In this paper, state-of-the-art power electronics and energy management solutions utilized in low-power (less than 5 mW), low-voltage (less than 3 V) energy harvesting powered wireless sensors for Internet of things related applications are detailed. All aspects of an energy harvesting powered sensor system are examined, including the challenges of low-power energy harvesting sources, energy management circuits including power converters and energy storage elements, as well as the impact of wireless sensor pulsed power profiles. In particular, this paper focuses on existing voltage step-up energy management techniques, including the issues of cold-start and maximum power point tracking, as well as energy storage which is necessary for wireless sensor operation. Both academic and commercially available energy harvesting powered systems are examined to provide a comprehensive analysis of existing solutions. Issues that are limiting the current system performance are identified to help define future developments needed to enable efficient and effective energy harvesting powered wireless sensor operation.

Review of Power Conversion and Energy Management for Low-Power, Low-Voltage Energy Harvesting Powered Wireless Sensors

A fully-integrated 18.5 kHz RC time-constant-based oscillator is designed in 65 nm CMOS for sleep-mode timers in wireless sensors. A comparator offset cancellation scheme achieves $4\times $ to $25\times $ temperature stability improvement, leading to an accuracy of ±0.18% to ±0.55% over −40 to 90 °C. Sub-threshold operation and low-swing oscillations result in ultra-low power consumption of 130 nW. The architecture also provides timing noise suppression, leading to $10\times $ reduction in long-term Allan deviation. It is measured to have a stability of 20 ppm or better for measurement intervals over 0.5 s. The oscillator also has a fast startup-time, with the period settling in 4 cycles.

An RC Oscillator With Comparator Offset Cancellation

This paper proposes a fully-integrated high-conversion-ratio dual-output voltage boost converter (VBC) with maximum power point tracking (MPPT) circuits for low-voltage energy harvesting. The VBC consists of two voltage generators that generate $V_{\mathrm { OUT1}}$ and $V_{\mathrm { OUT2}}$ . $V_{\mathrm { OUT1}}$ and $V_{\mathrm { OUT2}}$ are three and nine times higher than the harvester’s output $V_{\mathrm { IN}}$ , respectively. $V_{\mathrm { OUT1}}$ is used as a supply voltage for on-chip application circuits while $V_{\mathrm { OUT2}}$ is used as the charging voltage for a Li-ion secondary battery. The VBC achieves a high voltage conversion ratio (max. $\times \,\,9$ ) and a high power conversion efficiency. The MPPT circuits control the operating frequencies of the CPs to extract maximum power at each output. The measurement results demonstrated that the circuit converted a 0.59 V input to a 1.41 V output with 75.8% efficiency when the output powers of $V_{\mathrm { OUT1}}$ and $V_{\mathrm { OUT2}}$ were 396 and $0~\mu \text {W}$ , respectively, and a 0.62 V input to a 4.54 V output with 49.1% efficiency when the output powers were 0 and $114~\mu \text {W}$ , respectively.

Fully-Integrated High-Conversion-Ratio Dual-Output Voltage Boost Converter With MPPT for Low-Voltage Energy Harvesting

We present a low-power and low-energy level shifter (LS) circuit that can convert extremely low-voltage input into high-voltage output. The proposed LS consists of a pre-amplifier (pre-AMP) and an output latch. The pre-AMP employs a logic error correction circuit, which generates an operating current for the pre-AMP only when the logic levels of the input and output do not correspond. The pre-AMP generates complementary amplified signals, and the latch converts them into full-swing outputs. Measurement results demonstrated that the proposed LS fabricated in 0.18- $\mu \text{m}$ CMOS technology was able to convert an extremely low-voltage input of 80 mV into a high-voltage output of 1.8 V. The energy of the proposed LS was 0.35 pJ, when the low supply voltage, high supply voltage, and input pulse frequency were 0.4 V, 1.8 V, and 10 kHz, respectively. The static power dissipation without input was 0.12 nW.

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An 80-mV-to-1.8-V Conversion-Range Low-Energy Level Shifter for Extremely Low-Voltage VLSIs

This paper proposes an ultra-low power fully on-chip CMOS relaxation oscillator (ROSC) for a real-time clock application. The circuit has a distinctive feature in compensation architecture of a comparator's non-idealities caused by offset voltage and delay time. The ROSC can generate a stable and higher oscillation clock frequency without increasing power by using a low reference voltage and employing a novel compensation architecture for comparators. Measurement results in a 0.18-μm CMOS process demonstrated that the circuit can generate a stable clock frequency of 32.55 kHz. The power dissipation was 472 nW. Measured line regulation and temperature coefficient were 1.1%/V and 120ppm/°C, respectively.

A 32.55-kHz, 472-nW, 120ppm/°C, fully on-chip, variation tolerant CMOS relaxation oscillator for a real-time clock application

This paper proposes a fully integrated voltage boost converter with a maximum power point tracking (MPPT) circuit for ultra-low power energy harvesting. The converter is based on a conventional charge pump circuit and can deliver a wide range of load current by using nMOS and pMOS driver circuits for highly efficient switching operation. The MPPT circuit we propose dissipates nano-watt power to extract maximum power regardless of the harvester's power generation conditions and load current. The measurement results demonstrated that the circuit converted a 0.49-V input to a 1.46-V output with 73.6% power conversion efficiency when the output power was 348 µW. The circuit can operate at an extremely low input of 0.21 V. I. INTRODUCTION

A 0.21-V minimum input, 73.6% maximum efficiency, fully integrated voltage boost converter with MPPT for low-voltage energy harvesters

A compact and low-power current-mode RC oscillator (RCO) with process, voltage, and temperature (PVT) stability has been developed. The circuit employs a current-mode RCO architecture without using a conventional comparator based voltage-mode architecture. The current-mode architecture enables a compact RCO and faster switching speed to be achieved. By designing transistor sizes equally between one in a bias circuit and another in a voltage to current converter, the effect of process variation can be minimized. A prototype chip in a 0.18-µm CMOS process demonstrated that the RCO generates a stable clock frequency of 32.7 kHz with a small area of 0.19 mm2 and low-power dissipation of 54.2 nW at 0.85-V power supply, which achieves a figure of merit (FoM) of 1.66 nW/kHz. The measured temperature coefficient and line regulation were 99.5ppm/°C and 8.9ppm/mV, respectively.

A 1.66-nW/kHz, 32.7-kHz, 99.5ppm/°C fully integrated current-mode RC oscillator for real-time clock applications with PVT stability

A fully on-chip CMOS relaxation oscillator (ROSC) with a PVT variation compensation circuit is proposed in this paper. The circuit is based on a conventional ROSC and has a distinctive feature in the compensation circuit that compensates for comparator's non-idealities caused by offset voltage and delay time. We also developed a bias circuit consisting of positive and negative temperature coefficient resistors to obtain the temperature compensated clock frequency. Measurement results demonstrated that the circuit can generate a stable clock frequency of 6.66 kHz. The power dissipation was 940 nW. The measured line regulation and temperature coefficient were 0.98%/V and 56ppm/°C, respectively.

Keishi Tsubaki

Papers

A 32.55-kHz, 472-nW, 120ppm/°C, fully on-chip, variation tolerant CMOS relaxation oscillator for a real-time clock application

A 0.21-V minimum input, 73.6% maximum efficiency, fully integrated voltage boost converter with MPPT for low-voltage energy harvesters

A 1.66-nW/kHz, 32.7-kHz, 99.5ppm/°C fully integrated current-mode RC oscillator for real-time clock applications with PVT stability

A 6.66-kHz, 940-nW, 56ppm/°C, fully on-chip PVT variation tolerant CMOS relaxation oscillator

A Highly Efficient Switched-Capacitor Voltage Boost Converter with Nano-Watt MPPT Controller for Low-Voltage Energy Harvesting