A Practical Method for Transmitter Side Mutual Inductance Measurement Without Receiver Modifications in Wireless Charging Systems

A mutual inductance (M) estimation technique using only transmitter side measurements is presented, for use in inductive coupled charging systems. Knowledge of M is vital before the charging is initiated to optimize wireless power transfer. This is useful for inductive wireless charging systems, including vehicle charging applications. The input impedance seen from the transmitter side is a function of M and load. This makes estimating M difficult. The proposed method employs a new load isolation technique, which helps to resolve this problem. In this method, the transmitter side is excited at a very low voltage in a resonant frequency, slightly higher than the power transfer frequency. Also, the harmonic content in input current is monitored to determine the ON/OFF status of diodes, ensuring the load is decoupled. Unlike most of the existing techniques for M measurement, there is no need for any active switches in the receiver side and/or value of the battery voltage. Thus, no electrical changes are required in the existing vehicle side circuitry. The optimally selected measurement frequency range also ensures that the transmitter side parameters are measurable using simple instrumentation arrangement. To measure at resonance, the inverter frequency is controlled such that the phase of input current is zero. Utilization of a phase-sensitive detector for this ensures high signal to noise ratio. Using an equation derived based on the proposed technique, M is estimated, requiring only the circuit parameters and the measured resonant frequency. Results from simulation and experimental verification proved the practicality of the technique. The worst-case accuracy observed during experiment is <1.2% for a range of M from 7 to $22~\mu \text {H}$ .