Author Topic: Resistor and capacitor on mosfet gate?  (Read 5639 times)

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Offline fubar.grTopic starter

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Resistor and capacitor on mosfet gate?
« on: February 24, 2015, 03:42:17 pm »
What is the purpose of R4 in parallel with C3 driving the mosfet gate in this desulfator circuit? I've seen resistors in series with the gate before, but never with a capacitor in parallel.


Offline nuno

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Re: Resistor and capacitor on mosfet gate?
« Reply #1 on: February 24, 2015, 07:29:44 pm »
I think it's an old technique to help in fast turn off. When you have the FET ON, the voltage across the cap is 0V (R4 ensures that by shorting C3 and we're assuming some time for things to stabilize); then you switch the half-bridge to 0V (2N3904 -> off, 2N3906 -> on) which puts the capacitor in parallel with the FET's gate-source capacitance which is charged to Vcc, thus discharging it quickly to some point "in the middle" by having charge move from G-S to C3 without resistance (well, almost, there's the BJT + parasitics in the path). The faster the FET switch off, the higher the voltage transient obtained from the coil (dv = L (di/dt)).
« Last Edit: February 24, 2015, 07:33:14 pm by nuno »
 

Offline T3sl4co1l

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Re: Resistor and capacitor on mosfet gate?
« Reply #2 on: February 25, 2015, 02:11:09 am »
As should be expected for the purpose and mechanism, the circuit itself is also voodoo, so don't read too closely into it. :)

An R||C is actually useful for driving BJTs, in many cases, because of the lower input voltage.  Example:



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