mosfet
I am using the IRF9321 P channel MOSFET for controlling my BLDC motor.
The MOSFET is driven via a transistor which in turn is run by a freescale controller(3.3V pin). Now, when turning it on, it works fine. But, when turning it off its a disaster with the MOSFET taking almost 5ms to come to '0' Volts.
My circuit is shown below.
I have been through the datasheet and it says turn off delay time is 180nSecs (approx .1uSecs).
Am I missing something?
Impossible. You have to use one more PNP transistor for that. You cannot turn N-MOSFET only by NPN transistor without inverting input signal.
This is modified circuit that should work.
simulate this circuit – Schematic created using CircuitLab
When the input signal is 0V: Q2 is off, base of Q1 is pulled high, so Q1 is off. And M1 is pulled low, so mosfet is off.
When the input signal is 5V: Q2 is on, base of Q1 is pulled low, so Q1 is on. And M1 is pulled high, so mosfet is on.
Problem with your original circuit is that Q31 can be on only when emitter has lower voltage than base. But when you send 5V to input, Q31 is open and emitter is pulled to 12V and so turns Q31 off. Your circuit cannot work.
The circuit looks good.
If you want faster reset of the circuit move R7 to between 12V and GND and reduce its resistance... when the 12V is turned off C3 will discharge through D1 and the new R7
Best Answer
I presume you mean 5usec, not 5msec. If it is really 5msec, disregard the below and check for a bad (open) resistor, or perhaps one that is 270K not 270 ohms.
That's not far off what you'd expect discharging the gate charge, which could be relatively large (tens of nanocoulombs) with a 270 ohm resistor.
The MOSFET will only switch in 180nsec if you actually change the gate-to-source voltage fast.
To get it to switch quickly, you could use a gate driver capable of amperes of current such as the MCP1406/1407 which will quickly discharge the gate charge.