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
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?
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.
-- "I\'m never wrong, once i thought i was, but was mistaken" Real Programmers Do things like this. http://webpages.charter.net/jamie_5
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
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
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
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.
-- Ben Jackson AD7GD http://www.ben.com/
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