Fet G-S zener

Started by George Herold October 11, 2018
Well I just blew out the G-S junction of a fet. I'm going to redesign circuit 
a bit... but I was also thinking of adding a ~15V G-S zener. 
(V_GS max = +/- 20 V)  
Is there any downside I should think about?  
I guess I'd need some sort of current limit.  (Resistor at a minimum)  

George H. 
"George Herold" <gherold@teachspin.com> wrote in message 
news:6d28623e-8ace-49bb-902e-a2fb20f750a7@googlegroups.com...
> Well I just blew out the G-S junction of a fet. I'm going to redesign > circuit > a bit... but I was also thinking of adding a ~15V G-S zener. > (V_GS max = +/- 20 V) > Is there any downside I should think about? > I guess I'd need some sort of current limit. (Resistor at a minimum) > > George H.
You mean besides the obvious ones (conduction current, blocking leakage..)? Capacitance I guess, which can lead to gate oscillations, especially on modern FETs (SuperJunction) which like to sing at 300-400MHz (where even a 50pF zener is a fine resonant stub). A little gate resistance or ferrite bead as usual. Tim -- Seven Transistor Labs, LLC Electrical Engineering Consultation and Design Website: https://www.seventransistorlabs.com/
On Thursday, October 11, 2018 at 2:04:55 PM UTC-4, Tim Williams wrote:
> "George Herold" <gherold@teachspin.com> wrote in message > news:6d28623e-8ace-49bb-902e-a2fb20f750a7@googlegroups.com... > > Well I just blew out the G-S junction of a fet. I'm going to redesign > > circuit > > a bit... but I was also thinking of adding a ~15V G-S zener. > > (V_GS max = +/- 20 V) > > Is there any downside I should think about? > > I guess I'd need some sort of current limit. (Resistor at a minimum) > > > > George H. > > You mean besides the obvious ones (conduction current, blocking leakage..)? > > Capacitance I guess, which can lead to gate oscillations, especially on > modern FETs (SuperJunction) which like to sing at 300-400MHz (where even a > 50pF zener is a fine resonant stub). A little gate resistance or ferrite > bead as usual.
Right, thanks. like capacitance. So a little through hole, 1/4 W, 1n5xxx is about 20 pF on our lcr bridge. That seems about right. George H.
> > Tim > > -- > Seven Transistor Labs, LLC > Electrical Engineering Consultation and Design > Website: https://www.seventransistorlabs.com/
On 10/11/2018 12:37 PM, George Herold wrote:
> Well I just blew out the G-S junction of a fet. I'm going to redesign circuit > a bit... but I was also thinking of adding a ~15V G-S zener. > (V_GS max = +/- 20 V) > Is there any downside I should think about? > I guess I'd need some sort of current limit. (Resistor at a minimum) > > George H. >
Capacitance. I like to use the CB junction of a BFT25A between the gate and some reference voltage. (Keeping CPH3910s alive when connected to APDs running at -220V, for instance.) 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 https://hobbs-eo.com
On Thursday, October 11, 2018 at 3:36:28 PM UTC-4, Phil Hobbs wrote:
> On 10/11/2018 12:37 PM, George Herold wrote: > > Well I just blew out the G-S junction of a fet. I'm going to redesign circuit > > a bit... but I was also thinking of adding a ~15V G-S zener. > > (V_GS max = +/- 20 V) > > Is there any downside I should think about? > > I guess I'd need some sort of current limit. (Resistor at a minimum) > > > > George H. > > > > > Capacitance. I like to use the CB junction of a BFT25A between the gate > and some reference voltage. (Keeping CPH3910s alive when connected to > APDs running at -220V, for instance.)
Right, I think I can live with 20 pF more. But this CB junction thing. So I do have an N-Fet, I then drew the collector (of npn) attached to the gate, base to source... which leaves the emitter flapping around. I don't see how attaching the emitter to some reference V causes anything to turn on? Does the BFT25A have some predicable (lowish) Vcb? https://www.nxp.com/docs/en/data-sheet/BFT25A.pdf 8V? Have you tested that number? Then the reference voltage is in series with the cb junction? I don't need a lot of GS voltage so 8 V would be good enough. How hard (current-wise) can you zener a cb junction? George H.
> > 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 > https://hobbs-eo.com
On Thu, 11 Oct 2018 09:37:29 -0700 (PDT), George Herold
<gherold@teachspin.com> wrote:

>Well I just blew out the G-S junction of a fet. I'm going to redesign circuit >a bit... but I was also thinking of adding a ~15V G-S zener. >(V_GS max = +/- 20 V) >Is there any downside I should think about? >I guess I'd need some sort of current limit. (Resistor at a minimum) > >George H.
There are lots of fets with internal g-s zeners. They tend to conduct around 40 volts. You might have to limit the current; a resistor or parallel RC would work. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
> > Capacitance. I like to use the CB junction of a BFT25A between the gate > > and some reference voltage. (Keeping CPH3910s alive when connected to > > APDs running at -220V, for instance.) > Right, I think I can live with 20 pF more. > > But this CB junction thing. So I do have an N-Fet, I then drew the > collector (of npn) attached to the gate, base to source... which > leaves the emitter flapping around. I don't see how attaching the emitter > to some reference V causes anything to turn on? Does the BFT25A have some > predicable (lowish) Vcb? > https://www.nxp.com/docs/en/data-sheet/BFT25A.pdf > > 8V? Have you tested that number? Then the reference voltage is in > series with the cb junction? I don't need a lot of GS voltage so > 8 V would be good enough. How hard (current-wise) can you zener a > cb junction? > > George H.
George, I believe Phil was referring to the use of the CB junction as a low leakage, low capacitance diode. the emitter is left floating. there was a discussion about this a few months back, the thread title is "JFET used as a diode"
On Thursday, October 11, 2018 at 9:22:49 PM UTC-4, sea moss wrote:
> > > Capacitance. I like to use the CB junction of a BFT25A between the gate > > > and some reference voltage. (Keeping CPH3910s alive when connected to > > > APDs running at -220V, for instance.) > > Right, I think I can live with 20 pF more. > > > > But this CB junction thing. So I do have an N-Fet, I then drew the > > collector (of npn) attached to the gate, base to source... which > > leaves the emitter flapping around. I don't see how attaching the emitter > > to some reference V causes anything to turn on? Does the BFT25A have some > > predicable (lowish) Vcb? > > https://www.nxp.com/docs/en/data-sheet/BFT25A.pdf > > > > 8V? Have you tested that number? Then the reference voltage is in > > series with the cb junction? I don't need a lot of GS voltage so > > 8 V would be good enough. How hard (current-wise) can you zener a > > cb junction? > > > > George H. > > George, I believe Phil was referring to the use of the CB junction as a low
leakage, low capacitance diode. the emitter is left floating. there was a discussion about this a few months back, the thread title is "JFET used as a diode" Oh, that makes much more sense, thanks! George H.
On 11/10/2018 17:37, George Herold wrote:
> Well I just blew out the G-S junction of a fet. I'm going to redesign circuit
(Assuming you wrote about a MOSFET rather than a JFET then) Strictly speaking you didn't blow the "junction" because a mosfet gate has no junction, what happened was you broke down the gate insulation dielectric material. I think John Larkin reported that +/-20Vgs rated parts typically ruptured above 70V. piglet
On 10/11/18 11:16 PM, George Herold wrote:
> On Thursday, October 11, 2018 at 9:22:49 PM UTC-4, sea moss wrote: >>>> Capacitance. I like to use the CB junction of a BFT25A between the gate >>>> and some reference voltage. (Keeping CPH3910s alive when connected to >>>> APDs running at -220V, for instance.) >>> Right, I think I can live with 20 pF more. >>> >>> But this CB junction thing. So I do have an N-Fet, I then drew the >>> collector (of npn) attached to the gate, base to source... which >>> leaves the emitter flapping around. I don't see how attaching the emitter >>> to some reference V causes anything to turn on? Does the BFT25A have some >>> predicable (lowish) Vcb? >>> https://www.nxp.com/docs/en/data-sheet/BFT25A.pdf >>> >>> 8V? Have you tested that number? Then the reference voltage is in >>> series with the cb junction? I don't need a lot of GS voltage so >>> 8 V would be good enough. How hard (current-wise) can you zener a >>> cb junction? >>> >>> George H. >> >> George, I believe Phil was referring to the use of the CB junction as a low
leakage, low capacitance diode. the emitter is left floating. there was a discussion about this a few months back, the thread title is "JFET used as a diode"
> > Oh, that makes much more sense, thanks! > George H. >
Yes, sorry--I thought I'd expanded on this in another post, but it hasn't appeared. The CPH3910 JFET has a maximum V_GS of -15V. In my recent APD front end, I use a BAV99 series-connected dual diode with the anode connected to the source, the midpoint connected via a resistor to -5V, and the cathode connected via a BFT25A C-B junction to the gate. That bootstraps the already small capacitance and reduces the leakage to femtoamp levels. The reason for doing it is if somebody opens up the instrument with the APD bias attached, the gate of the FET would get pulled away towards -220V. The APD bias is current-limited to a few hundred microamps, so this is a complete solution. 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