I short that power supply to 
ground...or if the negative supply goes open and the voltage
gets dragged positive by the loads?"

There's no excuse for an un-keyed ribbon connector to reverse Vcc and Gnd,
if you reverse it or put it on "off-by-one".
It's amazing how many ribbon connector users put Vcc on one end
and ground on the other.
It doesn't cost anything to eliminate that possibility if you're
used to thinking about how the real world works.
Shit happens.  Design for it if it costs nothing.

They thought I was crazy, but after
the first few averted disasters, they decided that it wasn't such a dumb
idea after all.

There are plenty of disasters waiting to happen. There's little
reason to purposely introduce others.  Saying things like, "nothing
serious happens if the FET is destroyed," just adds insult to injury.

>>>
>>> The connectors and the PCB are within spec though.  Phew.
>>>
>>> Cheers
>>
>> That is NOT ACCEPTABLE and you are a BAD PERSON.
>
> Lol. I won't say a thing about unknown & thus effectively unspecified transistors then. Someone's in-house part numbers maybe.
>
> If there are transistors that can't do a few milliamps at a handful of volts I've not met them.
>
> Hi-rel electronics did teach me one thing: how inefficient it was & how to do it all cheaply.
>
>
> NT
>

On Wednesday, 11 April 2018 01:11:43 UTC+1, George Herold  wrote:
> On Tuesday, April 10, 2018 at 7:25:12 PM UTC-4, tabby wrote:
> > On Tuesday, 10 April 2018 20:39:41 UTC+1, John Larkin  wrote:
> > > On Tue, 10 Apr 2018 17:01:13 +0100, Clive Arthur
> > > <cliveta@nowaytoday.co.uk> wrote:
> > > >On 06/04/2018 20:18, mike wrote:
> > > >
> > > ><snip>
> > > >
> > > >> You guys talk like ignoring specifications is acceptable practice.
> > > >> IT IS NOT!
> > > >> Testing a few samples under controlled conditions is NOT reason to 
> > > >> exceed specifications.
> > > >
> > > >Oh dear.  My latest design has microprocessors, FPGAs and lots of analog 
> > > >stuff, almost all of which will be used outside the manufacturer's 
> > > >specifications, in some cases way outside.  Luckily, the customers are 
> > > >aware of this.
> > > >
> > > >The connectors and the PCB are within spec though.  Phew.
> > > >
> > > >Cheers
> > > 
> > > That is NOT ACCEPTABLE and you are a BAD PERSON.
> > 
> > Lol. I won't say a thing about unknown & thus effectively unspecified transistors then. Someone's in-house part numbers maybe.
> > 
> > If there are transistors that can't do a few milliamps at a handful of volts I've not met them.
> > 
> > Hi-rel electronics did teach me one thing: how inefficient it was & how to do it all cheaply.
> > 
> > 
> > NT
> 
> Can you say more about inefficient?  Do you mean there was testing and 
> lots of rejects?  
> 
> George H.

In what way is hi-rel not inefficient?
Eliminating some components ups the price
Eliminating most suppliers ups the price
Eliminating some circuit configs because you can't prove them in every conceiveable situation costs, eg asynchronous sm volt reg.
Training staff to follow numerous extra rules & restrictions consumes money & time & makes it harder to replace them
Then there are the many mechanical requirements. Eg what I worked on all had to have laced wire looms, laced in a relatively slow way of course, with every single wrap individually knotted.
Production needs far more controls, tests & checks
Some product designs get dropped because they don't meet some detail or other.

On and on it goes, everything is inefficient. Which is fine, sort of. But it opens the door for competitors selling stuff at a tenth the price.

The biggest issue I had with it all wasn't just the costs of the product parts, those are all justifiable for the target market. A similarly inefficient staffing setup was what bugged me most. Despite being very successful I became convinced that a new competitor would spring up & wipe them out. It happened. Success breeds inefficiency & cruft which leads to downfall. 


NT