Coss Loss Tangent of Field-Effect Transistors: Generalizing High-Frequency Soft-Switching Losses

The dissipated energy (Ediss) related to the resonant charging/discharging of a transistor output capacitance, becomes a dominant loss factor for power converters operating in the MHz range. A recent work has introduced a small-signal measurement method to quantify Ediss with a frequency-dependent small-signal resistance, Rs, and an effective small-signal output capacitance, Coss-eff. This work provides further insights on the effect of Rs and Coss upon the device losses in a broader sense. In particular, the Coss loss tangent, tan (δ), is introduced as a normalized Ediss to combine the roles of Rs and Coss together with the operating frequency into a single loss parameter. By evaluating commercial device families, it is demonstrated that tan (δ) is constant for a given family, independent of the device on-state resistance, RDS(on). It is shown that a minimum Ediss is achieved by having the lowest tan (δ) for a given stored energy (Eoss) in Coss. With accompanying guidelines, this work identifies tan (δ) as a powerful figure of merit to classify field-effect transistors for soft-switching applications, regardless of RDS(on) variations in devices within a family. The proposed concept provides a comprehensive method to characterize and benchmark power transistors for high-frequency applications.