Transformer-less bidirectional PWM converter integrating cell voltage equalizer using voltage multiplier for series connected energy storage cells

Abstract

Traditional energy storage systems consisting of lithium-ion battery cells or electric double-layer capacitors (EDLCs) connected in series require a cell voltage equalizer in addition to a bidirectional PWM converter. The system is prone to complexity as two converters (i.e., the equalizer and bidirectional converter) are separately necessary. To simplify the systems, this paper proposes the transformer-less bidirectional PWM converter integrating a cell voltage equalizer using a voltage multiplier. An inductor in a conventional bidirectional PWM converter is replaced with series-connected inductors in the proposed converter in order to produce square wave voltage with an arbitrary amplitude, by which the voltage multiplier is driven. The voltage multiplier in the proposed integrated converter is essentially a switchless circuit, and therefore, the bidirectional converter and voltage equalizer can be integrated with no additional switch, reducing both the system and circuit complexities. The charge-discharge cycling test for nice EDLCs connected in series was performed using the proposed integrated converter from a voltage-imbalanced condition. The voltage imbalance was gradually eliminated while charge-discharge cycling, demonstrating the integrated functions.