RF Energy Harvesting and Battery- Free Wireless Sensors

Transcription

1 RF Energy Harvesting and Battery- Free Wireless Sensors Pierre Mars, VP Applications Engineering, CAP-XX Charlie Greene, Head of Technology Platforms, Powercast Darnell nanopower Forum, May 2009

2 Overview About CAP-XX About Powercast Application Wireless Sensors System Overview System Performance Next Steps Summary 2

3 About CAP-XX World leader in thin, flat, small supercapacitors suitable for portable electronic devices Research-based, market-driven electronic components manufacturer. Founded in Australia in Listed on AIM in London, April 2006 Turn-key power design solutions Production in Sydney & Malaysia Significant sales to big brand customers in Europe, Asia and North America CAP-XX supercapacitor technology licensed to Murata in 2008 Distributors throughout USA, Europe and Asia 3

4 What is a supercapacitor? A supercapacitor buffers the load from the source. Source provides low average power, supercapacitor provides peak power to the load. C α A / d A, d = C Physical charge storage, Not electrochemical C: 210mF 1.8F ESR: 25m 85m, 2.75V/cell, -30 to +85 C ESR: 16m 50m, 2.25V/cell 2.25V/cell; -30 to +70 C No dielectric, working voltage determined by electrolyte 2 cells in series for 5.5V operation +ve ve Carbon coating: 2000m 2 / gm surface area Ions in Solvent Separation distance: Solid-liquid interface (nm) Separator Aluminum foil Basic Electrical Model 4

5 About Powercast Driving innovation in the transmission, reception, and conversion of RF Energy Wireless Power Systems Renewable Power Applications Wireless sensors, RTLS, Long-range RFID Device recharging Lighting Defense Powerharvester Module Privately held, Founded Product Showcase 5

6 Wireless Power Dedicated source transmits common radio waves Ambient sources also available: Mobile/cellular, TV, Radio, etc. Proprietary receiver captures the RF energy with an antenna efficiently converts the RF energy to the appropriate DC voltage Energy transfer is controllable and predictable by design 6

7 Wireless Sensors Applications Building automation Energy management Process monitoring Condition monitoring Location tracking Benefits of Wireless Power Reduced wiring Sealed devices Reduced maintenance Controllable power Difficult locations 7

8 Management Effort Issues with Primary Batteries in Wireless Sensor Networks Battery-Powered Battery Replacement Battery-Free 10s 100s 1000s Size of Sensor Network Intentional constraints to save power design, operation, application Reliability Temperature performance Battery replacement cost Limitations of scale Majority of energy is consumed in sleep mode

9 Battery-Free Concept Send power as needed - 1) continuously, 2) scheduled, or 3) on-demand Power Broadcast Capacitor Voltage V MAX V MIN GND Sensor Power Consumption Sensor Active Sensor Inactive 9

10 System Overview Simple 2 wire hardware integration for any RF module Sleeve Dipole Antenna Integrated, 915 MHz Front Back Powerharvester Module High Efficiency TI ez430-rf2500t SimpliciTI protocol Low Power CAP-XX GZ115 Small Form Factor 10

11 Powerharvester Module P MHz, Charge & Fire Features High Conversion Efficiency Internal Charge Management High Sensitivity Configurable Output Voltage 50mA Output Current Capacitor Overvoltage Protection Internally Matched to 50 ohms Low Quiescent Current (<1 A) Simple Integration Small Footprint 11

12 Output Current (ua) P2100 Charge Current P2100 Capacitor Charge Current at 1.1V Module Input Power (dbm) 12

13 Advantages of Low Supercapacitor Voltage The P2100 charges the superapacitor to only ~1.2V Supercapacitor cell voltage is limited by the voltage stability of the electrolyte, there is no dielectric. Cell voltage of supercapacitors with organic electrolytes < 2.3V to 2.7V depending on the electrolyte. A low supply voltage allows the use of a single cell supercapacitor. This avoids the need for balancing between cells No balancing circuitry No current drawn by balancing circuit Reduced rate of supercapacitor ageing at lower voltage Reduced leakage current at lower voltage 13

14 Leakage Current (microamps) Supercapacitor Leakage Current GZ115 Leakage 23 deg C, 2.3V Diffusion current: Ions migrate deeper into the pores of the carbon electrode. Leakage current behaviour means a minimum charge current is required to charge a supercapacitor. Leakage current increases exponentially with temp & decreases exponentially with voltage It takes many hours for diffusion current to decay and leakage current to settle to its equilibrium value Time (Hours) 14

15 Voltage Supercapacitors Need a Minimum Charge Current CAP-XX GZ115 Charge Rates for Low Charge Currents High diffusion current in the early phase of supercapacitor charging means a minimum charging current is required. Need at least 20 A to charge the supercapacitor. The larger the supercapacitor C, the greater the minimum current required. 10uA Cap 1 10uA Cap 2 10uA Cap 3 10uA Cap 4 15uA Cap 5 20uA Cap 6 20uA Cap 7 20uA Cap 8 30uA Cap 9 30uA Cap Time (Hours) 15

16 Capacitance (F) Supercapacitor Ageing: C Loss GW214@ 3.6V, 23C, Ambient RH y = 1.136E-01e E-05x R 2 = 9.889E-01 C Loss rate = 1.4%/1000hrs Time (hrs) 16

17 ESR (mohms) Supercapacitor Ageing: ESR Rise GW V, 23C, Ambient RH y = x R 2 = ESR rise rate of 2.7mOhms/1000hrs, or 3.4% of initial ESR/1000hrs Time (hrs) 17

18 Supercapacitor Ageing Allow for ageing when sizing the supercapacitor Ageing depends on operating temperature and voltage GZ115 initial values 0.15F, 60m. At 1.2V, 23 C: C loss after 10years 30% GZ115 C = 0.1F ESR increase after 10yrs 40% GZ115 ESR = 83m At 1.2V, 50 C: C loss after 10years 70% GZ115 C = 0.047F ESR increase after 10yrs 40% GZ115 ESR = 230m This still operates the wireless transmitter. Supercapacitor will discharge from 1.16V to 1.04V. As a rule of thumb, double ESR and reduce C by 1/3 when sizing the supercapacitor. 18

19 Sizing the supercapacitor If the supercapacitor is supplying a constant power load, such as a DC:DC converter, where supercapacitor current increases as supercapacitor voltage decreases, to maintain V x I constant, then supercapacitor ESR may become significant, and you should solve: V LOAD ESR V SUPERCAP C I LOAD P LOAD V P P I I LOAD LOAD LOAD 2 LOAD LOAD V V ( V ESR V V SUPERCAP LOAD I SUPERCAP SUPERCAP I LOAD I LOAD LOAD SUPERCAP V ESR ESR) I I 2 SUPERCAP LOAD 2 ESR P LOAD LOAD 0 4 ESR P If load current is very small, then I LOAD ESR << VSUPERCAP and can use an energy balance to size the supercapacitor. Otherwise, use a spread sheet to solve the above and simulate V & I over time, or use SPICE. 19

20 Output Voltage (V) Energy Storage Choosing the Supercap Value Energy Available 4 Voltage Window (Hysteresis) 3 2 Capacitor Value 1 C = 7.02E/e e 0.82 C = 8.57E = required load energy DC-DC conversion efficiency 0 V MIN Capacitor Voltage (V) V MAX GZ115 cap size = 0.16F (measured) Stored energy = 22.7 mj 20

21 Charge Time (sec) Energy Harvesting Performance Charge Time vs. Distance 3W EIRP Patch Antenna Transmitter 1000 (152 sec) 15 ft (103 sec) 100 (145sec) 10 1 Sleeve Dipole (G=1.5) Air Dipole (G=4.1) Yagi (G=6.1) Distance (ft) 21

22 TI ez430-rf2500t Start-up and Data V Initial start-up Data Note: V1.5 Software 22

23 Load energy = 3.7mJ Energy Storage From slide 20, C > 8.57 x > 31.7mF GZ115 = 0.15F >> 31.7mF, allows for ageing and more From the previous slide, max load current 25mA. Therefore max supercapacitor current = 25mA x 3.3/1.15/ mA. Voltage drop due to ESR (aged supercapacitor) = 120m x 90mA 11mV <<1.15V, so the energy calculation for min C is a good approximation. Solving the energy balance for min V = sqrt(vinit 2 2xE/(eC)) = 1.12V. This is verified by solving the quadratic eqn for constant power using C = 0.1F, ESR = 120m, Load power = 3.3V x 25mA/0.82, load duration = 50ms. With starting voltage = 1.16V, the supercapacitor discharges to 1.10V 23

24 System Summary Energy Calculations Stored energy = 22.7 mj Usable energy = 18.6 mj (current design) Initial start-up and data transmission = 3.7 mj Optimizing Performance Narrow the voltage window (hysteresis) of the Powerharvester module Adjust the capacitor size Improve DC-DC conversion efficiency Modify software start-up sequence 24

25 Next Steps Enable user configurable voltage hysteresis for additional flexibility Optimize software startup sequence to minimize the energy overhead Improve harvester input sensitivity to extend range Release system as a reference design 25

26 Summary Battery-free = maintenance-free Voltage window and cap sizing is important for operation cycle Low voltage charging increases life cycle performance of supercap 26

27 27