# An Inverter Topology for Wireless Power Transfer System with Multiple Transmitter Coils
## 1 Introduction  
Safety, convenience, reliability, weatherproof, and maintenance-free are the key advantages of wireless power transfer (WPT) compared with wired or conductive power transfer [1]. Nowadays, the WPT technique has been successfully adopted in many applications, such as biomedical implants [2], electronic devices [3], kitchen appliances [4], electric vehicles [5], etc. A two-coil system, i.e., a single transmitter coil and a single receiver coil, is commonly found in conventional WPT systems. Both coils usually have the same shape and dimension, which simplifies the design and implementation [6; 7]. The desired or ideal operation of the WPT system is that the receiver coil is always placed at the aligned position, where both the transmitter and receiver coils share the same center. The magnetic coupling coefficient is generally maximized at this position. Unfortunately, the misalignment between coupled coils is inevitable in practical operation. This leads to the magnetic coupling variation, which causes the output voltage or power to fluctuate. Moreover, the power transfer efficiency is drastically deteriorated with the reduction of the magnetic coupling coefficient [8; 9; 10]. Although the control of the duty cycle [11; 12] or switching frequency [13; 14] of the converter's switches has been reported to address these problems, the misalignment tolerance is still limited to a low value. To improve the sensitivity to magnetic coupling variation, developments related to the coupled coil [15; 16; 17], optimized design of the magnetic core [18], and new compensation topology [19; 20] are introduced. However, the transfer efficiency becomes problematic, because the magnetic coupling coefficient is reduced as the distance between coupled coils increases. Recently, WPT multi-coil systems have become increasingly popular, especially in applications with free-positioning features. The key advantages of the multi-coil system include improvements in the working area and system efficiency compared with conventional two-coil systems [21; 22; 23]. However, excitation circuits for multiple transmitter coils remain an issue in the multi-coil system. A straightforward approach to a multi-coil configuration is to adopt multiple sources, where each transmitter coil is energized by an inverter. This increases component counts, cost, and size to the system. To achieve an optimal operation, there is a need for current regulation on each transmitter coil [24; 25; 26]. This often yields complicated control methods. The transmitter coils to be energized must be carefully selected, as the position of the receiver coil may be varied during operation. Energizing transmitter coils with weak coupling to the receiver coil would result in little to no contribution to the power transfer, and the system efficiency is sacrificed. A proper transmitter coil, where magnetic coupling is maximized, should be determined before the chosen transmitter coil pattern is energized. Therefore, the detection of the coil's position and misalignment between coupled coils are indispensable. Pressure sensors [27], sensing coils [28], and tunneling magnetoresistive (TMR) sensors [29] have been adopted in past work. A tradeoff is that additional components and control are required, not to mention extra signal conditioner circuits to accommodate the sensors. To reduce the component counts and complexity of the conventional excitation circuit for multiple transmitter coils, this paper presents a topology of the inverter circuit with receiver coil position detection. Only single-source and (n+2) power switches for n transmitter coils are required. The proposed inverter circuit can supply the transmitter coils individually or collectively, depending on the determined coil pattern. Each transmitter coil current flows in the same direction to produce a magnetic field with the same polarity. The DC output voltage is regulated through the primary-side control by adjusting the angle \(\alpha\) of the inverter voltage.