Design Considerations for RF
Energy Harvesting Devices
Harry Ostaffe
Director, Marketing & Business Development
www.powercastco.com
©2010 Powercast Corporation
1
Overview
• RF energy is generally very low
– Direct-power at close range to a transmitter
– Energy accumulation for longer range
• Simple battery-recharge possible
• System level approach needed for optimal
implementation
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©2010 Powercast Corporation
2
Key System Elements
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•
•
•
•
•
•
Harvester
Antenna
Power Receiving
Antenna
Storage
MCU
RF-DC
Sensors
Radio
Comm. Protocol
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©2010 Powercast Corporation
Communication
Antenna
Energy
Storage
MCU
Radio
Sensors
3
System Parameters
• Operation
– continuous vs. intermittent
• RF power source
– distance, power, frequency
• Receiving antenna
– size, performance (gain)
• Operating voltage
• Energy Storage
• Sensors (active, passive)
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©2010 Powercast Corporation
4
Key RF Harvesting Characteristics
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•
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Peak Efficiency
Efficiency Range
Frequency Range
Sensitivity
Output Voltage
Performance Consistency
Implementation Scalability
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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
• Correct frequency
Deviation results in significantly
reduced efficiency
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©2010 Powercast Corporation
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
Standard Impedance (Powercast)
• Matched to 50Ω, Negligible matching loss
• No special RF equipment required
• Works with standard antennas
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©2010 Powercast Corporation
7
DC Matching Techniques
• Maximum Power Point Tracking (MPPT)
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–
–
–
Used in many other harvesting technologies
Requires monitoring of the DC operating point
Requires a voltage converter
Uses power (some designs require battery)
• Powercast technology
– Automatically adjusts to AC and DC operating
point
– No voltage converter required for harvesting
– Uses no power
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©2010 Powercast Corporation
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
Drawback
• Available RF energy is already low
• Active MPPT consumes power
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©2010 Powercast Corporation
9
DC Matching Technique - Powercast
P1110 Powerharvester™ Receiver
915 MHz, 3V Load
75
70
65
60
55
Efficiency (%)
50
45
40
35
30
• Wide operating range
• Automatically maximizes efficiency
• Uses no power
25
20
15
10
5
0
-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
Input Power (dBm)
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©2010 Powercast Corporation
10
Simplifying RF Design
Data
Custom Design
Drop-In Modules
Power
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©2010 Powercast Corporation
11
Battery-Free Sensor Module
Designed for Low-Power RF Harvesting
RF Powerharvester
Module Components & Features
•
•
•
•
•
•
•
Radio module
Powercast P2110 Powerharvester™ Receiver
MCU: Microchip PIC24 XLP
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
Microcontroller
Capacitor
Sensors
Temp/Humidity/Light
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©2010 Powercast Corporation
12
System Operation
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•
•
•
•
•
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Accumulate energy in capacitor
Power MCU upon reaching charge threshold
Power and read sensors
Measure RSSI
Format data packet
Transmit data packet (broadcast-only)
Turn off power (go dormant)
Components only powered when needed
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©2010 Powercast Corporation
13
Sensor Module System Voltage
3.3V
VMAX
Capacitor
Voltage
RESET
VMIN
Vmax
0V
Vmin
Regulated Output Voltage
RESET
Capacitor Voltage
Vmax = 1.25V
Vmin = 1.05V
GND
Power
Output
Sensor Inactive
“Zero Stand-By” Power
Sensor Active
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©2010 Powercast Corporation
14
P2110 Powerharvester™ Receiver
Pin Configuration
NC
1
14
NC
60
GND
2
13
DOUT
55
RFIN
3
12
VOUT
50
GND
4
11
GND
45
DSET
5
INT
6
7
XXXXXX
Functional Block Diagram
VSET
9
NC
8
VCAP
40
Efficiency (%)
RESET
P2110
10
35
30
25
20
15
915MHz
10
868MHz
950MHz
5
0
-12 -11 -10 -9
-8
-7
-6
-5
-4
-3
-2
-1
0
1
2
3
4
5
6
7
8
9
10 11
Input Power (dBm)
Measured at 1.2V charge on capacitor
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©2010 Powercast Corporation
15
Receiving Antenna
2”
• 915 MHz center frequency
• Directional Pattern
(122° horizontal, 68° vertical)
• Gain = 6.1 dBi
• FR4 material
7”
Antenna included with Powercast evaluation boards
Front
Side
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©2010 Powercast Corporation
16
Powercaster™ Transmitter
6.25” width
• 915 MHz center freq.
6.75”
height
• DSSS modulation
• 4 Watts EIRP
1.63” depth
• Directional Antenna
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©2010 Powercast Corporation
17
Complete Demo System
2.4 GHz
MRF24J40MA
915 MHz
Power
Transmitter
nanoWatt XLP 16-bit
Sensor Modules
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©2010 Powercast Corporation
Data Receiver
18
Battery-Free Sensor Module Performance
4W EIRP Power Transmitter
Patch RX Antenna G = 6.1dBi
120
Time between Packets (s)
100
80
60
40
20
0
0
5
10
15
20
25
30
35
40
45
Distance (ft)
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©2010 Powercast Corporation
19
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
to minimize power consumption
• Reduction in power consumption and
operating voltages will increase range
expand applications.
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©2010 Powercast Corporation
20
Questions
Harry Ostaffe
+1 412-923-4774
harry.ostaffe@powercastco.com
www.powercastco.com
©2010 Powercast Corporation
21