C$_{oss}$ Loss Tangent of Field-Effect Transistors: Generalizing High-Frequency Soft-Switching Losses

The dissipated energy ( $E_{\text{diss}}$ ) 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 letter has introduced a small-signal measurement method to quantify $E_{\text{diss}}$ with a frequency-dependent small-signal resistance, $R_{\text{s}}$ , and an effective small-signal output capacitance, $C_{\text{oss}}^{\text{eff}}$ . This letter provides further insights on the effect of $R_{\text{s}}$ and $C_{\text{oss}}$ upon the device losses in a broader sense. In particular, the $C_{\text{oss}}$ loss tangent, tan ( $\delta$ ), is introduced as a normalized $E_{\text{diss}}$ to combine the roles of $R_{\text{s}}$ and $C_{\text{oss}}$ together with the operating frequency into a single loss parameter. By evaluating commercial device families, it is demonstrated that tan ( $\delta$ ) is constant for a given family, independent of device on -state resistance, $R_{\text{DS(on)}}$ . It is shown that a minimum $E_{\text{diss}}$ is achieved by having the lowest tan ( $\delta$ ) for a given stored energy ( $E_{\text{oss}}$ ) in $C_{\text{oss}}$ . With accompanying guidelines, this letter identifies tan ( $\delta$ ) as a powerful figure of merit to classify field-effect transistors (FETs) for soft-switching applications, regardless of $R_{\text{DS(on)}}$ variations in devices within a family. The proposed concept provides a comprehensive method to characterize and benchmark field-effect transistors for efficient operation in high and very-high-frequency (VHF) applications.