New Synergetic Control of a 20kW Isolated VIENNA Rectifier Front-End EV Battery Charger

EV chargers with output power levels in the range of tens of kW are typically employing a front-end three-phase boost-type PFC rectifier stage for sinusoidal input current and DC-link voltage control, and a series-connected isolated DC/DC converter controlling the actual output/charging current or voltage. This paper explores a new synergetic control of both converter stages, which utilizes the DC/DC converter also for varying the DC-link voltage with six times the mains frequency, such that the currents of two mains phases are shaped sinusoidally. Accordingly, two bridge legs of the rectifier stage can remain clamped in 60°-wide intervals of the mains cycle and the pulse width modulation (PWM) can be restricted to the phase carrying the lowest current, i.e., only one of the three bridge legs is operated with PWM, designated as 1/3-PWM. Furthermore, the DC-link voltage that is switched by the operating rectifier phase is kept to the minimum and the system features high efficiency and low EMI, but still maintains boost capability, i.e., the option of conventional PWM of all three rectifier bridge legs (thus denominated as 3/3-PWM), which is advantageous in case a wide input or output voltage range needs to be covered. The new control concept is derived starting from a conventional approach with constant DC-link voltage, and is verified by simulations for a three-level Vienna Rectifier front-end and two cascaded DC/DC modules supplied from the halves of the symmetrically partitioned DC-link voltage. First, the operating behavior of the system utilizing the proposed control is described analytically. Next, the performance improvement achievable with the proposed control scheme is comparatively evaluated for a 20kW system designed for operation in a wide mains voltage range (260-530V
_rms
line-to-line) and an extremely wide DC output / battery voltage range (150-750V
_dc
), according to EV charging equipment supplier requirements of the State Grid Corp. of China. Finally, simulation results are presented which validate the operating principle of the proposed modulation and control scheme.