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Renesas Singapore - Knowledgebase

Why is a resistor in MOS FET if it operates on voltage?

Latest Updated:12/01/2011


There is a figure in the application circuits in the application note showing a resistor inserted at the MOS FET gate, but I'm not sure how to select the resistance value of this resistor (it seems to differ depending on the product).
The MOS FET operates on voltage, so I feel that this resistor is not necessary.
Why is it there?


Amongst other things, the FET gate resistor is inserted to:

- protect against parasitic oscillation,
- protect against surge on the gate,
- suppress the switching speed,
- make adjustments to match the current drive capacity of the previous stage.

The main purpose of this resistor is to protect against parasitic oscillation (resonance).
Parasitic resonance occurs when Q of the resonant circuit becomes large due to parasitic capacitance and parasitic inductance in the MOS FET.
Therefore by adding a several-10 to several-100 Ω resistor to the gate, the Q value is lowered and oscillation and ringing are reduced.

A resistor with an even higher resistance value is necessary to protect against surge, but be aware that a high resistance causes the operation speed to drop significantly.
We therefore advise you to determine an appropriate resistance value that accords with the specifications of your system.

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