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Soft-switching power converters based on wide-band-gap (WBG) transistors offer superior efficiency and power density advantages. However, at high frequencies, loss behavior varies significantly between different WBG technologies. This includes losses related to conduction and dynamic on-state resistance (RDS(ON)) degradation, also charging/discharging of input capacitance (CISS) and output capacitance (COSS). As datasheets lack such important information, we present measurement techniques and evaluation methods for soft-switching losses in WBG transistors which enable a detailed loss-breakdown analysis. We estimate the gate loss under soft-switching conditions using a simple small-signal measurement. Next, we use Sawyer–Tower and nonlinear resonance (NR) methods to measure large-signal COSS energy losses up to 40 MHz. Finally, we investigate the dependence of dynamic RDS(ON) degradation on off-state voltage using pulsed-IV measurements. We demonstrate an insightful comparison of soft-switching losses for various normally off gallium nitride (GaN) and silicon carbide (SiC) devices. A p-GaN-gated device exhibits the most severe RDS(ON) degradation and the lowest gate loss. Cascode arrangement increases threshold voltage for GaN devices and reduces gate losses in SiC transistors; however, it leads to higher C{OSS losses. The study facilitates the evaluation of system losses and selection of efficient WBG devices based on the trade-offs between various sources of losses at high frequencies. [ABSTRACT FROM AUTHOR]

Cascode topology which includes a high-voltage GaN and a low-voltage Si transistor is an attractive device concept, as it takes advantage of the low on-resistance of GaN while still being compatible with Si gate drivers. It demonstrates the highest threshold voltage among normally-off GaN devices, which is a favorable feature in power electronics. However, the observation of instability in power circuits using cascode devices highlights the need for a careful analysis to reveal the origins of this phenomenon, to fulfill the unique features of these devices in high efficiency and reliable power converters. In this letter, we demonstrate the presence of negative resistance in cascode devices, which explains the previously reported large-signal instabilities. We describe a theory to characterize this negative resistance behavior based on the subthreshold-voltage operation of the high-voltage GaN transistors, and propose guidelines to resolve this issue in cascode devices. Negative resistance behavior, although not favorable for a power transistor, is of great importance in high-frequency electronics, with applications in reflection-type power amplifiers. These results, together with the presented analysis, could guide the development of high-stability, low-loss devices for future efficient power converters. [ABSTRACT FROM AUTHOR]

The low on-resistance of wide-band-gap (WBG) transistors is a key feature for efficient power converters; however, the anomalous loss in their output capacitance (COSS) severely limits their performance at high switching frequencies. Characterizing COSS losses based on the large-signal measurement methods requires an extensive effort, as separate measurements are needed at different operation points, including voltage swing, frequency, and dv/dt. Furthermore, there is a practical tradeoff in the maximum voltage and frequency applied to the device. Here, we introduce a new circuit model, including an effective COSS and a frequency-dependent series resistance, along with a simple small-signal method to fully characterize COSS losses in WBG transistors. The method accurately predicts COSS losses at any voltage swing or frequency. Contrary to other methods, this technique directly leads to a general identification of COSS losses at different operation points, revealing new insights on COSS losses in WBG transistors, especially the dependence of EDISS on voltage and frequency. Based on the proposed approach, the issue of COSS losses in enhancement-mode GaN and SiC transistors was assigned to the limited quality factor of COSS. The precise characterization of COSS losses proposed in this letter is essential for designing efficient high-frequency power converters. [ABSTRACT FROM AUTHOR]