In-Circuit Inductance Measurement to Correct the Single-Pulse Avalanche Energy (Eas) of Transistor Under the Unclamped Inductive-Switching (UIS) Test

The single-mode avalanche energy ( ${E}_{\text {as}}\text {)}$ evaluated from the unclamped inductive-switching (UIS) test has been used in many application notes and literature as the ruggedness index of a transistor. The inductance is the key parameter for ${E}_{\text {as}}$ calculation. From the in-circuit measurement, the inductance at the channel turn-off time ( ${t}_{\text {off}}\text {)}$ is found smaller than the default inductance for the power transistor under the UIS test. However, most UIS testers still use the default inductance to calculate ${E}_{\text {as}}$ . In this article, whether the inductance can keep constant during ${t}_{\text {off}}$ for any transistor under any test condition of UIS and why it is smaller than the default inductance during ${t}_{\text {off}}$ for air core inductors is investigated. Without the ferrite core, the magnetization (M) of the air core inductor during the UIS test is caused by the eddy currents, which increase with increasing current and decrease with decreasing current linearly. Therefore, the differential magnetization (dM/dH) is constant, resulting in two different inductances during channel turn-on time ( ${t}_{\text {on}}\text {)}$ and turn-off time ${t}_{\text {off}}$ .