Drivers Fail for Understandable Reasons

When a gate driver misbehaves, the cause is usually not a random failure but one of a few systematic problems: excessive overvoltage from too fast a turn-off or a loose commutation loop, a nuisance protection trip from a mis-set threshold, a false turn-on from dv/dt coupling, or overheating from high switching frequency. This article presents a method for diagnosing the common issues in Power Integrations SCALE gate drivers, so an engineer can move from symptom to root cause quickly.

Overvoltage at Turn-Off

The most common problem is excessive collector-emitter overvoltage when a large IGBT turns off a high current. The overvoltage is set by the turn-off di/dt, the commutation-loop inductance and the clamping. If it is too high, first increase the turn-off gate resistor to slow the turn-off, then reduce the commutation-loop inductance by tightening the layout and moving the DC-link capacitor close to the module, and finally confirm that the active clamping network is correct and working. Measure the overvoltage at the IGBT terminals, because the bus measurement hides the spike the device sees.

Clamping Behaviour

Active clamping feeds a controlled current back to the gate when the collector voltage exceeds a threshold, which limits the overvoltage. Its effectiveness depends on the clamping components and the gate loop, so check both. A high-impedance gate loop weakens the clamping and the turn-off behavior together.

Nuisance Protection Trips

A driver that reports a fault when the IGBT is fine is tripping on its protection. Check the DESAT threshold and the blanking time against the module, and measure the on-state collector-emitter voltage at worst-case load; if the threshold is set too low, the protection trips on a normal voltage. Noise on the DESAT sense pin, coupled from the switching, also causes nuisance trips, so filter the sense and keep it short. Under-voltage lockout can also trip if the isolated supply dips, so confirm the supply and its decoupling.

False Turn-On

In a bridge, the fast dv/dt of the complementary switch can couple through the gate-drain capacitance and turn a device on when it should be off, causing shoot-through. A negative off-state gate voltage, a low-impedance gate loop and a small gate-source capacitor all resist this. Check the gate waveform at the device during the complementary switch's transition to confirm the mechanism, and improve the layout before adding capacitors.

Driver Overheating

The driver's own power dissipation scales with the switching frequency and the gate charge, so at high frequency and with a large module the driver can run hot. Check the switching frequency and the gate charge, confirm the isolated supply is within range, and provide adequate copper or airflow around the driver. If the driver is still hot, consider a higher-current driver or a plug-and-play board with a larger thermal margin.

A Systematic Method

Work from the simple to the complex: check the gate resistors and the overvoltage first, then the protection thresholds and the DESAT sense, then the false turn-on and the layout, then the driver temperature. Keep a reference board that is known good, and record the overvoltage and temperature of each new design on the bench, so a later change is immediately visible. BeiLuo supplies genuine Power Integrations drivers with import declaration, certificate of origin and RoHS documents, and our FAE team can help you diagnose a problem and choose a design change that resolves the root cause.