Resistors are often inserted between a CPU and MOSFETs. Why are these resistors necessary?

Gate resistors are used to control over-current in gate drivers and to reduce overshoot between the drain and source during switching (EMI noise-reduction).

For MOSFET operation, the switching time (rise and fall time) of MOSFET varies depending on the resistor of the connected gate.
When the switching time is short or long, the following problems may occur. Please set the switching time suitable for the system.

  • If the resistance of the gate is small,
    Switching time is reduced and ringing (damped oscillation) may occur.
    Ringing may result in oscillations or EMI noises.
  • If the resistance of the gate is large,
    Switching time becomes longer, switching loss increases, and more heat is generated.

In the bridge circuit, a short circuit may occur across the upper and lower MOSFETs by combination of the gate resistances.
Therefore, it is necessary to consider the optimum gate resistance.

For details on how to suppress parasitic oscillations in MOSFET parallel operation, see what are the considerations when using MOSFETs in parallel?

What are the considerations when using MOSFETs in parallel?

Fig. 1: Output waveform: Example with small gate resistance
Fig. 1: Output waveform: Example with small gate resistance
Fig. 2: Output waveform: Example when the gate resistance is appropriate
Fig. 2: Output waveform: Example when the gate resistance is appropriate
Fig. 3: Output Waveform: Gate Resistance Large Example
Fig. 3: Output Waveform: Gate Resistance Large Example

See also the pages and documentation below.

Back to FAQs of MOSFETs / Bipolar Transistors / IGBTs

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