Unfolding PV Microinverter Current Control: Rectified Sinusoidal vs Sinusoidal Reference Waveform

Photovoltaic (PV) microinverter technology has become a popular solution in small-scale PV applications. The most used commercial microinverter topology is a two-stage converter composed by a dc-dc converter followed by a full-bridge unfolding inverter. The dc-dc-stage is in charge of performing MPPT and producing a rectified sinusoidal current waveform synchronized with the grid voltage, while the inverter stage only unfolds the current into an ac waveform. In this way no dc-link capacitor is needed (improving reliability), and only the dc-dc stage is performing PWM while the inverter stage is switching at fundamental frequency (improving efficiency). Since the reference current for the dc-dc stage is a rectified sinusoidal waveform, multi-resonant controllers are necessary. This type of controller is complex to design and requires more computational power compared to classic linear controllers, which is limited in PV microinverters, where lower-end micro-controllers are used to reduce cost. In this paper, a simple current control method is proposed, by following the opposite approach, in which a sinusoidal current waveform is controlled with easier controller design and implementation, and then folded or rectified in terms of the modulation in the dc-dc stage. The inverter stage unfolding function remains the same. In order to validate the proposed control, the interleaved flyback and push-pull unfolding PV microinverters are simulated, and the main features and differences of the traditional and proposed control are analyzed.