Mosfet gate resistor
Mosfet gate resistor
(OP)
Hi,
I have a basic question:
When I use as digital circuit to control a power Mosfet transistor (used as a switch) do I have to put a resistor in series with the mosfet gate or not?
Do circuits used others not. Wich is the bast practice?
How to calculate the resistor?
Thanks in advance
Joaquim
I have a basic question:
When I use as digital circuit to control a power Mosfet transistor (used as a switch) do I have to put a resistor in series with the mosfet gate or not?
Do circuits used others not. Wich is the bast practice?
How to calculate the resistor?
Thanks in advance
Joaquim





RE: Mosfet gate resistor
RE: Mosfet gate resistor
You need to look at the capabilities of the driver, particularly peak current rating and power dissipation, the total input capacitance of the MOSFET, and the switching speed and frequency.
Any significant series inductance in the circuit combined with gate capacitance could create ringing. That can be dangerous from the perspective of gate breakdown voltage if it is uncontrolled. Ferrite beads could be an alternative solution to damp ringing.
There is no simple answer, each situation needs to be assessed, and a resistor value selected to do the job.
RE: Mosfet gate resistor
Thanks for the awnsers.
I'm driving IRF7104 Hexfet (P channel) from a microcontroller (5V). The frequency is at the most 1Khz.
Input capacitance is about 210 / 290 pF.
Maybe a 47R resistor is adequate. Do you thin I need a pull down resistor?
Joaquim
RE: Mosfet gate resistor
I'd skip the gate resistor, the output resistance from the MCU I/O pins is probably higher anyway.
But I see a problem: you should select a logic-level FET for this job. The IRF7104 will not turn fully on from 5 V.
Second, be careful how you power this. If your FET and the MCU have separate supplies, the FET will turn on if you cut power to the MCU.
Regards.
RE: Mosfet gate resistor
A)Should the MOSFET fail, the resistor may prevent damage to the uP.
B)Some uPs do not like driving capacitive loads.
I once had a N MOSFET being driven by a Z8 through at ~100 Hz through a 1K resistor, but failed FCC because the processor insisted on radiating it's 4 MHz clock everywhere. Changed the resistor to 10K and the problem went away. Board had all the proper bypasses and a solid ground plane. This was a tricky one to find because the 4 MHz was not the real problem but the uP driving the MOSFET was.
RE: Mosfet gate resistor
in fact the threshold voltage of the irf7104 is 3V. It's a bite high for logic levels.
The power is same.
I will follow your advice, skip the gate resistor ang go for a IRLML6401 transistor (Threshold 0,55V typ.)
Ragarding radiation, I will use an AVR micro. I never experince any problems of that kind, but thanks for the tip
Comkid!
Morcego
RE: Mosfet gate resistor