Mosfet gate resistor

[Morcego]

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

 

[OperaHouse]

If you go slow, it doesn't matter. Think of it this way, it is just like driving a capacitor. Some mosfets easily have 4500pf of capacitance. In some cases almost all fet heating is is the result of turn on and turn off transition delays.

 

[Warpspeed]

The gate resistor performs two functions, peak current limiting and damping. These may or may not be important in particular circumstances.

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.

 

[Morcego]

Hi,

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

 

[benta]

I take it you are driving a ground-referenced load since you are using a P-channel.
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.

 

[Comcokid]

There are other reasons to possibly have a 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.

 

[Morcego]

Thanks benta for pointing out the logic level.
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

 

[itsmoked]

Morcego; I've never used AVRs, but have used all the others. Don't forget that as most micros start up they float their digital outputs until your code declares them OUTPUTS and drives them low. During this time the leakage currents can turn the FETS on. This can be real annoying depending on what the FET is controlling... e.g. robot wheels.. the robot jumps a little every time you turn it on, type stuff. Also if the leakage is "just right" the FET can go into linear mode and dissipate a large amount of power, in some cases killing itself. Good design, dictates you generally put in that pull down resistor.