Analysis and Design of a Wireless Power Transfer System with Dual Active Bridges
Nowadays, work on Wireless Power Transfer (WPT) systems with dual active bridges is attracting great attention due to their low conduction losses, power regulation, load transformation and reactance compensation. However, in these studies limitations such as overall analysis, design and realization techniques of the system were not considered properly. To address the aforementioned issues, this paper presents a detailed analysis, design and realization of a Series-Series (SS) WPT system with
... WPT system with dual active bridges, which will improve the overall performance. Three independent Phase Angles (PAs) have been analyzed and designed in this study, one PA on the primary side and the other two PAs on the secondary side. This Multiple Degrees of Phase Control (MDPC) method can achieve additional reactance compensation, load transformation and output regulation simultaneously. To realize the proposed method in practice, key implementation techniques have been investigated in detail, including additional reactance estimation, mutual inductance estimation, phase detection and synchronization. The feasibility and effectiveness of the proposed system is verified through simulation and experimental results. 2 of 20 occur, the additional reactance increases apparent power and causes additional losses. Thus, extra input power is required at the same load which causes a lower efficiency. Maximum power transfer efficiency is achieved when the load matches with the system [7, 11] . In a Series-Series (SS) WPT system, a small loading resistance results in most losses consumed by the receiving coil, whereas a large loading resistance leads to a small reflected resistance and most of the energy is wasted in the transmitting coil. In [9,12], a cascaded buck-boost converter is utilized for load transformation. An improved efficiency of 78% with a 16 W transferred power at a distance of 10 cm is achieved in  , whereas  realizes an efficiency of 65% when delivering 4.5 W power. In , a two-stage switching converter is applied to modulate the load, an efficiency of 60.2% is obtained for a 20-W load at 20 cm. In  , the concept of phase-shift and amplitude control is introduced to modify the equivalent secondary-side load impedance for efficiency enhancement and extractable power optimization. The amplitude control is realized by an additional buck converter on the receiver side. The measured maximum efficiency is 77.3% when the coupling factor is 0.6. In , a front-end boost circuit and a rear-end buck circuit are employed for voltage regulation and load transformation, respectively. The optimal efficiency is 79% when 100 W power is delivered at 20 cm, with 6% power loss measured in DC-DC converters. Compared with traditional systems, these topologies help to transform the load to the optimal value and contribute to improving the efficiency. However, the control of multi-stage system is complex and additional losses in the converters are inevitable. In addition, they can increase the size of the receiver. Greater focus on finding a better solution is needed. Adding an active rectifier on the receiver side has been proposed in [14, 16] . Furthermore, researchers have found that Phase Control (PC) can be employed for conduction loss reduction and bidirectional power flow regulation    . In  , two Phase Angles (PAs) are applied to regulate dual-side resonant currents equally to reduce losses. However, the de-tuning of the WPT system is not taken into consideration and parasitic resistances are omitted in the analysis. Researchers in [14, 20, 21] realized that PC can change the effective voltage applied on the resonant tank and the resulting equivalent impedance seen from the AC side. In  , the equivalent impedance is regarded as an optimization function and the mathematical expression of the equivalent impedance is not presented. In , a semi-bridgeless active rectifier is investigated and the output power can be regulated by secondary PC. In addition, the equivalent impedance of the semi-bridgeless active rectifier is derived. Both of  and  majorly focus on the proposal of the concept and its derivation, whereas the implementation techniques are not rigorously studied. Researchers in  demonstrate PC from the standpoint of load transformation and additional reactance compensation that achieves a maximum efficiency rise of 10% compared with traditional diode rectifier. An Auxiliary Measurement Coil (AMC) along with a decoupling transformer is adopted to detect the additional reactance. The decoupling transformer is used to cancel out the voltage induced by the receiver side current to the measurement circuit. Therefore, two mutual inductances, one between the receiving coil and AMC and the other of the transformer, should be identical. However, mutual inductance is strongly affected by the inductances, the position of the coils and its surroundings. It is difficult to ensure that two mutual inductances have equal values in experiments. Discrepancy of mutual inductances can bring about an estimation error in principle. In addition, the measurement of mutual inductance, which is an important parameter for optimal load calculation, is not presented. Although a real-time mutual inductance estimation method has been proposed in  , it assumes that the WPT system is tuned completely. In  , mutual inductance estimation is implemented by detecting primary PA. Nevertheless, when the system operates at secondary resonant frequency, a large estimation error appears. Further work on additional reactance and mutual inductance estimations is required. Synchronization is an important technique in the control of secondary PC. Researchers in  proposed an AMC for synchronization. However, over 20% phase error occurs at 20 kHz when the load or the mutual inductance changes. In  , a current sensor is used to capture the secondary resonant current for synchronization at 30 kHz. Since the current measurement and filtering process involve a significant time delay, a phase-shift circuit should be adopted to correct the resulting phase error.