The Role and Selection Guide of Gate Resistor Rg in Power Semiconductor IGBT/MOSFET Applications

Published: 2022-05-17 Views: 次

The Role of Gate Resistor Rg in Power Semiconductor IGBT/MOSFET Applications and Selection Guide

I. Role of Gate Resistor Rg
1. Eliminating gate oscillation
　　The gate-emitter (or gate-source) terminals of insulated-gate devices (IGBT, MOSFET) form a capacitive structure, and the parasitic inductance of the gate loop is unavoidable. Without a gate resistor, the gate loop will produce strong oscillation when excited by the driver's drive pulse, so a resistor must be connected in series to quickly damp it.
2. Transferring the power loss of the driver
　　Capacitors and inductors are reactive components. Without a gate resistor, most of the drive power will be dissipated in the output transistors inside the driver, causing a significant temperature rise.
3. Adjusting the switching speed of the power switching device
　　A small gate resistor results in fast switching and low switching loss; conversely, a large one results in slow switching and high switching loss. However, too fast a drive speed will greatly increase the voltage and current change rates of the switching device, thereby generating greater interference and, in severe cases, preventing the entire device from working. Therefore, a balanced compromise must be made.

II. Selection of Gate Resistor
1. Determining the resistance value of the gate resistor
　　Under various considerations, the selection of the gate resistor may vary greatly. For initial trials, it can be selected as follows:

IGBT modules from different brands may have their own specific requirements; adjustment can be made around the recommended values in their parameter manuals.

2. Determining the power rating of the gate resistor

　　The power of the gate resistor is determined by the gate drive power of the IGBT. Generally, the total power of the gate resistor should be at least twice the gate drive power.

IGBT gate drive power P = FVQ, where:
F is the operating frequency;
V is the peak-to-peak value of the drive output voltage;
Q is the gate charge, which can be found in the IGBT module parameter manual.
For example, a common IGBT driver (such as Beijing LMY's TX-KA101) outputs a positive voltage of 15V and a negative voltage of -9V, so U = 24V.
Assuming F = 10KHz, Q = 2.8uC,
P = 0.67W can be calculated. A 2W resistor should be selected for the gate resistor, or two 1W resistors connected in parallel.

III. Other Precautions for Setting the Gate Resistor

1. Minimize the inductive impedance of the gate loop. Specific measures include:
a) Place the driver close to the IGBT to reduce lead length;
b) Twist the gate and emitter leads of the drive together, and do not use excessively thick wires;
c) Keep the two drive traces on the PCB as close together as possible;
d) Use non-inductive resistors for the gate resistor;
e) If inductive resistors are used, several can be connected in parallel to reduce inductance.

2. Select different gate resistors for IGBT turn-on and turn-off
　　Usually, to achieve better drive performance, different drive speeds can be adopted for IGBT turn-on and turn-off, and it is often necessary to select Rgon and Rgoff (also called Rg+ and Rg-) separately.
　　Some IGBT drivers have separate outputs for turn-on and turn-off control; simply connect Rgon and Rgoff respectively.
　　Some drivers have only one output, in which case a series network of a resistor and a diode must be connected in parallel with the original Rg to adjust the drive speed in both directions.
3. Connect a resistor Rge = 10–100K between the gate and emitter of the IGBT to prevent the IGBT from being burned out through the Miller capacitance if high main-circuit voltage is accidentally applied when the drive leads are not connected. Even if Rge is already present, considering the above factors, users are advised to additionally install Rge between the gate and emitter of the IGBT or between the gate and source of the MOSFET.