Institutionen för systemteknik

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Institutionen för systemteknik

Department of Electrical Engineering

Examensarbete

Study of CMOS Rectifiers for Wireless Energy

Scavenging

Examensarbete utfört i Elektroniska komponenter vid Tekniska högskolan i Linköping

av

Aiysha Ali Khalifa

LiTH-ISY-EX--10/4359--SE Linköping 2010

Department of Electrical Engineering Linköpings tekniska högskola

Linköpings universitet Linköpings universitet

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Scavenging

Examensarbete utfört i Elektroniska komponenter

vid Tekniska högskolan i Linköping

av

Aiysha Ali Khalifa

LiTH-ISY-EX--10/4359--SE

Handledare: Atila Alvandpour

isy, Linköpings universitet Examinator: Atila Alvandpour

isy, Linköpings universitet Linköping, 21 November, 2010

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Division of Electronic Devices Department of Electrical Engineering Linköpings universitet

SE-581 83 Linköping, Sweden

2010-11-21 Språk Language ¤Svenska/Swedish ¤Engelska/English ¤ £ Rapporttyp Report category ¤Licentiatavhandling ¤Examensarbete ¤C-uppsats ¤D-uppsats ¤Övrig rapport ¤ £

URL för elektronisk version

http://www.ek.isy.liu.se http://www.ep.liu.se ISBN — ISRN LiTH-ISY-EX--10/4359--SE Serietitel och serienummer Title of series, numbering

ISSN —

Titel Title

Trådlös Energi Sophantering

Study of CMOS Rectifiers for Wireless Energy Scavenging

Författare Author

Aiysha Ali Khalifa

Sammanfattning Abstract

In recent years, there has been recent increase in the deployment of wireless sensor networks. These sensors are typically powered by a battery which has limited life span. This problem can be overcomed by using energy scavenging or power harvesting which is the process of converting ambient energy from the environment into usable electrical energy. It can be used in applications such as remote patient monitoring, implantable sensors, machinery/equipment monitoring and so on. The thesis presents the RF scavenging system and mainly focuses on the study of the rectifier architectures which is one of the key components in the RF scavenging system. The thesis also provides the design challenges while implementing the different rectifier structures, which are PMOS bridge rectifier, CMOS differential rectifier and charge pump. The functionality of the rectifier structures are studied by simulation using RF signal of 900 MHz and implemented in 0.35µm and 65 nm technologies to compare the results. The simulation results shows that there is a tradeoff between high output DC voltage and high power efficiency.

Maximum DC output voltage of 1 V is obtained from input amplitude level of 0.16 V using 7-stage charge pump rectifier. In the other hand maximum power efficiency of 23 % is obtained using CMOS differential rectifier.

Nyckelord

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Abstract

In recent years, there has been recent increase in the deployment of wireless sensor networks. These sensors are typically powered by a battery which has limited life span. This problem can be overcomed by using energy scavenging or power harvesting which is the process of converting ambient energy from the environment into usable electrical energy. It can be used in applications such as remote patient monitoring, implantable sensors, machinery/equipment monitoring and so on. The thesis presents the RF scavenging system and mainly focuses on the study of the rectifier architectures which is one of the key components in the RF scavenging system. The thesis also provides the design challenges while implementing the different rectifier structures, which are PMOS bridge rectifier, CMOS differential rectifier and charge pump. The functionality of the rectifier structures are studied by simulation using RF signal of 900 MHz and implemented in 0.35µm and 65 nm technologies to compare the results. The simulation results shows that there is a tradeoff between high output DC voltage and high power efficiency.

Maximum DC output voltage of 1 V is obtained from input amplitude level of 0.16 V using 7-stage charge pump rectifier. In the other hand maximum power efficiency of 23 % is obtained using CMOS differential rectifier.

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Acknowledgments

Firstly, I would like to thank Allah for giving me the strength and blessings to carry out this study.

I would like to express my sincere gratitude to my examiner and supervisor Atila Alvandpour for offering me this opportunity and giving me valuable guidance and encouragement throughout the work.

I am grateful to Sara Qazi for been a great friend and supporter all along the master studies.

I am very thankful to my parents who have supported me unconditionally through-out my life.

Finally, I express my gratitude to all who supported me in any respect during the completion of the master thesis.

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4.1.4 Effect of load resistance on output voltage and PCE . . . . 36 4.2 Differential CMOS Rectifier . . . 38 4.2.1 Effect of input amplitude on output voltage and PCE . . . 39 4.2.2 Effect of transistor size on output voltage and PCE . . . . 40 4.2.3 Effect of load resistance on output voltage and PCE . . . . 42 4.2.4 Effect of frequency on PCE . . . 43 4.3 Charge Pump . . . 46 4.3.1 Effect of transistor size on output voltage and PCE . . . . 47 4.3.2 Effect of number of stages on output voltage and PCE . . . 48 4.3.3 Effect of frequency on output voltage and PCE . . . 49

5 Comparison of Results and Conclusion 51

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1.1 General block diagram of an energy harvesting system. . . 4

1.2 Piezoelectric power generation [12]. . . 5

1.3 Diagram of electromagnetic power generator. . . 8

2.1 Communication link between base station ans sensor in passively powered sensor network. . . 14

2.2 Block diagram illustrating the RF-DC power conversion system in a passively powered sensor. . . 16

2.3 Diode-connected PMOS transistor. . . 17

3.1 Diode bridge rectifier. . . 22

3.2 CMOS bridge rectifier. . . 22

3.3 MOS transistor transient equivalent circuits. . . 22

3.4 Conventional CMOS rectifier circuit[21]. . . 23

3.5 External-Vth-cancellation CMOS rectifier circuit[17]. . . 24

3.6 Self-Vth-cancellation CMOS rectifier circuit[21]. . . 24

3.7 Differential-drive CMOS rectifier[21]. . . 25

3.8 Rectifier formed by cascading N rectifying cells in series[14]. . . 26

3.9 Dickson charge pump . . . 27

3.10 cascade charge pump . . . 28

3.11 Single stage charge pump. . . 28

4.1 PMOS bridge rectifier. . . 31

4.2 Output voltage dependence on input amplitude. . . 32

4.3 PCE dependence on input amplitude. . . 33

4.4 Output voltage dependence on transistor size. . . 34

4.5 PCE dependence on transistor size . . . 34

4.6 PCE dependence on operating frequency. . . 35

4.7 Output voltage dependence on load resistance. . . 36

4.8 PCE dependence on load resistance. . . 37

4.9 Schematic of differential CMOS rectifier. . . 38

4.10 Output voltage dependence on input amplitude. . . 39

4.11 PCE dependence on input amplitude. . . 40

4.13 PCE dependence on transistor size. . . 41

4.14 Output voltage dependence on load resistance. . . 42

4.15 PCE dependence on load resistance. . . 43

4.16 PCE dependence on input amplitude at various frequencies. . . 44

4.17 7-stage charge pump test bench. . . 46

4.18 Output voltage generated with and without load resistance. . . 46

4.20 Output voltage for different number of stages. . . 48

5.1 Input amplitude vs output voltage for different rectifier architectures. 51 5.2 Input amplitude vs PCE for different rectifier architectures. . . 52

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List of Tables

1.1 Power density of different types of energy scavenging[22] . . . 10 1.2 Comparison of the mechanical converters . . . 11 4.1 Summary of the achieved result of the output voltage and PCE for

different values in 65 nm technology. . . 37 4.2 Summary of the achieved result of the output voltage and PCE for

different values in 0.35µm technology. . . 38 4.3 Summary of the achieved result of the output voltage and PCE for

different values in 65 nm technology . . . 45 4.4 Summary of the achieved result of the output voltage and PCE for

different values in 0.35µm technology. . . 45 4.5 Summary of the achieved result of the output voltage and PCE for

different values in 65 nm technology. . . 49 4.6 Summary of the achieved result of the output voltage and PCE for

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Chapter 1

Introduction

1.1 Motivation for Energy Scavenging

The vast advancement of wireless technology and low power electronics have in-creased the deployment of wireless sensor nodes. Batteries have been the primary source of power for the wireless sensor. As the technology scales down, it is ex-pected that the size of the battery decreases. However, the battery technology has evolved very slowly compared to electronic devices [18]. The limited life time and large physical dimension introduce an unwanted maintenance burden of replace-ment or recharging. The greatest potential however, lies in a new class of devices that will be battery-free and thus enable applications that would have been pro-hibitively expensive due to the maintenance cost and repeated battery replacement [18]. It is highly desirable to have an alternate power sources which overcome the above limitation. This limitation can be solved through energy harvesting.

Power harvesting or energy scavenging is the process of acquiring ambient en-ergy from the surrounding environment (vibration, thermal enen-ergy, light enen-ergy or RF radiation) and converting it to a usable electrical energy. The potential benefit of energy harvesting mechanism is that the life time of the electronic sensor is not limited by the life time of the energy source. However, most of the harvestable energy sources are not continuous due to the variation of the environmental con-ditions. Thus the amount of energy harvested may not be sufficient to power the electronic sensor node. Hence, it is necessary to accumulate the energy for further use and also the electronic device should have low power consumption. The com-bination of an energy harvester with a small-sized rechargeable battery or other storage device is the best approach to power electronic sensor node over the entire lifetime.

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1.2 Generic Block Diagram for Energy

Scaveng-ing

Figure 1.1. General block diagram of an energy harvesting system.

The four main building blocks of an energy harvesting system:

• Transducer

The transducer converts a harvestable energy which can be in the form of solar energy, thermal energy, vibration or RF energy into electrical energy, through an antenna, solar cell, a piezoelectric device, or other various forms. The produced output from the transducer can be in a DC form or in AC form depending on the energy source.

• Power conversion

The power conversion circuit can be a rectifier or DC-DC converter which can converts the obtained energy into a usable DC voltage. The efficiency of the circuit is an important factor which indicates the amount of the useful energy that can be utilized by the application.

• Power management

Depending on the application and the voltage available at the power conversion circuit, the output voltage of the power conversion circuit can be regulated to a stable DC voltage using buck or boost converter or it can limited by voltage limiter. The power management circuit controls the conduction path between the device and energy harvester [5].

• Charge storage

The charge storage is used to hold the charge and store it in a capacitor or a rechargeable battery or other storage element. When selecting a rechargeable ele-ment, it is important to consider the ability of the battery/capacitor to withstand

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a high number of charge/discharge cycles and maintain its performance character-istics. Super-capacitors, traditional capacitors, and thin film batteries are known for their ability to retain performance even after a high number of charge/discharge cycles [22].

1.3 Energy Scavenging Techniques

Energy scavengers can be obtained from different energy sources, then converted to the usable electrical energy using specific circuits. In this section some of the energy scavengers are discussed:

1.3.1 Piezoelectric energy scavenging

Piezoelectric energy scavenger is one of the promising energy scavenging techniques for micro-power generators, which generates an electric charge as the result of a force exerted on a piezoelectric material. Before subjecting the material to some external stress, its internal structure is in neutral. While exerting some pressure on the material, its internal structure can be deformed which causes polarization of the material and hence, generates an electric field. Piezoelectric effect is the conversion of mechanical energy such as vibration into electrical energy. Piezoelectric effect is a reversible process where it also produces mechanical strain when electrical energy is applied to the piezoelectric material. Due to these bi-directional effects, piezoelectric materials are widely used for making sensors and actuators [12]. The widely used piezoelectric material is Lead Zirconate Titanate (PZT).

Figure 1.2. Piezoelectric power generation [12].

The figure above shows the mechanical setup of a piezoelectric generator with a cantilever and proof mass, where the piezoelectric material is mounted on a long thin cantilever beam and directly connected between the mass and the frame. When the beam/mass structure oscillates, the piezoelectric structure deforms and causes a charge to be displaced across the capacitor electrodes positioned on the top and bottom surfaces of the piezoelectric elements, a voltage then appears across the capacitor and a current will flow through the load. The power generated from this system is proportional to the proof mass, to the square of acceleration, and inversely proportional to the resonant and excitation untitled folder frequency [12].

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Most conventional power supplies have low internal impedance, but the piezo-electric generator has high impedance. This restricts the amount of the output current which relatively reduces the output voltage, hence makes it challenging in designing power conversion circuitry.

1.3.2 Solar energy scavenging

Photovoltaics(PV) is the direct conversion of light into electricity. Some mate-rials exhibit a property known as the photoelectric effect, that causes them to absorb photons of light and release electrons. Movement of these free electrons re-sults in an electric current. PV cells are made of semiconductor materials such as crystalline which behave as a voltage limited current source. The output current depends on the extent of light radiation, the size of the PV cell and on the time varying load impedance as the PV cell has current source-like behavior. PV con-version has potentially long life time, higher output level(10mW/cm2₎_compared

to other types of energy harvesting. However, the varying light intensity affects the output power.

1.3.3 Thermoelectric energy scavenging

Thermoelectric energy scavenger makes use of the thermal energy that is ubiqui-tous in the environment. To harvest thermal energy, thermoelectric devices are required to convert the temperature difference between two junctions to electric-ity. The direct conversion of temperature difference to electric voltage and vice versa is called as thermoelectric effect. There are three kinds of thermoelectric ef-fects which include: the Seebeck effect, the Peltier effect and the Thomson effect. The Seebeck effect is responsible for the production of a voltage by a temperature gradient.

By applying a temperature difference across the junctions of a conducting material, a voltage V will be generated in the circuit,

V =αabδT (1.1)

whereδT = (TH−TC)is the temperature difference across the two junctions and

αabis the Seebeck coefficient. The generated voltage is proportional to the

temper-ature difference across the junction, hence large thermal gradient is necessary to obtain reasonable voltage. The most attractive feature of a thermoelectric device is its capability of converting body heat into electricity which can power medical implants, wireless devices and other electronics devices. A successful example in this attempt is the thermoelectric wristwatch developed by Seiko [2] and Citizen [1] are successful examples of thermoelectric generator which convert heat from the wrist into electrical energy. The thermoelectric generator produces a power of at least1.5µW when the temperature difference is in the range of1◦_C_to₃◦_C_[15].

1.3.4 Electrostatic energy scavenging

Electrostatic energy scavenger converts a mechanical energy into electrical energy by moving a part the pre-charged plate of capacitors. There are two possible

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energy conversion cycles, charge constrained conversion and voltage constrained conversion [16]. The conversion is based on which property (charge or voltage) is held constant during the conversion process while the other changes in response to a varying capacitance.

In charge constrained, the charge is held constant. Moving apart the capacitor plates due to the applied vibration decreases the capacitance, this necessarily increases the voltage across the device to maintain constant charge.

Qconst= 1

2CV (1.2)

Increase in the voltage increases the net energy stored in the capacitor, as shown in the equation below,

E= 1 2CV

2 _(1.3)

However, the increased voltage can be very large compared to the breakdown limit of the CMOS technology, for instance, a 100-to-1 pF variation produces a 2.7-to-270 V change across the capacitor [16]

In the voltage-constrained method, the voltage across the capacitor is held constant. When the capacitance decreases due to separation of the plates of the capacitor, the charge flows out of the capacitor.

Q=1

2CVconst (1.4)

Mechanical energy is required to move the capacitor plates, and thereby the charge that flow out of the capacitor is harvested and stored for further use. However, to maintain constant voltage in the capacitor, a voltage source is required, which is not optimal,

1.3.5 Electromagnetic energy scavenging

Electromagnetic energy harvesting is based on Faraday’s electromagnetic induc-tion. It converts mechanical energy into electrical energy. A coil is attached to an oscillating mass which traverses magnetic field produced by a stationary magnet as shown in the figure below:

When a coil travels through a varying magnetic flux, a voltage is induced. Sim-ilarly, a voltage is induced when the coil is kept fixed and the magnetic structure is moved. The voltage or electromotive force (emf) induced in the circuit is directly proportional to the time rate of change of flux linkage of the circuit.

V =−dφ

dt (1.5)

where, V is the induced voltage andΦis the flux linkage. In this implementation, the circuit consists of a coil of multiple turns and permanent magnet which creates the magnetic field. The voltage induced in N turns coil is given as:

V =−NdΦ

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Figure 1.3. Diagram of electromagnetic power generator.

Φ =N BA (1.7)

where, N is the number of turns in a coil, B is the magnetic field flux density over the area A. The voltage induced can also be expressed as the product of flux linkage and the velocity,

V =−NdΦ dt =−N dΦ dt dx dt (1.8)

The power can be extracted by connecting the coil terminal to load resistance

RL and allowing the current to flow through it. The interaction between the field

from the induced current and the magnetic field give rise to a force which opposes the motion. By acting against the electromagnetic force Fem, the mechanical

energy is converted to electrical energy. The electromagnetic force is expressed as the product of electromagnetic dampingDem and velocity.

Fem=Demdx

dt (1.9)

The power extracted from the electromagnetic force is the product of the electro-magnetic force and the velocity.

P=Femdx

dt (1.10)

The power dissipated in the coil and load impedance is given as:

P = V2

RL+Rc+jwLc

(1.11) where,Rc is the coil resistance andLc is the coil inductance. Using equation 1.8,

1.9, and 1.10 the electromagnetic damping can be given as:

Dem=

N2₍dφ dt)2

RL+Rc+jwLc

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From the above equations, it is shown that to extract maximum power, the electromagnetic damping must be increased, that is by maximizing the flux linkage and minimizing the coil impedance.

1.3.6 RF energy scavenging

RF energy scavenger is one of the most popular power extraction methods for pas-sively powered devices. It harvest power from propagating radio frequency (RF) signals [6], which can be generated by cellular phones, local area network and other wireless devices. RF powered devices are often part of telemetry systems to remotely measure and report data back to a central processing unit [6]. It is also used in applications such as structural monitoring, access control, equipment monitoring and personal identification. Devices powered by propagating RF waves are most often used in passive radio frequency identification (RFID) or passive RF tag[6][9]. The harvested power depends on the distance of the harvester from the power source, another phenomenon like multipath fading, reflection and absorp-tion affect significantly the RF input power level. RF energy scavenging will be discussed in detail in the later sections.

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1.4 Comparison of Energy Scavenging Methods

The main performance metrics that are commonly considered when selecting an energy harvester are:

• Power density

The harvestable energy source tends to provide highly fluctuating amount of en-ergy, hence, it is characterized by its power density as it depends on how long the source is in operation [9]. The power density determines the life span of the device.

• Efficiency

The efficiency measures the effectiveness of the energy scavenging system in con-verting the environmental energy into electrical energy.

• Integration

It is the ability of the energy harvester to be integrated on a chip.

A comparison of different types of energy harvesting is given in Table 1.1. Table 1.1. Power density of different types of energy scavenging[22]

Energy Source Characteristics Power Density Efficiency Integration

Radio Frequency GSM 900 MHz 0.1µW/cm2 _50% _Easy

WiFi 2.4 GHz 1nW/cm2

Light Outdoor 100mW/cm2 _10-50% _Possible

Indoor 100µW/cm2

Vibration Humans-Hz 6µW/cm2 _25-50% _Difficult

Machines-KHz 800µW/cm2

Thermal Human 60µW/cm2 _0.1% _Difficult

Industrial 100 mW/cm2 _3%

The power density obtained from the solar energy has the maximum power density, but it decreases with the variation of environmental condition. Since the power density of the harvestable energy is not constant, it is necessary to store significant amount of the harvested energy to make it a feasible power source.

From the above harvestable energy, piezoelectric, electromagnetic and electro-static energy require mechanical energy (vibration) to provide electrical energy. The comparison of those mechanical converters is given in the table below.

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Table 1.2. Comparison of the mechanical converters

Mechanism Advantage Disadvantage

Piezoelectric No voltage source is required More diffcult to in-tegrate in micro-systems

Electrostatic Easy to integrate in micro-systems Separate voltage source is required Electromagnetic No voltage source is required Difficult to operate

at optimum condi-tion when the size is scaled down

1.5 Organization of the Thesis

The thesis discusses the design and study of RF-DC converter architectures and presents some of the various energy scavenging methods. Chapter 2 discusses the RF scavenging system and how it is possible to transmit RF energy wirelessly. It also discusses the parameters that affect the RF transmission and estimates the available RF energy required for harvesting using link budget.

Chapter 3 discusses about rectification and the main parameters of the rectifier. Chapter 4 provides the various rectifier architectures and their analysis.

Chapter 5 presents the simulation results of the rectifier architectures and the effects of various parameters on the output voltage and on the power conversion efficiency of the rectifier.

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Chapter 2

RF Scavenging System

In the earlier chapter various forms of harvestable energy have been discussed. One of the most popular power extraction methods for passively powered devices is to harvest power from propagating radio frequency (RF) signals [6].

2.1 Near and Far Field Propagation

RF energy harvesting is widely used in passive RFID (Radio Frequency Iden-tification) system. Passive RFID stores and retrieve data via electromagnetic transmission to a radio-frequency compatible integrated circuit without using a battery. Energy transfer is the way by which passive RF devices are powered [3]. The energy transfer mechanism is based on near-field propagation and far-field propagation of electromagnetic waves. Near field propagation employ inductive and capacitive coupling, where, inductive coupling is the transfer of energy be-tween two electronic circuits due to mutual inductance bebe-tween them. Similarly, capacitive coupling is the transfer of energy between two electronic circuits due to mutual capacitance between them [3].

Inductive coupling uses the magnetic field produced by the information signal to induce current in a coiled antenna. The current induced charges the capacitor to provide the operating voltage and power for the device. Near field coupling generally operates in the LF (Low Frequency) and HF (High Frequency) band with relatively short reading distance. It has been used in transformers, electric motors, medical telemetry, RFID and so on.

In far field coupling the antenna is comparable in size to the wavelength. It is applicable for longer reading distances as it operates in UHF and microwave bands. In far field system, the propagation energy drops off rapidly with distance.

2.2 RF Power Transmission

RF powered devices harvest their power from RF wave radiation transmited from a radiant source or base station. Fig 2.1 illustrates the communication link between

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Figure 2.1. Communication link between base station ans sensor in passively powered sensor network.

the radiating source which transmit RF signal and the multiple sensors. The maximum power transmitted decreases as the distance from the transmitter and the receiver increases. The operating range vary with the power contained in the wave transmitted, sensitivity of the receiving equipment, environment through which the wave travels and presence of interferences. The transmitting station for RFID typically operates in the ISM band (industrial, scientific or medical) which is controlled by FCC (Federal Communications Commission). FCC regulations limit the amount of power that can be transmitted.

2.2.1 Link budget

To estimate the available energy that can be carried by a RF signal and how much of its energy can be harvested by the wireless device, it is essential to calculate the power budget. Link budget accounts for all the gains and losses from the transmitter through the medium to the receiver. Generally the received power is calculated as,

received power[dBm] =Transmitted power[dBm]−Losses[dB]+Gains[dB] (2.1) The amount of power that can be transmitted between 902 and 928 MHz ISM band is 30 dBm before the antenna according to FCC regulations. If 6 dB gain of antenna is added, then the maximum allowed power is 36 dBm which correspond to 4 W. However, most of the power is lost in the transmission medium, which is called free space loss. The free space loss is governed by Friis transmission equation which gives an ideal estimate of the received powered, provided the transmitted power and the operating distance from the transmitter to the receiver are known.

Pr=PtGrGt( λ

4πR)

2 _(2.2)

where,Pr andPtare the received and the transmitted power respectively,Grand

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λ is the wavelength of the transmitted signal and R is the distance between the transmitter and the receiver. From the Friis formula, the free space loss is given as:

F SL= (4πRf

c )

2 _(2.3)

For example if 900 MHz frequency and 10 m operating distance are used, then the free space loss is found to be 51.52 dB. Thus the received power calculated from the above equation is -15.5 dBm corresponding to 28 µW. The estimated voltage that can be received is less than 74 mV, if 50 ohm antenna is used.

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2.3 System Overview of RF Scavenging System

Figure 2.2. Block diagram illustrating the RF-DC power conversion system in a pas-sively powered sensor.

Block diagram of the RF scavenging system is shown in figure 2.2. It illustrates the method of conversion of ambient RF energy emitted by various sources such as TV signal, wireless radio networks, base stations onto a feasible DC voltage.

The main components of RF energy scavenging system are:

• Antenna

The antenna design is critical in designing RF scavenging system since it must captures RF signals generated by multiple RF sources. The antenna is designed to have high gain and small return loss over wide range of frequencies. The antenna should also have a small size so it can be integrated on a chip.

• Matching network

As the received RF signal is very small, impedance matching between the antenna and the input of the rectifier circuit is required to obtain the maximum achievable power. Passive voltage amplification is also achieved by using high Q impedance matching.

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• Rectifier

The harvested RF signal is converted into a DC voltage by using RF-to-DC con-verter (rectifier). It will be discussed in detail in later sections.

• Power Storage and Management

The ambient RF signal source is not always readily available, hence, it cannot provide steady power all the time. Therefore, power management is required to provide a regulated output voltage. The DC-DC converter is often used to provide a regulated voltage or to boost the voltage level. The obtained DC voltage can be stored in a battery or a super capacitor for later use.

2.4 Rectification- RF to DC Conversion

An efficient power conversion system is required to convert power from the incident electromagnetic waves to a DC voltage. The rectifier is the basic part of the DC voltage generation circuit, which comprises mainly of a diode. Large losses in the rectifier must be avoided, because the typical output power of the energy harvester (antenna) is in the micro-watt range. It is also obvious that with decrease in power level, the input voltage also decreases. Hence, it is challenging to build a rectifier that extract very low level of power from the incident EM radiation.

In this thesis the focus is on CMOS rectifier. Diode is the main component in a rectifier, which allows one-way flow of electrons. PN junction diode has a threshold voltage of 0.7 V, thus the input must exceed the threshold voltage to operate the rectifier. Hence, this is not suitable for rectifying low level signals. Whereas, schottky diode has low threshold voltage, low conduction resistance, low junction capacitance, and large saturation current. However, Schottky diode is incompatible with standard CMOS circuits. While implementation of CMOS rectifier, the diode is replaced by a diode-connected transistor i.e connecting the gate and the drain terminal. When considering a diode-connected PMOS, the

Figure 2.3. Diode-connected PMOS transistor.

substrate is connected to its source. When the source has a higher voltage than the drain, there will be a current flowing from source to drain, which means that the diode turns on when it is forward-biased. On the other hand, when drain has a higher voltage than the source, the diode is reverse-biased. By connecting the substrate of PMOS to the highest voltage and substrate of NMOS to the lowest voltage, the leakage currents and latch up can be avoided.

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2.5 Critical Issues in RF Scavenging System

2.5.1 Threshold of the scavenger

In a CMOS rectifier circuit, the typical threshold voltage of a MOS transistor ranges between 0.2-0.6 V. The power required to overcome the threshold voltage with typical 50 ohm input impedance can be calculated from the equation below.

P = V

2 rms

R (2.4)

The power required to turn on the transistor translates to 0.4 mW to 3.6 mW, which corresponds to the dead zone of the rectifier. The minimum power required to overcome the rectification dead zone is called the power-up threshold of the harvester [4].

According to Friis equation, to obtain the required power to turn on the tran-sistor, the distance of operation is calculated to be 2.65 m-0.88 m at 900 MHz frequency. Those distances are considered to operate in the near field. In order to harvest power for longer operating distances, the threshold voltage should be minimized.

From the previous section, the maximum input voltage that can be received from 3.6 mW input power is 0.074 V, which is very small to turn on the CMOS transistor. There is also some other solutions used to reduce the threshold voltage which are the floating gate devices[13]. Impedance transformation boosts the voltage level of the input signal, by employing matching network between the antenna and the rectifier circuit.

2.5.2 Power conversion efficiency

Power conversion efficiency (PCE) is defined as the ratio of the power dissipated by the load to the total energy consumed by whole circuit. It can also defined as the ratio of output DC power to the input RF power.

P CE= Pout

Pin ∗100 (2.5)

The RF power available for extraction is in the range of micro-watt, thus it is required to utilize the power without losses. But according to maximum power transfer theorem, half of the power is available for delievery to the load under matched input conditions. Impedance matching network between the antenna and the rectifier increases the PCE of the system.

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Chapter 3

RF CMOS Rectifier

Architectures

The RF rectifier is a key component in a wireless sensor networks and RFID applications. It converts the input RF signal into DC voltage to power up the system. Rectifiers can be broadly categorized in two types, half wave rectifier and full wave rectifier. The simplest of all the rectifier circuits is the half-wave rectifier which uses a single diode that conducts current in one direction. It is inefficient as it just uses one half of each complete sine wave of the RF signal to convert it to a DC voltage. In the other hand, full wave rectifier makes use of the whole input signal to deliver a DC signal. There are several ways of connecting diodes to make a rectifier, but bridge rectifier is the most popular version. In this section conventional CMOS bridge rectifiers and voltage multiplier are going to be discussed.

3.1 Bridge Rectifier

A full wave bridge rectifier along with a capacitor converts both polarities of the input signal to a DC signal. The arrangement requires four diodes as shown in fig 3.1, whereby, a pair of diodes is responsible for rectification of each signal cycle. When the input voltage is higher than the output, a diode conducts to deliver power to the load and the other regulates the current path from the load to the ground [8]. This structure benefits from high power efficiency and smaller output ripple compared to half wave rectifier. However, threshold voltage drop of two diodes are lost in each signal cycle.

The rectified output voltage is given by,

Vout= 2Vin−2Vth (3.1)

As the MOS transistor is the main part of the rectifier structure, it is worth to analyze the equivalent circuit of the MOS transistor. The power consuming

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Figure 3.1. Diode bridge rectifier. Figure 3.2. CMOS bridge rectifier.

Figure 3.3. MOS transistor transient equivalent circuits.

elements in the equivalent circuit are mainly drain diode, substrate-source diode and channel resistor. So most of the power is consumed by the channel resistance which is the resistance between the source and the drain, MOS diodes and the load resistance. To increase the power efficiency of the rectifier, the turn on voltage of the transistor should be small and the channel resistance also should be decreased. To obtain small channel resistance, larger transistor size is required which contribute to large CMOS diodes and parasitic capacitances which can introduce greater leakage current and parasitic losses.

3.1.1 Conventional CMOS rectifier

A variety of conventional full wave rectifier have been addressed in [8][23] [7] [20] [10] [21] [11]. A conventional CMOS rectifier circuit [11], which is composed of series connection of diode-connected NMOS and PMOS transistors is shown below. The RF input is applied through the coupling capacitor Cc. During the positive

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Figure 3.4. Conventional CMOS rectifier circuit[21].

half cycle of the RF voltage signal, forward current flows to the output load. When negative half cycle of the RF voltage signal is applied, almost no current flows. The output voltage that is developed across the load is given as,

Vout = 2Vrf−Vdrop (3.2)

Most of the power losses of the integrated rectifier circuit originate from the on-resistance of the transistor. The PCE of the rectifier circuit is affected by circuit topology, diode parameters, and input RF signal level [20][10]. Small ON-resistance and small reverse leakage current are the main parameters of the MOS diode which can increase the PCE of the rectifier. Generally, small on-resistance of the MOS-diode is achieved by small turn on voltage of the transistor that is the threshold voltage of the transistor. It is thus desirable for the threshold voltage of the transistor to be as small as possible to decreases the losses in the rectifier circuit. The voltage drop across a MOS diode is given as,

δV =Vth+

s 2LI

CoxW µ (3.3)

where, W and L are the width and length of the transistor, I is the current flowing andCox is process related product andVthis the threshold voltage of the

transis-tor. The voltage drop not only related to the threshold voltage, but also depend on the overload voltage, which linearly increases with square root of the current. As the threshold voltage is the main parameter which can degrade the perfor-mance of the rectifier, appropriate Vth-cancellation mechanism is applied, which are External-Vth-cancellation (EVC) scheme [21], Self-Vth-cancellation (EVC) scheme [11] [20], and Internal-Vth-cancellation (IVC) scheme [17].

External-Vth-cancellation (EVC) scheme

The output voltage that can be obtained from conventional rectifier as discussed above is given as:

Vout= 2∗(Vrf−Vth) (3.4)

Maximum output voltage is obtained when threshold voltage is negligible. In this scheme a bias voltage is added between the gate and drain of the transistor.

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Figure 3.5. External-Vth-cancellation CMOS rectifier circuit[17].

Thereby the threshold voltage changes toVth−Vbias. The output voltage can be

rewritten as:

Vout= 2∗(Vrf−Vth+Vbias) (3.5)

The threshold voltage is expected to reduce to zero when Vth is nearly equal to

Vbias, then the output voltage can be approximated as,

Vout= 2Vrf (3.6)

Therefore, it is possible to rectify small RF signals with this structure. However, it is not optimal due to the additional batteries used.

Self-Vth-cancellation (SVC) scheme

The developed self-Vth-cancellation (SVC) CMOS rectifier circuit [11] [20] is shown figure 3.6. In self-Vth-cancellation rectifier, the gate of the NMOS and PMOS transistors are cross-connected such that the NMOS transistor and The transistors

Figure 3.6. Self-Vth-cancellation CMOS rectifier circuit[21].

are connected to the output terminal and ground terminal respectively as shown in the figure. This connection increases the gate-source voltage of the transistor which equivalently decreases the threshold voltage.

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This scheme is simple and does not require any additional power circuitry. Gate-source voltages of the NMOS and PMOS transistors are statically biased using the output DC voltage, thus reducing the effectiveVth of the MOS

transis-tors [20], which result in large PCE. In this configuration, the energy loss mostly depends on the on-resistance of the transistor. When the threshold voltage is too small, it may cause reverse leakage current which will reduce the PCE. Hence it is not possible to achieve small on-resistance and small leakage current in self-Vth-cancellation rectifier. As a result, PCE of the SVC rectifier circuit will first increase with the increase in input power, but then decrease with the further increase in input power, exhibiting some peak value in between [10].

CMOS differential rectifier

According to SVC scheme rectifier it is not possible to achieve small on-resistance and small leakage current simultaneously. In order to solve the problem

differential-Figure 3.7. Differential-drive CMOS rectifier[21].

drive CMOS rectifier circuit has been developed [20] [10]. It consist of a cross-coupled differential CMOS with a bridge structure. In this differential structure, the gate of the transistor is biased by a differential signal. From the figure,Vxand

Vy are the differential input RF signal. When Vx is negative then the transistor

MN1 is forward biased and whenVy is positive which correspond to positive gate

voltage bias of MN1 transistor, which decreases the threshold voltage of the tran-sistor. This results in small on-resistance. Whereas, when Vx is positive and Vy

is negative, the transistor is reversed biased and the gate voltage decreases which increase the threshold voltage, resulting in reduction in reverse leakage current.

This kind of rectifier performs better than the MOS-diode based rectifier. It is also called four-transistor cell according to [14] and called negative voltage converter according to [19]. The structure consists of combination of two cross-connected gate structure which provide complimentary bridge rectifier. In the circuit, the PMOS transistor delivers highest voltage to the load, whereas the NMOS-transistor provide the lowest voltage. The transistor operates in the triod

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region which behaves as a switch, thereby having smaller voltage drop compared to MOS-diode. The output voltage is given as,

VDC = 2VRF−Vdrop (3.7)

where, VRF is the amplitude of the differential signals and Vdrop is the losses

due to swich resistance and reverse conduction. The maximum output voltage is limited to2VRF. To increase the output voltage, N cells of the structure can be

cascaded. Differentials signal of the first stage is directly connected to to the RF source whereas, the proceeding stages are capacitivly coupled to the RF source. This structure behaves as a charge pump voltage multiplier [14], as the expected output voltage at the N’th stage is VDC =N(2VRF −Vdrop)but in practice the

output voltage is lower because theVdropincreases with the increase of the number

of cells due to increase of the body bias of the transistor.

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3.2 Charge Pump

3.2.1 Dickson charge pump

Charge pump circuit generates higher output voltage from lower input signal. It has been widely used in non-volatile memories such as EEPROM and flash memories. The basic charge pump architecture is proposed by Dickson in 1976.

Figure 3.9. Dickson charge pump

This kind of charge pump consists of diodes and two non-overlapping pumping clocksφ1φ2, whose amplitude is the supply voltageVφand is capacitively coupled

to alternate nodes along the diode chain. It is operated by pumping charges through the diodes and thereby charging the capacitors each clock cycle. At each node the voltage increases progressively from the input to the output of the diode chain byVφ. However, the total output voltage is affected by a voltage dropVD

of the diode. The output voltage of a dickson charge pump is given as:

Vout=Vin+N(Vφ−VD)−VD (3.8)

where, N is the number of stages of the charge pump. The extra VD in the

equation is because of the last diode available out of the chain. This kind of charge pump implementation is not optimal for passive wireless devices, as it require clock generation module which consume more power and area. To avoid the clock generation module, the RF input signal is used to pump the charges through the circuit where it rectifies the AC signal and increases the DC level.

3.2.2 Voltage multiplier

The N-stage voltage multiplier or cascade charge pump is based on the dickson charge pump except that, it does not require a clock signals. The N-stage voltage multiplier consists of a cascade of peak-to-peak detectors as shown in the figure below. A peak detector is a series connection of a diode and a capacitor which outputs a DC voltage equal to the peak value of the applied AC signal.

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Figure 3.10. cascade charge pump

To describe the operation of the charge pump, a single stage charge pump is analyzed. During the negative half cycle of the RF signal, whenVn−1> Vin+Vth,

Figure 3.11. Single stage charge pump.

the transistorMn−1 turns on while transistor Mn turns off as Vc < VN and the

charge transfers from capacitorCV(N−1) to capacitorCc. At this cycle the node

voltage atVc is given asVin+Vn−1−Vth. When the input changes to the positive

half cycle the transistorMn−1turns off while transistorMnturns on and the charge

transfer fromCc to capacitorCn. The available voltage atCn is given as

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The maximum voltage that can be achieved in a single stage charge pump is twice the RF amplitude minus twice the threshold voltage of the MOS diode. The output voltage of N’th stage charge pump is given as:

Vn= 2N(Vin−Vth) (3.10)

From the above equation it is obvious that the threshold voltage has a drastic effect on the the output voltage. To obtain a decent voltage from small input RF signal, the threshold voltage must be minimized. Other way to increase the output voltage is by increasing the number of stages. However, increasing the number of stages causes to degradation of power dissipation and power conversion efficiency. The threshold voltage also increases linearly with the increase of number of stages due to the body effect, thus limiting the number of stages that can be cascaded.

The MOS transistor in the charge pump operate in saturation region due to the short connection between gate and darin, the drain current in this region is given as: ID=β 2.(VGS−Vt) 2 _(3.11) β=µn.Cox.W L (3.12)

According to the equation to ensure pumping of low input voltage, low threshold voltage and small on-resistance of the transistor are required. To obtain small resistance of the transistor, large transistors have to be used, leading to larger parasitic capacitance and thereby increasing the leakage current. In the other hand, if the transistor size is too small, then the charge transfer is incomplete which leads to small output voltage and hence low efficiency. Hence to obtain maximum output voltage and high efficiency, low threshold voltage and optimal transistor size should be selected.

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Chapter 4

Simulation and Evaluation

Results

In this section RF rectifiers are simulated in cadence environment using 0.35µm and 65 nm technology. The results have been analyzed, and effect of various parameters on the output voltage and power conversion efficiency.

4.1 Bridge Rectifier

As discussed in section 3, the full wave bridge rectifier consists of four diode con-nected MOS-transistors concon-nected in bridge form. The transistors can be NMOS or PMOS, however, PMOS transistor is preferred to reduce the variation of thresh-old voltage due to body effect. The bridge rectifier suffers from low voltage and efficiency not only because of the threshold voltage drop of two diode connected transistor, in addition to the leakage current of two MOS diodes at every half cycle. The effect of different parameters on the output voltage and on the power conversion efficiency is given below.

Figure 4.1. PMOS bridge rectifier.

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4.1.1 Effect of input amplitude on output voltage and PCE

According to the simulation results, it is shown that increasing the input voltage increases the output voltage linearly as shown in figure 4.2. Similarly, the PCE increases for larger input voltage as shown in figure 4.3.

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Figure 4.3. PCE dependence on input amplitude.

4.1.2 Effect of transistor size on output voltage and PCE

It is noticed from the graph 4.4 and 4.5 that the output voltage increases with the transistor size since larger transistor size has small channel resistance. Similarly, the PCE increases with larger transistor size.

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Figure 4.4. Output voltage dependence on transistor size.

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4.1.3 Effect of frequency on PCE

The PCE decreases with higher frequency ranges. This rectifier structure is not suitable for higher frequency as it suffer from lower PCE wit increasing the fre-quency.

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4.1.4 Effect of load resistance on output voltage and PCE

Figure 4.7. Output voltage dependence on load resistance.

It is shown from the figure 4.7 and 4.8 that the PCE goes up with the load resistance, but still the PCE cannot be very high for larger load resistance. This is because of the increased power consumption. However, larger load resistance gives higher output voltage.

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Figure 4.8. PCE dependence on load resistance.

Summary of the achieved result of the output voltage and PCE for different values of input amplitude for 0.35 µm and 65 nm technology is given below: Table 4.1. Summary of the achieved result of the output voltage and PCE for different values in 65 nm technology.

Input amplitude (V) Output voltage (V) Power conversion efficiency(%)

0.4 0.152 15.63

0.35 0.104 12.17

0.3 0.064 8.3

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Table 4.2. Summary of the achieved result of the output voltage and PCE for different values in 0.35µm technology.

1 0.387 0.6

0.8 0.218 0.329

0.65 0.1 0.16

0.6 0.056 0.1

4.2 Differential CMOS Rectifier

The schematic of differential CMOS rectifier or four transistor cell rectifier is shown below and it has been simulated in 0.35 µm and 65 nm technology. Differential input signal with a 50 ohm resistance as of the antenna is used. The output voltage obtained when input signal amplitude of 0.25 V is applied at 900 MHz frequency. Capacitance of 10 pF and resistance of 8 k ohm are used as a filter which reduces the ripple voltage to obtain a smoother output.

Figure 4.9. Schematic of differential CMOS rectifier.

Higher efficiency is obtained comparing to PMOS bridge rectifier. This is because when the diode connected transistor is reversed biased, the source appear connected to the gate. A large leakage current is available at Vgs =0. Whereas, when a pair of cross connected transistor is reversed biased, the leakage current produced is very small.

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4.2.1 Effect of input amplitude on output voltage and PCE

From figure 4.10, increasing the input voltage level increases the output voltage. However, figure 4.11 shows that the PCE increases first and decreases with further increase of the input voltage. The reason behind the decrease in PCE for higher input voltage level is that higher input voltage level increases theVGSwhich causes

decrease in threshold voltage. In this case, reverse leakage current of the rectifier increases which causes extra energy dissipation.

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Figure 4.11. PCE dependence on input amplitude.

4.2.2 Effect of transistor size on output voltage and PCE

The output voltage increases with the increase of the transistor size according to figure 4.12. This is due to increase of transistor size decreases the channel resistance of the transistor and hence increases the output voltage. However, larger transistor sizes lead to larger parasitic capacitance which will lead to parasitic losses. Hence, the PCE decreases for larger transistor sizes as it has been shown in the figure 4.13.

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4.2.3 Effect of load resistance on output voltage and PCE

Figure 4.14 shows the output voltage increases linearly with the load resistance. However, figure 4.15 shows that with larger load resistance, the Peak PCE can be obtained at a smaller input power. The peak PCE increases slightly with the increase of the load resistance.

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Figure 4.15. PCE dependence on load resistance.

4.2.4 Effect of frequency on PCE

The dependence of the PCE on the operating frequency is illustrated in figure 4.16. As the operating frequency increases, the PCE decreases. This is because with the increase of the frequency, the energy loss increases due to increase of the reactance component.

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Summary of the achieved result of the output voltage and PCE for different values of input amplitude for 0.35 µm and 65 nm technology is given below Table 4.3. Summary of the achieved result of the output voltage and PCE for different values in 65 nm technology

0.4 0.297 11.56

0.35 0.262 14.14

0.3 0.224 17.9

0.25 0.18 23

0.2 0.124 26.78

Table 4.4. Summary of the achieved result of the output voltage and PCE for different values in 0.35µm technology.

0.55 0.309 9.9

0.5 0.222 6.3

0.45 0.122 2.4

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4.3 Charge Pump

From the previous section, it was shown that as the number of stages in the charge pump is increased, the output voltage is also increased, but this will also results increase in internal impedance thereby decreasing the output drive current. To increase the output current, the capacitance and the size of the transistor can be increased, but these contribute to parasitic capacitance. Hence optimal number of stages of the charge pump should be chosen. In the simulation, 7-stage charge pump is used. The simulation setup has been shown in the figure below:

Figure 4.17. 7-stage charge pump test bench.

A sinusoidal signal is applied to the charge pump with a 50 ohm resistance as of the antenna. A load resistance of 1 M ohm is used to represent a transmitter with high impedance connected to the charge pump. The output voltage obtained from 7-stage charge pump using 0.15 V input amplitude of the signal is given below:

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The generated output voltage is nearly 1 V but when output load of 1 M ohm is connected 0.25 V output voltage is obtained, however, this voltage can vary with various parameter like transistor size, number of stages of the charge pump, capacitance value, load resistance.

4.3.1 Effect of transistor size on output voltage and PCE

How the transistor size can affect the output voltage and PCE can be noticed from figure 4.18. It is seen that as the transistor size increases the output voltage

increases. This is due to the fact that large transistor sizes cause small resistance of the transistor and thereby increasing the output current. Similarly, the power conversion efficiency can be improved with large transistor size but not signifi-cantly.

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4.3.2 Effect of number of stages on output voltage and PCE

Figure 4.20. Output voltage for different number of stages.

As the number of stages of the charge pump increases the output voltage also increases as shown in equation 3.10 and figure 4.20. However, as the numbers of stages increases, the source voltage of the transistor increases, thereby increasing the VSB voltage. Thereby increasing the threshold voltage, which can lead to

degradation of the power conversion efficiency.

Vth=Vtho+ (

p

|2ΦF+VSB| −

p

|2ΦF|) (4.1)

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4.3.3 Effect of frequency on output voltage and PCE

To increase the output voltage, the number of stages need to be increased as discussed earlier but this increases the internal impedance as represented in the equation below.

Rs= N

C∗f (4.2)

where,Rsis the internal impedance, N is the number of stages, C is the capacitance

and f is the operating frequency of the charge pump. The optional way to achieve higher output voltage is to increase the frequency or the capacitance as increas-ing them reduces the internal impedance. A larger capacitor allow more charge to be transferred and thus increases the output voltage, but larger capacitance introduces parasitic capacitance, this can lead to a higher power consumption,

P =Cf V2_{. Hence, the power efficiency of the charge pump decreases with the}

increased output voltage.

Summary of the achieved result of the output voltage and PCE for different values of input amplitude for 0.35 µm and 65 nm technology is given below: Table 4.5. Summary of the achieved result of the output voltage and PCE for different values in 65 nm technology.

Input amplitude (V) Output voltage without load (V) PCE with 1 M ohmRL(%)

0.37 2.7 13.3

0.16 1 2.8

0.1 0.5 0.78

0.06 0.22

-Table 4.6. Summary of the achieved result of the output voltage and PCE for different values in 0.35µm technology.

Input amplitude (V) Output voltage without load (V) PCE with 1 M ohmRL(%)

1 1.375 0.3

0.8 0.743 0.1

0.7 0.446 0.08

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-Chapter 5

Comparison of Results and

Conclusion

The choice of rectifier structure depends on the application in use. Charge pump and the differential CMOS rectifier are the most promising. Charge pump is suitable to generate the preferred output voltage level from too low input voltage, but it suffers from low power efficiency due to the more stages incorporated which increases the power consumption.

Figure 5.1. Input amplitude vs output voltage for different rectifier architectures.

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Figure 5.2. Input amplitude vs PCE for different rectifier architectures.

On the other hand, to achieve better PCE the differential CMOS rectifier or NMOS-PMOS cross-coupled structure is suitable but requires larger input voltage compared to charge pump. It is also noticed that the differential CMOS rectifier has higher efficiency for lower input voltage, but the PCE decreases for larger input voltage.

The comparison between PMOS-bridge rectifier, differential CMOS rectifier, and the charge pump are shown in Figure 5.1 and 5.2. It is noticed from the figure that the charge pump can achieve the highest output voltage of 2.7 V using input amplitude of 0.35 V. However, the maximum PCE is achieved using differential CMOS rectifier of 23% at 0.25 V input amplitude. On the other hand, the PMOS-bridge rectifier has the lowest output voltage and PCE.

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