A Systematic Approach

IGBT bridge problems usually show up as excess heat, EMI or intermittent failure rather than an obvious fault, which makes them easy to misdiagnose. This article presents a systematic procedure for diagnosing STPOWER IGBT bridge designs, using the STGW60H65DFB and STGW40V60DF as references, so that the cause is found by measurement rather than by guessing.

Step 1: Separate Conduction and Switching Loss

Total loss is the sum of conduction loss, which depends on current and saturation voltage, and switching loss, which depends on frequency and the switching transitions. Measure the device temperature at a low switching frequency and again at a high one. If the temperature rises steeply with frequency, the loss is switching-related and the gate drive or the diode is the place to look. If the temperature is high even at low frequency, the conduction loss or the thermal path is the problem.

Thermal Path

Before blaming the device, confirm the thermal path. The package must be mounted on a heatsink or a sufficient copper area with a good thermal interface, and the junction temperature must be calculated from the dissipation and the thermal resistance. A design that is electrically correct can still overheat through a poor thermal path.

Step 2: Examine the Gate Drive

The gate resistor controls the switching speed and the di/dt, balancing loss against EMI. A gate resistor that is too small accelerates the transition, reducing loss but increasing voltage overshoot and EMI; one that is too large slows the transition, reducing EMI but increasing loss. Measure the collector voltage overshoot at turn-off and the gate waveform, and adjust the gate resistance in small steps until the overshoot is within the device rating and the loss is acceptable.

Parasitic Turn-On

In a bridge, the fast dv/dt of one device can couple through the Miller capacitance of the other and lift its gate above threshold, causing cross-conduction. The classic fixes are a negative off-state supply, a lower-impedance gate pull-down, or a lower gate resistance for the off transition. Measure the gate voltage of the low-side device during a high-side transition to see whether the spike exceeds the threshold.

Step 3: Check the Antiparallel Diode

The antiparallel diode conducts during freewheeling, and its recovery behaviour sets part of the switching loss and the EMI. A hard-recovery diode produces a large reverse-recovery current that adds to the loss and radiates noise; a soft-recovery diode, such as the one paired with the HB-series IGBT, reduces both. Confirm that the diode's recovery is compatible with the switching frequency and the gate drive.

Step 4: Verify Dead Time

Dead time prevents shoot-through between the high-side and low-side devices, but too much dead time distorts the output and adds loss in the diode. Set the dead time from the switching speed and the gate drive, and verify it under worst-case temperature, because switching times vary with temperature.

When to Escalate

If the bridge still underperforms after the thermal path, gate drive, diode and dead time are corrected, escalate with the measured data: the switching waveforms, the overshoot, the device temperature and the diode recovery. BeiLuo's FAE team can review the design against the reference and characterize the bridge in our lab so the cause is identified before a board respin.