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TitleCMOS Indoor Light Energy Harvesting System for Wireless Sensing Applications
File Size9.0 MB
Total Pages221
Table of Contents
1 Introduction
	1.1 Motivation and Context
	1.2 Book Organization
2 Energy Harvesting Electronic Systems
	2.1 Available Energy Sources
		2.1.1 Mechanical Energy Electromagnetic Conversion Piezoelectric Conversion Electrostatic Conversion
		2.1.2 Thermal Gradients
		2.1.3 Radio Frequency Electromagnetic Energy
		2.1.4 Human Generation
		2.1.5 Microbial Fuel Cells
		2.1.6 Light
	2.2 Comparison of Harvestable Energy Sources
	2.3 Energy Harvesting-Based Sensor Networks
		2.3.1 Introduction
		2.3.2 Energy Neutrality
		2.3.3 Examples of WSN Powered by Harvested Energy Health Condition Monitoring Forest Surveillance and Monitoring Energy and Environment Monitoring in Buildings WSNs in Automotive Applications Structural Health Monitoring (SHM) Wireless Networks for Localization or Study of Animals
	2.4 Conclusions
3 Photovoltaic Cell Technologies
	3.1 Introduction
	3.2 Concepts and Parameters Regarding PV Cells
		3.2.1 Standard Illumination Conditions
		3.2.2 Fill Factor
		3.2.3 Efficiency
		3.2.4 Peak Watt
	3.3 Generation of Electric Power in Semiconductor PV Cells
		3.3.1 Efficiency Limit According to Shockley and Queisser
	3.4 Types of PV Cells
		3.4.1 First-Generation PV Cells Monocrystalline PV Cells Polycrystalline PV Cells Emitter Wrap Through (EWT)
		3.4.2 Second Generation PV Cells Amorphous Silicon Cadmium Telluride or Cadmium Sulfide/Cadmium Telluride Copper Indium Diselenide or Copper Indium Gallium Diselenide
		3.4.3 Third-Generation PV Cells Compound Semiconductor Dye-Sensitized Cells Organic Cells Carbon Nanotubes Quantum Dots
		3.4.4 Comparison of the Different PV Technologies
	3.5 Integrated CMOS PV Cell Prototype
		3.5.1 Electrical Model of a CMOS PV Cell
		3.5.2 Development and Layout of an Integrated CMOS PV Cell
		3.5.3 Experimental Results of the Prototyped Integrated PV Cell
		3.5.4 Conclusions About the Integrated PV Cell
	3.6 Indoor Light Energy Availability Study
		3.6.1 Light Power Intensity Measurements
		3.6.2 Conclusions
4 Voltage Step-up Circuits
	4.1 Types of Voltage Converters
		4.1.1 Linear Converters
		4.1.2 Switched Converters
	4.2 Inductor-Based Converters
		4.2.1 Voltage Step-up Circuits Boost DC--DC Voltage Converter Current-Fed Bridge DC--DC Voltage Converter Inverse Watkins--Johnson DC--DC Voltage Converter
		4.2.2 Voltage Step-down Circuits
		4.2.3 Voltage Step-up/Step-down Circuits Buck--Boost DC--DC Voltage Converter Non-Inverting Buck--Boost DC--DC Voltage Converter 010Cuk DC--DC Voltage Converter SEPIC DC--DC Voltage Converter Zeta DC--DC Voltage Converter
	4.3 Switched-Capacitor (SC) DC--DC Voltage Converters
		4.3.1 Voltage Step-up Converter Using the Ladder Topology
		4.3.2 Voltage Step-up Converter Using the Cockcroft--Walton Topology
		4.3.3 Voltage Step-up Converter Using the Dickson Charge Pump Topology
		4.3.4 Voltage Step-up Converter Using the Fibonacci Topology
		4.3.5 Voltage Step-up Converter Using the Parallel--Series Topology
		4.3.6 Voltage Step-up Converter Using the Voltage Doubler Topology
	4.4 Energy Storing Devices
		4.4.1 Batteries
		4.4.2 Supercapacitors
	4.5 Maximum Power Point Tracking (MPPT) Techniques
		4.5.1 Introduction
		4.5.2 Quasi-MPPT Techniques
		4.5.3 True MPPT Techniques
		4.5.4 Critical Analysis
	4.6 Conclusions
5 Proposed Energy Harvesting System
	5.1 Introduction
	5.2 SC Voltage Doubler
		5.2.1 SC Voltage Doubler with Charge Reusing Switch Sizing
	5.3 Phase Controller
		5.3.1 MPPT Regulation Using the Fractional Open-Circuit Voltage
		5.3.2 Asynchronous State Machine (ASM) Circuit Determination of the Optimum Circuit Parameters
	5.4 Local Supply
	5.5 Start-up
		5.5.1 Electrical Structure and Operating Principle
	5.6 Voltage Limiter Circuit
		5.6.1 Motivation and Background
		5.6.2 Voltage Limiter Circuit Architecture
		5.6.3 Voltage Reference Circuit
		5.6.4 Differential Voltage Amplifier
		5.6.5 Stability Analysis
		5.6.6 Simulated Performance of the Voltage Limiter
	5.7 Conclusions
6 Layout of the System
	6.1 Introduction
	6.2 SC Voltage Doubler
		6.2.1 Switches
		6.2.2 MOSFET Capacitors
	6.3 Phase Controller
		6.3.1 Logic Gates
		6.3.2 Delay Circuits
		6.3.3 Voltage Divider and Its Respective Decoupling
		6.3.4 Comparator Circuits
	6.4 Local Supply
	6.5 Start-up Circuit
	6.6 Voltage Limiter Circuit
		6.6.1 Voltage Reference Circuit
		6.6.2 Differential Voltage Amplifier
		6.6.3 Complete Layout of the Voltage Limiter Circuit
	6.7 Overall Circuit and Pin Assignment
	6.8 Extracted Layout Simulation
	6.9 Conclusions
7 Experimental Evaluation of the Prototype
	7.1 Experimental Prototype
	7.2 Experimental Results
		7.2.1 Experimental Evaluation of the Start-up Circuit
		7.2.2 Experimental Evaluation of the DC--DC Converter
		7.2.3 Experimental Evaluation of the MPPT Controller
		7.2.4 Experimental Results Using the PV Cells
8 Conclusions and Future Perspectives
	8.1 Summary and Achievements
	8.2 Future Perspectives
Appendix A: Light Power Measuring Device
Appendix B: Description of the Manufactured PV Cell
Appendix C: Computation of Power in a Circuit with a Switched-Capacitor
Document Text Contents
Page 1

Carlos Manuel Ferreira Carvalho
Nuno Filipe Silva Veríssimo Paulino

CMOS Indoor Light
Energy Harvesting
System for
Wireless Sensing

Page 2

CMOS Indoor Light Energy Harvesting System
for Wireless Sensing Applications

Page 110

There are some emerging technologies using materials such as LiCoO2 or
graphite, that in conjunction with poly vinylidene fluoride–ionic (PVDF–Ionic)
electrolyte, have already given promising results, just like technologies based on the
other types of materials and electrolytes [34].

If it is intended to use Lithium-ion or Lithium-polymer batteries, their charging
process is not trivial, so a specific charging circuit is often used. This circuit is
especially designed to guarantee that there is not any overcharging or overdis-
charging that could cause damage to the battery or even set it on fire [29]. Thus,
these battery technologies are demanding with respect to this issue. The role of the
circuit that controls the charging process is also to guarantee that the battery is
provided with a high pulsating charging current. This is why the charging method
in Table 4.2, for the Li-ion or Li-polymer types, is referred to as pulse charging. In
addition to the controlling circuit, an auxiliary battery can be used to provide the
charge for the current charging pulses.

The other battery technologies only require that batteries are trickle charged.
Trickle charging means that the batteries can be directly connected to an energy
source, without requiring any complex circuits controlling the charge by current

As it can be seen from Table 4.2, all technologies have advantages and disad-
vantages, which must be weighted according to the final application requirements
and deployment conditions.

Besides being electric charge buffers, batteries serve as voltage stabilizers,
providing a constant voltage at the output of the regulator circuit. Examples of
harvesting systems that make use of batteries to store harvested energy can be
found, for instance, in [35, 36].

The dimensions of such a device can be fairly reduced, like 5.8 mm × 31 mm ×
52 mm (thickness × width × length), representing the Ultralife UBP053048, one of
the devices present in [37].

4.4.2 Supercapacitors

Supercapacitors (or ultracapacitors) are also known as electric double-layer
capacitors (DLC). These exhibit particular characteristics that make them differ-
ent from ordinary capacitors. The DLC consists of activated carbon particles that
act as polarizable electrodes. These strongly packed particles are immersed in an
electrolytic solution forming a double-layer charge distribution along the contact
surface between carbon and electrolyte. The electrical model of a supercapacitor is
not simply a high-valued capacitor, but instead, a set of several branches with
different time constants [38]. Such a model is depicted next in Fig. 4.25.

This model is based on the electrochemistry of the interface between two
materials in different phases, so that the double-layer charge distribution, of dif-
ferential sections of the interface, is modeled as a series RC circuit. The resistive
element represents the resistivity of the materials forming the double-layer charge

100 4 Voltage Step-up Circuits

Page 111


distribution, mainly the resistivity of the carbon particles. The capacitive element
represents the capacitance between the two materials, which are carbon and elec-
trolyte. According to [39], in supercapacitors, the electric charge stored at a metal or
electrolyte interface is exploited to construct a storage device. The high content of
energy stored by supercapacitors comes from activated carbon electrode material,
having an extremely high surface area and a short distance of charge separation
created by the opposite charges in the interface between electrode and electrolyte.

Randomly distributed ions in electrolyte move toward the electrode surface of
opposite polarity under an electric field when charged. This is a purely physical
phenomenon rather than a chemical reaction, and hence, it is an easily reversible

These features result in high power, high life cycle, long shelf life, and in a
maintenance-free product.

Although the physical definition could indicate a large number of RC time
constants describing the microscopic physical structure of the DLC, the electric
behavior can be reduced to that of the circuit in Fig. 4.25. These time constants can
model how the device behaves, in response to the application of a voltage at its

The model shown in Fig. 4.25 tries to keep the number of branches to a mini-
mum (for practical reasons), includes an existing nonlinear relation between
capacitance and terminal voltage in only one of the branches, and also includes the
effect of self-discharge. However, in the practical voltage range of the device, the
DLC capacitance varies linearly with the capacitor terminal voltage.

Each of the three branches has a distinct time constant differing from the others
in more than an order of magnitude. In Fig. 4.25, starting from left, the first branch
is called the immediate branch. It contains elements Ri and Ci0, as well as the
voltage-dependent capacitor (identified as Ci1, whose units are in F/V), which
depends from Vci. This branch dominates the immediate behavior of the DLC in the
time range of seconds, in response to a charging action. The second is the delayed
branch, with parameters Rd and Cd, which dominates the behavior of the device in
the range of minutes. The third is the long-term branch, with parameters Rl and Cl.
It determines the behavior for times longer than 10 min. To reflect the voltage
dependence of the capacitance, the first branch is modeled as a voltage-dependent
differential capacitor. The differential capacitor consists of a fixed capacitance Ci0

Ri Rd Rl


Ci0 Ci1×Vci Cd Cl


Fig. 4.25 Equivalent circuit model for a double-layer capacitor

4.4 Energy Storing Devices 101

Page 220


These derivations depend on the fact that voltage and current are proportional
between themselves through RL, which is resistive. For reactive loads, one will have
a different situation.

To prove these conclusions, consider the following circuit, which has been
simulated in Spectre:

The values considered in the simulation are indicated in Table C.1.
Theoretically, the average power provided by the voltage source is given by

�Px ¼ Vx � Ic ¼ VxC

¼ 1� 10� 10�12 1
0:1� 10�6 ¼ 100 lW: ðC:13Þ

The average power in the capacitor is given by

�Pc ¼ Vc � �Ic ¼ Vc � 0 ¼ 1� 0 ¼ 0: ðC:14Þ

Finally, the average power in the load resistance is given by

�PRL ¼




dt ¼ 1
T � RL



V2RLdt ¼

T � RL





� �2

¼ 1
T � RL





� �
dt ¼ 1

T � RL



� �T

¼ � C

RLC � 1

� �
¼ C

1� e� 2TRLC

� �
¼ 50 lW:


The data extracted from simulation are presented next in Table C.2, in which the
computation of the power on the three circuit elements (switches are excluded) is
performed by three different methods, by using the auxiliary tools provided by the
results browser of the simulator. Only one of them is correct.

Table C.1 Values of the components of the circuit of Fig. C.2

f Vx C RL
10 MHz 1 V 10 pF 100 Ω

φ 1 φ 2




Fig. C.2 Switched-capacitor circuit simulated in Spectre

Appendix C: Computation of Power in a Circuit with a Switched-Capacitor 215

Page 221

According to the various possible computation methods which are presented, the
one that shows coherence with the values obtained theoretically is the one in the
first line. Thus, given the computation objectives, this is the one that should be

Table C.2 Results for the power computation, according to three different methods

Method Px Pc PRL ðlWÞ
Average(i(t) × v(t)) 99.82 μW 696.4 nW 49.2

rms(i(t) × v(t)) 3.906 mW 2.13 mW 494.4

rms(i(t)) × rms(v(t)) 3.906 mW 2.807 mW 49.2

216 Appendix C: Computation of Power in a Circuit with a Switched-Capacitor

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