Andrew Stillwell with advisor R.Pilawa-Podgurski

This research explores a bidirectional 4-level flying capacitor multi-level (FCML) converter for high voltage (> 1 kV) step-down/step-up conversion needed in hybrid rail and electric vehicle applications. Previous work achieved similar input voltage requirements through increasing the number of levels in the FCML, at the cost of additional transistors, flying capacitors and control complexity. In this work, we investigate the application of 650 V GaN transistors and apply a cascaded bootstrap method to supply the isolated gate drivers. The design also employs a single-sided PCB to improve the design flexibility for back-side heat sinking, as shown in Figure 1. To further improve efficiency, we explore zero voltage switching (ZVS) and derive the operating conditions over which ZVS is possible for FCML converters. We show how flying capacitor ripple and inductor saturation limit the ZVS operating range (shown in Figure 2) and how design for ZVS can extend the ZVS operating range.  Funding is provided by CRRC (

Figure 1: The converter prototype utilizes a single sided design.

Figure 2: ZVS is only possible for some regions of operating duty cycle and load conditions. Other regions are limited by inductor saturation, flying capacitor ripple or gate driver power operation (depicted in yellow).