Analysis of Gate Oscillation of MOSFET and Suppression Measures
Analysis of Gate Oscillation of MOSFET and Suppression Measures
Gate oscillation is a common abnormal phenomenon in power circuit debugging. It usually appears at the moment when the MOSFET is turned on or off. The gate waveform produces high-frequency ringing, which may cause false turn-on, increased switching loss and device overheating. In severe cases, the device will be damaged by breakdown. This problem is not necessarily caused by the poor quality of the MOSFET, but is closely related to the parasitic parameters of the drive loop and peripheral components.
Comparison of Different Drive Schemes
| Drive Scheme | Oscillation Risk | Characteristic Description |
| Direct drive without gate resistor | High | Fast switching speed, large di/dt and dv/dt, prone to obvious ringing |
| Single gate resistor drive | Medium | Most widely used scheme, the resistance value needs to be adjusted reasonably |
| Gate resistor plus diode branch | Low | Separate control of turn-on and turn-off speed, flexible suppression effect |
| Gate RC absorption network | Low | Good for high-frequency strong oscillation, but will bring extra loss |
Main Causes of Gate Oscillation
Parasitic inductance of the drive loop: long gate traces form stray inductance, which resonates with gate capacitance
Excessive switching speed: the drive current is too large, resulting in excessive dv/dt and coupling interference
Insufficient power supply decoupling: the fluctuation of the drive power supply aggravates the waveform ringing
Millar capacitance coupling: the voltage change of the drain terminal is coupled to the gate through Cgd, triggering oscillation
Adverse Effects Caused by Oscillation
Continuous gate oscillation will repeatedly charge and discharge the gate capacitor, increasing the switching loss and the temperature of the power tube. The false turn-on caused by ringing will lead to the straight-through of the upper and lower bridge arms of the half-bridge circuit, which is a fatal hidden danger. High-frequency ringing also radiates electromagnetic noise, making the EMC test fail.
Common Suppression Methods
The most conventional method is to add a suitable gate resistance in series. Too small resistance cannot suppress oscillation, while too large resistance will seriously reduce the switching speed and increase the loss. For half-bridge and full-bridge topologies, an independent turn-off diode branch can be added to realize fast turn-off and slow turn-on. It is also necessary to place the decoupling capacitor close to the power supply pin of the driver chip.
Key Points of Layout and Debugging
The gate drive loop area should be minimized in PCB layout. The gate wire should be short and thick, and avoid parallel wiring with the drain power loop. When testing with an oscilloscope, the ground wire of the probe should be as short as possible to avoid misjudging the oscillation caused by the probe itself. It is necessary to balance the suppression effect and switching loss instead of blindly increasing the gate resistance.