Multi-Frequency Control of Single-Stage Converters using High-Frequency Isolation Transformers

Abstract

Galvanic isolation using transformers is an important feature of many power converters, providing additional safety by separating the ground potential of two or more sections of an electrical system. High-frequency (HF) transformers increase the power density of converters by reducing the core size and weight. A further increase in power density can be achieved by using a single power conversion stage. Two single-stage power converter systems offering HF isolation are presented: a dc to ac converter and a multiport topology. The converters take advantage of the differential (DM) and common-mode (CM) currents that can be driven using center-tapped transformers connected to dual inverters. The DM currents are generated as HF three-phase sinusoidal currents to achieve power control through the transformers, introducing a straightforward alternative approach to the conventional dual active bridge (DAB) concept, where square-wave voltages are phase shifted to control HF transformer power transfer. The CM currents are generated as low-frequency (LF) currents for ac grid power exchange. A multi-frequency pulse-width modulation (PWM) scheme is employed to generate these current components. Independent control of HF transformer power and LF ac power can be realized using well-established control techniques, such as dq-frame current control, simplifying the implementation of the converter concept. Both converter systems are experimentally validated, showing that decoupled control of ac grid power and HF transformer power can be attained. Moreover, the converters are shown to have reduced ac output filtering requirements than two-level voltage-source converters. The multiport converter concept is among a very limited number of single-stage converters offering HF isolation that can realize ac-ac power conversion.