Small-Signal Approach for Precise Evaluation of Gate Losses in Soft-Switched Wide-Band-Gap Transistors

High-frequency switching is favorable for fast transient response, small size of passive components and superior power density, especially in soft-switching topologies. At high frequencies, power dissipation due to consecutive charging/discharging of gate capacitance is considerably large. As presented in this work, the actual gate charge of a transistor can be very different from the typical values reported in manufacturer datasheet, which leads to errors in estimation and modeling of gate loss based on datasheets. Furthermore, the reported QG values in datasheets correspond to a hard-switching test condition, and are not a good representative of the losses in soft-switched transistors. Here, we propose a simple method to precisely evaluate gate loss in soft-switched transistors for high-frequency applications. A small-signal input-capacitance measurement is used to derive gate loss in two commercial Gallium-Nitride (GaN) transistors. The estimated losses are then verified by results from accurate thermal modeling based on a matrix of temperatures, when the transistors are driven up to 30 MHz. The results are of great significance to the modeling and accurate measurement of gate losses at high frequencies. It is instrumental to a proper cooling design to avoid device and gate driver thermal runaway and failure. Also, the more accurate gate capacitance measurement enables an accurate dead time adjustment to achieve synchronized turn ON between various transistors in soft-switching topologies

Calibration-Free Calorimeter for Sensitive Loss Measurements: Case of High-Frequency Inductors

Wide-band-gap technologies enable ultra-high efficiencies in high-frequency power conversion. However, inadequate accuracies in electrical measurements could lead to spurious efficiency measurements, even above 100%. Furthermore, bandwidth limitations, particularly for current probing, delay mismatches between current and voltage probes, loading effects and electromagnetic interferences make electrical techniques inappropriate for measuring extremely-low losses in switches with high dv/dt and di/dt values and magnetic components with high-frequency excitations. Calorimetric methods overcome this issue by a direct loss measurement through the generated heat. Nevertheless, limited ranges and accuracies of existing systems hinder their application in sensitive measurements. Moreover, time-consuming calibrations with extensive data processing impede rapid design assessments. In this work, we further investigate a previously proposed closed-type double-chamber calorimeter and present its high accuracy for the evaluation of low levels of losses in high-frequency power inductors. The system provides adjustable cooling and can measure losses as low as 500 mW, enabling evaluation of high-performance power converters and their components with no dependencies on the geometries of the evaluated devices/systems