design considerations for rf energy harvesting devices
DESCRIPTION
Useful guide in designing a system that will RF signals to use as power sourceTRANSCRIPT
Design Considerations for RF Energy Harvesting Devices
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Energy Harvesting Devices
Harry OstaffeDirector, Marketing & Business Development
1
Overview
• RF energy is generally very low
– Direct-power at close range to a transmitter
– Energy accumulation for longer range
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• Simple battery-recharge possible
• System level approach needed for optimal implementation
2
Key System Elements
• Harvester
• Antenna
• Storage
• MCU
Power Receiving
AntennaCommunication
Antenna
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• MCU
• Sensors
• Radio
• Comm. Protocol
3
RF-DC
Sensors
MCU RadioEnergy
Storage
System Parameters
• Operation
– continuous vs. intermittent
• RF power source
– distance, power, frequency
• Receiving antenna
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• Receiving antenna
– size, performance (gain)
• Operating voltage
• Energy Storage
• Sensors (active, passive)
Key RF Harvesting Characteristics
• Peak Efficiency
• Efficiency Range
• Frequency Range
• Sensitivity
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• Sensitivity
• Output Voltage
• Performance Consistency
• Implementation Scalability
5
What makes an RF harvester efficient?
• RF Matching
– Harvester is non-linear
• Proper loading (DC match)
– Generally requires a specific discrete or emulated resistance
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emulated resistance
• Correct frequency
Deviation results in significantly
reduced efficiency
6
RF Matching Techniques
Rectenna (Rectifying Antenna)• No matching network, No matching loss (assuming
lossless antenna dielectric)
• Difficult to measure diode complex impedance
• Requires specialized antenna design
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• Requires specialized antenna design
Standard Impedance (Powercast)• Matched to 50Ω, Negligible matching loss
• No special RF equipment required
• Works with standard antennas
7
DC Matching Techniques
• Maximum Power Point Tracking (MPPT)– Used in many other harvesting technologies– Requires monitoring of the DC operating point– Requires a voltage converter– Uses power (some designs require battery)
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• Powercast technology– Automatically adjusts to AC and DC operating point
– No voltage converter required for harvesting– Uses no power
8
DC Matching Technique - MPPT
Example RF Harvester Efficiency
Challenge
• Narrow operating band for each load
Solution: Max. Power Point Tracking
• Used by other harvesting technologies
• Active monitoring of the operating point
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• Active monitoring of the operating point
Drawback
• Available RF energy is already low
• Active MPPT consumes power
DC Matching Technique - Powercast
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Effi
cien
cy (
%)
P1110 Powerharvester™ Receiver915 MHz, 3V Load
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©2010 Powercast Corporation 10
0
5
10
15
20
25
30
35
40
45
-10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Effi
cien
cy (
%)
Input Power (dBm)
• Wide operating range
• Automatically maximizes efficiency
• Uses no power
Simplifying RF Design
Data
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Custom Design
Drop-In Modules
Power
Battery-Free Sensor Module
Designed for Low-Power RF Harvesting
Module Components & Features
• Powercast P2110 Powerharvester™ Receiver
• MCU: Microchip PIC24 XLP
• Radio module: Microchip MRF24 (802.15.4)
Radio module
RF Powerharvester
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• Radio module: Microchip MRF24 (802.15.4)
• System power: 3.3V
• Capacitor: 50mF (as low as 3300uF)
• Discrete sensors: Temp, Humidity, Light
• Wireless protocol: MiWi™ P2P
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SensorsTemp/Humidity/Light
MicrocontrollerCapacitor
System Operation
• Accumulate energy in capacitor
• Power MCU upon reaching charge threshold
• Power and read sensors
• Measure RSSI
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• Measure RSSI
• Format data packet
• Transmit data packet (broadcast-only)
• Turn off power (go dormant)
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Components only powered when needed
Sensor Module System Voltage R
eg
ula
ted
Ou
tpu
t V
olt
ag
e
3.3V
0V
RESET
Capacitor
VoltageVMIN
RESETVMAX
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Vm
ax
Vm
in
Capacitor Voltage
Re
gu
late
d O
utp
ut
Vo
lta
ge
0V
Sensor Inactive“Zero Stand-By” Power
Power
Output
Sensor Active
GND
Vmax = 1.25V
Vmin = 1.05V
P2110 Powerharvester™ Receiver
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4
3
2
1
8
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DOUT
VOUT
NC
GND
VSET
NC
VCAP
NC
GND
RFIN
GND
DSET
INT
RESET
P2110
XXXXXX
Pin Configuration
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35
40
45
50
55
60
Effi
cien
cy (
%)
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Functional Block Diagram
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Effi
cien
cy (
%)
Input Power (dBm)
915MHz
868MHz
950MHz
Measured at 1.2V charge on capacitor
Receiving Antenna
• 915 MHz center frequency
• Directional Pattern
(122° horizontal, 68° vertical)
• Gain = 6.1 dBi
2”
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• Gain = 6.1 dBi
• FR4 material7”
Front Side
Antenna included with Powercast evaluation boards
Powercaster™ Transmitter
• 915 MHz center freq.
• DSSS modulation
6.25” width
6.75”
height
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• 4 Watts EIRP
• Directional Antenna
17
1.63” depth
height
Complete Demo System
MRF24J40MA
915 MHz
2.4 GHz
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Power
Transmitter Sensor Modules Data Receiver
nanoWatt XLP 16-bit
915 MHz
Battery-Free Sensor Module Performance
80
100
120
Tim
e b
etw
ee
n P
ack
ets
(s)
4W EIRP Power Transmitter
Patch RX Antenna G = 6.1dBi
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0
20
40
60
0 5 10 15 20 25 30 35 40 45
Tim
e b
etw
ee
n P
ack
ets
(s)
Distance (ft)
Conclusion
• Optimal performance results from system design that focuses on minimizing power
• Every component must be selected based on power consumption
Wireless protocol must also be implemented
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• Wireless protocol must also be implemented to minimize power consumption
• Reduction in power consumption and operating voltages will increase range expand applications.
20
Questions
Harry Ostaffe
+1 412-923-4774
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