Duty-Cycle Compensation in Flying-Capacitor Multi-Level Converters
Andrew Stillwell with adviser R. Pilawa-Podgurski
Flying-capacitor multi-level (FCML) converters are attractive solutions for compact power conversion with a large step-down ratio. For many of these applications, there is a need for low-voltage auxiliary supplies to power control logic, gate drivers and/or subsystems. Traditionally, an additional converter with a low output voltage is implemented, but it needs to operate from the high-voltage input. The large step-down nature of the auxiliary converter poses a challenge in obtaining a compact and low-cost auxiliary supply. One approach is to extract control power from the converter output for applications where the output voltage is much lower than the input voltage. This does not work for applications involving an inverter or a dc-dc convertor with a wide output voltage range.
This research demonstrates an alternative approach by extracting the power from a low-voltage node, shown in Figure 23, within the FCML converter itself. Since the low-voltage node has a maximum voltage at a fraction of the input voltage, the voltage rating and thus the size of the auxiliary supply converter can be significantly reduced. However, extracting additional current from a node within the FCML converter disrupts the natural self-balancing of the flying capacitors, which (if left unchecked) can increase the switch stress and capacitor rating requirements of the components. Previous work demonstrated the use of duty-cycle control to enhance the natural self-balancing. We propose a method of duty-cycle compensation, in which the duty cycle of the individual switches is controlled to compensate for the induced imbalance from the auxiliary load. This method requires no additional switches or topology changes and is implemented with one measurement on the affected capacitor voltage and proportional control. The effectiveness of the approach is demonstrated with both simulation and experimental results. This work is funded by Sandia National Laboratories and the Army Research Laboratory.