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Critical Conduction Mode Based High Frequency Single-Phase Transformerless PV Inverter

Schematic of full bridge inverter
Fig. 1. Single-phase, transformerless, full-bridge inverter in a PV System
   In a residential PV system, single-phase transformerless invert- ers have been widely used. Even without a line frequency transformer, the power density of products in the market is low due to the low switching frequency with Si-based power devices. Critical condition mode (CRM) operation with wide-bandgap (WBG) devices has been deemed a reasonable solution for high frequency applications by achieving zero voltage switching (ZVS). This paper explores a CRM-based full-bridge inverter with bipolar pulse-width modulation (PWM) in detail. Also, how CRM operations affect leakage current is analyzed, and a new switching modulation strategy is introduced to minimize the leakage current.
   Based on the full-bridge inverter in Fig. 1, Fig. 2 illustrates waveforms of a CRM full-bridge inverter in bipolar mode. The small difference of iL1 and iL2 due to the line frequency leakage current iLK_L brings about a zero-crossing timing mismatch for inductor currents. The resonance starting point of each switching node VaN and VbN then becomes inconsistent to the point that a hump appears in the common mode voltage Vcm (green shaded area). Eventually, the non- constant Vcm becomes the source of high-frequency leakage current in the system. On account of the inherent operating status in CRM, resonant period, common mode voltage variation occurs. The hump in Vcm is largest at the ac voltage zero crossing since the line frequency leakage current is the largest and the resonant period is also the longest, resulting in a large high-frequency leakage current.
   In general, CRM leads to a wide switching frequency range, especially a high frequency near zero crossing. To avoid large switching-related loss, the maximum switching frequency is limited by operating the inverter in discontinuous conduction mode (DCM). However, instead of using an operation to clamp switching frequency and to shorten the resonant period near ac voltage zero crossing, a triangular current mode (TCM) operation is employed in this paper. By combining CRM and TCM, a full-bridge inverter with bipolar PWM can soothe the common mode voltage and suppress the leakage current with a narrow switching frequency range.
   Experimental results with a 2.5 kW laboratory prototype built with an E-mode GaN device validate its performance. The peak efficiency of the inverter is slightly higher than 98%, and the leakage current is suppressed below 300 mA.
Waveform of inverter operation in bipolar mode
Fig. 2. Waveforms of a CRM full-bridge inverter in bipolar mode