佐藤 之彦

This paper proposes a quasi-MPPT (maximum power point tracking) control for integrating multiple energy harvesters of different characteristics. Energy harvesting (EH) technology is a promising technology and is expected to be widely used in various fields. However, the amount of the harvested power is inherently limited and strongly depends on the weather and other conditions. Therefore, multiple energy harvesters are often integrated to increase total harvested energy. In the case of integrating multiple energy harvesters of different characteristics, the control tends to be complicated due to the different characteristics of the energy harvesters. The proposed quasi-MPPT control enables the multiple energy harvesters to work near their MPP voltages by referring to the command voltages derived from preliminary identification of the energy harvesters and measurement of the open-circuit voltages. It achieves harvesting of approximately maximum powers of the multiple energy harvesters by a simple control strategy using a power management circuit. The effectiveness is validated by using an experimental system that consists of photovoltaic (PV) and thermoelectric (TE) energy harvesters.

The voltage drop in GaN HEMTs during reverse conduction is larger than that of Si-MOSFETs, causing an increase in dead-Time loss and a rise in the bootstrap capacitor voltage when a bootstrap circuit is applied. We propose connecting a low-side device in parallel with a Schottky barrier diode as a solution. This allows the bootstrap capacitor to be charged at the appropriate voltage because the voltage drop is lower. This also reduces dead-Time losses but increases the losses due to capacitance across the connected diode terminals. These losses increase with frequency and are not negligible in ultrahigh frequency switching operations such as 1MHz. This study examines the effect of using anti-parallel diodes for GaN devices.

Carrier frequency of N-level flying capacitor converter (FCMLC) is equivalently (N-1) times higher than that of the 2-level converter with the same carrier frequency. In consequence, its control bandwidth is extended. However, updating the command value in that equivalent carrier period causes capacitor voltages to deviate from predetermined voltages. It is caused by mismatches between charging and discharging times of flying capacitors. Imbalanced flying capacitor voltages cause problems such as increase of harmonics in the output current and applied voltages to the devices. In this study, we derive the discrete-time state-space equation of FCMLC considering flying capacitor voltages and analyze stabilities of flying capacitors. Depending on the analysis, a compensation method for capacitor voltages is proposed, and the effectiveness of it is verified by the experiments.