This document summarizes a master's thesis presentation on the design of power management for autonomous wireless monitoring systems. It discusses motivation for the work, the state-of-the-art in power management techniques, measurements of RF energy harvesters to characterize performance and derive specifications, design of the power management circuit based on the specifications, and recommendations and conclusions. The work aims to design an efficient power management system that can operate from low power harvested by RF rectennas.

1. Design of Power Management for
Autonomous Wireless Monitoring
Systems
Master Thesis Presentation
By
Mayur Sarode
Supervisors
TU/e : P.G.M Baltus, Dusan Milosevic
imec/Holst center :Valer Pop

2. RF ENERGY HARVESTING
RF-DC DC-DC Energy
PM
converter converter
circuit
Storage WATS
Device
RECTENNA
Horn antenna DC-DC converter e.g. EOG tracking
Microstrip patch based
antenna , Ni-MH battery Eye system
Diode based voltage
doubler
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3. PROJECT OUTLINE
MOTIVATION
STATE-OF-THE-ART-PM*
HARVESTER
MEASUREMENTS
PM CIRCUIT DESIGN
PM CIRCUIT
MEASUREMENTS
RECOMMENDATIONS &
CONCLUSIONS
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4. MOTIVATION
Architectural level: Multi supply  one-voltage domain system
9% 22%
<1% 11% Radio
<1% MCU
PM 63μW PM 33μW
31% 39%
(704 μW) ADC (151μW)
Sensor&R-out 1%
78% PM 4%
Vdd[V] Component Vdd [V]
3 Radio 1.2
2.3 MCU 1.2
2.7 ADC 1.2
3 Sensor & R-out 1.2
2.9 Battery 1.5
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5. STATE-OF-THE-ART OF PM
Inductive vs Capacitive Converter topology
PWM/PFM control strategy
Size
Efficiency
Quiescent current
Start-up voltage
Converter specs Io(max),Vdc(max), fs and Vbatt
Open-loop resistor –emulation optimum control strategy
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6. STATE-OF-THE-ART PM ; IMEC/HOLST CENTER
AC-DC buck-converter Inductive boost- converter Integrated capacitive DC-DC
buck-boost converter
Specifications
Specifications Specifications ▸ Input voltage 1~5VDC
▸ Integrated AC-DC rectifier ▸ Adaptive MPP Control ▸ Output 10~300 μW
▸ Input voltage 4~42VRMS ▸ Input voltage 1~2VDC ▸ Active Efficiency 80~87%
▸ Output 10μW~5mW @ 3VDC ▸ Output 10μW~5mW & up to100% in direct charge
▸ DC-DC Efficiency 87 - 94% ▸ End to end efficiency 60~70%
Technology
Technology Technology ▸ Indoor Photo Voltaic
▸ Vibrational Harvesting ▸ Indoor Photo Voltaic
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7. HARVESTER MEASUREMENTS; CHARACTERIZATION
Harvester characterization  Power management specifications
Parameter Value Unit
Load No load, 10, 100, 10K, 100K Ω
Rectenna
Transmitted power 0 ,14 ,20 dBm
Distance 1, 10, 20, 30, 50 cm
Height 10 cm
Orientation of Broadside /Vertical
Rectenna
Configuration Line of Sight, 45o ------
Rectifier
Pinc -15 to 10 dBm
 Find optimum load resistance
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8. HARVESTER MEASUREMENTS; RESULTS
Parameter Value (EIRP: 100 mW) Unit
Distance , R 1 10 20 30 50 cm
Voltage , Vdc 1.2 0.6 0.3 0.2 0.12 mV
Power, Pdc 1886 292.6 82 44 15 μW
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9. HARVESTER MEASUREMENTS; LOSSES
Impedance matching losses ZL [Ω] Г
ZS [Ω] Pinc [dBm]
2
Z L Z s*  35+40j Ω
-15
0
2.5-55j Ω
35-40 j Ω
0.78
0
Z L Zs
ZL - Load Impedance
ZS – Source Impedance
 Rectenna efficiency  varies with available power
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10. HARVESTER MEASUREMENTS; CONCLUSIONS
Rectenna A Rectenna B
Input power to the converter < 500 μW
Maximum input voltage to converter Vdc(max) ~ 0.4 V
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11. HARVESTER MEASUREMENTS; CONCLUSIONS
Rectenna A Rectenna B
Optimum load resistance varies with input power MPPT
Approximated to a constant resistance (Rdc) for resistor emulation
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12. HARVESTER MEASUREMENTS;DERIVED SPECS.
Parameter Value/ Functionality Unit
Harvester
Distance 0.2 – 0.6 m
Rectenna Broadside in LOS* ---
EIRP (max) 4 W, 50% Duty Cycled W
Power management Circuit
Input voltage Vdc 0.1 – 0.5 V
Output voltage Vbatt Dependent on the battery (~1) V
Input impedance Rdc 220 (reconfigurable) Ω
Input power Pdc 1 - 500 μW
Choice of a lower Rdc rectenna for resistor emulation
 Choice of optimum PM circuit components *LOS- Line of Sight
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13. RF MEASUREMENTS; RECTENNA MODELING
 Friis model Rectifier measurements
Parameter Value Unit
PT 0.004 – 1.24 W
GT 3.2 ---
GR 3.1 ---
λ 0.1244 m
R 0.16 – 0.60 m
PT(14dBm) ~ EIRP(80.64 mW)
Based on Spline interpolation
GT – Gain of the transmitter antenna
GR – Gain of the receiver antenna
Used for predicting autonomy λ – wavelength
R - Distance from the transmitter
EIRP - Effective Isotropic Radiated Power
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14. PM CIRCUIT DESIGN; DEFINING VARIABLES
Harvester Terminology
Variable Details
Pin( Vin) Incident power(voltage)on the rectenna
Pdc(Vdc) Input power(Input voltage) to the
converter
Pout Harvested Power
ηconverter (Pout/Pdc)
ηharvester (Pout/Pin)
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15. PM CIRCUIT DESIGN; SPECIFICATIONS
Specifications Comments Unit
Input impedance (Rdc) 220 (rectenna B) Ω
Switching frequency, fs ----- kHz
Input voltage, Vdc 0.1 - 1 V
Output voltage, Vbatt 1 – 1.3 V
Output current , Io(max) 1 mA
Under Lock-out voltage ----- V
Over lock-out voltage 1.3 V
Input Ripple voltage 20% Vdc V
output Ripple voltage ,ΔVo 1 mV
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16. PM CIRCUIT DESIGN; POWER STAGE
ton D
M1
Vdc
Rdc
RECTENNA
Ds RL
Rin
Cin Cout
Ls
Vin Vbatt
Buck-Boost converter topology
D
Relating input/output voltage Vbatt Vdc
2 Ls
RLTs
On-time is calculated by 2 Ls
ton
f s Rdc
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17. PM CIRCUIT DESIGN; SELECTION OF M1
MOSFET power loss modeling
MODELED MEASURED
Choice dependant on Ron , tr , tf and Cgs of the MOSFET
Verified with measurement results
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18. PM CIRCUIT DESIGN; SELECTION OF Ds
Schottky Diode forward voltage drop (Vf )
& Continuous Reverse Current( Is)
Power losses at Vin 0.3 V (model)
Verified with SPICE simulations
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19. PM CIRCUIT DESIGN; SELECTION OF Ls
Conduction losses  DCR
Trade off between inductor and diode conduction time
80
COnverter Efficiency [%]
70
60
50 900uW
180uW
100uW
40
1000
1500
1800
2200
220
68
 Sweeping Ls for Rdc of 220Ω
Inductance [μH]
 Ls between 1 – 1.5mH is optimal
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20. PM CIRCUIT DESIGN; SELECTION OF Cin
Cin  Reduce ripple voltage
BEFORE AFTER
Input capacitance was selected to be 10 μF(ESR 5 mΩ)
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21. PM CIRCUIT DESIGN; SELECTION OF COUT
Cout  Charge battery when M1 is ON
 Reduce output voltage ripple
BEFORE AFTER
 10μF low ESR selected
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22. PM CIRCUIT DESIGN; SELECTION OF fS
 Selected for minimum converter loss
 Optimum switching frequency increases with input power
 PWM designed to reduce losses at low input power levels
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23. PM CIRCUIT DESIGN; OSCILLATOR DESIGN
RC relaxation oscillator Low voltage comparator
Vdd, Vss
R2
R1
+
-
R3 D1 D2
Rh
Rl
Cosc
Vdc Observed oscillation frequency at Vin :0.9 volt
Duty cycle scales with step-up ratio
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24. PM CIRCUIT DESIGN; PROTECTION CIRUCIT
Overcharging protection MAX9064
Under lock out protection MAX9063
Vbatt
Vref
Vdc
Vref
R10
- R7
Vin2 +
R8 V1
Comp_U Comp_H V2
+ Dc
Vin1 -
R6
R9
Vbatt
Under-lock out  150 mV , Vdc
Over-charging protection  1.3 V, Vbatt
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25. PM CIRCUIT DESIGN; LOSS ANALYSIS
Modeling Converter losses at different power levels
250
Ploss [μW]
Leakge
Oscillator
200 Switch
Diode
Inductor
150
100
50
0
102 410.58 924
Diode major contributor Pin [μW]
Leakage losses dominate at lower power levels
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26. PM CIRCUIT DESIGN; HARDWARE DEVELOPMENT
PCB
testing
Layout  Expedition
PCB tool*
Schematic Design Capture tool*
Circuit Verification on Breadboard
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27. PM MEASUREMENTS
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28. PM MEASUREMENT; RESULTS
Comparing Efficiency present & new generation PM
Efficiency and output power for Vbatt 1.030 and 3.5 volt for 220 Ω rectenna
Higher efficiency at lower rectenna voltages Vin
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29. PM MEASUREMENTS; WITH RECTENNA
Comparing Autonomy
80 80
Efficiency [%]
Efficiency [%]
10 cm 20 cm Present generation
New generation
60 60
40 40
20 20
0 0
converter harvester converter harvester
Measured efficiency at distance of 10 and 20 cm
Increase in Autonomy of the harvester EIRP:100 mW
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30. PM MEASUREMENT; RESULTS
Start-up voltage varies with battery voltage
Start-up voltage of 210 mV Vin for Vbatt of 1.03 volt
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31. PM CIRCUIT ; OVERVIEW
DCM, non synchronous buck-boost converter
Over-Charging protection / under lockout protection
Quiescent current of 27 μA
Compact Design (2X2 cm 2 layer PCB board )
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32. RECOMMENDATIONS
TPS22902 load switch, Ron 146 mΩ
Quiescent Current Distribution
Reducing Quiescent current at under-lockout voltage levels
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33. RECOMMENDATIONS
 Higher ηconverter  PWM-PFM control strategy
Higher ηconverter with adaptive PWM
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34. SUMMARY
 1 V battery charging  lowest among commercial solutions
ηconverter ~ 68% @ 900 mV available voltage
Start-up voltage ~ 0.210 V
Quiescent current ~~ 1 V IC solutions
Protection circuits
Reconfigurable for any arbitrary rectenna ( Rmpp 800Ω, 2.6 kΩ)
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35. THANKS ….
Valer Pop , Prof. Peter Baltus , Dusan Milosevic
My family and friends
Audience present today
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36. CIRCUIT VERIFICATION
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37. WATS ARCHITECTURE
Application electronics
Sensor ADC Processor Radio
Power transfer DC-DC
converter
Data transfer
Energy storage
RF-DC DC-DC
converter converter - Battery
- Super capacitor
Rectenna PM
Energy Harvester
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38. SCHEMATIC DESIGN
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39. LAYOUT
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40. PCB TESTING
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