Beyond PWM: Active balancing, start-up and ZVS for multi-level converters with applications in renewable energy systems

This dissertation addresses many of the operational challenges of flying capacitor multi-level (FCML) converters and introduces new techniques to improve performance. First, start-up and a new method to extract power from within an FCML converter at a naturally occurring low-voltage node are investigated with a 5-level FCML converter. The introduction of the auxiliary power supplied induces an undesirable imbalance in the flying capacitors, which increases switch stress and induces additional harmonics in the output current. To reduce the induced imbalance, active balancing is implemented with a single voltage measurement of one specific flying capacitor. To correct for flying capacitor imbalance for more generic conditions, valley current detection has previously been proposed. However, prior work did not account for light load conditions, which we show to be challenging for traditional duty cycle compensation. Here we derive the light load conditions that lead to instability, and propose a new constant effective duty cycle compensation method for active balancing across the full load range. The effectiveness of the method is successfully demonstrated on a 4-level FCML prototype in light load and full load conditions. Quasi square wave zero voltage switching (ZVS) reduces switching losses and has been demonstrated at specific duty cycles for FCML converters in prior work. Here, we derive the fundamental limitations imposed by the FCML topology and show how the ZVS operating range can be extended through specific design choices. The results are demonstrated on an experimental 4-level hardware prototype, achieving step-down operation with a 1 kVdc input. The 4-level prototype also serves as an early stage proof of concept for high-voltage operation with 650 V GaN switches, as previous FCML designs utilized a higher number of FCML levels with lower voltage rated switches. As a final application of these techniques, a 1.5 kV, 15 kW, 3-phase solar photovoltaic (PV) inverter has been designed and implemented. The 5-level converter achieves a peak efficiency of 98.5% and demonstrates 3-phase operation with a Total Harmonic Distortion (THD) of less than 0.25%