**1. Introduction**

Energy harvesting (EH) is technology for harvesting power for IoT edge devices from environmental energy using energy transducers [1]. Eliminating the replacement of batteries based on EH can reduce the total cost of IoT devices. Electrostatic energy transducers (ES-ETs) can convert vibration energy into electronic power [2,3]. Due to the high output impedance of 1 MΩ or larger, open circuit voltages have to go beyond 10 V to generate power of 10 µW or larger. In [4], a battery charger is proposed using two variable capacitors based on ES-ETs. Capacitance varies with vibration, resulting in variable voltage at the capacitor node. Two capacitors vary out of phase. Thus, the diode connected with those two capacitors flows current from one to the other. The latter capacitor is connected to the battery via another diode. As a result, with sufficient amplitude in the voltage at the capacitor node, the battery can be charged with vibration energy. Another power converter is proposed in [5,6] based on a full bridge rectifier (FBR) followed by a DC-DC buck converter, as shown in Figure 1a. An HV rectifier is composed of four diodes for converting the AC power of ES-ETs into DC power to the converter. As the DC voltage is much higher than the maximum voltage acceptable to sensor CMOS ICs, power management circuits (PMC) in DC/DC converters need to be fabricated using a BCD process, which provides an HV CMOS operating even at high voltages of 10 V or higher. Buck converters require external components such as inductors, capacitors, and resistor (LCRs) to convert the DC input voltage of an order of 10 V into an output voltage of an order of 1 V. The priority in design was power conversion efficiency at a power of 1 mW rather than the cost and form factor. What if the priority should be the cost and form factor? In this work, we focus on the full integration of a converter into the same chip for a sensor/RF, as shown in Figure 1b. Section 2 discusses key design features. Power-on-reset (POR) is a critical block to starting up the operation. A built-in POR with no additional power is proposed. The circuit was

**Citation:** Ishida, Y.; Tanzawa, T. A Fully Integrated AC-DC Converter in 1 V CMOS for Electrostatic Vibration Energy Transducer with an Open Circuit Voltage of 10 V. *Electronics* **2021**, *10*, 1185. https://doi.org/ 10.3390/electronics10101185

Academic Editor: Bor-Ren Lin

Received: 11 April 2021 Accepted: 14 May 2021 Published: 15 May 2021

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fabricated in 65 nm low-Vt CMOS. Experimental results are shown in Section 3. Section 4 compares the circuit features of the proposed circuit with previously reported converters.

**Figure 1.** IoT edge device with an ES-ET and (**a**) a buck converter with FBR and PMC in a BCD process [6] or (**b**) a proposed shunt regulator integrated into an IoT chip.
