• ## the use of resistors in the gate of an FET

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• If we believe that the current through the gate of an FET is zero, then what is the use of puting a resistor in the gate?
Message 1 of 5 , Nov 4, 2002
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If we believe that the current through the gate of an
FET is zero, then what is the use of puting a resistor
in the gate?

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• The current into the gate is not zero but the resistance is near infinite. The FET gate is capacitive and so an alternating current can pass thought the gate.
Message 2 of 5 , Nov 4, 2002
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The current into the gate is not zero but the resistance is near
infinite. The FET gate is capacitive and so an alternating current
can pass thought the gate. Gate resistors are used for different
things in different circuits. It may be there to protect the driver
circuit from large negative spikes caused by the FET source being
driven quickly negative by an inductor. It may be there to reduce EMI
or to change the frequency characteristics inother ways.

o What circuit are you analyzing?

--- In Electronics_101@y..., Zeinab Ghofrani <zghofrani@y...> wrote:
> If we believe that the current through the gate of an
> FET is zero, then what is the use of puting a resistor
> in the gate?
>
>
> __________________________________________________
> Do you Yahoo!?
> Y! Web Hosting - Let the expert host your web site
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• Hi, A power MOSFET has significant gate to source capacitance (500pF to 3,000pF or more). A resistor (typically 10 to 100 Ohms) in series with the gate forms
Message 3 of 5 , Nov 6, 2002
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Hi,

A power MOSFET has significant gate to source capacitance (500pF to
3,000pF or more). A resistor (typically 10 to 100 Ohms) in series
with the gate forms an RC time constant. This slows down the
switching speed of the MOSFET and keeps di/dt and dv/dt from reaching
potentially destructive levels.

As an example, an inch of wire is about 1nH. A di/dt of 10A/ns would
generate a 10V potential across that wire. If it were in the source
to ground circuit, it would have the potential to damage the MOSFET.

Mariss

--- In Electronics_101@y..., Zeinab Ghofrani <zghofrani@y...> wrote:
> If we believe that the current through the gate of an
> FET is zero, then what is the use of puting a resistor
> in the gate?
>
>
> __________________________________________________
> Do you Yahoo!?
> Y! Web Hosting - Let the expert host your web site
> http://webhosting.yahoo.com/
• Dear Marris, Then what is the use of slowing down the switching speed? Thank you, Zeinab Ghofrani ... __________________________________________________ Do you
Message 4 of 5 , Nov 6, 2002
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Dear Marris,

Then what is the use of slowing down the switching
speed?

Thank you,
Zeinab Ghofrani

--- mariss92705 <mariss92705@...> wrote:
> Hi,
>
> A power MOSFET has significant gate to source
> capacitance (500pF to
> 3,000pF or more). A resistor (typically 10 to 100
> Ohms) in series
> with the gate forms an RC time constant. This slows
> down the
> switching speed of the MOSFET and keeps di/dt and
> dv/dt from reaching
> potentially destructive levels.
>
> As an example, an inch of wire is about 1nH. A di/dt
> of 10A/ns would
> generate a 10V potential across that wire. If it
> were in the source
> to ground circuit, it would have the potential to
> damage the MOSFET.
>
> Mariss
>
> --- In Electronics_101@y..., Zeinab Ghofrani
> <zghofrani@y...> wrote:
> > If we believe that the current through the gate of
> an
> > FET is zero, then what is the use of puting a
> resistor
> > in the gate?
> >
> >
> > __________________________________________________
> > Do you Yahoo!?
> > Y! Web Hosting - Let the expert host your web site
> > http://webhosting.yahoo.com/
>
>

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• 3 reasons. Prevent damage to the MOSFET, reduce current requirements for the gate driver and to reduce unecessary EMI (electro-magnetic interference). Mariss
Message 5 of 5 , Nov 6, 2002
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3 reasons. Prevent damage to the MOSFET, reduce current requirements
for the gate driver and to reduce unecessary EMI (electro-magnetic
interference).

Mariss

--- In Electronics_101@y..., Zeinab Ghofrani <zghofrani@y...> wrote:
> Dear Marris,
>
> Then what is the use of slowing down the switching
> speed?
>
> Thank you,
> Zeinab Ghofrani
>
>
>
> --- mariss92705 <mariss92705@y...> wrote:
> > Hi,
> >
> > A power MOSFET has significant gate to source
> > capacitance (500pF to
> > 3,000pF or more). A resistor (typically 10 to 100
> > Ohms) in series
> > with the gate forms an RC time constant. This slows
> > down the
> > switching speed of the MOSFET and keeps di/dt and
> > dv/dt from reaching
> > potentially destructive levels.
> >
> > As an example, an inch of wire is about 1nH. A di/dt
> > of 10A/ns would
> > generate a 10V potential across that wire. If it
> > were in the source
> > to ground circuit, it would have the potential to
> > damage the MOSFET.
> >
> > Mariss
> >
> > --- In Electronics_101@y..., Zeinab Ghofrani
> > <zghofrani@y...> wrote:
> > > If we believe that the current through the gate of
> > an
> > > FET is zero, then what is the use of puting a
> > resistor
> > > in the gate?
> > >
> > >
> > > __________________________________________________
> > > Do you Yahoo!?
> > > Y! Web Hosting - Let the expert host your web site
> > > http://webhosting.yahoo.com/
> >
> >
>
>
> __________________________________________________
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> HotJobs - Search new jobs daily now
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