A gate driven too little, too much, or not at all

Tags: Gate drive

Symptom

A switching stage runs hotter than its loss budget predicts; or an eGaN transistor fails after its gate rings above its maximum gate-source voltage; or the switch conducts for a moment while the controller starts or resets, with a current spike or, in a half-bridge, a shoot-through.

Why

Three things set the gate’s window:

• Too little. A MOSFET turns fully on only at the gate voltage it is made to be driven with, and a higher gate voltage lowers its on-resistance further: a logic-level MOSFET is made for 5 V, a standard-level one needs at least 10 V [Nexperia 2025]. A drive that only just exceeds the threshold leaves the switch partly on, at a higher resistance and loss than its data sheet’s on-resistance.
• Too much. The maximum allowable gate-source voltage of an eGaN FET is low compared with silicon MOSFETs [Efficient Power Conversion (EPC), eGaN FET Drivers and Layout Considerations], and the ringing of the gate loop’s inductance adds to the drive (gate drive), so the drive and its ringing together must stay below that maximum.
• Not at all. With the driver unpowered, nothing drives the gate and its voltage is undefined. A gate-source resistance in the kilohm range keeps a MOSFET off when the gate drive has a fault or no power is applied [Nexperia 2025]. A powered driver whose input floats while the controller starts up does drive the gate, to whatever level its floating input reads, and the resistor at the gate cannot hold that off.

How to confirm

Measure the gate-source voltage at the transistor’s own terminals, with a short ground connection (probes), at the switching edges and while the controller powers up and resets. A switch whose source is not at the oscilloscope’s earth, such as a high-side switch or any switch on an off-line primary, needs a differential probe: a probe’s ground lead is tied to the oscilloscope’s earth (gate drive). Compare the gate’s high level with the drive voltage the transistor’s data sheet gives its on-resistance at, its peaks with the gate’s maximum, and look for any pulse while the controller is not yet running.

Fix

Drive the gate at the voltage the data sheet gives the on-resistance at, from a driver whose undervoltage lockout stops it before its supply sags to where the switch is only partly on. Keep the gate loop short, so that its ringing stays well below the maximum. Place a resistor from gate to source for the driver without power [Nexperia 2025], and for the driver with power hold its input at the level that keeps its output low (a pull-down resistor on a non-inverting input), unless the driver’s data sheet says its input does so by itself. The gate currents at the edges are worked through in gate drive (gate current at turn-on).

References

• [Nexperia 2025]: the gate voltage a MOSFET needs to be fully on, and a gate-source resistor that keeps it off without drive.
• [Efficient Power Conversion (EPC), eGaN FET Drivers and Layout Considerations]: an eGaN FET’s low maximum gate-source voltage.
• Gate drive and Probes and measurement bandwidth.