Rectifiers: analysis and optimization for radiative wireless energy transfer

Wireless energy transfer can be divided in two categories: one that provides energy to relatively low power sensor devices and typically bridges distances of several meters, and a second approach based on inductive transfer, which can typically be used at distances below a meter at various power levels for applications such as electric toothbrushes and electric vehicles.

The first category makes use of antennas and electromagnetic waves, and is therefore called radiative wireless energy transfer, while the second category uses inductors to transfer a dominantly magnetic field and is referred to as inductive wireless energy transfer. Hans Pflug has explored radiative wireless energy transfer as part of his PhD research, with insights from this approach then applicable to inductive wireless energy transfer

Rectifier importance

An essential element in an energy receiver is the rectifier, which is needed to convert the alternating current, which is used for the energy transfer, into a direct current. In his research, Pflug focused on the rectifying circuit, making use of semiconductor discrete diodes in the process.

To design an optimal rectifier circuit, insight on the rectifier operation is required. The non-linear nature of a rectifying device calls for the use of a circuit simulator, with the important quantity in wireless energy transfer being the rectifier efficiency.

Major findings

Over the course of his PhD research, Pflug was able to draw a series of major conclusions on the research. First, shorting the non-linear harmonic currents generated in the rectifier diode resulted in a higher energy transfer efficiency and a constant rectifier input impedance.

Second, most designs in the literature are based on the SPICE diode model. Pflug’s research shows the shortcomings of this model for use in wireless energy transfer. He provides two alternative models that could solve these issues.

Third, an accurate diode model and insight into the rectifier input impedance does not guarantee that an optimal input impedance matching network can be designed. Pflug shows that an adaptive signal source is required to obtain an optimal load resistance and the corresponding efficiency of energy transfer for a specific input power level.

Fourth, a simple load resistance experiment shows that the rectifier input impedance is dependent on the diode type for low input power levels and on the rectifier load resistance for high input power levels. Also, the intuitive idea that a high temperature and large diode saturation current results in a high rectifier efficiency appears to be incorrect. Pflug finds that it is the other way around, which is confirmed by working out the corresponding formulas.

In addition, comparing different rectifier topologies shows that they do not differ much in terms of efficiency. Instead, they differentiate on output voltage and optimal load resistance. Finally, Pflug shows that, in the design of a rectifier, the choice of using an optimal load resistance or a fixed load resistance depends on the chosen input power range.

Title of PhD thesis: Rectifiers – Analysis and Optimization for Radiative Wireless Energy Transfer. Supervisors: Huib Visser and Martijn van Beurden.