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Universita degli Studi di Pavia

Facolta di Ingegneria

Dottorato di ricerca inMicroelettronica

XXII ciclo

Integrated magnetic sensor interfacecircuits and photovoltaic energy

harvester systems

Tutor:

Chiar.mo Prof. Piero Malcovati

Coordinatore del Corso di Dottorato:

Chiar.mo Prof. Rinaldo Castello

Tesi di Dottorato

di Ferri Massimo

Alle ambizioni

Contents

Introduction 1

1 Magnetic Sensor Interface Circuits 3

1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.2 Magnetic Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.2.1 SQUID . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

1.2.2 Search-coil . . . . . . . . . . . . . . . . . . . . . . . . . 6

1.2.3 Magneto-inductive sensor . . . . . . . . . . . . . . . . . 6

1.2.4 Magneto-resistance . . . . . . . . . . . . . . . . . . . . . 7

1.2.5 Hall sensor . . . . . . . . . . . . . . . . . . . . . . . . . 7

1.2.6 Fluxgate sensor . . . . . . . . . . . . . . . . . . . . . . . 9

1.3 Digital Compass System Characterization . . . . . . . . . . . . . 16

1.3.1 Magnetic field measurement system . . . . . . . . . . . . 18

1.3.2 Automated acquisition system . . . . . . . . . . . . . . . 25

1.3.3 Dedicated software . . . . . . . . . . . . . . . . . . . . . 30

1.3.4 Acquisition system optimization . . . . . . . . . . . . . . 32

1.3.5 Experimental results . . . . . . . . . . . . . . . . . . . . 34

1.4 Re-Design of the Fluxgate Magnetic Sensor Interface Circuit . . . 36

1.4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 37

1.4.2 Excitation circuits . . . . . . . . . . . . . . . . . . . . . 38

1.4.3 Read-out chain . . . . . . . . . . . . . . . . . . . . . . . 44

i

Contents

1.4.4 Experimental Results . . . . . . . . . . . . . . . . . . . . 59

2 Energy Harvesting 65

2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

2.2 Micro Energy Harvesting . . . . . . . . . . . . . . . . . . . . . . 68

2.3 Photovoltaic Energy Harvesting Process . . . . . . . . . . . . . . 71

2.3.1 Optical absorption . . . . . . . . . . . . . . . . . . . . . 72

2.3.2 Solar cells . . . . . . . . . . . . . . . . . . . . . . . . . . 74

2.4 Integrated Micro-Solar Cell Structures for Harvesting Supplied

Microsystems in 0.35-m CMOS Technology . . . . . . . . . . . 78

2.4.1 Solar cells characterization . . . . . . . . . . . . . . . . . 79

2.4.2 Power management system chip . . . . . . . . . . . . . . 84

2.4.3 Miniaturized solar cell model . . . . . . . . . . . . . . . 85

2.4.4 Ring oscillator and charge pump . . . . . . . . . . . . . . 87

2.4.5 Power monitoring circuit . . . . . . . . . . . . . . . . . . 88

2.4.6 Hysteresis comparator . . . . . . . . . . . . . . . . . . . 88

2.4.7 Voltage level shifter . . . . . . . . . . . . . . . . . . . . . 90

2.4.8 Voltage regulator . . . . . . . . . . . . . . . . . . . . . . 90

2.4.9 Storage capacitor sizing . . . . . . . . . . . . . . . . . . 92

2.4.10 Experimental results . . . . . . . . . . . . . . . . . . . . 93

2.5 Integrated Stabilized Photovoltaic Energy Harvester . . . . . . . . 95

2.5.1 Micro solar cells . . . . . . . . . . . . . . . . . . . . . . 96

2.5.2 Bandgap Reference Circuit . . . . . . . . . . . . . . . . . 98

2.5.3 LDO Circuit . . . . . . . . . . . . . . . . . . . . . . . . 101

2.5.4 Simulation Results . . . . . . . . . . . . . . . . . . . . . 103

2.5.5 Temperature Sensor . . . . . . . . . . . . . . . . . . . . 105

2.5.6 Experimental Results . . . . . . . . . . . . . . . . . . . . 105

2.5.7 Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . 107

ii

Contents

A PIC 16F877 Datasheet 111

Conclusions 113

iii

List of Figures

1.1 Classification of magnetic sensors . . . . . . . . . . . . . . . . . 5

1.2 Hall effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

1.3 Structure of a Fluxgate magnetic sensor . . . . . . . . . . . . . . 11

1.4 Effect of the magnetic field on the Fluxgate sensor output . . . . . 12

1.5 Structure of a planar Fluxgate magnetic sensor . . . . . . . . . . . 15

1.6 Structure of a planar Fluxgate sensor with an external magnetic field 16

1.7 Measurement and acquisition systems interaction . . . . . . . . . 17

1.8 Fluxgate sensor micro-photograph . . . . . . . . . . . . . . . . . 19

1.9 Block diagram of the integrated read-out circuit. . . . . . . . . . . 20

1.10 Effect of the magnetic field on the sensor output . . . . . . . . . . 22

1.11 Microphotograph of the integrated front-end circuit . . . . . . . . 24

1.12 Layout of the magnetic sensor interface circuit board . . . . . . . 24

1.13 Photograph of the magnetic sensor interface circuit board . . . . . 24

1.14 Example of stepper motor . . . . . . . . . . . . . . . . . . . . . . 26

1.15 Driver adopted to excite a single solenoid of the stator . . . . . . . 28

1.16 Layout of the motor driver board . . . . . . . . . . . . . . . . . . 28

1.17 Plastic tower . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

1.18 Board of the microcontroller-based interface circuit . . . . . . . . 30

1.19 Front panel of the software . . . . . . . . . . . . . . . . . . . . . 31

1.20 Angular accuracy as a function of the acquisition system evolution 33

v

Contents

1.21 Angular accuracy achieved with the automated acquisition system 34

1.22 Data acquired from the sensor over 360with the automated ac-

quisition system . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

1.23 Linearity of the complete system . . . . . . . . . . . . . . . . . . 36

1.24 System block diagram . . . . . . . . . . . . . . . . . . . . . . . . 38

1.25 Schematic of the 3.3 V excitation circuit . . . . . . . . . . . . . . 39

1.26 Excitation current waveform obtained in simulation with the 3.3-

V excitation circuit . . . . . . . . . . . . . . . . . . . . . . . . . 39

1.27 Schematic of the 5-V excitation circuit . . . . . . . . . . . . . . . 40

1.28 Triangular waveform generator . . . . . . . . . . . . . . . . . . . 41

1.29 Waveform obtained in simulation at the output of the triangular

waveform generator . . . . . . . . . . . . . . . . . . . . . . . . . 42

1.30 Schematic of the voltage-driven current generator . . . . . . . . . 43

1.31 Current waveform delivered to the H-bridge obtained in simulation 44

1.32 Full H-Bridge circuit scheme . . . . . . . . . . . . . . . . . . . . 45

1.33 Current waveform delivered to the sensor obtained in simulation . 45

1.34 Block diagram of the read-out chain . . . . . . . . . . . . . . . . 46

1.35 Effect of the external magnetic field over the sensor . . . . . . . . 46

1.36 Charge injection . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

1.37 Clock feed-through . . . . . . . . . . . . . . . . . . . . . . . . . 49

1.38 Schematic of the switches . . . . . . . . . . . . . . . . . . . . . . 49

1.39 Schematic of the operational amplifiers . . . . . . . . . . . . . . 50

1.40 Equivalent circuit of the operational amplifier . . . . . . . . . . . 51

1.41 Relation between gm and ID obtained with the circuit simulator . . 53

1.42 Bode diagram of the operational amplifiers used before the de-

modulator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

vi

Contents

1.43 Bode diagram of the operational amplifiers used after the demod-

ulator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

1.44 Instrumentation amplifier . . . . . . . . . . . . . . . . . . . . . . 56

1.45 Instrumentation amplifier transient response to an ideal Fluxgate

output signal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

1.46 Schematic of the Sallen-Key filter . . . . . . . . . . . . . . . . . 58

1.47 Frequency response of the Sallen-Key filter . . . . . . . . . . . . 59

1.48 Programmable gain amplifier . . . . . . . . . . . . . . . . . . . . 60

1.49 Programmable gain amplifier logic circuit scheme . . . . . . . . . 60

1.50 Microphotograph of the interface circuit chip . . . . . . . . . . . 61

1.51 Maximum relative linearity error of the system as a function of

the full-scale magnetic field . . . . . . . . . . . . . . . . . . . . . 62

1.52 Transfer characteristic of the system for a full-scale magnetic field

of 100 T (50 T) . . . . . . . . . . . . . . . . . . . . . . . . . 62

1.53 Relative linearity error of the system for a full-scale magnetic field

of 100 T (50 T) . . . . . . . . . . . . . . . . . . . . . . . . . 63

2.1 Trend of power dissipation in microprocessors design field . . . . 66

2.2 Optically generated electron-hole pair formation in a semiconductor 73

2.3 Photon intensity versus distance for two absorption coefficients . . 75

2.4 A p-n junction solar cell with resistive load . . . . . . . . . . . . 76

2.5 I-V characteristics of a p-n junction solar cell . . . . . . . . . . . 77

2.6 Maximum power rectangle of the solar cell I-V characteristics . . 78

2.7 Geometries and dimensions of the realized micro solar cells . . . . 79

2.8 Cross-section and equiva