A Rapidly Reconfigurable DC Battery for Increasing Flexibility and Efficiency of Electric Vehicle Drive Trains

Abstract

Traditional electric vehicle (EV) battery drive trains comprise hard-wired batteries forming a fixed high-voltage DC link, and a main inverter. This paper proposes a novel topology to break hard-wired batteries into smaller subunits interfaced by low-voltage field effect transistors (FET). The new DC link, named <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">reconfigurable DC battery</i> , can offload a great portion of the switching duty from the main inverter. Several benefits follow: <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1)</i> The proposed topology reduces the total loss despite the added components. The net loss is reduced because the reconfigurable DC battery can generate an optimal high-frequency voltage waveform to spare up to 2/3 of the switching actions of the main inverter—i.e., delegating the modulation duty from the main inverter to the reconfigurable DC battery. The added semiconductor loss is negligible compared to that of the main inverter, thanks to small voltage steps of multilevel conversion and the latest low-voltage transistors. <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2)</i> The main inverter’s output waveform has less distortion, e.g., down to 50% compared to conventional space-vector modulation. This distortion reduction is a consequence of large segments of the overall output voltage (particularly the apex) being entirely formed by the reconfigurable DC battery with multilevel precision. <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3)</i> The multilevel output qualities substantially reduce the voltage transients d <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">v</i> /d <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">t</i> of the inverter output, which is known to reduce insulation and bearing stress in motors. <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4)</i> The topology eliminates the vulnerability of large hard-wired battery packs to the weakest cells. <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5)</i> The constant presence of the high voltage in conventional hard-wired batteries and the associated issues, e.g., during manufacturing, maintenance, and crashs are avoided because of normally-off FETs. We demonstrate the proposed motor drive on a 3-kW setup with eight battery modules.