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?
Exceeding Vgs rating
Started by ●April 6, 2018
Reply by ●April 6, 20182018-04-06
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?Take 30 of these transistors and drive them to failure. See where they actually fail. That may help in your decision process if you think you can get away with exceeding the rating. Do it at cold and hot temp too
Reply by ●April 6, 20182018-04-06
On Friday, 6 April 2018 11:01:50 UTC+1, 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?Failure voltage with bipolars is sometimes much higher than rated. I presume the same is true of fets. You could always feed microamps through it to see what voltage it breaks down at harmlessly. A lot of leeway is needed if silicon is to survive well enough in use, don't push it too far. NT
Reply by ●April 6, 20182018-04-06
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.
Reply by ●April 6, 20182018-04-06
George Herold <gherold@teachspin.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?> Is there some reason you can't pick a fet with higher Vgs?> George H.Good call. Regardless of the Vgs rating, if the drain goes to an outside circuit, I have found a zener from gate to source is needed to protect against ESD. In this case, it might also be useful to avoid exceeding the Vgs rating. Although JL has measured the actual Vgs destruction voltage and found it exceeded the rating by a wide margin. ISTR around 70 volts. So a zener clamp might be good insurance.
Reply by ●April 6, 20182018-04-06
<djlocher56@gmail.com> wrote in message news:86b153ee-d046-4efa-8c38-09502d248558@googlegroups.com...> > Take 30 of these transistors and drive them to failure. See where they > actually fail. That may help in your decision process if you think you > can get away with exceeding the rating. Do it at cold and hot temp too >It's trickier than a one-shot test, actually: at high voltages (above rating), ionic diffusion occurs, and Vgs(th) drifts. (Back in the day, one of the difficulties in realizing MOSFETs was getting the process clean enough so that mobile sodium ions weren't forever moving around in the oxide, shifting Vgs(th) depending on Vgs history. There are still ions and defects present today, but they aren't as mobile -- except in a strong electric field.) And yeah, temperature matters too, so you could test them above Vgs(max) while monitoring gate leakage current and checking Vgs(th) periodically. High temperature is probably the most important, but there might be something about lower temperature giving lower leakage, allowing higher voltages and therefore stresses? Not sure. Tim -- Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website: https://www.seventransistorlabs.com/
Reply by ●April 6, 20182018-04-06
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. -- John Larkin Highland Technology, Inc lunatic fringe electronics
Reply by ●April 6, 20182018-04-06
On Fri, 6 Apr 2018 05:09:12 -0700 (PDT), tabbypurr@gmail.com wrote:>On Friday, 6 April 2018 11:01:50 UTC+1, 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? > >Failure voltage with bipolars is sometimes much higher than rated. I presume the same is true of fets. You could always feed microamps through it to see what voltage it breaks down at harmlessly. A lot of leeway is needed if silicon is to survive well enough in use, don't push it too far. > > >NTOvervoltage on non-protected mosfet gates seems to be destructive, even through megohms of source impedance. That happens at roughly 6 * Vgs_max. I recently tested some EPC GaN fets for Vds limits. They fold back at roughly 2.2 * Vds_max, nondestructive if the current is limited. -- John Larkin Highland Technology, Inc lunatic fringe electronics
Reply by ●April 6, 20182018-04-06
On Fri, 6 Apr 2018 06:01:13 -0700 (PDT), George Herold <gherold@teachspin.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? > >Is there some reason you can't pick a fet with higher Vgs? > >George H.Might not be available. Numbers like 8, 10, 12 volts max are common on low voltage, low Ron fets. -- John Larkin Highland Technology, Inc lunatic fringe electronics
Reply by ●April 6, 20182018-04-06
On 04/06/18 10:11, Tim Williams wrote:> <djlocher56@gmail.com> wrote in message > news:86b153ee-d046-4efa-8c38-09502d248558@googlegroups.com... >> >> Take 30 of these transistors and drive them to failure.� See where >> they actually fail.� That may help in your decision process if you >> think you can get away with exceeding the rating.� Do it at cold and >> hot temp too >> > > It's trickier than a one-shot test, actually: at high voltages (above > rating), ionic diffusion occurs, and Vgs(th) drifts.� (Back in the day, > one of the difficulties in realizing MOSFETs was getting the process > clean enough so that mobile sodium ions weren't forever moving around in > the oxide, shifting Vgs(th) depending on Vgs history.� There are still > ions and defects present today, but they aren't as mobile -- except in a > strong electric field.) > > And yeah, temperature matters too, so you could test them above Vgs(max) > while monitoring gate leakage current and checking Vgs(th) periodically. > > High temperature is probably the most important, but there might be > something about lower temperature giving lower leakage, allowing higher > voltages and therefore stresses?� Not sure. > > Tim >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. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
