How to use a MOSFET transistor as a switch to drive a motor?

Hi
I want to use a MOSFET transistor as a switch to drive a motor..
My question is: Should I connect a resistance to the gate of the MOSFET, or not ??
As I know, The MOSFET has an isolated gate. But I have seen many designes with a resistance connected to the gate of the MOSFET!!!!
any comment

protect gate mosfet pull down resistor

There is many reason to connect a "small" value resistor in series with the Mosfet gate, a resistor act as a limiter for the peak current into the MOSFET gate then the MOSFET will switch slower than if you don't put the resistor. This behaviour may be useful for EMI emission and to prevent parasitics high frequency oscillation (see later), but for hard switching application (like PWM DC-DC converter) this has a drawback because slow down the MOSFET commutation increase the commutation loss of the MOSFET (this will be proportional to the MOSFET commutation time).
But there is also another very important reason to add a small resistor and this is to tur off spurius oscillation that can arise between the gate input capacitors and the parasitics inductance of the driving stage (parasitics oscillation can turn off and on the MOSFET at high frequency with a lot of power loss and noise), from this last point of view you can search the best compromise from commutation time (MOSFET speed when used as a switch in hard-switch commutation circuit like PWM DC-DC converter) and the parasitic high frequency oscillation (basically you've to calculate the resistor value in order to keep the Q of the resonant circuit equal to 1 then sqrt(L/Cgs), for most application a 10÷15 Ohm shoud be enough to prevent oscillation).

Just a first reply, hope it help.

Cheers
F.F. a.k.a. Powermos

irfz44n inductive load

Cheers is right.
Aslo,a pull-down resistor which can reduce the input resistance of the MOSFET to avoid static.

mosfet gate pull down resistor

I also have heard that if you do not connect any resistor than that is a floating point and floating point creates problem for circuits.

rcd snubber circuit for mosfet

Thank you for replying

I will use IRFZ44 N-Channel MOSFET to switching on and off 10 A DC motor
I will not control the speed of the motor (I will not use PWM )
Am I still need the gate resistor??

Another question, Is there what is called snubber circuit for MOSFET transistor??
and for what this circuit is used??

mosfet pull-down resistor calculate

Snubber circuit used to reduce (or contrl) dv/dt and di/dt in transistor. it is because of protect transistor from this distructive effects.
It is usualy consist of a capacitor (0.01-0.1 nF) which is series with a resistor (10-100 ohm), this combination located between drain-source of MOSFET. In some cases one diode connect to snubber circuit to clip excessive voltage, and protect transistor from over voltage in inductive load.

oscillate when mosfet turn on

Hi Gameelgamal:
Choose IRFZ44N to control a 10A load may cause it too hot. I think you'd better to choose a new model.
A pull-down resistor(10K 1/4W) is necessary.

connecting snubber to mosfet drain and source

There is many type of snubber network (RCD, resonant, ecc...) that can be used, I've attached a paper that should be able to solve all your question and drive you into the design stage, take care of it is really a piece of knowledge in SMPS design from Unitrode.

Some other example here below:

**broken link removed**

**broken link removed**

for resonant snubber type read also:

https://www.onsemi.com/pub_link/Collateral/AND8296-D.PDF

I think for you application that can be enough a simple RCD snubber, btw you are using the mosfet to make a simple motor driver then a simple diode put on antiparallel to the motor winding should be enough to prevent extravoltage on MOSFET turn-off.

About the power loss estimation you need also to know what is the switching frequency used to drive the motor in order to compute the switching loss, also for conduction loss estimation you need to keep a low rdson MOSFET (to better estimate the MOSFET conduction power loss take a value that is double of the nominal one presented on the datasheet).

Hope it help

Cheers
Powermos