Efficient Power Management Circuit: Thermal Energy Harvesting to Above-IC Microbattery Energy Storage

Abstract

Autonomous devices that are self-powered over a full lifetime, by extracting their energy from the environment, are crucial for applications such as ambient intelligence, active security in smart cards or monitoring. As the energy availability and power dissipation are not constant over time, energy management becomes a key function and determines the potential for information processing. All these challenging constraints have been taken into account to develop an autonomous system enabling thermal energy harvesting and power storage in the microwatt range.The microsystem architecture, illustrated in Fig. 3.1.1, is comprised of two power sources, RF and thermoelectric, a microbattery used as a storage unit and integrated circuits to transform and manage the harvested energy and interface the microbattery. Both sources are managed by the ICs: the microbattery being charged either using thermal energy harvested by the thermogenerator associated with the DC/DC converter or using external RF power converted by the RF converter. The state of charge of the storage device is monitored periodically.Thermoelectric power generators have three main advantages: no human intervention is required throughout their lifetime, they are highly reliable and quiet since there are no moving mechanical parts and the materials used are environment friendly. Micro and nanotechnologies enable production of the small power generators required to match the decreasing dimensions of standard wireless sensor modules. The thermal micro-generator illustrated in Fig. 3.1.2 has an output power of 4µW/cm 2 per degree C, a 90Ω series resistance and generates 1V for a temperature difference of 60°C.A micropower up-converter switching power supply is used to convert the available power from the thermogenerator into a regulated power supply ( Fig. 3.1.3). The difference between one part of the boost filter output voltage (α*Vout) and a voltage setpoint (SP) is amplified, then modulated into pulse density information for control of the MOS power switch. A sub-1V bandgap voltage reference [1] is used as the voltage reference (570mV). The error amplifier is comprised of a 50dB gain OTA and a buffer. The innovative pulse density modulation (PDM) is based on an asynchronous passive ∆Σ modulator instead of the traditional PWM controller for simplicity of implementation (2 RC filters and 3 inverters), very low power consumption (1µW) and spectral spread of the switch noise. A low-voltage, high-performance charge pump, composed of one clock booster and two stages of voltage doublers [2], is used to increase the PDM signal voltage four-fold. This allows a large decrease in the equivalent Ron of the MOS switch.The RF converter is composed of a limiter, a rectifier and a control loop to provide a stabilised DC output. In standard 13.56MHz RFID applications, RF power and conversion efficiency depend on the distance between the RF source and the IC; the input RF power is much greater than the needed power and the superfluous current is diverted through ballast MOS...

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“…However, the output of energy harvester is not suitable for main power supply application because of its variations in the power and voltage overtime. Therefore, a power management circuit is required together with energy buffer such as battery for practical applications [161].…”