mosfet bias question

Started by default July 8, 2018
I plan to use a mosfet in a dawn to dusk switching circuit with a 12V
regulated power supply.  In the application notes they give the gate
source absolute maximum voltage as +/- 10V Gate to Source. (page 1)

On page 6 of the same application note they show a "Gate charge test
circuit." with a 12 volt battery between the gate and source with a 50
Kohm variable resistor for the current source circuit.

Aren't they exceeding their own absolute maximum G-SV when the wiper
is turned to the +12 side of the battery?

https://www.vishay.com/docs/91328/91328.pdf

P/N IRLZ44

My Circuit is simple with just a photo transistor gate to source and a
4-10 megohm resistor to the gate and +12V biasing it on when the photo
transistor is dark.  The load is a 12V ten watt LED spotlight.

I've incorporated several of these and never had a failure and they've
been working outdoors for >5 years and I just now noticed that I'm
doing something illegal according to the app note.

As I understand mosfets a G-S breakdown, even through a megohm value
resistor, would kill it forever.  Is this wrong?
On Sun, 08 Jul 2018 15:22:44 -0400, default <default@defaulter.net>
wrote:

>I plan to use a mosfet in a dawn to dusk switching circuit with a 12V >regulated power supply. In the application notes they give the gate >source absolute maximum voltage as +/- 10V Gate to Source. (page 1) > >On page 6 of the same application note they show a "Gate charge test >circuit." with a 12 volt battery between the gate and source with a 50 >Kohm variable resistor for the current source circuit. > >Aren't they exceeding their own absolute maximum G-SV when the wiper >is turned to the +12 side of the battery?
Only of they turn the pot all the way up, which they wouldn't. That's just a test circuit.
> >https://www.vishay.com/docs/91328/91328.pdf > >P/N IRLZ44 > >My Circuit is simple with just a photo transistor gate to source and a >4-10 megohm resistor to the gate and +12V biasing it on when the photo >transistor is dark. The load is a 12V ten watt LED spotlight. > >I've incorporated several of these and never had a failure and they've >been working outdoors for >5 years and I just now noticed that I'm >doing something illegal according to the app note. > >As I understand mosfets a G-S breakdown, even through a megohm value >resistor, would kill it forever. Is this wrong?
Most mosfets die at around 70 volts g-s, except that the protected-gate ones zener in the low 40's. 12 volts won't hurt your fet. But you might fry it from power dissipation. Check your worst-case dissipation and heat sinking. -- John Larkin Highland Technology, Inc lunatic fringe electronics
On Mon, 09 Jul 2018 08:24:59 -0700, John Larkin
<jjlarkin@highlandtechnology.com> wrote:

>On Sun, 08 Jul 2018 15:22:44 -0400, default <default@defaulter.net> >wrote: > >>I plan to use a mosfet in a dawn to dusk switching circuit with a 12V >>regulated power supply. In the application notes they give the gate >>source absolute maximum voltage as +/- 10V Gate to Source. (page 1) >> >>On page 6 of the same application note they show a "Gate charge test >>circuit." with a 12 volt battery between the gate and source with a 50 >>Kohm variable resistor for the current source circuit. >> >>Aren't they exceeding their own absolute maximum G-SV when the wiper >>is turned to the +12 side of the battery? > >Only of they turn the pot all the way up, which they wouldn't. That's >just a test circuit. > >> >>https://www.vishay.com/docs/91328/91328.pdf >> >>P/N IRLZ44 >> >>My Circuit is simple with just a photo transistor gate to source and a >>4-10 megohm resistor to the gate and +12V biasing it on when the photo >>transistor is dark. The load is a 12V ten watt LED spotlight. >> >>I've incorporated several of these and never had a failure and they've >>been working outdoors for >5 years and I just now noticed that I'm >>doing something illegal according to the app note. >> >>As I understand mosfets a G-S breakdown, even through a megohm value >>resistor, would kill it forever. Is this wrong? > >Most mosfets die at around 70 volts g-s, except that the >protected-gate ones zener in the low 40's. 12 volts won't hurt your >fet. > >But you might fry it from power dissipation. Check your worst-case >dissipation and heat sinking.
Worst case is when it is in the linear region. Heating is insignificant. While daylight is fading or the sun is rising, full on to full off and visa versa takes about 15-20 seconds. The difference in light is imperceptible, it surprised me. I figured it would dim gradually but it is a lot faster than I was expecting.
On 7/8/2018 2:22 PM, default wrote:
> I plan to use a mosfet in a dawn to dusk switching circuit with a 12V > regulated power supply. In the application notes they give the gate > source absolute maximum voltage as +/- 10V Gate to Source. (page 1) > > On page 6 of the same application note they show a "Gate charge test > circuit." with a 12 volt battery between the gate and source with a 50 > Kohm variable resistor for the current source circuit. > > Aren't they exceeding their own absolute maximum G-SV when the wiper > is turned to the +12 side of the battery? > > https://www.vishay.com/docs/91328/91328.pdf > > P/N IRLZ44 > > My Circuit is simple with just a photo transistor gate to source and a > 4-10 megohm resistor to the gate and +12V biasing it on when the photo > transistor is dark. The load is a 12V ten watt LED spotlight. > > I've incorporated several of these and never had a failure and they've > been working outdoors for >5 years and I just now noticed that I'm > doing something illegal according to the app note. > > As I understand mosfets a G-S breakdown, even through a megohm value > resistor, would kill it forever. Is this wrong? >
Years ago, QST had a Class E amplifier (Author Sokal) I think they used a +/- 18Vpp signal to turn the FETs on and off, even though they were rated at 10V.