Energy Harvesting Reference Design Technical Overview

Transcription

1 Energy Harvesting Reference Design Technical Overview

2 Agenda Energy Harvesting reference design overview Energy harvesting concepts Energy Harvesting reference design operating example Using the Energy Harvesting reference design Summary Appendix: devices used on the reference design Appendix: battery considerations 2

3 Energy Harvesting Solution

4 Why Do We Want to Harvest Energy? No need to use conventional batteries Batteries are expensive Inconvenient to replace Unreliable under certain operating conditions Energy harvesting is environmentally friendly Natural energy resources are inexhaustible Conventional energy generation produces greenhouse gases We have reached a tipping point Energy harvesters have become reasonably cheap and efficient Ultra low power Wireless MCUs can now operate at low enough power levels to implement a wireless sensor node using harvested energy 4

5 Emerging Energy Harvesting Applications Wireless sensor nodes Home and building automation Industrial control systems Infrastructure sensing systems Security systems Agricultural monitoring systems Asset tracking 5

6 Energy Harvesting Reference Design Implemented as a Silicon Labs ToolStick TM daughter card Enables code development for custom applications Single chip Silicon Labs Si1012 wireless MCU Integrated low power MCU plus wireless transceiver 65 na MCU + RF transceiver sleep mode Integrated low power wireless transceiver +13 dbm output power Thin film battery Deeply embeddable battery technology 0.7 mah battery capacity 30 ma discharge current capability Battery charging solution Low quiescent current extends battery life Automatic switching for shunt mode to protect battery and start charge cycle Energy harvesting using solar energy 42 µa at 200 lux light output 6

7 Energy Harvesting Reference Design Features Wireless Sensor Board Wireless USB Adapter Debug connector front Si1012 wireless MCU Sanyo Solar cell Printed antenna back IPS Thin film battery C8051F342 USB MCU running HID firmware Si4431 radio running EZMacPRO LTC4071 Battery power management 7

8 Solar Power Energy Harvesting Terms Energy harvesting ability to create electrical energy from physical sources such as light, vibration, wind or other sources Illumination Amount of light energy that hits the surface of the solar panel in lux Indoor light levels range from 50 to 1000 lux Direct sunlight is in the range of 100,000 lux C rate charge and discharge rate of a battery 100 mahr battery can charge/discharge 100 ma in 1 hour (1C) 100 mahr battery can charge/discharge 50 ma in 2 hours (C/2) Energy capacity of a battery (typical capacity) Stored energetic content of the battery (Joules) typically specified as ma-hr (i.e. 620 ma-hr for Lithium ion coin cell) Activity profile time representation of system tasks and the current requirements for each mode of operation Sleep mode is the lowest power mode used when the system is required to do no work Active mode is the highest current mode used to carry out system functions when awake 8

9 The Photovoltaic Effect Solar cells implement the photovoltaic effect Silicon releases electrons when light energy is introduced When a load is present, a current is generated Current can be used to charge an energy storage device such as a battery or supercapacitor Load P P P P P P P P P Solar Cell P P P N Channel P Channel P -Photons Electrodes 9 The Photovoltaic Effect

10 Battery Operation and Charging Thin film battery parameters Operating voltages: fully charged is 4.1 V, nominal output is 3.9 V Discharge voltage of 2.1 V (discharging below rated voltage could cause damage) Very low leakage provides a long shelf life 50 Ω internal resistance Battery charging Low internal resistance enables low current charging Can effectively charge with as little as 1 µa Current shunt method effectively terminates the charge cycle Charger provides low quiescent current to minimize battery discharge I Operating Solar Panel + - LTC4071 Battery Charger I Shunt Switch I Charge + Battery Capacitor Si10xx Wireless MCU 10 Battery Disconnect Sample Circuit

11 Operating Modes and Power Consumption LP sleep mode of the wireless MCU consumes 65 na and uses the pin port match to wake up (2 µs) The wireless MCU in RTC sleep and the RF transciever in standby is 350 na na, respectively Voltage regulator and associated circuitry consumes 3 µa of leakage current No Sleep Button Press? RTC on Yes Wake-up every 1 s Transmit light level Send temp & charge level every 60 s In Sleep mode, ENERGY-HARVEST-RD consumes ~ 3 ua (leakage from battery management chips) In RTC mode, ENERGY-HARVEST-RD consumes ~ 3.8 ua (MCU RTC adds 800 na to existing leakage) In Active mode, ENERGY-HARVEST-RD consumes ~ 51 ua (3 ua of this is battery management leakage) Active mode turns on the MCU and the RF transceiver 29 ma for transmit current at +13 dbm 19 ma receive current at -121 dbm sensitivity Yes RTC on for 3 minutes? No EH Flow Diagram 11

12 Average Current Average current is a good metric for determining the power efficiency of an embedded system There are two contributors to the average current: Active mode current Inactive mode current The total average current is the sum of both contributors Total Average Current = Active Current Active Time + Inactive Current Inactive Time Total Time 12

13 Calculating EH Battery Life (1 of 2) Active mode of the Energy Harvesting reference design Device remains in sleep mode until a button is pressed (V REG and battery charger I Q ) Device transmits measurements once a second for 3 minutes after button press and receives an acknowledgement for each transmit Example assumes device is transmitting and receiving periodically for given time period 29 ma 19 ma Active Mode Tx Active Mode Rx 800 na associated with wireless MCU, the rest is due to supporting circuits 3 µa RTC Sleep RTC Sleep like the regulator 1mS 1mS 998 ms 1mS 1mS 998 ms 1mS 1mS Active Mode Tx Active Mode Rx Active Mode Tx Simplified EH Activity Profile While Transmitting Active Mode Rx MEC101 Thin Film Lithium Battery, 0.7mA-Hr rated capacity Mode Duration (s) Current (I) Battery Current RTC Sleep 998E-3 3.0E E-6 Active Mode Tx 1.0E E-3 29E-6 Active Mode Rx 1.0E E-3 19E-6 Calculate average current : ((.003mA.998s) + (29mA.001s) + (19mA.001s)) AvgI = 1.0s =.051mA 13

14 Calculating EH Battery Life (2 of 2) Let s assume in an example system data is sent every 20 minutes and we will use the RTC sleep mode instead of the LP sleep mode RTC has higher current than LP sleep mode so this is worst case 29 ma 19 ma 3 µa 1mS 1mS 998 ms 1mS 1mS 998 ms 1mS 1mS 51 µa 3 µa LP Sleep Active Mode LP Sleep Active Mode 17 minutes 3 Minutes 17 Minutes Overall EH Activity Profile While Transmitting 3 Minutes MEC101 Thin Film Lithium Battery, 0.7mA-Hr rated capacity Mode # of Cycles per Hour Duration (s) Current (I) RTC Sleep s.0038 ma Active Mode s ma Calculate average current : 3 ((.0038mA 1020s) + ( s)) AvgI = 3600s =.0109mA Battery capacity is 0.70mA - hr 0.70mA hr BatteryLife =.0109mA = 64hrs hour battery life on a 0.7 ma-hr battery!!!

15 Putting it All Together (1 of 4) The solar panel can provide 42 µa with a 200 lux light source Case 1: the battery and capacitor are not fully charged Current from the solar cell is directed to the capacitor Capacitor is used to supplement the current during the transmit cycle due to the internal resistance of the battery System is off V BAT 2.1 V V BAT Shut-Off Threshold TIME I Solar Solar Panel LTC4071 Battery Charger Battery I Charge Capacitor Si10xx Wireless MCU 15 Sample Circuit

16 Putting it All Together (2 of 4) The solar panel can provide 42 µa with a 200 lux light source Case 2: the battery is not fully charged, but the capacitor has reached the threshold to enable charging the battery Current from the solar cell is directed to the battery Once threshold is reached the solar cell can supply current to the system 4.1 V 2.1V V BAT Shut-off Threshold TIME I Solar I System Solar Panel LTC4071 Battery Charger + I Charge - + Battery Capacitor Si10xx Wireless MCU Sample Circuit 16

17 Putting it All Together (3 of 4) The solar panel can provide 42 µa with a 200 lux light source Case 3: the battery is fully charged and the battery charging circuit shunts excessive current to ground to protect the battery Current from the solar cell is directed to the system and excess is to ground 4.1 V V BAT 2.1V Shut-off Threshold TIME I Solar I System Solar Panel LTC4071 Battery Charger I Shunt Battery Capacitor Si10xx Wireless MCU Sample Circuit 17

18 Putting it All Together (4 of 4) Case 4: no light available and the battery supplies the current to the system Current is drawn from the battery and consumed by the system High current pulses are sourced by the capacitor charge When light is present again the battery charger begins to recharge the battery 4.1 V 2.1V V BAT Shut-off Threshold TIME I System Solar Panel LTC4071 Battery Charger + I Battery - + Battery Capacitor Si10xx Wireless MCU Sample Circuit 18

19 Simplified Current Profile The solar panel can provide 42 µa with a 200 lux light source Using linear region of solar cell 1000 lux provides 210 µa The radio system requires 11 µa to operate At 200 lux I CHARGE during system operation is 31 µa A fully discharged battery takes 23 hours to charge while operating ChargeTime = 22.5hrs 0.7mA hr =.031mA A fully discharged battery takes 17 hours to charge while in ship mode (I SYSTEM = 0 ma) ChargeTime = 16.6hrs = 0.7mA hr ma I Solar I System Solar Panel LTC4071 Battery Charger I Charge Battery Capacitor Si10xx Wireless MCU 19 Sample Circuit

20 Daily Profile (1 of 2) What is a Power Budget? A quick first order calculation that gives you a ballpark figure of the total average current supported by your power source Required information for making a power budget: How long must the system operate without replacing batteries? How much capacity can I expect from my battery? Power budget calculation (assuming 12 hour life): Battery Capacity [ma - H] Maximum Average Current [ma] = Re quired Battery Life[H] = 0.7mA hr 12hrs = 0.058mA Our system consumes 0.011mA 20

21 Daily Profile (2 of 2) Assume there are 12 hours of daylight and 12 hours of darkness Solar panel can supply 42 µa with a 200 lux light source With the example activity profile the system consumes 16 µa 28% of the battery capacity is discharged during dark hours Discharge = = 19% 0.011mA 12hr 0.7ma - hr 100% With 26 µa available the time to charge the battery is: Charge = = 4.3hrs 0.011mA 12hr 0.031mA The battery will never fully discharge under these situations. The energy harvesting solution will provide a perpetual source of data to the system. 21

22 Sample Charging Times of the EH RD To fully charge an empty battery with no system load Electric Light Lux Office Window 1000 Lux Direct Sunlight 100K Lux USB Charging Limited to 3 ma 24 hours 6 hours 2 hrs 30 minutes 22

23 Alternative Harvested Energy Sources The reference design can be powered by different sources of harvested energy Bottom side pads provide connection points for RF, vibration, thermal inputs 23

24 Energy Harvesting Firmware

25 Energy Harvesting RD Based on EZMacPRO EZMacPRO TM covers the PHY, MAC and network layer Physical layer (PHY) The physical layer defines the relationship between a device and a physical medium Provides the means of sending and receiving data on a carrier Media Access Control (MAC) layer Detect and avoid packet collisions Addressing mechanism Auto acknowledgment Application... EZMacPRO Network Media Access Control Physical Network layer Responsible for end-to-end (source to destination) packet delivery Packet forwarding 25

26 The Energy Harvesting Firmware Application layer handles measuring, packaging and delivering energy harvesting parameters to the EZMacPRO stack Light level Temperature Battery voltage The application layer carries out the specific function of the device The network layer protocol can perform such functions as packet forwarding and packet filtering etc. The application functions call the MAC layer that controls the interface to the RF IC and the upper application layer The MAC firmware calls the low level read and write routines that access the registers and the FIFOs of the RF IC SPI The RF handles the physical layer to the RF network 26

27 EZMacPRO Packet Format Preamble used to synchronize the PLLs for the transmission Sync data that starts the packet reception Header set of bytes that define the lower layer control and addressing Control (CTRL) used for auto-acknowledgement and packet forwarding Customer ID (CID) avoids unexpected interactions between systems installed in close proximity Sender ID (SID) identify transmitting node address Destination ID (DID) identifies the receiving node address Payload length (PL) number of bytes in the payload (maximum 64 bytes) CRC recognize if there are any bad data bits in the packet Preamble Syn c CTRL CID SID DID PL Data CRC Header Payload 27

28 Packet Forwarding Forwards a packet that is not intended for the received node Method to increase the range of data transmission Settings found in the header control byte The Radius field specifies how many times the packet can be forwarded Decremented when the packet is forwarded Packets can be identified by the sender ID and the sequence number (SEQ) Avoids retransmitting the same already-forwarded packet with lower radius Packet forwarding done automatically Packet forwarding callback function is called to enable modifying the data payload Control byte (CTRL) Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0 SEQ[7:4] ACK ACKRQ RAD[1:0] 28

29 Auto Acknowledgement Provides feedback on whether the transmitted packet arrived at the destination Transmit side Sets the ACKRQ bit in the control register Waits for acknowledgement packet for a predefined time Receive side Receives packet that has matching DID and the ACKRQ bit set Generates an acknowledgment packet and sends it to the transmitting node Handled automatically by EZMac PRO Control byte (CTRL) Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0 SEQ[7:4] ACK ACKRQ RAD[1:0] Preamble Syn c ACK Set CID DID SID PL 0x00 CRC 29

30 Using the Energy Harvesting Reference Design

31 The Energy Harvesting Reference Design Wireless sensor node Wireless USB adapter Wireless sensor network GUI Solar powered Measures temperature, light level and charge level Single chip Si1012 wireless MCU controller Transmits data to wireless USB adapter Interfaces wireless sensor node to PC MCU runs HID USB software as well as EZMacPRO wireless network stack Optimized RF & USB implementation Displays data from sensor node No special drivers required 31 Part number: ENERGY-HARVEST-RD

32 Getting Started with WDS Start WDS 32

33 The Application Manager Window Connect the USB dongle to the PC The Application Manager window should show up Available applications for the selected hardware are displayed Update the firmware on the dongle by clicking Firmware Upgrade 1 33

34 Update the Dongle Firmware Select the desired 2 firmware image 1 Select Continue to access the firmware download Select Download to start 3 the boot loading process 34

35 Rerun Firmware Upgrade You may see this message pop up. If If so, repeat the firmware upgrade steps Success! 35

36 Running the Demo 1 Select Harvesting Demo 2 Select Select Application to start the demo 36

37 The Application Wizard (1 of 3) The Application Wizard provides a check of the hardware components prior to launching the application 1 Select Start Wizard to begin the hardware verification 37

38 The Application Wizard (2 of 2) The firmware of the connected device is is verified 1 Select Continue to launch the application The firmware and hardware of the connected device is is verified 38 Select Launch Application to start the energy harvesting network demo application 2

39 The Energy Harvesting Network Demo The demo starts with no nodes associated to the dongle and all of the images are grayed out Three tabs are used to display network diagrams, packet information and specific node data Make sure the power switch is in the correct position and press the button on the energy harvesting board to start the network attach and data transfer 39

40 Communicating with the Board When the button is pressed the node attaches to the network Data begins transmitting to the dongle for display The Logic tab shows the connected devices on the network The node attached to the network and the address image becomes active. The sun represents the lux value (high lux has the image of the sun. Low lux has the image of the moon). Received Signal Strength Indicator (RSSI) values displayed graphically 40

41 The Packet Tab The packet tab provides info on the packet format of the data transfers Energy harvesting demo is based on the EZMacPRO TM firmware Packet list shows all of the packets transfers and the data associated with them Packet Extraction selects a single packet and breaks down the fields within the packet I.e. CTRL is the control byte that defines sequence number, ACK request and radius Packet filtering addresses Packet length Packet payload Data transmitted in the payload of the packet that is specific to the demo Light levels Battery level Temperature readings 41

42 The Node Info Tab The node info tab provides info on the data payload specific to the demo application Data transmitted in the payload of the packet that is specific to the demo Light levels Battery level Temperature readings 42

43 Other Silicon Labs Low Power Solutions

44 The C8051F99x MCU Low power MCU Low MCU active current (150 μa / MHz) Low MCU sleep current with brownout detector disabled (10 na) Low MCU sleep current with brownout detector enabled (50 na) Low MCU sleep current using internal RTC operating (300 na) Fastest analog settling time from wake-up (1.5 us) Integrated smartclock oscillator Human interface functionality Integrated capacitive sensing module 16-bit capacitance to digital converter with no external components required Fast capacitive measurements that can wake the CPU from low power mode Autonomous auto-scanning Capable of internally connecting multiple channels for a single measurement Additional features 12-bit ADC with autonomous burst mode and auto-averaging accumulator Integrated low drop out (LDO) voltage regulator to maintain ultra-low active current at all voltages Tiny 3x3 mm package 44

45 Broad Portfolio of Ultra Low Power MCUs 64 kb F93x DC-DC, 10b ADC F93x DC-DC, 10b ADC Si100x RF, DC-DC, ADC 32 kb F92x DC-DC, 10b ADC F92x DC-DC, 10b ADC Si100x RF, DC-DC, ADC 16 kb F91x DC-DC, 12b ADC Si101x RF, DC-DC, ADC F90x DC-DC, 12b ADC F90x DC-DC, 12b ADC Si101x RF, DC-DC, ADC 8kB F99x 13 ch CDC, 12b ADC F99x 13 ch CDC, 12b ADC F99x 13 ch CDC, 12b ADC 4kB F98x 12b ADC F98x 12b ADC F98x 12b ADC F98x 12b ADC F98x 12b ADC F98x 12b ADC NEW! 2 kb F98x 20p QFN 3 mm x3 mm 24p QFN 4 mm x 4 mm 24p QSOP 32p QFN / TQFP 42p QFN 5 mm x 7 mm Core Features Cap touch DC-DC Wireless 45

46

47 Appendix

48 Components Used on the Energy Harvesting Reference Design

49 Introducing the LTC4071 Shunt Battery Charger System with Low Battery Disconnect

50 50 LTC4071: What is It? Shunt Battery Charger + Integrated Pack Protection Shunt Battery Charger System With Low Battery Disconnect Low Operating Current: 550nA 1% Float Voltage Accuracy Over Full Temperature and Shunt Current Range 50mA Maximum Internal Shunt Current Pin Selectable Float Voltage Options: 4.0V, 4.1V. 4.2V 2006 Linear Technology

51 51 LTC4071: Key Technical Features Integrated Pack Protection in One IC 2006 Linear Technology Overvoltage (zener) and Undervoltage (Low Battery Disconnect) Low Operating Current: 550nA Pin Selectable Low Battery Disconnect Level: 2.7V or 3.2V 1% Float Voltage Accuracy Over Full Temperature and Shunt Current Range 50mA Maximum Internal Shunt Current Pin Selectable Float Voltage Options: 4.0V, 4.1V. 4.2V Ultralow Power NTC Float Voltage Conditioning for Li- Ion/Polymer Protection Suitable for Very Low Power (Intermittent or Continuous) Charging Sources High Battery Status Output Thermally Enhanced, Low Profile 8-Lead DFN (2mm x 3mm x 0.75mm) and MSOP Packages

52 52 LTC4071: Features and Benefits 2006 Linear Technology

53 53 LTC4071: Features and Benefits - NTC (cont.) 2006 Linear Technology

54 54 LTC4071: Features and Benefits NanoPower (cont.) When the input supply is removed and the battery voltage is below the high battery output threshold, the LTC4071 consumes just 550nA from the battery. This enables the device to draw or harvest power from previously unusable low current, intermittent or continuous power sources Linear Technology

55 55 LTC4071: Typical Application Circuits Simple Single Cell Li Charger 2006 Linear Technology

56 56 LTC4071: Typical Application Circuits (cont.) 2-Cell Stacked Battery Charger Photovoltaic (Solar) Charger 2006 Linear Technology

57 57 LTC4071: Positioning Linear Tech Linear Tech Linear Tech Part # LTC4071 LTC4070 LTC4065L Topology Shunt-based Charger Shunt-based Charger Linear Charger - current-limited LDO Battery Chemistry Li-Ion/Polymer Li-Ion/Polymer Li-Ion/Polymer Shunt/Float Voltage 4.0V, 4.1V, 4.2V 4.0V, 4.1V, 4.2V 4.2V 50mA (10uA min) Charge Current Maximum Shunt Current 50mA (10uA min) (500mA w/ ext PFET) 250mA max Quiescent Current 550nA (TYP) 1.2uA (max) 450nA (TYP) 1.04uA (max) 120uA (TYP) 250uA (max) Over-voltage Protection (zener) Yes Yes No Under-voltage Protection (Low Battery Disconnect) Yes, internal & programmable Yes, fixed, ext PFET + R + 2 inverters No Load Disconnect Yes, load & batt ext PFET (load only) No Battery Stack Compatible Yes Yes No Status Indicators HBO LBO, HBO /CHRG Thermal Battery Qualifier/NTC Yes Yes No (thermal reg) Package 2mmx3mm DFN-8, MSOP-8 2mmx3mm DFN-8, MSOP-8 2mmx2mm DFN-6 1k Price $2.20 $2.06 $ Linear Technology

58 58 LTC4070/1: End Markets/Applications Low Power Li-Ion/Polymer Battery Back-Up Energy Scavenging/Harvesting Solar Power Systems with Back-Up Memory Back-Up Embedded Automotive Thin Film Batteries 2006 Linear Technology

59 59 Thin Film Battery

60 Battery Considerations

61 Battery Specifications (1 of 2) Nominal voltage Voltage of the battery cell as measured across the positive and negative terminals (i.e. 3.6V for Lithium ion) Energy capacity (typical capacity) Stored energetic content of the battery (Joules) typically specified as mahr (i.e. 620 ma-hr for Lithium ion coin cell) Energy density The amount of useful energy stored in a given system or region of space per unit volume (size to energy ratio) 61

62 Battery Specifications (2 of 2) Self-Discharge Internal chemical reactions reduce the stored charge of the battery without any connection between the electrodes Dynamic Considerations Variations in temperature, output impedance, duty cycle and energy delivery will affect battery loading conditions Internal resistance (IR) Opposition to the flow of current within the battery Composed of resistivity of actual materials (metal caps, internal components) Composed of ionic resistance (temperature, electrode surface area and conductivity) Large component of output voltage swing under load 62

63 The Supercapacitor Electrochemical double-layer capacitors (EDLCs) Highly porous activated carbon materials generate internal structures with large surface areas Fast charge times High charge capacity Higher self discharge than batteries Super capacitors are used in conjunction with batteries, they do not replace them Can supply the high current pulse charge necessary for some applications Reducing the current needed from the battery reduces the battery voltage drop due to internal resistance Example: high current flash and auto-focus in digital cameras, high current transmitters 63

64 Battery Use Serial vs. parallel battery configurations Serial increases the battery voltage while the discharge rate remains the same Paralleling batteries keep the nominal cell voltage the same while increasing system operation lifetime Pulsed operation vs. continuous current drain Positive ions are consumed by the cathode electrolyte interface and are replenished via diffusion from the anode If the diffusion rate is lower than the ion consumption the terminal voltage decreases Batteries can recover if allowed to reduce the charge transfer System level pulsed operation can increase battery life by providing the idle period necessary 64