Digital Control of a Wireless Power Transfer System for Electrical Vehicles

Abstract

Wireless power transfer (WPT) is becoming a popular choice for charging of batteries in various applications. Particularly, the increase in popularity of electrical vehicles (EVs) and the pursuit of user convenience for charging the batteries makes WPT an attractive solution. The usage of WPT in e-transportation is not straightforward because the current standardization limits the allowed frequency operating range and magnitude of the irradiated magnetic field. Additionally, a fast control loop with high current or voltage sensing bandwidth is necessary to guarantee soft-switching of the inverter’s semiconductors which are commonly employed as part of an equivalent DC-DC resonant converter. The resonant circuit typically comprises of a H-bridge inverter, two passive compensation network one at each side of the air-gapped transformer formed by a transmitter and a receiver coil, and a diode-bridge rectifier. The control of the resonant converter can be cumbersome particularly in case of dynamic charging condition because of the natural change in equivalent load and coil misalignment conditions which cause themagnetic coupling to change. The main goals of this thesis are: to understand the challenges involved with the control of WPT for EVs; to propose and implement a digital control strategy for WPTs which not only maintain soft-switching but also realize other functionalities such as soft start-up, soft shut-down and over-current protection. With respect to power efficiency, it is important to operate the inverter with a frequency whichmakes the resonant converter to function in the inductive region enabling soft-switching turn-on of the implemented active semiconductors, such as MOSFETs. For that reason the inverter is typically operated at a frequency slightly above the resonance, however this must avoid the effect of bifurcation. Studies in the literature have found that the efficiency of the system is typically the highest at maximal magnetic coupling between the coils and when the resonant circuit impedance match the so-called optimal load resistance. Furthermore, the current available standards have set narrow requirements on the frequency range, minimum power efficiency and restriction of irradiated magnetic field for both, safety of living beings and electromagnetic compatibility. All these requirements makes the control implementation of WPT very challenging, especially for high power applications. In this work, the closed-loop control is successfully integrated into a TI microcontroller where the soft-switching mechanism is derived with a selective band zero crossing detection circuit of the inverter AC current. The current band is used to tune the instant where the semiconductor bridge turns from ON to OFF state because there is a minimal current that need to be impressed into the inverter to reach the desired soft-switching by compensating the accumulated charges across the parasitic MOSFET capacitances. This is also important to compensate the signal processing delays of the closed-loop control. This functional block is implemented externally to the TI digital controller with the existing high bandwidth current sensing and commercial analog logic ICs. Moreover, an over-current protection is incorporated into the zero crossing block, achieving a reaction time of 66ns. Finally, several strategies are proposed and tested for the soft start-up and a concept is made for the soft shut-down of aWPT experimental setup available in the laboratory.