*4.1. Comparison with Previously Reported Converters*

Circuit features are compared in Table 3. In [6], the AC-DC converter was composed of FBR and a buck DC-DC converter. To allow for a high voltage input of 60 V at the peak, a BCD process was used, which provided 60 V transistors. Discrete diodes for FBR were required in addition to a converter chip. A high power efficiency of 85% was realized by the buck converter. In [10], another AC-DC converter was presented, which was composed of capacitor divider, switched capacitor converter, and FBR to convert power from 120 V mains. Wiring capacitors were used to manage a high voltage of 168 V without adding extra process steps or devices. As capacitance density was quite low, the converter size needed to be as large as 9.8 mm<sup>2</sup> . In [11], another AC-DC converter was proposed to generate a standard CMOS-compatible voltage of 2 V from the magnetostrictive energy harvester (MS-EH) with a peak open circuit voltage of 0.5 V. Due to the on-chip oscillator running at 5 MHz to drive a charge pump circuit (CP), the control circuit consumed power of 18 µW. In [12], a DC-DC charge pump was developed for piezo-electric energy harvesting. The voltage conversion ratio was varied for energy efficient power conversion according to VA. Area per maximum output power was realized with 3.1 [mm2/mW]. On the other hand, a shunt regulator was used instead of the buck converter in this work at the cost of a reduction in PCE. However, when the transducer can generate sufficient power for the IoT chip even with the AC-DC converter with 43% PCE, it can be integrated into the same IoT chip without additional discrete components and the buck converter chip. Area per maximum output power was realized with 8.1 [mm2/mW] in measurement and 0.81 [mm2/mW] in simulation under the conditions of R<sup>S</sup> = 100 kΩ, R<sup>L</sup> = 10 kΩ, and V<sup>A</sup> = 30 V.

μ

Ω Ω


(\*1) The data were taken from the condition of VOUT = 2 V and V<sup>A</sup> = 3 V. (\*2) The data were simulated under the conditions of R<sup>S</sup> = 100 kΩ, R<sup>L</sup> = 10 kΩ, and V<sup>A</sup> = 30 V. Ω Ω
