A 6.78-MHz Wireless Power Transfer System with Voltage-/Current-Mode 0X/1X Regulating Rectifier and Global-Loop Power Control

Wireless power transfer (WPT) has great potential to be adopted in various applications, such as implantable medical devices (IMDs) and radio frequency identification (RFID) tags. However, the transmission distance and received power level vary due to coil misalignment and environmental conditions. Figure 1 compares the conventional WPT system with the proposed design. [1] employs a full-bridge rectifier with local-loop regulation to eliminate the regulation stage in the receiver (Rx). The global-loop power control (GLPC) is also combined to improve the power conversion efficiency (PCE), but the Rx voltage conversion efficiency (VCE) is less than one, limiting the transmission distance and input power range. Resonant-type receivers [2]–[6] resonate the LC tank for several cycles to accumulate the energy, which improves the VCE and minimum input power level of the Rx. [2] presents the resonant voltage mode operation to isolate the Rx coil from the output for resonating, but an extra off-chip inductor is needed. [3] demonstrates a resonant current mode (RCM) Rx at 13.56 MHz. However, VCE improvement is limited, and the output voltage is not regulated. [4], [5] present the voltage-current mode (VCM) Rx that combines the RCM and voltage mode (VM) operations to extend the input-voltage range while regulating the output, but the operating frequency is only 1 MHz. Without GLPC, transmitters deliver too much power than the required output under a light load condition, which limits efficiency.