Bidirectional Nonisolated Fast Charger Integrated in the Electric Vehicle Traction Drivetrain

Abstract

Integrated chargers, where the traction inverter is used as the primary charging interface, have emerged as a solution for reducing the cost and footprint of electric vehicle (EV) charging. This article proposes a bidirectional nonisolated integrated charger topology that also provides benefits to motor lifetime and reliability through a reduction in bearing currents and voltages. This is achieved by adding to an EV drive: an <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$LC$ </tex-math></inline-formula> filter per phase with a common-mode (CM) voltage control; a CM inductor for additional leakage current filtering; contactors to switch between traction and charging modes; and a residual current device to satisfy standards for transformerless charging. To obtain high efficiency and make the filter small for traction applications, variable frequency critical soft switching is leveraged. Traction mode experimental validation demonstrates torque and speed steps with no degradation compared to a standard drive, a decrease in leakage currents and shaft voltages of more than 90%, and an increase in motor drive efficiency of 0.6% at 5-kW output power. Charging mode validation shows active and reactive power steps and peak efficiency of 99.4% and 98.4% at a rated power of 11 kW. Charging mode leakage current is measured to be 22 mA, which satisfies standards permitting the transformerless operation.