Analysis and Control of Critical Conduction Mode High-Frequency Single-Phase Transformerless PV Inverter

This article presents a critical conduction mode (CRM) single-phase transformerless full-bridge inverter in a residential photovoltaic system. The CRM full-bridge inverter in bipolar mode features zero-voltage switching capability for the whole line cycle. This enables the inverter to push switching frequency up to hundreds of kHz and achieve high power density with high efficiency. However, CRM operation incurs nonconstant common mode (CM) voltage during the resonant period, causing high frequency leakage current. To minimize the leakage current, a new switching modulation strategy is proposed introducing triangular current mode (TCM) near the ac voltage zero crossing. The switching modulation scheme alleviates the CM voltage by shortening the resonant period and consequently reduces the leakage current. Then, discontinuous conduction mode is inserted between the CRM and TCM operation regions to improve light load efficiency. In this article, the basic operation principle of the CRM full-bridge inverter, impacts of CRM operation on the leakage current, and details of the proposed switching modulation method are discussed. Experimental results with a 2.4-kW prototype built with GaN devices validate its performance.