Hi
I've got a high-speed switching project using a MOSFET. I'm trying to delay the switching just a little bit (too little to do with a 555 timer).
Does anyone know if increasing the gate resistor of a MOSFET (ie in series with the gate) combines with the gate capacitance to form an RC (time) constant?
e.g. for a gate with 10 pF capacitance, if I put a 100k resistor in the line to the gate, will I get a delay propagating the switching signal of 100 x 10^5 x 10 x 10^12 = 1 x 10^6 = 1 microsecond ?
It seems to me that "normal" electronic theory says that it should.
I do understand that RC constants are the time for only part (66% ?) of what is a negative expontial rate of change. I'm not worried about that part. Just RC / delay question.
thanks for any help.
Hi again
I've found out
(1) that there is an RC network (of course :icon_redface:)
(2) that the few pF capacitance of MOSFETs varies with charge and other things, so the RC varies all over the place
What I'll do is add a regular capacitor of about 100pF, and a resistor of 47k, in series, to get the few microseconds of delay that I need (RC~ 6 us for a MOSFET with C of 25 pF).
cheers
It's worse. There's another problem that you may have depending on what it is you're switching.
If you're switching high voltages and/or high currents/power, you will run into power dissipation issues. If you put an RC on a MOSFET gate, it not only delays switching, it slows down the transition when it does switch. The integral of volts times amps in the transition between on and off makes for big spikes of power. This can overheat MOSFETs in power switching.
If you're doing signal switching, the region between on and off is also a region where the switch is affecting the signal it's switching, so you may have to do selective blanking or sampling to get outside the on/off transition region.
Thanks RG. You're so very helpful around here.
Motorola had a good MOSFET handbook that showed a number of MOSFET drive circuits.
This might help
http://www.ti.com/lit/ml/slup169/slup169.pdf (http://www.ti.com/lit/ml/slup169/slup169.pdf)