Power Control of a Capacitively-isolated Bidirectional DC-DC Converter with Auxiliary Converters

This paper verifies an improved control method of a capacitively isolated bidirectional DC-DC converter with auxiliary converters, designed for electric railway systems. The converter consists of four unidirectional power devices, two capacitors for galvanic isolation, two inductors, and two auxiliary converters. Each auxiliary converter comprises three full-bridge cells connected in cascade. To downsize the isolated capacitors, as well as the filter capacitors and inductors on the primary and secondary sides, the switching frequency of the main converter should be set higher than 500 Hz, as used in [1]. However, it leads increased losses in the auxiliary converters because the sinusoidal inductor currents used for archiving power balance between the auxiliary converters. To address the above issue, this paper proposes an inner power control method based on triangular inductor currents, allowing the switching frequency of the main converter to be set higher. The operating principles of this inner power control method are validated experimentally using a 1.2-kW, 120-V down-scaled model, in which the switching frequency of the main converter can be set at 4 kHz.