Modelling of Capacitor Voltage Imbalance and Dynamics in Flying Capacitor Three-level Single-phase and Dual-phase DC-DC Converters

Abstract

A main challenge of operating multi-level flying-capacitor (FC) converters is balancing the FC voltages to the ideal values to avoid inefficient operation or failure. The thesis investigates the FC voltage balancing challenge in the three-level, DC-DC converters of single-phase and dual-phase with inter-phase transformer, which may form part of electric vehicle powertrain or photovoltaic renewable energy systems. To predict the steady-state voltage imbalance in practical systems, the thesis models and analyses the FC voltage influencing effects that are thought to be most significant, which are the natural balance effect of the inductor switching harmonics, the high-side impedance, the device parasitic output capacitance and on-state resistance, and the duty-ratio mismatch between devices. The analysis is based on the frequency domain and derives average-value equivalent circuits for the first time to provide visual insight on the voltage imbalance mechanisms. Due to the interaction between phases, the dual-phase converter benefits from an extra voltage balancing mechanism, making it less sensitive to the unbalancing effects by orders of magnitude than the single-phase converter. The extra balancing mechanism is enabled by using an appropriate PWM interleaving strategy and is enhanced by the inter-phase transformer, whose effects are also incorporated to the model. A generalised average-value model for the FC converter with arbitrary numbers of phases and levels is derived for the first time, which can be useful for analysing the FC voltage behaviour and designing balancing controllers. The thesis also derives an accurate small-signal linearised model for the dual-phase converter with inter-phase transformer, including the natural dynamic effect for the first time, to investigate the FC voltage dynamic characteristics. The model is compared with the standard small-signal model that neglects the natural dynamics. The FC voltages are modelled as damped, second order nonminimum-phase system in the proposed model, whereas they are modelled as integrators in the standard model leading to significant prediction errors under open- and closed-loop operations. The natural dynamic effect causes significant voltage overshoots and reduces the stability range under closed loop. The model predictions of the voltage steady-state imbalance and dynamic transients are validated with SABER simulation of the switched circuits and with experimental measurements from a 100 kW, 1200 V SiC DC-DC converter prototype over a wide range.