Energy-Harvesting for Micro-Power Portable Applications

1. Energy-Harvesting for Micro-Power Portable Applications Erick O. Torres, Student Member, IEEE, and Gabriel A. Rincón-Mora, Senior Member, IEEE Georgia Tech Analog and Power IC Design Lab E-mail: ertorres@ece.gatech.edu, rincon-mora@ieee.org Website: http://www.rincon-mora.com

2. Applications: Low-Duty Cycle Operation Ad-Hoc Transceiver Micro-Sensors Bio-Implantable Devices e.g. Neural Implants Structure-Embedded Sensors Remote Metering Desired Specifications: Self-Powered and Self-Sustaining Long Operational Life Stable DC Voltage (power source) Capable satisfying peak load demands Motivation

3. Problem Space constraints Limited stored energy available Short operational lifetime Solution Harvest energy from the environment to replenish consumption Long-lasting renewable energy source Challenge

4. Proposed System

5. Sources of Energy in the Environment Vibration-based: moderate power levels, on-chip integration

6. Vibrations Electromagnetic Piezoelectric Electrostatic • Pros: • Higher power and voltage levels • Cons: • Rectification • Power conditioning • Piezoelectric materials difficult to align properly • Pros: • Simple concept (Faraday’s Law) • Cons: • Small voltage levels • Rectification and boosting • Bulky magnet and transformer • Pros: • Moderate power levels • Compatible with IC/MEMS technology • Cons: • Synchronization and stability issues Electrostatic: better on-chip integration, moderate power levels

7. Electrostatic Energy Harvesting Charge-Constrained: Assume Cmin = 1 pF, Cmax = 100 pF pre-charged to 4 V Q = 400 pC, VMax = 400 V !!!!, Enet = 79.2 nJ

8. Electrostatic Energy Harvesting Voltage-Constrained: Assume Cmin = 1 pF, Cmax = 100 pF pre-charged to 4 V Q = 360 pC, EHarvest = 1.584 nJ, Enet = 792 pJ

9. Proposed Harvesting Scheme Voltage-Constrained Harvester: Step 1: Pre-Charge Step 2: Harvest Step 3: Recover Net Energy Gain:

10. Harvester Schematic Non-Idealities: • Resistive Losses • Switches • ESRs • Parasitic Capacitances • Charge Leakage • Synchronization • Voltage Mismatch

11. Simulation Results IHarvest = 40.0 µA EInvested + ELosses = 1 nJ EHarvested = 1.58 nJ Net Energy Harvested ~ 580 pJ/cycle Assuming 15 µs period vibrations: PHarvest = 38.7 µW

12. Conclusions – Next Steps • Build Prototype: • Design and fabricate MEMS variable capacitor • Confirm simulation results • Other Designs Issues: • Synchronization with vibrations: • Self-adaptive scheme vs. Non-resonant • Accurate voltage matching • Low-power control circuitry

13. Questions? Thank you.