I need to switch 8 IRF9540 (P-Channel Mosfets) with about 32kHz (so 1 MOSFET about 4kHz) so that only one is On at one time. I thought about using the decade counter 4017 for this, and the 555 Timer for driving the 4017. Because of the low output current of the 4017 the Mosfet would take a long time to switch completly or am I thinking wrong? Is this a possible layout for a simple Mosfet driver for switching the mosfet faster than without the npn transistor?
EDIT: Maybe it's helping I have to switch 4A at maximum. It should switch with about 32kHz (so 1 MOSFET about 4kHz)
simulate this circuit – Schematic created using CircuitLab
respectively what is the differnce between these two schematics in switching speeds?
Best Answer
Yes, the transistor-resistor driver is helping.
A quick look at the datasheet of the MOSFET show Qin = 38nC and Cin = 860pF.
With I=2mA direct drive, switch time is t=Q/I=19us. With your driver, pulling up, the RC time constant is t=R*C=40ns (way faster), and pulling down, that BJT must have some current gain. So yes, it appears to help quite a bit.
That is the "back of the envelope" approximation. You can simulate it very easily to check for other "2nd order" effects.