Efficient DC-DC power-conversion with wide-span voltage-regulation is crucial to a sustainable and robust power electronics system. Dual-active-bridge (DAB) offers straightforward regulation and its transformer enables voltage stepup/down required for many applications, such as battery chargers and bus converters for DC distribution systems. However, losing soft-switching at light loads or when operating at voltage gains far from the turns ratio severely degrades the efficiency of DAB, especially at high switching frequencies. In this work, we demonstrate an enhanced DAB (E-DAB) topology which employs an adjustable-tap transformer to extend the softswitching over wider voltage gains and increase the powertransfer capability. By a proper tap adjustment and with single phase-shift modulation, the proposed GaN-based converter achieved a peak efficiency of 97.4% with an overall efficiency greater than a conventional DAB for voltage gains of up to 2.8 times higher. Employing a quasi-planar matrix transformer with integrated leakage inductance at 300 kHz allowed for an extremely high power density of 10 kW/l (7.5 kW/l with cooling). The tapped transformer did not incur extra losses to the topology. The gain versus power-transfer characteristic for softswitching operation was derived for the E-DAB and its improvement in efficiency was experimentally verified over a wide power range.
Soft-switching power converters based on wideband-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-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 (ST) and Nonlinear Resonance (NR) methods to measure largesignal 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 COSS 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.
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