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Question about mosfet- is resistance needed at gate terminal

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gameelgamal

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Question about mosfet

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
 

Re: Question about mosfet

The gate resistance is used to slow down switch operation to a defined di/dt or du/dt rate. Also unwanted RF oscillations during switching are reduced. Basically increasing the gate resistor gives slower current and voltage transition at the output, less overvoltage but increased switching losses. Gate resistor dimensioning depends on MOSFET size, respectively gs and gd capacitances. Could be a few ohms up to several 100 ohms.
 

Question about mosfet

A resistor in series with the gate of a Mosfet must be mounted very close to the gate pin. It stops the Mosfet from oscillating at a very high frequency which might make it too hot.
 

Re: Question about mosfet

FvM said:
The gate resistance is used to slow down switch operation to a defined di/dt or du/dt rate. .
Click to expand...

Could you tell me What do you mean by 'switching rate'??
Do you mean turnning on and turnning off time??

Another question: when should I change the switching rate?? In other words How to know If I should connect a resistor or I shouldnt??
 

Question about mosfet

If you don't connect a resistor in series with the gate of a Mosfet right at the gate pin then it can oscillate at a high frequency which might cause it to over-heat.
 

Question about mosfet

As it was said before, it's to prevent overheat so you'll realize you should put it once you see smoke... :)
 

Re: Question about mosfet

Ok..I will use IRFZ44 MOSFET transistor to control a 3A DC motor
I will control the speed of the motor using a pulse width modulation signal (PWM) with a frequency up to 1 KHz
Should I connect a resistance or not???
If I should..What is the optimum value of the resistor??

The datasheet of IRFZ44
**broken link removed**



Audioguru said:
A resistor in series with the gate of a Mosfet must be mounted very close to the gate pin. It stops the Mosfet from oscillating at a very high frequency which might make it too hot.
Click to expand...
Could you explain what do you mean by oscillating...
 

Question about mosfet

I klooked in Google at Mosfet Gate resistor.
Fairchild have a switching power supply (pretty high frequency) and they say to use 4.7 ohms in series with each gate. They show the switching time slowing down with a gate resistor of 20 ohms.

Fairchild says that a mosfet can oscillate at up to 100MHz and overheat if it doesn't have a series gate resistor.

Your PWM is very slow so a gate resistor of 10 ohms to 68 ohms will be fine.
 
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