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EDNE MARCH 2013

LHighly resonant wireless power ate in 2012, Efficient Power Conversion (EPC) made available a demonstration board, or more correctly a suite of boards, that act as a demonstrator and development tool for a wireless power transfer scheme. EPC is a maker of gallium nitride power transistors, and the wireless power application is one vehicle the company employs to make the case for using GaN devices where a combination of high frequency, voltage and power is required. This short article is not so much a tear-down – no tearing down is needed, the boards are a development tool, not a packaged product - more an overview of the elements needed to assemble a wireless power transfer system. Many existing solutions for wireless power transfer use relatively low frequencies, typically 100 to 300 kHz – effectively, transferring power purely via magnetic coupling. This board set uses the architecture devised by WiTricity, which licenses intellectual property to product developers; which operates at a much higher frequency, with highly-resonant coupled coils. WiTricity believes this approach is capable – among other advantages – of transferring power without excessive losses over much greater distances. EPC’s EPC9104 Highly Resonant Wireless Power Transfer system (Figure 1) is capable of delivering 15W DC to a load from 24 VDC input to the source. It consists of A an amplifier switching at 6.78 MHz, a source resonator B with impedance matching network, a capture resonator C with impedance matching network, and rectifier with high frequency filtering D. 6.78 MHz is an attractive operating frequency for wireless power transfer as it enables the power transfer coils to be fabricated as extremely thin printed circuits so they can be integrated into mobile devices. 6.78 MHz happens to be the lowest non licensed frequency band available designated for industrial, scientific, and medical (ISM) equipment, and is an excellent choice for many wireless power transfer applications. One challenge for such wireless technology is efficient power transfer across a significant distance between the source and capture resonators. Highly resonant, impedance matched circuits on both the source and capture devices provide the highest possible coil to coil efficiency. In order to maximise the total efficiency of a wireless power transfer system, the designer must also seek to minimise switching losses of the amplifier that drives the source resonator. At high switching frequencies such as 6.78 MHz, EPC eGaN FETs deliver the required efficiency. The EPC9104 delivers 15W across one inch of air at over 70% end-to-end efficiency. The primary configuration of the demo is open-loop so VOUT varies with VIN as shown in Figure 2. Underneath the heatsink on the amplifier, we find the “power plant” of the system shown in Figure 3. It features the EPC2014 in a half-bridge configuration (Q40 and Q41) driven by the LM5113 half bridge driver for enhancement mode GaN FETs from Texas Instruments (U20). The EPC2014 is a 40-V, 16-mΩ (maximum @ VGS = 5V), eGaN FET. Critical specifications are shown in Table 1. The key to the efficiency performance is the ultra-low RDS(ON) x QG and RDS(ON) x QGD Figures of Merit (FOMs), and low RDS(ON) x QOSS FOM, along with the low inductance of the 1.7 x 1.1 mm wafer-level package. Two challenges that face the power switches in this application are low threshold and limited VGS overhead. The LM5113 is ideally suited to driving the eGaN FETs because it has a low impedance turn off, separate turn on and turn off pins, bootstrap clamp, minimal and matched A B Figure 2 System efficiency plotted against total power delivered over the link. 26 EDN Europe | MARCH 2013 www.edn-europe.com


EDNE MARCH 2013
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