mosfet bias question

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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?


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?

Re: mosfet bias question

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Only of they turn the pot all the way up, which they wouldn't. That's
just a test circuit.

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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

But you might fry it from power dissipation. Check your worst-case
dissipation and heat sinking.


John Larkin         Highland Technology, Inc

lunatic fringe electronics  

Re: mosfet bias question
On Mon, 09 Jul 2018 08:24:59 -0700, John Larkin

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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.

Re: mosfet bias question
On 7/8/2018 2:22 PM, default wrote:
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   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.

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