[SOLVED] MOSFET Gate to Source Resistor Calculation and Importance

[Eshal]

Hello experts!

What is importance of using the resistor at gate to source of the MOSFET and how to calculate its value? What is the formula?

Thank you

 

[yassin.kraouch]

this resistor is used in high switching frequency, Usually a gate to source resistance is provided to prevent accidental turn-on due to noise or other factors or gate-to-drain internal capacitance (Miller capacitance).
you can refer to this thread https://www.edaboard.com/threads/166905/

 

[Ogu Reginald]

Gate to Source resistor creates a known level on the gate in the absence
of an input signal. The gate is a high impedance
node, and when left floating, can assume any
voltage due to parasitic coupling, noise and other
factors. The resistor prevents the bad stuff from
affecting your circuit.

- - - Updated - - -

Concerning the value of gate to source resistor, a resistor of between 1k and 2.2k is sufficient for the purpose.

 

[mrinalmani]

Once a MOSFET is switched on, it will not switch off again even if the voltage is removed, because the parasitic capacitance between gate and source (1nF to 5nF typical) will store charge and will maintain the gate-source voltage even when the signal is removed. To remove this charge quickly, a resistor is required in parallel with this capacitor. Depending on how fast you want the MOSFET to switch off, the resistance value is chosen.

Consider the following:
Let gate source signal voltage = 10V
Let gate-source capacitance = 1nF
Let R = 1K ohm
T = RC = 1uS
So with 1K ohm resistor the gate voltage will decay to 3.7V within one time constant ie. 1uS. The MOSFET will practically switch off within two time constants.
If we used a larger resistor, say 10k, then it would take about 20 uS for the MOSFET to switch off completely after the signal is removed

 

[Eshal]

@all 3 replies.

Your answer is just for high frequency input signal. What about low frequency input signal? Does low frequency input also activate the Gate-to-Source capacitance?

@mrinalmani

How did you calculate 3.7V for one time constant?

 

[mrinalmani]

Whether high frequency or low, once a capacitor is charged, it needs to be discharged.
In one time constant a capacitor's voltage falls down to 1/e or 36.7%

 

[FvM]

I guess the OP is asking of additional gate-source resistors used with push-pull gate drivers, as suggested by some publications.

If so, no RC time constant calculation applies. The gate will be driven most of the time by a low impedance driver, the resistor would be only effective during rising or falling supply voltage in a small voltage range where the driver doesn't yet pull-down the gate voltage (e.g. below 3-4 V). Or situations where the driver might be disconnected.

I believe that gate-source resistors can serve a purpose in some situations where the DC bus is powered before the driver supply. It shouldn't be expected to help against all possible power-on sequence issues. It can also cause additional problems with bootstrap circuit design.

 

[Ogu Reginald]

Consider using totem pole.

 

[Eshal]

Thank you all of you.