MOSFET driving

[Frederic]

Hello everyone,

I'm a bit confused about the use of a small series resistor at the gate of a
power MOSFET. I've seen this in some schematics.
I have the gate of a IRFD120 MOSFET pulled up to 13V (with respect to
source) with a 100K resistor. This gate is then pulled down by the
"discharge pin" of a TLC555 (CMOS version of a NE555), in few hundred
nanoseconds at most.
Is a small resistor needed in series with the gate? What's the purpose?
Which value? How is it calculated???

Fred

 

[John Popelish]

Hello everyone,

I'm a bit confused about the use of a small series resistor at the gate of a
power MOSFET. I've seen this in some schematics.
I have the gate of a IRFD120 MOSFET pulled up to 13V (with respect to
source) with a 100K resistor. This gate is then pulled down by the
"discharge pin" of a TLC555 (CMOS version of a NE555), in few hundred
nanoseconds at most.
Is a small resistor needed in series with the gate? What's the purpose?
Which value? How is it calculated???

I doubt that you need the additional series resistance,
since the discharge pin has considerable resistance,
already. Such a series resistor is often needed when the
gate is driven by a very low impedance source that doesn't
damp the resonance created by the gate to drain capacitance
and the lead and trace inductance. Such cases risk becoming
oscillators.

A more important question might be, how fast do you need the
MOSFET to turn on when the discharge pin goes high
impedance, and the gate capacitance is being charged
positive only by the 100k pull up resistor?

 

[Jamie]

Hello everyone,

I'm a bit confused about the use of a small series resistor at the gate of a
power MOSFET. I've seen this in some schematics.
I have the gate of a IRFD120 MOSFET pulled up to 13V (with respect to
source) with a 100K resistor. This gate is then pulled down by the
"discharge pin" of a TLC555 (CMOS version of a NE555), in few hundred
nanoseconds at most.
Is a small resistor needed in series with the gate? What's the purpose?
Which value? How is it calculated???

Fred

that's a common method used to remove parasitics
sort of lowers the Q in the circuit among other
things like, help saving some major components if an
attached component fails and could cause over drive for
which the R could take the heat instead.
there are to many reasons popping in my head at the moment.
it simply depends on the application.
Another one is to reduce High freq noise..
i'll stop now.

 

[Terry Given]

I doubt that you need the additional series resistance, since the
discharge pin has considerable resistance, already. Such a series
resistor is often needed when the gate is driven by a very low impedance
source that doesn't damp the resonance created by the gate to drain
capacitance and the lead and trace inductance. Such cases risk becoming
oscillators.

A more important question might be, how fast do you need the MOSFET to
turn on when the discharge pin goes high impedance, and the gate
capacitance is being charged positive only by the 100k pull up resistor?

an even better question is do you care if Cgd takes on your 100k pullup
resistor, and wins? I have seen a commercial gatedrive circuit using 47k
pullups and BC547 pulldown transistor actually latch at Vth, causing
enormous power loss

Cheers
Terry

 

[Frederic]

I doubt that you need the additional series resistance, since the
discharge pin has considerable resistance, already. Such a series
resistor is often needed when the gate is driven by a very low impedance
source that doesn't damp the resonance created by the gate to drain
capacitance and the lead and trace inductance. Such cases risk becoming
oscillators.

A more important question might be, how fast do you need the MOSFET to
turn on when the discharge pin goes high impedance, and the gate
capacitance is being charged positive only by the 100k pull up resistor?

It doesn't need to be charged fast. In fact I count on it to delay the
turn-on for roughly 50uS or so, not critical.
Thanks for your insight!
Fred

 

[Robert Scott]

Hello everyone,

I'm a bit confused about the use of a small series resistor at the gate of a
power MOSFET. I've seen this in some schematics.
I have the gate of a IRFD120 MOSFET pulled up to 13V (with respect to
source) with a 100K resistor. This gate is then pulled down by the
"discharge pin" of a TLC555 (CMOS version of a NE555), in few hundred
nanoseconds at most.
Is a small resistor needed in series with the gate? What's the purpose?

We use a 560 Ohm resistor to drive the gate of a IRL3103 to soften the turn-on
and turn-off switching a little to prevent EMI. It is still fast enough to
drive the PWM solenoids we are driving without dissipating any significant heat.


Robert Scott
Ypsilanti, Michigan

 

[Frederic]

We use a 560 Ohm resistor to drive the gate of a IRL3103 to soften the
turn-on
and turn-off switching a little to prevent EMI. It is still fast enough
to
drive the PWM solenoids we are driving without dissipating any significant
heat.


Robert Scott
Ypsilanti, Michigan

I think I got the picture now.

Thanks!

Fred

 

[Ben Jackson]

I'm a bit confused about the use of a small series resistor at the gate of a
power MOSFET. I've seen this in some schematics.

In HF amps this is to avoid VHF oscillations. You can put a bead in
instead, but I imagine they are less common because they are more
expensive.