Load-dependent power transfer efficiency for on-chip coils

This paper presents a theory for the power transfer efficiency of printed circuit board coils to integrated circuit coils, with focus on load-dependence for low-power single-chip systems. The theory is verified with electromagnetic simulations modelled on a 350 nm CMOS process which in turn are verified by measurements on manufactured integrated circuits. The power transfer efficiency is evaluated by on-chip rectification of a 151 MHz signal transmitted by a spiral coil on a printed circuit board at 10 mm of separation to an on-chip coil. Such an approach avoids the influence of off-chip parasitic elements such as bond wires, which would reduce the accuracy of the evaluation. It is found that there is a lower limit for the load below which reducing the power consumption of on-chip circuits yield no increase in voltage generated at the load. For the examined process technology, this limit appears to lie around 56 k Ω . The paper is focused on the analysis and verification of the theory behind this limit. We relate the results presented in this work to the application of wireless single-chip temperature monitoring of power semiconductors and conclude that such a system would be compatible with this limit.