You can place all of the resistors you want in series with a MOSFET gate, but it won't prevent the gate from charging to the applied voltage.

The proper way to limit the gate voltage is usually done with a "clamp" or a divider.

A gate resistor and a pull resistor can form a divider.

 

A small series gate resistor is to eliminate possible MOSFET oscillations, not to protect the gate.
I don't think that's a problem here, but you can add one if you like.
Since the gate will never be floating on the PCB, I see no reason to add a pull-down resistor.

Thank you for your reply. I think I will add the gate resistor just in case, does 1k Ohm sound good enough? would the 1k ohm effect the pwm speed much (frequency)? Do you suggest another value?
For the pull-down resistor, I agree with the PCB there should be no floating with the gate.

Appreciate your input and help.
Fady

 

You can place all of the resistors you want in series with a MOSFET gate, but it won't prevent the gate from charging to the applied voltage.

The proper way to limit the gate voltage is usually done with a "clamp" or a divider.

A gate resistor and a pull resistor can form a divider.

Thank you ElectricSpidey for your reply.
Yes the gate resistor and the pull resistor would act as a divider. But actually I don't think I will need the pull resistor as crutschow mentioned in the case of the PCB. I think I will only add the gate resistor to prevent the mosfet oscillation. As for the mosfet gate it is supposedly rated at 20V so fingers crossed the 12V wouldn't be a problem for it.
Thank you.
Fady

 

The proper way to limit the gate voltage is usually done with a "clamp" or a divider.

I don't see that 12V it too high for a 20V rated gate voltage, which is derating to 60% of the max.
Typically 10V is applied to fully turn a standard MOSFET fully on.

does 1k Ohm sound good enough?

For eliminating oscillations 10-100Ω is typically used.
1kΩ could reduce the rise and fall time, increasing the transistor dissipation.

 

I don't see that 12V it too high for a 20V rated gate voltage, which is derating to 60% of the max.

Neither do I...so what is your point?

I was just providing general purpose information to counter the idea that a series resistor can protect a MOSFET gate.

What is your problem, anyway?

 

What is your problem, anyway?

None to speak of.
Why do you think I have one?

I thought your post was implying that the max gate voltage needed to be reduced.
Sorry if I offended you.

 

I don't see that 12V it too high for a 20V rated gate voltage, which is derating to 60% of the max.
Typically 10V is applied to fully turn a standard MOSFET fully on.
For eliminating oscillations 10-100Ω is typically used.
1kΩ could reduce the rise and fall time, increasing the transistor dissipation.

Hello crutschow,
Thank you for the explanation. I will use the gate resistor for the oscillation with the value 100Ohm. I will finalize the PCB and hopefully have it soon. I will keep you posted about the out come.
Thank you again for all the help.
Fady

 

Hello @crutschow

It has been a while but I finally got he circuit made and the results are rather interesting. So I decided to make the circuit using a moset called " KIA50N03BD" since it was what's available and at a good price. When I tested the circuit the output went first full voltage (12v) and then went to 7V (it is supposed to go to 3.3V). The mosfet gets super hot almost instantly even tho it is supposed to be able to handle 50AMP (the load is around 2 or 2.5 AMP). I decided to change the mosfet to another mosfet I had "IRF3205" and to my surprise the circuit worked as it is supposed to with no issues!

My question is, aren't all N-channel mosfets created equal? In the sense of, both mosfets are capable of handling more than needed in terms of AMP and drain to source voltage. Why are they acting differently in the circuit.

I would really appreciate your help since curiosity is killing me!

Thank you in advance.
Fady

 

do you check the power supply unit

 

Hello flyBird,
Yes I did, it is a very stable 12V power supply with over 5 Amp capacity.

Thank you