On Fri, 06 Apr 2018 07:21:02 -0700, John Larkin <jjlarkin@highlandtechnology.com> 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.Agreed! The manufacturers cover their asses with lots of margin. ...Jim Thompson -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | STV, Queen Creek, AZ 85142 Skype: skypeanalog | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | Understanding is a fountain of life to one who has it, But the instruction of fools is folly. Proverbs 16:22
Exceeding Vgs rating
Started by ●April 6, 2018
Reply by ●April 6, 20182018-04-06
Reply by ●April 6, 20182018-04-06
On 4/6/2018 6:31 PM, George Herold 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? > > Is there some reason you can't pick a fet with higher Vgs? > > George H. >Not really. I just like to keep the option of using a 12V type open. As JL says, low-V, Low-Ron types with 12V or lower Vgs rating are more common than those with, say, 20V Vgs(max).
Reply by ●April 6, 20182018-04-06
On 4/6/2018 7:51 PM, 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. >That's what I thought. I understand that no one can guarantee a product's behaviour outside the manufacturer's specs. An informed estimate would have been welcome even if it was negative.
Reply by ●April 6, 20182018-04-06
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".
Reply by ●April 6, 20182018-04-06
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. 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. George H.
Reply by ●April 6, 20182018-04-06
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. NT
Reply by ●April 6, 20182018-04-06
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. If you read the fine print, VGSmax is usually specified as "guaranteed by design"... that is, it's NOT measured during manufacture, so there's a HUGE margin. ...Jim Thompson -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | STV, Queen Creek, AZ 85142 Skype: skypeanalog | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | To those of us in my age bracket... GREEN means inexperienced and/or incompetent.
Reply by ●April 6, 20182018-04-06
On Friday, April 6, 2018 at 7:47:01 AM UTC-7, Phil Hobbs wrote:> C-B breakdown in BJTs is pretty benign unless it results in dumping a > lot of energy into the junction. MOSFET gate breakdown is terminal.The C-B breakdown has the damaging high field buried in the device, far from surfaces and mobile contaminant ions. B-E breakdown, as used in the negative generator trick, makes near-surface field stresses, and some devices age badly when you do that. It used to be, you could buy oscillator transistors that were durable in this respect, but modern transistors aren't oscillator-rated. And, a MOSFET gate is always at a surface, so there's no burial-like protection. How tight are the specs on protective diodes? For the MOSFETs that have them, can they ever be substitutes for Zener clamps? JL says 'zenered at 40', but you'd really want thresholds at 10V or so, if that didn't bollix the device during recombination times.
Reply by ●April 6, 20182018-04-06
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!Acceptable to who?>Testing a few samples under controlled conditions is NOT reason to >exceed specifications.It's a judgement call. Sometimes pushing parts results in serious performance improvements. Sometimes context, like duty cycles or cooling, influence how close to abs max you can run... under or over. Some parts, like tantalum caps, should almost never be run at abs max voltage. Some parts are perfectly reliable at 3x abs max. Too often, important things are not to be found on a data sheet. So we test parts to destruction and make up our own rules.> >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?That is not allowed in our company. Engineering assigns acceptable manufactuers parts to company stock numbers.> >I've never attended a seminar on >"it won't cause a disaster if the transistor fails". >It's impressive how rare parts failures are nowadays. The early days of transistors and mosfets and TTL were horrible. -- 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
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. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
