The evolution and optimisation of wireless power transfer

The benefits of wireless charging – convenience (no plugs or special connectors) and robustness (no contacts that can break and the ability to completely seal the product for use in harsher conditions) – are magnified further in applications that depend on mobility, especially as applications become more autonomous.

WPT overview A typical WPT product comprises a transmitting (TX) base unit that transmits the power and a mobile receiving (RX) unit that receives this power wirelessly and uses it to charge a battery. 

The TX base unit is the static piece of the WPT system; it takes power from the AC input and generates a magnetic or electric field that transfers power wirelessly. The TX unit connects to the utility grid and comprises a front-end AC/DC power factor correction (PFC) converter that converts the utility AC voltage to a known DC voltage. Following the AC/DC converter, you can optionally employ a DC/DC stage to help maintain resonance across the voltage ranges that the receiving load may demand. A DC-AC converter then converts the DC voltage to a high-frequency AC waveform. 

This high-frequency inverter is typically anywhere from 80-500 kHz, depending on the power level and end application. For example, the Qi standard, widely used in personal electronics, calls for a 110-205 kHz switching frequency, whereas in some high-power applications such as electric vehicles (EVs), it’s common to use a lower frequency, like 50-80kHz. 

With the advent of wide-bandgap devices such as silicon carbide (SiC) and gallium nitride (GaN), designers are starting to use higher switching frequencies for wireless power systems. The high-frequency inverter (DC-AC) connects to a TX coil and a compensation network that acts as an antenna. The TX coil is responsible for generating the desired electric and/or magnetic fields that couple the energy to the receiving side through a wireless medium. 

The RX coil and compensation network then receive the magnetics and/or electric fields transmitted over the wireless medium, which ultimately results in rectified voltages/currents. Most designs use a passive diode-based rectifier. However, given the trend toward higher efficiency, synchronous rectification scheme are increasingly being considered in the design and used. The rectification of the coupled voltage generates a DC voltage that is used by the battery system of the RX unit to charge the battery

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