A 13.56MHz Wireless Power Transfer System with Adaptive Mode-Switching Insensitive to Coupling Variations

Abstract

Reconfigurable Wireless Power Transfer (WPT) systems utilizing a Series-Parallel (SP) matching network are being actively researched to achieve high efficiency for Implantable Medical Devices (IMDs). Among these, hysteretic control methods that regulate the Transmitter (TX) mode based on Receiver (RX) mode transitions within a hysteresis window are gaining prominence. These methods offer instant transient response times against load variations and ensure stable output voltage ripples defined by the preset window. However, conventional hysteretic WPT systems face several limitations: 1) the necessity for reference adjustment of the current sensor to accommodate coupling coefficient (k) variations, 2) energy loss during mode transitions between the TX and RX, and 3) significant power dissipation in the TX low-power mode. These issues necessitate unnecessary trimming during operation and lead to a degradation in overall End-to-End (E2E) efficiency. To overcome these limitations, this paper proposes a WPT system featuring adaptive mode switching that is insensitive to variations. The proposed system enhances k-insensitive sensing capabilities and E2E efficiency through three key techniques: 1) a fixed-reference sensor topology utilizing filtered integrated sensor outputs, 2) an adaptive mode switching scheme synchronizing RX 0X-to-1X transitions with TX 0X-to-1X mode transitions, and 3) optimized switch sizing for the low-power mode to minimize power dissipation. The proposed system was designed and fabricated using the TSMC 0.18 μm CMOS process. Operating at 13.56 MHz with a TX input of 5 V and an RX output of 3.3 V, the system supports an output power range of 5–147 mW. Simulation results demonstrate a peak E2E efficiency of 64.9% and efficiency improvement of 44.8% verify stable operation within a fixed-reference k range of 0.1 to 0.4.