Can anyone explain me what is the use of adding resistor across gate to source for MOSFET and how to calculate the value. I am using IRFP460
Dear Bharath,
It is not normal practice to shunt the gate to source of a power transistor with a resistor when operating it as a switch. Normally it is driven by a low source resistance driver to affect high speed switching from the off to the on state and vice verse. The switching time is about the time constant of Cgs Rs where Cgs the source resistance and Cgs is the gate to source capacitance, which is a transistor parameter and you can find it in the data sheet.
However, if you want to protect the gate to source of the transistor from electrostatic discharge you can shunt it by a zener diode with the highest allowed voltage at the gate to source junction. However you can use a shunting resistor to leak the the electrostatic charge. This resistance can be as high as 10 Kohms. It must be much higher than the Rs the source resistance to avoid appreciable current bleeding from the driver source.
If you drive the transistor with uni polar current source the you have to discharge the Cgs by a shunting resistance. It is the discharge time which sets the value resistor.
In case of using the the transistor as an amplifier in class A operation then the gate must be biased by a potential divider between VDD an the ground with the gate at the mid point of the divider. Th value of the resistor depends on the required gate bias and VDD.
Dear Zekry
Thank you for your replay.
In below paper on page 15 for dv/dt protection they have given one condition that
http://www.radio-sensors.se/download/gate-driver2.pdf
one resistor has to be place across gate to source to limit the dv/dt. But it is not matching with the 10kohm.
Regards
Dear Bharath,
The dV/dt problem is not treated in my comment. The dv/dt is a dynamic protection.
It is so that the MOST can be also made on by a rapid increase of the drain to source voltage with time, that is unintentional on of the transistor not through the gate pulses but through a too rapid increase of Vds with time., dVds/dt. Such rate of rise of Vds will cause an equal rate of rise of the Vgs. Which in turn cause a displacement capacitive current to flow in Cgs charging it. If Cgs is charged to >= Vth the transistor will unintentionally turn on while the voltage is high on it. And so can be damaged except one has to see for the right protection. The right protection is partially discharge Vgs across a shunt resistance.
For sake of simplification and assuming that most of the displacement current passes through the shunting resistance Rsh , then Rsh i= Rsh Cgd dVds /dt
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