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I. Role of the Gate Resistor Rg
1. Eliminating gate oscillation
The gate-emitter (or gate-source) terminals of insulated-gate devices (IGBTs, MOSFETs) form a capacitive structure, and parasitic inductance in the gate loop is unavoidable. Without a gate resistor, the gate loop will oscillate strongly when excited by the driver's drive pulses, so a resistor must be connected in series to quickly damp the oscillation.
2. Transferring the driver's power loss
Capacitors and inductors are reactive components. Without a gate resistor, most of the drive power would be dissipated in the driver's internal output transistors, causing a significant temperature rise.
3. Adjusting the switching speed of the power switching device
A small gate resistance makes the switching device switch fast with low switching loss; conversely, a large resistance makes it slow with high switching loss. However, too fast a drive speed will greatly increase the voltage and current change rates (dv/dt and di/dt) of the switching device, generating significant interference and, in severe cases, preventing the entire system from working. Therefore, a balanced trade-off must be made.
II. Selection of the Gate Resistor
1. Determining the gate resistance value
Depending on various considerations, the choice of gate resistance can vary widely. As a starting point, select as follows:
IGBT rated current (A) | 50 | 100 | 200 | 300 | 600 | 800 | 1000 | 1500 |
Rg range (Ω) | 10~20 | 5.6~10 | 3.9~7.5 | 3~5.6 | 1.6~3 | 1.3~2.2 | 1~2 | 0.8~1.5 |
IGBT modules from different brands may have their own specific requirements; you can fine-tune around the recommended values in their datasheets.
2. Determining the gate resistor power rating
The power rating of the gate resistor is determined by the IGBT gate drive power. Generally, the total power rating 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 datasheet.
For example, a common IGBT driver (such as the TX-KA101 from Beijing LMY) outputs a positive voltage of 15V and a negative voltage of -9V, so V = 24V.
Assuming F = 10 kHz and Q = 2.8 uC,
P = 0.67 W can be calculated. A 2 W resistor should be selected for the gate resistor, or two 1 W resistors 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, connect several in parallel to reduce inductance.
2. Use different gate resistors for IGBT turn-on and turn-off
Usually, to achieve better drive performance, different drive speeds can be used 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 should be connected in parallel with the original Rg to adjust the drive speed in the two directions.
3. Connect a resistor Rge = 10–100 kΩ between the gate and emitter of the IGBT to prevent the IGBT from being damaged through the Miller capacitance if high main-circuit voltage is accidentally applied when the drive leads are not connected. Although Rge is already present, considering the above factors, users are advised to add an additional Rge between the gate and emitter of the IGBT or between the gate and source of the MOSFET.
