## 1 Introduction
This research effort can be generally classified as load impedance estimation and load impedance matching control, which requires modifying the rectifier side circuit and implementing dual side control of the DC/DC resonant converter. A simple solution is to add a post-regulation DC/DC converter after the diode rectifier to regulate the effective load impedance of the resonant circuit when the battery charging condition is changed. In [14], dynamic maximum-efficiency tracking control is proposed for S-S resonant converter wireless charging system by adding a pre-regulation DC/DC converter before the inverter on the transmitter side and a post-regulation DC/DC converter after the diode rectifier on the receiver side. On top of battery voltage/current/power regulation in inverter control, the duty cycles for pre-regulation DC/DC converter and post-regulation DC/DC converter are adjusted based on \"perturb & observer\" methods to minimize the measured efficiency. In [15], a post-regulation buck-boost converter is added after the diode rectifier bridge for the LCC resonant converter wireless charging system. The diode rectifier output DC load resistance is regulated around a reference load resistance value by duty cycle control of the buck-boost converter to improve efficiency in CV charging mode operation. The CC/CV/power regulation is realizedin inverter control using model-based power/current calculation from the estimated inverter AC load resistance. In [16], a pre-regulation buck converter is added before the inverter. A semi-active rectifier replaces the diode bridge. The battery charging current is controlled by rectifier side phase shift PWM modulation with voltage pulse synchronized with measured receiver coil current. The inverter phase shift angle is kept at maximum value. The buck-boost duty cycle is adjusted to optimize the system efficiency based on the \"perturb & observer\" method. The drawback of this extra pre/post-DC/DC converter solution is that the efficiency improvement is compromised by the power loss generated by the additional power circuit. The wireless charging system also becomes more bulky, costly, and less reliable. Recently, intensive research interests are raised in using DAB resonant converter control to improve the efficiency of the wireless charging system by simply replacing the diode bridge with an active rectifier bridge [7; 17]. The advantage of DAB resonant converter topology is that it does not increase power device component count and opens the possibility to use low loss Sic MOSFET devices. For the DAB DC/DC resonant converter, it is crucial to ensure that both the inverter and rectifier work in ZPA conditions to minimize the reactive power input and output to the resonant circuit. The resonant compensation circuit design based on inverter ZPA constraint was originally developed for unidirectional DC/DC resonant with the diode rectifier. The underline assumption for unity power factor operation of the inverter is that the resonant circuit output has a purely resistive load. However, DAB resonant converter control can operate the active rectifier bridge in either a capacitive or inductive power factor by advancing or delaying the rectifier AC voltage relative to the current. It is essential that DAB DC/DC resonant converter control takes care of pure resistance AC load assumption and ensures that the DAB rectifier bridge operates at unity power factor for the DAB inverter bridge operating at ZPA condition. This restriction on DAB resonant converter control freedom has yet to be explained and discussed in publications. In [7], the method to implement triple phase shift (TPS) control of the DAB LCC resonant converter wireless charging circuit is reported. The input/output current/voltage waveform relationship is utilized to derive the inverter phase shift angle \(\beta\)p and rectifier phase shift angle \(\beta\)s according to the load power demand.