On Friday, April 6, 2018 at 5:19:58 PM UTC-4, John Larkin wrote:> On Fri, 6 Apr 2018 12:59:25 -0700 (PDT), George Herold > <gherold@teachspin.com> wrote: > > >On Friday, April 6, 2018 at 3:19:36 PM UTC-4, mike wrote: > >> On 4/6/2018 7:21 AM, John Larkin wrote: > >> > On Fri, 6 Apr 2018 16:31:38 +0530, Pimpom <Pimpom@invalid.invalid> > >> > 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? > >> > > >> > The few mosfets that I've tested failed at Vgs around 70 volts. There > >> > could be long-term effects. > >> > > >> > Gate-protected ones zenered in the low 40's. > >> > > >> > Test a couple to destruction. It's fun to blow things up. > >> > > >> > I'd estimate that a couple of volts over Vgs_max will have no effect > >> > on reliability. > >> > > >> > > >> 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. > >> > >> Have you never underestimated the consequences of a decision? > >> Have you never had a vendor make a design change that still meets > >> the original specification? > >> Have you never had a part go obsolete and get replaced by an > >> "equivalent" part? > >> Have you never had a purchasing manager switch parts on you > >> without even telling you so he could save a buck and get a bigger bonus? > >> > >> I've never attended a seminar on > >> "it won't cause a disaster if the transistor fails". > > > >Grin, I've done all of that.. or had it happen to me. > >Still... I'm hardly an expert, but there are many specs that > >are listed on the conservative side. Maybe it's too hard > >to measure, leakage currents less than ~1uA. > > Most small bipolars leak picoamps. Some cheap RF transistors have fA > c-b leakage when used as diodes... their leakage isn't even specified! > > > > >Or just lazy, > >(All LED's have a 5V Vrev.) And then other specs that you better > >beat by a large margin.. voltage rating on tants. I'm not sure how > >you learn this stuff, other than word of mouth, or doing it yourself. > > I've seen LEDs and optocouplers that have reverse zener voltages in > the 35v sort of range. The 5V spec is probably to permit multiplexing.Sure, Specs are weird things, 99% of the time you just stay within limits and don't worry about it. But sometimes you'd like to explore the edges... low leakage diodes are a good example. I didn't know RF parts were good... but it figures, they'd have a small area. //story, years ago... I was talking with the guy who designed our Earth's Field NMR. He was lamenting the leakage of some diode into the signal chain and I suggested the c-b junction of a transistor. (I think it needed 50 or 60V of Vrev.) He tried and liked it. // end story George H.> > > -- > > John Larkin Highland Technology, Inc > picosecond timing precision measurement > > jlarkin att highlandtechnology dott com > http://www.highlandtechnology.com
Exceeding Vgs rating
Started by ●April 6, 2018
Reply by ●April 6, 20182018-04-06
Reply by ●April 6, 20182018-04-06
On Fri, 6 Apr 2018 16:31:13 -0700 (PDT), George Herold <gherold@teachspin.com> wrote:>On Friday, April 6, 2018 at 5:19:58 PM UTC-4, John Larkin wrote: >> On Fri, 6 Apr 2018 12:59:25 -0700 (PDT), George Herold >> <gherold@teachspin.com> wrote: >> >> >On Friday, April 6, 2018 at 3:19:36 PM UTC-4, mike wrote: >> >> On 4/6/2018 7:21 AM, John Larkin wrote: >> >> > On Fri, 6 Apr 2018 16:31:38 +0530, Pimpom <Pimpom@invalid.invalid> >> >> > 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? >> >> > >> >> > The few mosfets that I've tested failed at Vgs around 70 volts. There >> >> > could be long-term effects. >> >> > >> >> > Gate-protected ones zenered in the low 40's. >> >> > >> >> > Test a couple to destruction. It's fun to blow things up. >> >> > >> >> > I'd estimate that a couple of volts over Vgs_max will have no effect >> >> > on reliability. >> >> > >> >> > >> >> 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. >> >> >> >> Have you never underestimated the consequences of a decision? >> >> Have you never had a vendor make a design change that still meets >> >> the original specification? >> >> Have you never had a part go obsolete and get replaced by an >> >> "equivalent" part? >> >> Have you never had a purchasing manager switch parts on you >> >> without even telling you so he could save a buck and get a bigger bonus? >> >> >> >> I've never attended a seminar on >> >> "it won't cause a disaster if the transistor fails". >> > >> >Grin, I've done all of that.. or had it happen to me. >> >Still... I'm hardly an expert, but there are many specs that >> >are listed on the conservative side. Maybe it's too hard >> >to measure, leakage currents less than ~1uA. >> >> Most small bipolars leak picoamps. Some cheap RF transistors have fA >> c-b leakage when used as diodes... their leakage isn't even specified! >> >> >> >> >Or just lazy, >> >(All LED's have a 5V Vrev.) And then other specs that you better >> >beat by a large margin.. voltage rating on tants. I'm not sure how >> >you learn this stuff, other than word of mouth, or doing it yourself. >> >> I've seen LEDs and optocouplers that have reverse zener voltages in >> the 35v sort of range. The 5V spec is probably to permit multiplexing. >Sure, >Specs are weird things, 99% of the time you just stay within limits >and don't worry about it. But sometimes you'd like to explore the >edges... low leakage diodes are a good example. I didn't know RF >parts were good... but it figures, they'd have a small area. > >//story, years ago... >I was talking with the guy who designed our Earth's >Field NMR. He was lamenting the leakage of some diode >into the signal chain and I suggested the c-b junction >of a transistor. (I think it needed 50 or 60V of Vrev.) >He tried and liked it. >// end story > >George H.You can buy a PAD-1 for $9. It's a really crappy 1 pA diode, actually a jfet characterized as a diode. Or you can buy a BFT25A RF transistor for 30 cents, and use it as a fA diode. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by ●April 6, 20182018-04-06
"John Larkin" <jjlarkin@highland_snip_technology.com> wrote in message news:hmofcdlgdpec7qe47nqgjagtggtuejfvgk@4ax.com...> I've seen LEDs and optocouplers that have reverse zener voltages in > the 35v sort of range. The 5V spec is probably to permit multiplexing. >I've tested red LEDs anywhere from 20V to over 200V. Very inconsistent between sources. Green (GaP) go similarly. Blue (and white, and high intensity green and cyan, they're all InGaN) die suddenly around 20-40V. Tim -- Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website: https://www.seventransistorlabs.com/
Reply by ●April 6, 20182018-04-06
On 4/6/2018 1:33 PM, Jim Thompson wrote:> On Fri, 06 Apr 2018 12:18:32 -0700, mike <ham789@netzero.net> wrote: > >> On 4/6/2018 7:21 AM, John Larkin wrote: >>> On Fri, 6 Apr 2018 16:31:38 +0530, Pimpom <Pimpom@invalid.invalid> >>> 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? >>> >>> The few mosfets that I've tested failed at Vgs around 70 volts. There >>> could be long-term effects. >>> >>> Gate-protected ones zenered in the low 40's. >>> >>> Test a couple to destruction. It's fun to blow things up. >>> >>> I'd estimate that a couple of volts over Vgs_max will have no effect >>> on reliability. >>> >>> >> 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. >> >> Have you never underestimated the consequences of a decision? >> Have you never had a vendor make a design change that still meets >> the original specification? >> Have you never had a part go obsolete and get replaced by an >> "equivalent" part? >> Have you never had a purchasing manager switch parts on you >> without even telling you so he could save a buck and get a bigger bonus? >> >> I've never attended a seminar on >> "it won't cause a disaster if the transistor fails". >> > > You're just exhibiting your inexperience.You're correct. I'm inexperienced at designing systems that exceed the specifications of components. I taught my engineers to derate components. This statement gave me chills: The very low frequency gate drive may, on rare >>>> occasions, exceed the max Vgs rating of 12V by about 1V, possibly 2V. So kinda maybe we might, on rare occasions see 1V...maybe 2V beyond the spec. But don't worry, it ain't no big thing if it fails. Why not 3V on rarer occasions? How about 10V? How rare? The OP is asking for encouragement, permission, to build in a failure mode and supporting it with kinda/maybe/possibly... If an engineer came to me with that, I'd send him back to the drawing board. "We'll discuss it when you come back with words like absolutely, definitely, here are the options and costs..." If you read the fine print,> VGSmax isusually I split out that word so you can't miss it. Curiosity isn't the only thing that can kill a cat. specified as "guaranteed by design"... that is, it's> NOT measured during manufacture, so there'susually? a HUGE margin.> > ...Jim Thompson >I once was forced to build a logic system to operate in an environment with limited power availability. We literally designed in what would fit the power budget. Problem was that it was TTL and there was a huge difference between typical and maximum power specs. If you used the max numbers, we were over the power budget by 20% or so. Prototypes ran at about 20% under the max. The QA dept refused to sign off on the design. I did a Monte Carlo power analysis that included batch effects per part showing that the probability of excess was near zero. QA signed off on the condition that power was measured at final test and that we supply test fixtures to the service centers and included the power measurement after every repair. I don't think we ever had a non-conforming unit. I didn't "kinda/maybe". I did the rigorous analysis. Validation procedures were in place. The process worked.
Reply by ●April 7, 20182018-04-07
On Saturday, 7 April 2018 02:21:53 UTC+1, mike wrote:> On 4/6/2018 1:33 PM, Jim Thompson wrote: > > On Fri, 06 Apr 2018 12:18:32 -0700, mike <ham789@netzero.net> wrote: > >> On 4/6/2018 7:21 AM, John Larkin wrote: > >>> On Fri, 6 Apr 2018 16:31:38 +0530, Pimpom <Pimpom@invalid.invalid> > >>> 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? > >>> > >>> The few mosfets that I've tested failed at Vgs around 70 volts. There > >>> could be long-term effects. > >>> > >>> Gate-protected ones zenered in the low 40's. > >>> > >>> Test a couple to destruction. It's fun to blow things up. > >>> > >>> I'd estimate that a couple of volts over Vgs_max will have no effect > >>> on reliability. > >>> > >>> > >> 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. > >> > >> Have you never underestimated the consequences of a decision? > >> Have you never had a vendor make a design change that still meets > >> the original specification? > >> Have you never had a part go obsolete and get replaced by an > >> "equivalent" part? > >> Have you never had a purchasing manager switch parts on you > >> without even telling you so he could save a buck and get a bigger bonus? > >> > >> I've never attended a seminar on > >> "it won't cause a disaster if the transistor fails". > >> > > > > You're just exhibiting your inexperience. > > You're correct. I'm inexperienced at designing systems that exceed > the specifications of components. I taught my engineers to derate > components. > > This statement gave me chills: > The very low frequency gate drive may, on rare > >>>> occasions, exceed the max Vgs rating of 12V by about 1V, possibly 2V. > > So kinda maybe we might, on rare occasions see 1V...maybe 2V beyond the > spec. But don't worry, it ain't no big thing if it fails. > Why not 3V on rarer occasions? How about 10V? How rare? > The OP is asking for encouragement, permission, to build in a failure > mode and supporting it with kinda/maybe/possibly... > > If an engineer came to me with that, I'd send him back to the drawing > board. "We'll discuss it when you come back with words like absolutely, > definitely, here are the options and costs..." > > If you read the fine print, > > VGSmax is > > usually > I split out that word so you can't miss it. > Curiosity isn't the only thing that can kill a cat. > > > specified as "guaranteed by design"... that is, it's > > NOT measured during manufacture, so there's > usually? > a HUGE margin. > > > > ...Jim Thompson > > > I once was forced to build a logic system to operate in > an environment with limited power availability. > We literally designed in what would fit the power budget. > Problem was that it was TTL and there was a huge difference > between typical and maximum power specs. > If you used the max numbers, we were over the power budget > by 20% or so. Prototypes ran at about 20% under the max. > > The QA dept refused to sign off on the design. I did a Monte Carlo > power analysis that included batch effects per part showing > that the probability of excess was near zero. > > QA signed off on the condition that power was measured at > final test and that we supply test fixtures to the service > centers and included the power measurement after every repair. > I don't think we ever had a non-conforming unit. > > I didn't "kinda/maybe". I did the rigorous analysis. Validation > procedures were in place. The process worked.That's all a reflection of the type of products you and many folk here design. Cheap consumer electronics is a different market. NT
Reply by ●April 7, 20182018-04-07
On 4/6/2018 8:34 PM, tabbypurr@gmail.com wrote:> On Saturday, 7 April 2018 02:21:53 UTC+1, mike wrote: >> On 4/6/2018 1:33 PM, Jim Thompson wrote: >>> On Fri, 06 Apr 2018 12:18:32 -0700, mike <ham789@netzero.net> wrote: >>>> On 4/6/2018 7:21 AM, John Larkin wrote: >>>>> On Fri, 6 Apr 2018 16:31:38 +0530, Pimpom <Pimpom@invalid.invalid> >>>>> 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? >>>>> >>>>> The few mosfets that I've tested failed at Vgs around 70 volts. There >>>>> could be long-term effects. >>>>> >>>>> Gate-protected ones zenered in the low 40's. >>>>> >>>>> Test a couple to destruction. It's fun to blow things up. >>>>> >>>>> I'd estimate that a couple of volts over Vgs_max will have no effect >>>>> on reliability. >>>>> >>>>> >>>> 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. >>>> >>>> Have you never underestimated the consequences of a decision? >>>> Have you never had a vendor make a design change that still meets >>>> the original specification? >>>> Have you never had a part go obsolete and get replaced by an >>>> "equivalent" part? >>>> Have you never had a purchasing manager switch parts on you >>>> without even telling you so he could save a buck and get a bigger bonus? >>>> >>>> I've never attended a seminar on >>>> "it won't cause a disaster if the transistor fails". >>>> >>> >>> You're just exhibiting your inexperience. >> >> You're correct. I'm inexperienced at designing systems that exceed >> the specifications of components. I taught my engineers to derate >> components. >> >> This statement gave me chills: >> The very low frequency gate drive may, on rare >> >>>> occasions, exceed the max Vgs rating of 12V by about 1V, possibly 2V. >> >> So kinda maybe we might, on rare occasions see 1V...maybe 2V beyond the >> spec. But don't worry, it ain't no big thing if it fails. >> Why not 3V on rarer occasions? How about 10V? How rare? >> The OP is asking for encouragement, permission, to build in a failure >> mode and supporting it with kinda/maybe/possibly... >> >> If an engineer came to me with that, I'd send him back to the drawing >> board. "We'll discuss it when you come back with words like absolutely, >> definitely, here are the options and costs..." >> >> If you read the fine print, >>> VGSmax is >> >> usually >> I split out that word so you can't miss it. >> Curiosity isn't the only thing that can kill a cat. >> >> >> specified as "guaranteed by design"... that is, it's >>> NOT measured during manufacture, so there's >> usually? >> a HUGE margin. >>> >>> ...Jim Thompson >>> >> I once was forced to build a logic system to operate in >> an environment with limited power availability. >> We literally designed in what would fit the power budget. >> Problem was that it was TTL and there was a huge difference >> between typical and maximum power specs. >> If you used the max numbers, we were over the power budget >> by 20% or so. Prototypes ran at about 20% under the max. >> >> The QA dept refused to sign off on the design. I did a Monte Carlo >> power analysis that included batch effects per part showing >> that the probability of excess was near zero. >> >> QA signed off on the condition that power was measured at >> final test and that we supply test fixtures to the service >> centers and included the power measurement after every repair. >> I don't think we ever had a non-conforming unit. >> >> I didn't "kinda/maybe". I did the rigorous analysis. Validation >> procedures were in place. The process worked. > > That's all a reflection of the type of products you and many folk here design. Cheap consumer electronics is a different market. > > > NT >I disagree with that mindset. Start with the mindset that everything should work as advertised and be reliable. Stated another way...put on your customer hat. What do you expect from your product? When you're done, go back and try to cut cost. Weigh the savings against the loss of reliability. If you start with slipshod design, it's hard to fix little things like overvoltaging a FET. Thought experiment: Do two designs. Start one with a quality mindset. Start the other with a minimum cost mindset. I'd bet that if you massage each design toward the middle, you'll end up at price parity with the quality design resulting in a higher quality product. And your next design will be better for it. If you came here for validation, you already know you shouldn't be doing it.
Reply by ●April 7, 20182018-04-07
tabbypurr@gmail.com writes:> On Friday, 6 April 2018 20:19:36 UTC+1, mike wrote: >> On 4/6/2018 7:21 AM, John Larkin wrote: >> > On Fri, 6 Apr 2018 16:31:38 +0530, Pimpom <Pimpom@invalid.invalid> >> > 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? >> > >> > The few mosfets that I've tested failed at Vgs around 70 volts. There >> > could be long-term effects. >> > >> > Gate-protected ones zenered in the low 40's. >> > >> > Test a couple to destruction. It's fun to blow things up. >> > >> > I'd estimate that a couple of volts over Vgs_max will have no effect >> > on reliability. >> > >> > >> You guys talk like ignoring specifications is acceptable practice. >> IT IS NOT! > > it's done all the time in bottom end products. Why else do you think > consumer products often have such short lives? I would be in less of a > hurry to do it in mil hardware. Everything in engineering is tradeoffs > & compromises.Also in "top-end" products, sometimes, where the part is limiting the performance that can be achieved. -- John Devereux
Reply by ●April 7, 20182018-04-07
On Saturday, 7 April 2018 06:10:59 UTC+1, mike wrote:> On 4/6/2018 8:34 PM, tabbypurr wrote: > > On Saturday, 7 April 2018 02:21:53 UTC+1, mike wrote: > >> On 4/6/2018 1:33 PM, Jim Thompson wrote: > >>> On Fri, 06 Apr 2018 12:18:32 -0700, mike <ham789@netzero.net> wrote: > >>>> On 4/6/2018 7:21 AM, John Larkin wrote: > >>>>> On Fri, 6 Apr 2018 16:31:38 +0530, Pimpom <Pimpom@invalid.invalid> > >>>>> 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? > >>>>> > >>>>> The few mosfets that I've tested failed at Vgs around 70 volts. There > >>>>> could be long-term effects. > >>>>> > >>>>> Gate-protected ones zenered in the low 40's. > >>>>> > >>>>> Test a couple to destruction. It's fun to blow things up. > >>>>> > >>>>> I'd estimate that a couple of volts over Vgs_max will have no effect > >>>>> on reliability. > >>>>> > >>>>> > >>>> 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. > >>>> > >>>> Have you never underestimated the consequences of a decision? > >>>> Have you never had a vendor make a design change that still meets > >>>> the original specification? > >>>> Have you never had a part go obsolete and get replaced by an > >>>> "equivalent" part? > >>>> Have you never had a purchasing manager switch parts on you > >>>> without even telling you so he could save a buck and get a bigger bonus? > >>>> > >>>> I've never attended a seminar on > >>>> "it won't cause a disaster if the transistor fails". > >>>> > >>> > >>> You're just exhibiting your inexperience. > >> > >> You're correct. I'm inexperienced at designing systems that exceed > >> the specifications of components. I taught my engineers to derate > >> components. > >> > >> This statement gave me chills: > >> The very low frequency gate drive may, on rare > >> >>>> occasions, exceed the max Vgs rating of 12V by about 1V, possibly 2V. > >> > >> So kinda maybe we might, on rare occasions see 1V...maybe 2V beyond the > >> spec. But don't worry, it ain't no big thing if it fails. > >> Why not 3V on rarer occasions? How about 10V? How rare? > >> The OP is asking for encouragement, permission, to build in a failure > >> mode and supporting it with kinda/maybe/possibly... > >> > >> If an engineer came to me with that, I'd send him back to the drawing > >> board. "We'll discuss it when you come back with words like absolutely, > >> definitely, here are the options and costs..." > >> > >> If you read the fine print, > >>> VGSmax is > >> > >> usually > >> I split out that word so you can't miss it. > >> Curiosity isn't the only thing that can kill a cat. > >> > >> > >> specified as "guaranteed by design"... that is, it's > >>> NOT measured during manufacture, so there's > >> usually? > >> a HUGE margin. > >>> > >>> ...Jim Thompson > >>> > >> I once was forced to build a logic system to operate in > >> an environment with limited power availability. > >> We literally designed in what would fit the power budget. > >> Problem was that it was TTL and there was a huge difference > >> between typical and maximum power specs. > >> If you used the max numbers, we were over the power budget > >> by 20% or so. Prototypes ran at about 20% under the max. > >> > >> The QA dept refused to sign off on the design. I did a Monte Carlo > >> power analysis that included batch effects per part showing > >> that the probability of excess was near zero. > >> > >> QA signed off on the condition that power was measured at > >> final test and that we supply test fixtures to the service > >> centers and included the power measurement after every repair. > >> I don't think we ever had a non-conforming unit. > >> > >> I didn't "kinda/maybe". I did the rigorous analysis. Validation > >> procedures were in place. The process worked. > > > > That's all a reflection of the type of products you and many folk here design. Cheap consumer electronics is a different market. > > > > > > NT > > > I disagree with that mindset. Start with the mindset that everything > should work as advertised and be reliable. Stated another way...put > on your customer hat. What do you expect from your product? > When you're done, go back > and try to cut cost. Weigh the savings against the loss of reliability. > > If you start with slipshod design, it's hard to fix little things like > overvoltaging a FET. > > Thought experiment: > Do two designs. > Start one with a quality mindset. > Start the other with a minimum cost mindset. > I'd bet that if you massage each design toward the middle, > you'll end up at price parity with the quality design resulting in a > higher quality product. > And your next design will be better for it. > > If you came here for validation, you already know you shouldn't be > doing it.I think you misunderstand the basics of the process. There's nothing slipshod about it, engineers in a free market can't afford to screw up on product life expectancy. One single product that fails often during warranty can be a financial disaster. You need to know how far you can push each type of part, to understand the tradeoffs. A substantial percentage of consumer electronics involves such tradeoffs. It's normal engineering. NT
Reply by ●April 7, 20182018-04-07
On 4/7/2018 12:48 AM, mike wrote:> On 4/6/2018 7:21 AM, John Larkin wrote: >> On Fri, 6 Apr 2018 16:31:38 +0530, Pimpom <Pimpom@invalid.invalid> >> 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? >> >> The few mosfets that I've tested failed at Vgs around 70 volts. There >> could be long-term effects. >> >> Gate-protected ones zenered in the low 40's. >> >> Test a couple to destruction. It's fun to blow things up. >> >> I'd estimate that a couple of volts over Vgs_max will have no effect >> on reliability. >> >> > 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. > > Have you never underestimated the consequences of a decision? > Have you never had a vendor make a design change that still meets > the original specification? > Have you never had a part go obsolete and get replaced by an > "equivalent" part? > Have you never had a purchasing manager switch parts on you > without even telling you so he could save a buck and get a bigger bonus? >Now hold your horses. This was not a memo to my boss asking for permission to implement something (I have no boss). I was not "asking for encouragement, permission,....." as you claim in your other post. I said "What do you think?", which is a request for informed opinions. How would you put a question in a more neutral tone? The design is not for production. Nor is it to be a part of a more complex system.> I've never attended a seminar on > "it won't cause a disaster if the transistor fails". >How did you interpret that statement? It simply meant what it said. If the transistor fails, there's no chance of its damaging anything else. It will not cause any great inconvenience to the user (me).
Reply by ●April 7, 20182018-04-07
On 4/7/2018 5:01 AM, George Herold wrote:> Specs are weird things, 99% of the time you just stay within limits > and don't worry about it. But sometimes you'd like to explore the > edges...Exactly. It's fun and enlightening, provided that it's done under the right circumstances. It seems "mike" never does that kind of thing. Pity.
