High weighted efficiency in single-phase solar inverters by a variable-frequency peak current controller

This paper discusses a control method that achieves high weighted efficiency in solar microinverters. A challenge in microinverters is to achieve high efficiency over a range of output powers. To address this challenge, the proposed controller presents two primary benefits that enable such an efficiency profile, a switching frequency that scales with power, and a low peak current that enables efficient magnetic design of the inductor. At high powers, the switching frequency increases to minimize the root-mean-square (rms) current, and at low powers, the switching frequency decreases to minimize the switching loss. Since the peak inductor current is low, the inductor may be designed with fewer turns of wire, or with lower flux density, and is thus highly efficient. The proposed constant peak current switching scheme is implemented by a cycle-by-cycle predictive controller that uses a fast integrator to control the switching period, achieving high bandwidth and stability. This controller senses only the peak inductor current and, therefore, does not require expensive average current sensors. We demonstrate a low-cost inverter prototype with a 300-W solar panel. The prototype uses standard silicon devices and a small inductor of 360 μH to achieve a weighted efficiency of 99.15%.