Tag Archives: A4WP

Using eGaN FETs in Wireless Power Transfer Systems

Highly resonant wireless power transfer systems such as the A4WP use loosely coupled coils operating at the standard 6.68 MHz or 13.56 MHz unlicensed industrial, scientific and medical ISM bands. The popularity of such wireless energy transfer is increasing over the last few years specifically for applications targeting charging of portable devices. Usually, such solutions for wireless energy transfer for portable devices demand features such as lightweight, high efficiency, low profile and robustness to varying operating conditions.

Such features call for efficient designs capable of operating without bulky heat sinks and able to handle a wide range of load variations and couplings. Only a few amplifier topologies can meet such extreme demands and these are the current mode class-D, the voltage mode class-D and class-E. Of these, class-E is the most popular choice for several types of wireless energy solutions, chosen for its ability to operate with very high conversion efficiency.

As compared to regular MOSFETs, eGAN FETs have demonstrated superior performance when using voltage mode class-D topologies in a wireless energy transfer application In fact, eGAN FETs showed higher peak efficiencies of more than four percentage points. At output power levels beyond 12 W, regular MOSFETs required the addition of a heat sink to provide the necessary cooling for the switching devices and their gate drivers.

Moreover, in the traditional class-D topology, the resonant coils needed to be operated above resonance for them to appear inductive to the amplifier. Operating the coils above resonance reduced the coil transfer efficiency resulting in high losses in matching the inductor because of its reactive energy.

Working in class-E topology, eGAN FETs were able to deliver as much as 25.6 W of power to the load while operating at 13.56 MHz. Transferring wireless energy with high load resistance of about 350 ohms made sure the system had a high Q resonance. Measuring the system efficiency gave a figure higher than 73%, which included gate power consumption.

In a single-ended class-E circuit, the series capacitance resonates with the reactive component of the load yielding only the real portion of the coil circuit to the amplifier. The design of the matching network works for a specific load impedance and establishes the necessary conditions of zero voltage and current switching.

In tests comparing the performance of MOSFETS and eGAN FETs, temperatures were kept well below 50C, when operating in an ambient temperature of 25C. No forced-air cooling or heat sinks were used during the tests, which used the same gate driver for driving both the eGAN FET and the MOSFET.

Measurements show the eGAN FET requires significantly lower gate charge for the same operating conditions and this is an important consideration for low power converters. Gate power forms a significant portion of the total power processed by the amplifier. Additionally, as the eGAN FET has a 33% higher voltage rating compared to a MOSFET, it can be operated at higher voltages for higher output power.
Therefore, the simple and efficient class-E topology, coupled with eGAN FETs, is well suited for wireless transfer converters.