Wouldn't it be easy if you drop your phone on your desk and it automatically gets charged? Wireless power transfer will make this happen. Is it magic? Is it difficult to understand? Well, actually not. All you need, are two resonating LC-circuits trimmed at the same frequency and you can send energy from one circuit to the other. Isn't that called radio transmission? Yes it is, but now we are working at lower frequencies and we aim at transferring power over a short distance compared to radio where we want to transmit information of a longer distance.
The key in designing wireless power transfer are the coupled circuits that are trimmed to a frequency typically between 100 and 200kHz. Simulations will show that using these coupled circuits, energy can be transferred as long as both circuits are resonating at the same frequency. The primary and secondary winding consist of an air coil in series with a capacitor. The figure below shows a typical simulation model for this in Caspoc.
The numerical values for the inductance of the cores is something that can be calculated in the 3D BEM/FEM solver Amperes. The fact that we are using air-coils, really makes this application suitable for Boundary elements as no space surrounding the coils has to be discretized.
The power electronics is modeled in Caspoc and mostly consists of a full bridge on the primary side and a rectifier on the secondary side.
The efficiency is depending on the resonant frequency of the- waveform created with the primary converter. This resonant frequency changes with changing distance between primary and secondary coil.
Typically air coils are used for the primary and secondary side. Both circular winding and square windings are employed.
To find out exactly the resonant frequency of the coupled circuits, a frequency sweep i nthe transient simulation in Caspoc has to be undertaken. It will reveal mostly two resonant frequencies, from which the one with the highest efficiency is used. In the simulation below the inductor values of the two coils were first investigated and included in the simulation. The frequency sweep revealed the two resonant frequencies.