Exceeding Vgs rating

On Thu, 12 Apr 2018 16:11:09 -0700, mike <ham789@netzero.net> wrote:

>On 4/12/2018 2:59 PM, John Larkin wrote:
>> On Thu, 12 Apr 2018 14:37:47 -0700, mike <ham789@netzero.net> wrote:
>>
>>> On 4/12/2018 10:29 AM, John Larkin wrote:
>>> snip
>>>> There's nothing incompetent about
>>> Split the line so you couldn't miss it
>>> testing and understanding parts.
>>> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>>>
>>> Never said anything about testing/qualifying parts and taking
>>> calculated risk that your samples will continue to perform
>>> beyond their specification for the life of the product
>>> (or the vendor of said product).
>>>
>>> I suggested that it was irresponsible to encourage someone
>>> to IGNORE a Vgs spec on some unspecified part in some vague
>>> application that may or may not exceed the spec by one, maybe two,
>>> whoknowswhatthehellthenumbermightbe volts.
>>>
>>>> There are some opamps that I wouldn't run anywhere close to the data
>>>> sheet abs max supply voltage, and there are caps that I run at 1/4
>>>> spec sheet voltage, and others at twice.
>>>>
>>>> I
>>> There's that word again...
>>> tested
>>>
>>> some polymer aluminum caps at 4x rated voltage, and at
>>>> (totally unspecified) reverse voltage... cycled a bunch of them for
>>>> over a month. Now I know.
>>>>
>>>> RF parts are woefully unspecified for time-domain applications. So I
>>> See a trend here?
>>>> test
>>>
>>> them and
>>> Oooo, be still my heart...
>>> derate
>>>
>>> my own measurements.
>>>>
>>>>
>>> We appear to be in complete agreement.  Pissing contest not required.
>>>
>>> Now that we've established that testing is required, we can look at the
>>> cost/benefit ratio.
>>>
>>> YOU seem to work at or beyond the state of the art.
>>> Testing and creative application of components can yield significantly
>>> better performance that's worth the risk.
>>>
>>> If the OP can buy a better FET, or a protection diode for a penny or three,
>>> how much testing can he afford?  How much risk should he take?
>>>
>>> I submit that the cost of engineering resources wasted on this thread
>>> might have bought a lifetime supply of diodes.
>>>
>>> We know nothing about the system that started this thread.
>>> A holistic approach can often yield significant benefit.
>>>
>>> I once attended a design review of two subsystems designed
>>> by different engineers.  Took about 10 seconds to ask the question,
>>> "What happens if the assembler gets distracted and fails to plug
>>> in that cable?"  Took another ten minutes to get them to appreciate
>>> that it stressed a component and another ten minutes for them to
>>> appreciate that I wasn't gonna allow release to manufacturing with
>>> that design defect.
>>> I knew how to fix it, so I insisted that they work together to achieve
>>> a solution.  They were pissed, but eventually realized that all they
>>> had to do was to move a component from one board to the other.
>>> And they became better engineers for the exercise.
>>>
>>> Your holistic
>>> vision becomes more acute when you can expect your manager to
>>> come around with clip leads and short a power supply to ground.
>>> Or if there was a plug that could reach two sockets of the same
>>> format, I was gonna plug it onto the wrong one and hit the power switch.
>>> Cascading failures become less common. ;-)
>>> I almost never got the call from manufacturing to send some engineers
>>> over to clean up their mess.
>>>
>>
>> If a mosfet is rated for 12 volts max Vgs, I am completely confident
>> that an occasional excursion to 14 or 15 will not affect reliability.
>>
>> As I noted, mosfet gates tend to blow out around 70 volts.
>That's good information, but I'd still verify by using the actual parts.
>>
>> I would not run an aluminum electrolytic past specified Vmax, even
>> after testing some. Bad stuff can happen long-term.
>
>Sounds like you have a good mindset for an engineer.
>I'm bitching about general statements implying that you can ignore
>specs without testing on the ACTUAL parts you're gonna use.
>
>Nobody has mentioned binning.
>Many manufacturing processes have great variability.  And the
>parts that test better can be sold at a higher price as a different
>part number, or selected from the same part number.
>What happens when the vendor decides to obsolete a process
>and produce the part on a different machine that still meets
>the specs?  Or someone else special orders high spec parts.
>Or my favorite...some purchasing agent decides to save half a cent.

Most resistors are laser trimmed nowadays, on a super-high speed
production line; they are not generally binned, and the average 1%
resistor (we only buy 1%s) is a lot better than 1%.

Some ICs may be binned. Mosfets possibly. But nobody is going to bin
for max gate voltage, because measuring Vgs-max destroys the part.


>
>Back in the day, when the ink on my diploma was still wet,
>I needed a 1% resistor for a test.
>I had a few dozen carbon resistors in the drawer and expected
>I'd be able to select one close enough for the test.
>Well, there weren't any.
>So, I went to the engineering stockroom and borrowed the whole supply of 
>them.
>There were far fewer than would be suggested by a bell-shaped curve.
>So, I took a few minutes to bin a bunch.  There was a deep hole
>in the distribution around 5%.  "So that's where 5% resistors come from."
>That was my first realization that expecting parts to be better than
>spec was risky. Browsing the approved parts catalog confirmed
>that we had quite a few parts under different internal part numbers selected
>by the vendor for specific characteristics.  And the same hole
>in the distribution on the generic ones.
>
>What would you find if you ordered a bunch of 2N3904 transistors
>from different sources?  I'd bet that most of them exceed specification,
>but in very different ways.
>
>If you're lucky, the parts you get tomorrow will still work in the
>design you did yesterday.  The probability of that goes way up
>when you stay within ALL the specifications.  Don't worry,
>there will still be plenty of unspecified issues to keep you busy.
>
>I believe that calculated risks are a critical component
>of a business strategy,
>but only if you actually do the calculations.
>

I don't always believe the data sheets. We have one fast DAC that we
buy from TI. We asked them about some timing ambiguities on the data
sheet, and they said "That's an old Burr-Brown part; nobody knows
anything about it."

Probably some junior test engineers come up with the data sheet max
limits anyhow. What did some famous person say, "Trust but verify."

And it's fun to blow up parts.


-- 

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement 

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

On 4/12/2018 5:39 PM, krw@notreal.com wrote:

> How would a "purchasing agent" change the specs on a part?
>>
That's exactly the point!

The "purchasing agent"
sources a part that MEETS the specification, period!
They have a whole engineering team that verifies specs.
Cost reduction is a huge deal for them.
If your requirement exceeds the specification, you
may have a problem.

I had a production line shut down.  After a lot of head
scratching, we determined that one of the components
wasn't doing what was expected.  Funny, it didn't even
look the same.

Corporate had placed a new demand on purchasing:
All components will have multiple sources.
Purchasing complied.  They wanted the quantity
price break, so we had a year's supply of parts
that didn't work.  Stock was rotated, so there weren't
any working parts.

I went to purchasing and had them pull their copy
of the corporate specification.
Right on the first page, in BIG letters, it said,
"DO NOT APPROVE OTHER VENDORS FOR THIS PART WITHOUT
CONSULTING PRODUCT ENGINEERING."

They pulled out the memo and said something equivalent to,
"Suck it.  Come back with a waiver from corporate."
Of course, it got resolved with only a few bruised knuckles
on each side...including sales who couldn't meet deliveries...
and production who had people sitting around with partial assemblies
then later had to pay them overtime to catch up...
and finance who wanted the cash flow we didn't get and and and ...

It was a cascading fubar that wasn't really anybody's fault.
But the root CAUSE was using a part beyond its specification.

Bottom line is that shit happens no matter how
smart you think you are or how
explicit your instructions that violate the norm.

Let me try to anticipate your next salvo...
If you change the spec to what you need and the vendor
refuses to test a part to those specs at a price
you're willing to pay, that's a clue that you shouldn't
be doing that.

Calculated risk is great in the aggregate.
In the specific instance of failure, it's a bitch.
Nobody appreciates your successes.  But they
sure do complain about your failures.

This thread has brought back a lot of old memories...
Good times...

On Thursday, 12 April 2018 22:42:33 UTC+1, Tim Williams  wrote:

> Doesn't matter much for today's logic, what with TVS diodes being useless 
> under 5V.  I suppose they would've worked just fine back in the day, a "5V" 
> TVS protecting TTL or HC CMOS (or a 12V TVS protecting CD4000, but probably 
> not a 15 or 18V TVS!).
> 
I once tested a 5V logic device with higher power supply voltages.  It
lasted only a few minutes at 6.5V.  I can't remember exactly what it was,
or even why I was doing it, but it was most likely a GAL programmable
logic device.
John

<jrwalliker@gmail.com> wrote in message 
news:1cd22915-9697-44aa-921f-c66b8e104baf@googlegroups.com...
> On Thursday, 12 April 2018 22:42:33 UTC+1, Tim Williams  wrote:
>
>> Doesn't matter much for today's logic, what with TVS diodes being useless
>> under 5V.  I suppose they would've worked just fine back in the day, a 
>> "5V"
>> TVS protecting TTL or HC CMOS (or a 12V TVS protecting CD4000, but 
>> probably
>> not a 15 or 18V TVS!).
>>
> I once tested a 5V logic device with higher power supply voltages.  It
> lasted only a few minutes at 6.5V.  I can't remember exactly what it was,
> or even why I was doing it, but it was most likely a GAL programmable
> logic device.
> John

Once had a 3.3V PIC overvolted by a mis-spec'd LDO, it was simmering about 
3.8-4.2V and 100-200mA.  Not a bad LV zener, I suppose.  It was still 
"thinking", mostly, but USB wasn't behaving -- probably because of substrate 
current zapping the analog bits (PLL?) or something.

74LVC is technically rated for 5V, but running it that hot gives me a bit of 
the creeps.

Tim

-- 
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Contract Design
Website: https://www.seventransistorlabs.com/ 

On 13/04/2018 08:36, Tim Williams wrote:

<snip>

> Once had a 3.3V PIC overvolted by a mis-spec'd LDO, it was simmering 
> about 3.8-4.2V and 100-200mA.&#4294967295; Not a bad LV zener, I suppose.&#4294967295; It was 
> still "thinking", mostly, but USB wasn't behaving -- probably because of 
> substrate current zapping the analog bits (PLL?) or something.

Quite possibly temperature stuffing the PLL IME.

> 74LVC is technically rated for 5V, but running it that hot gives me a 
> bit of the creeps.
> 
> Tim
> 

Cheers
-- 
Clive

On Friday, April 13, 2018 at 9:36:32 AM UTC+2, Tim Williams wrote:
> <jrwalliker@gmail.com> wrote in message 
> news:1cd22915-9697-44aa-921f-c66b8e104baf@googlegroups.com...
> > On Thursday, 12 April 2018 22:42:33 UTC+1, Tim Williams  wrote:
> >
> >> Doesn't matter much for today's logic, what with TVS diodes being useless
> >> under 5V.  I suppose they would've worked just fine back in the day, a 
> >> "5V"
> >> TVS protecting TTL or HC CMOS (or a 12V TVS protecting CD4000, but 
> >> probably
> >> not a 15 or 18V TVS!).
> >>
> > I once tested a 5V logic device with higher power supply voltages.  It
> > lasted only a few minutes at 6.5V.  I can't remember exactly what it was,
> > or even why I was doing it, but it was most likely a GAL programmable
> > logic device.
> > John
> 
> Once had a 3.3V PIC overvolted by a mis-spec'd LDO, it was simmering about 
> 3.8-4.2V and 100-200mA.  Not a bad LV zener, I suppose.  It was still 
> "thinking", mostly, but USB wasn't behaving -- probably because of substrate 
> current zapping the analog bits (PLL?) or something.
> 
> 74LVC is technically rated for 5V, but running it that hot gives me a bit of 
> the creeps.
> 

On that thought, standard LVC logic has 5.5V recommended supply voltage, and 6.5V absolute max

Page 6 of this app note gives some insight:

http://www.ti.com/lit/an/szza036c/szza036c.pdf

So, really any function is not guaranteed at max abs limits, just no destruction

But, why then specify it. Maybe some funny application where sometimes a pulse comes along and the VDD ramps up to abs max, but then the function of that application is not guaranteed. I cannot really think of a function in which I would use it like that

I suspect it's specmanchip. The device probably works up to abs max, but the fine print specification is no longer valid (leakage, voltage levels etc)

Cheers

Klaus

On Friday, 13 April 2018 00:12:15 UTC+1, mike  wrote:
> On 4/12/2018 2:59 PM, John Larkin wrote:
> > On Thu, 12 Apr 2018 14:37:47 -0700, mike <ham789@netzero.net> wrote:

> > If a mosfet is rated for 12 volts max Vgs, I am completely confident
> > that an occasional excursion to 14 or 15 will not affect reliability.
> >
> > As I noted, mosfet gates tend to blow out around 70 volts.
> That's good information, but I'd still verify by using the actual parts.
> >
> > I would not run an aluminum electrolytic past specified Vmax, even
> > after testing some. Bad stuff can happen long-term.
> 
> Sounds like you have a good mindset for an engineer.
> I'm bitching about general statements implying that you can ignore
> specs without testing on the ACTUAL parts you're gonna use.

As Mr. Larkin said, not in these words, doing the testing yourself instead of the mfr doing it is just fine. It seems everyone now agrees on that. However what you criticise there is also done and also fine for some products - and not for some others. If I were doing mil spec hardware, absolutely not. 

Snip a lot for brevity. I'm sure we all know really what the uspides & downsides of using parts past their specs are. That processes can change etc is only news to students.


> I believe that calculated risks are a critical component
> of a business strategy,

Precisely!


> but only if you actually do the calculations.

Have you bought any consumer goods lately? And noticed how well they last? You really think it's all running within mfr spec in there?

Some things you need to test. Some you really don't, for some products. I've used transistors that don't even come with any spec. They're just 'transistors.' The numbers on them I don't get any useful hits online for. I don't care, any random small low cost transistor can handle 6v 10mA, and for the low cost goods I design for the downside to that is fine.

Yes there are 4v rated trs out there, but not at that price.


NT

On Thu, 12 Apr 2018 21:07:06 +0100, "Kevin Aylward"
<kevinRemovAT@kevinaylward.co.uk> wrote:

>>"Pimpom"  wrote in message news:cAHxC.142146$cC7.99542@fx19.ams1...
>
>>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?
>
>
>There is "by the book" design and reality.
>
>Manufactures have to put in  safety factor. The probability that a mos rated 
>for 12V will blow at 14 is very low.
>
>For example, a typical IC process for a "5.5V" rated Vgs (a nominal 5V part 
>at 10% tol) will typically data sheet spec a typical breakdown of around 
>14V, with 7.5V as a minimum.
>
>Even the ESDs devices have to operate at a nominal voltage higher than the 
>rated voltage of the part, otherwise they might activate when the system is 
>sitting at its rated voltage due to the tolerance of the ESD protector.
>
>So, don't worry, shit don't always happen...
>
>-- Kevin Aylward
>http://www.anasoft.co.uk - SuperSpice
>http://www.kevinaylward.co.uk/ee/index.html 

I recently tested some TinyLogic gates and flops. They are rated 4.6V
abs max, 3.6 recommended opearting max, and do weird non-destructive
things at 7 volts. I think they have ESD zeners inside, and those are
pretty tightly controlled.

I may run them at 5 volts. Or just have a 4.6 volt "5 volt" rail.

I may have a Vcc ladder, logic powered from 3.3, next stage 4.2, final
stage 5.0, to keep up the speed but avoid shoot-through current.


-- 

John Larkin         Highland Technology, Inc

lunatic fringe electronics 

On Thu, 12 Apr 2018 21:59:41 -0700, mike <ham789@netzero.net> wrote:

>On 4/12/2018 5:39 PM, krw@notreal.com wrote:
>
>> How would a "purchasing agent" change the specs on a part?
>>>
>That's exactly the point!
>
>The "purchasing agent"
>sources a part that MEETS the specification, period!
>They have a whole engineering team that verifies specs.
>Cost reduction is a huge deal for them.
>If your requirement exceeds the specification, you
>may have a problem.

That's complete bullshit!  "Purchasing agents" wouldn't know a spec if
it bit their ass completely off.  They buy what the system tells them
to buy, when it needs to be bought.  No more.  No less.  If you can't
tell them what to buy (and *only* what to buy) then your company is
completely broken.

>I had a production line shut down.  After a lot of head
>scratching, we determined that one of the components
>wasn't doing what was expected.  Funny, it didn't even
>look the same.

...and you blame a lowly purchasing agent FOR YOUR MISTAKE?  Amazing.

>Corporate had placed a new demand on purchasing:
>All components will have multiple sources.

With no input from engineering?  Sell your stock.  That one's going
under.

>Purchasing complied.  They wanted the quantity
>price break, so we had a year's supply of parts
>that didn't work.  Stock was rotated, so there weren't
>any working parts.

Find a new employer.  Your paycheck is about to bounce.

>I went to purchasing and had them pull their copy
>of the corporate specification.
>Right on the first page, in BIG letters, it said,
>"DO NOT APPROVE OTHER VENDORS FOR THIS PART WITHOUT
>CONSULTING PRODUCT ENGINEERING."

And who didn't follow orders?
>
>They pulled out the memo and said something equivalent to,
>"Suck it.  Come back with a waiver from corporate."
>Of course, it got resolved with only a few bruised knuckles
>on each side...including sales who couldn't meet deliveries...
>and production who had people sitting around with partial assemblies
>then later had to pay them overtime to catch up...
>and finance who wanted the cash flow we didn't get and and and ...
>
>It was a cascading fubar that wasn't really anybody's fault.
>But the root CAUSE was using a part beyond its specification.
>
>Bottom line is that shit happens no matter how
>smart you think you are or how
>explicit your instructions that violate the norm.

No, the bottom line is that your company is completely out of control
and will probably go under.
>
>Let me try to anticipate your next salvo...
>If you change the spec to what you need and the vendor
>refuses to test a part to those specs at a price
>you're willing to pay, that's a clue that you shouldn't
>be doing that.

Wrong all around.

>Calculated risk is great in the aggregate.
>In the specific instance of failure, it's a bitch.
>Nobody appreciates your successes.  But they
>sure do complain about your failures.
>
>This thread has brought back a lot of old memories...
>Good times...

Good time?  Working for a shit company?  I don't _think_ so.

On Saturday, 14 April 2018 03:49:51 UTC+1, k...@notreal.com  wrote:
> On Thu, 12 Apr 2018 21:59:41 -0700, mike <ham789@netzero.net> wrote:
> >On 4/12/2018 5:39 PM, krw@notreal.com wrote:
> >
> >> How would a "purchasing agent" change the specs on a part?
> >>>
> >That's exactly the point!
> >
> >The "purchasing agent"
> >sources a part that MEETS the specification, period!
> >They have a whole engineering team that verifies specs.
> >Cost reduction is a huge deal for them.
> >If your requirement exceeds the specification, you
> >may have a problem.
> 
> That's complete bullshit!  "Purchasing agents" wouldn't know a spec if
> it bit their ass completely off.  They buy what the system tells them
> to buy, when it needs to be bought.  No more.  No less.  If you can't
> tell them what to buy (and *only* what to buy) then your company is
> completely broken.
> 
> >I had a production line shut down.  After a lot of head
> >scratching, we determined that one of the components
> >wasn't doing what was expected.  Funny, it didn't even
> >look the same.
> 
> ...and you blame a lowly purchasing agent FOR YOUR MISTAKE?  Amazing.
> 
> >Corporate had placed a new demand on purchasing:
> >All components will have multiple sources.
> 
> With no input from engineering?  Sell your stock.  That one's going
> under.

for a corporation to ever be buying parts that doesn't have multiple sources is a clear hazard. It may be acceptable if there's no other way to get your bleeding edge gear done, but normally it's a big no go. So why was this not implemented from the start?
And am I understanding right that it was implemented part way through a production run, thereby stopping that production, causing the problem it was intended to prevent?

> >Purchasing complied.  They wanted the quantity
> >price break, so we had a year's supply of parts
> >that didn't work.  Stock was rotated, so there weren't
> >any working parts.
> 
> Find a new employer.  Your paycheck is about to bounce.
> 
> >I went to purchasing and had them pull their copy
> >of the corporate specification.
> >Right on the first page, in BIG letters, it said,
> >"DO NOT APPROVE OTHER VENDORS FOR THIS PART WITHOUT
> >CONSULTING PRODUCT ENGINEERING."
> 
> And who didn't follow orders?
> >
> >They pulled out the memo and said something equivalent to,
> >"Suck it.  Come back with a waiver from corporate."
> >Of course, it got resolved with only a few bruised knuckles
> >on each side...including sales who couldn't meet deliveries...
> >and production who had people sitting around with partial assemblies
> >then later had to pay them overtime to catch up...
> >and finance who wanted the cash flow we didn't get and and and ...
> >
> >It was a cascading fubar that wasn't really anybody's fault.
> >But the root CAUSE was using a part beyond its specification.
> >
> >Bottom line is that shit happens no matter how
> >smart you think you are or how
> >explicit your instructions that violate the norm.
> 
> No, the bottom line is that your company is completely out of control
> and will probably go under.
> >
> >Let me try to anticipate your next salvo...
> >If you change the spec to what you need and the vendor
> >refuses to test a part to those specs at a price
> >you're willing to pay, that's a clue that you shouldn't
> >be doing that.
> 
> Wrong all around.
> 
> >Calculated risk is great in the aggregate.
> >In the specific instance of failure, it's a bitch.
> >Nobody appreciates your successes.  But they
> >sure do complain about your failures.
> >
> >This thread has brought back a lot of old memories...
> >Good times...
> 
> Good time?  Working for a shit company?  I don't _think_ so.

It totally reminds me of my time in bigcorp. Incompetents making key decisions, no-one noticing there was any kind of problem. And anyone that noticed and said anything got the blame for anything they can think up. No surprise that the mortality rate of big successful businesses is so high.


NT