Small payment accessories (e.g., watches, bracelets, rings) are becoming the future of "contactless" payment. Designing these devices as passive (batteryless) in terms of power supply is faced with challenges concerning miniaturization and compliance to standards. To evaluate performance, we introduce a simulation framework that can predict a design's minimum operating magnetic field strength (H-min) with an accuracy under 0.1 A/m. The framework combines S-parameter models of the device's antenna and the ISO-standardized setup (the ISO test PCD assembly) with a data-based nonlinear model of the device's IC. Techniques for optimizing the energy transfer are discussed (tuning and power matching) and backed by analytical and practical examples. We also demonstrate how to use the simulation framework to determine the impact of the device's structure on energy transfer. Two designs of small payment accessories are ultimately compared, both as models in the framework and as fabricated samples. By applying power matching instead of tuning, the second design's size can be reduced by approximately half, without significant change in H-min. As the predicted Hmin values match the measurements, the results show that multiple design parameters can be varied within the framework to determine their effect on H-min, which is of great assistance for finding the optimal design.
