In Situ Detection of Bond Wire Lift-Off Events in Operational SiC MOSFETs

Bond wire lift-off is one of the main failure mechanisms for silicon carbide mosfets. This occurs when the metallurgical weld between the bond-wire and the device source breaks creating an open circuit. The higher current on the remaining wires subjects them to additional thermal stress creating a positive feedback loop that can quickly lead to device failure if left unchecked. By detecting lift-off events during power converter operation damaged devices can be de-rated and equipment operators can be alerted to replace the power module before a failure occurs. This article presents an online, in situ monitoring circuit that can detect the incremental change in on-state inductance (L_ds) caused by the redistribution of bond-wire current after a lift-off occurs. Unlike on-state resistance (R_ds), the inductance does not depend on temperature, gate-oxide health, or gate-bias level. In addition, since the inductive impedance is frequency dependent, any bias related to converter operation can be suppressed by increasing the measurement frequency well above the switching frequency. Experimental results show that the proposed circuit can accurately measure inductive changes down to 10 pH, providing sufficient sensitivity to detect single wire lift-off events in test devices.