TECH SERIES
High Efficiency Wireless Power Transfer Amplifier Topologies Two high efficiency amplifier topologies will be analyzed, namely the ZVS Class-D and the single-ended Class E. The schematics and ideal operating waveforms for each of the amplifier topologies is shown in figure 2. The ZVS Class-D topology makes use of a non-resonant ZVS tank circuit to allow the switch-node to self-commutate between switching transitions, which effectively eliminates the output capacitance (COSS) associated losses of the devices, from a Class-D implementation. The single device Class-E topology makes use of a resonant circuit Le and Csh, whose resonant frequency differs from the operating frequency, to establish the conditions necessary for ZVS. In this design, the output capacitance (COSS) is effectively connected in parallel with Csh and thus becomes part of the resonant circuit needed to establish ZVS. In some cases, the design of the Class E will be limited to the value of COSS as the value of the external capacitor Csh reduces to zero. Device Comparison A Wireless Power Transfer figure of merit (FOMWPT), as defined in [5], was used to compare the eGaN FETs with best-in-class MOSFETs and is shown in figure 3. Superior devices will have lower values of FOMWPT. It is clear from the FOMWPT that eGaN FETs inherently exhibit potentially superior performance in both the amplifier topologies.
Experimental Verification Four amplifiers were constructed in this example, two of each type, where one was fitted with an eGaN FET and the other a MOSFET. Each of the amplifier designs was carefully adjusted based on the small differences in COSS of each device, which ensured that operating conditions were the same in each case. A special load was also constructed for testing that could be discretely configured for various real and imaginary load settings. This load was calibrated using a VNA and the discrete load range settings were varied from +30j 立 through -30j 立 and 1 立 through 56 立. Some of measurements are shown in figure 1 for real impedance variations at various imaginary settings. The measured values were then used to accurately determine the power delivered to the load during power testing.
Figure 3. Wireless Power Figure of Merit Comparison for the Class E and ZVS Class D topologies between eGaN FET and MOSFETs.
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