CMOS RF Power Rectifier Design Author
CMOS RF Power Rectifier Design
Author:
Law Carlos
Student I.D.:
03614511
Supervisor:
Professor K.K.Cheng
Associate Examiner:
Professor H.P.Ho
A project report presented to the Chinese University of Hong Kong In partial fulfilment of the Degree of Bachelor of Engineering
Department of Electronic Engineering The Chinese University of Hong Kong
April, 2006
i
Abstract A set of design criteria for the radio frequency section of a passive RF identification (RFID) transponders operating at 900MHz industrial, scientific and medical (ISM) range is derived in this thesis report. Particular focus is put on the analysis, verification and IC implementation of the voltage multiplier and the power matching sections. A reader-to-tag read range of around 4m is achieved with a 4W Effective Isotropic Radiated Power (EIRP) from the base station. A voltage multiplier sensitivity of –14dBm is achieved with proper power matching to give at the output of the voltage multiplier an approximate 1.5V and 1.5A to drive the digital section of the RFID passive transponder with a 10% conversion efficiency. Two IC designs are submitted to Austriamicrosystems for manufacturing. The first design includes the complete set of the voltage multiplier circuit. The second has the input capacitors omitted to see the effect of such an omission to conversion efficiency and size reduction.
ii
Acknowledgements The author would like to thank his supervisor, K.K. Cheng, for his patience and kind suggestions during the course of the project undertaking, and his insights in assigning an experienced tutor to assist in the guidance and troubleshooting of the CADENCE design tool. He would also like to thank F.L. Wong, W.F. Chung, and C.F. Au Yeung for their invaluable technical support in the project. H.Y. Yim, C.P. Kong and K.K. Tse have also been very helpful in the microwave laboratory.
iii
Contents Abstract
i
Acknowledgements Contents 1.
iii
Introduction A.
1
Radio Frequency Identification (RFID) i.
Brief History
1
ii.
Key Features
2
B.
Types of RFID Tag i.
Passive
ii.
Semi-passive
iii.
Active
C.
3 5
9
Rectifiers
B.
Voltage Multiplier (VM)
9 12
Traditional Voltage Multipliers
ii.
CMOS Voltage Multipliers
iii.
Parameter Analysis Matching
D. Antenna E.
16
32
Limiter and Regulator
Rectifiers
34 40
40
12 15
28
Simulation Results A.
6
7
A.
i.
3.
3
Radio Frequency Identification Application
Theory
C.
1
5
D. Overview 2.
ii
iv
i.
Comparison
40
ii.
Conclusion
45
B.
C.
Voltage Multiplier i.
Schematic
46
ii.
Layout
51
Matching
57
D. Proposed Layout 4.
Discussions
46
60 61
A.
Comparison of performance
B.
Design Limitations
C.
Further Improvement Work
5.
Conclusions
64
6.
References
65
61 62 63
1
1. Introduction A. Radio Frequency Identification (RFID) Radio Frequency Identification (RFID) is an automatic identification method, relying on storing and remotely retrieving data using devices called RFID tags or transponders. An RFID tag is a small object that can be attached to or incorporated into a product, animal, or person. RFID tags contain silicon chips and antennas to enable them to receive and respond to radio-frequency queries from an RFID transceiver. Passive tags require no internal power source, whereas active tags require a power source. [1]
i.
Brief History In 1945 Léon Theremin invented an espionage tool for the Soviet government which retransmitted incident radio waves with audio information. Even though this device was a passive covert listening device, not an identification tag, it has been attributed as the first known device and a predecessor to RFID technology. The technology used in RFID has been around since the early 1920s according to one source. [1]
CMOS RF Power Rectifier Design
2
ii.
Key Features [2] •
Not line of sight WID tags do need to be visible to be read / Written.
•
Robust Because they don't need to be visible, they can be encased within rugged materials protecting them from the environment they are being used in. This means they can be used in harsh fluid and chemical environments and rough handling situations.
•
Read speed Tags can be read from significant distances (especially the active variety) and can also be read very quickly. This is especially useful when the items needing to be identified are moving quickly for example on a conveyor.
•
Reading multiple items A number of tagged items can be read at the same time within a RF field. This cannot be done as easily with "visual" identifiers.
•
Security Because tags cm be enclosed, they are much more difficult to tarnper
CMOS RF Power Rectifier Design
3
with, A number of tag types now also come programmed with a unique identifier (Serial Identification) which is guaranteed to be unique throughout the world. •
Programmability Many tags are read / write capable, rather than read only. This means that information can be written to the tag, perhaps to show that the item being tagged has gone through a particular process, or that it's condition or status has changed somehow. Or in some instances to store information about the tagged items e.g. the results of a test that it has undergone.
B. Types of RFID Tag RFID tags can be classified as passive, semi-passive, or active. i.
Passive tag Passive RFID tags have no internal power supply. The minute electrical current induced in the antenna by the incoming radio frequency signal provides just enough power for the CMOS integrated circuit (IC) in the tag to power up and transmit a
CMOS RF Power Rectifier Design
4
response. Most passive tags signal by backscattering the carrier signal from the reader. This means that the aerial (antenna) has to be designed to both collect power from the incoming signal and also to transmit the outbound backscatter signal. The response of a passive RFID tag is not just an ID number (GUID); the tag chip can contain nonvolatile EEPROM for storing data. Lack of an onboard power supply means that the device can be quite small: commercially available products exist that can be embedded under the skin. Passive tags have practical read distances ranging from about 2 mm (ISO 14443) up to a few meters (EPC and ISO 18000-6) depending on the chosen radio frequency and antenna design/size. Due to their simplicity in design they are also suitable for manufacture with a printing process for the antennae. Passive RFID tags do not require batteries, and can be much smaller and have an unlimited life span. Because passive tags are cheaper to manufacture and have no battery, the majority of RFID tags in existence are of the passive variety. [1]
CMOS RF Power Rectifier Design
5
ii.
Semi-passive Tag Semi-passive RFID tags are very similar to passive tags except for the addition of a small battery. This battery allows the tag IC to be constantly powered, which removes the need for the aerial to be designed to collect power from the incoming signal. Aerials can therefore be optimized for the backscattering signal. Semi-passive RFID tags are faster in response and therefore stronger in reading ratio compared to passive tags.
iii.
Active Tag Unlike passive and semi-passive RFID tags, active RFID tags have their own internal power source which is used to power any ICs and generate the outgoing signal. They may have longer range and larger memories than passive tags, as well as the ability to store additional information sent by the transceiver. At present, the smallest active tags are about the size of a coin. Many active tags have practical ranges of tens of meters, and a battery life of up to 5 years.
CMOS RF Power Rectifier Design
6
C. Radio Frequency Identification Application •
RFID are used as replacement of barcode tags
•
High-frequency RFID tags are used in library book or bookstore tracking, pallet tracking, building access control, airline baggage tracking, and apparel and pharmaceutical item tracking. High-frequency tags are widely used in identification badges, replacing earlier magnetic stripe cards. These badges need only be held within a certain distance of the reader to authenticate the holder.
•
Systems for prepaying for unlimited public transport have been devised, making use of RFID technology. The design is embedded in a credit-card-like pass, that when scanned reveals details of whether the pass is valid, and for how long the pass will remain valid.
•
UHF RFID tags are commonly used commercially in case, pallet, and shipping container tracking, and truck and trailer tracking in shipping yards.
•
Microwave RFID tags are used in long-range access control for vehicles.
•
RFID tags are used for electronic toll collection at toll booths. The tags are read remotely as vehicles pass through the booths, and tag
CMOS RF Power Rectifier Design
7
information is used to debit the toll from a prepaid account. The system helps to speed traffic through toll plazas as it records the date, time, and billing data for the RFID vehicle tag. •
During the 2006 NASCAR racing season, the Goodyear Tire and Rubber Company began testing RFID tags provided by Advanced ID Corporation embedded in racing tires. It is expected that these tags will be commercially available by the end of 2006.
•
A number of ski resorts, particularly in the French Alps, have adopted RFID tags to provide skiers hands-free access to the lift system.
E. Overview The architecture of a passive microwave RFID transponder is shown in Fig. 0. The coupling element is an antenna, which typically is a dipole or a patch antenna. A voltage multiplier converts the input alternating voltage into a dc voltage, which is used by a series voltage regulator to provide the regulated voltage required for the correct operation of the transponder. The voltage multiplier is matched with the antenna in order to ensure the maximum power transfer from the transponder’s antenna to the input of the voltage
CMOS RF Power Rectifier Design
8
multiplier. A backscatter modulator is used to modulate the impedance seen by the transponder’s antenna, when transmitting. The RF section is then connected to the digital section, which typically is a very simple microprocessor or a finite-state machine able to manage the communication protocol.
Fig. 0 A Passive transponder architecture [7] The objective of this thesis is to design a RF power-rectifying unit that suits in the application of a RFID passive transponder and to maximize the operating read range. Special attention is given to the voltage multiplier design and the matching network. The rectifier is expected to give 1.5V and 1.5uA to the digital section with reasonable efficiency as shown in Fig. 0.
CMOS RF Power Rectifier Design
9
2. Theory A. Rectifier Rectification is a process whereby alternating current (AC) is converted into direct current (DC). Rectifiers are devices that perform this duty. Almost all rectifiers comprise a number of diodes in a specific arrangement for more efficiently converting AC to DC than is possible with just a single diode. Rectification is commonly performed by semiconductor diodes. Rectifiers can have various configurations and are chosen depending on applications.
Half-wave Rectifiers In a half-wave rectifier (Fig. 9), either the positive or negative half of the AC wave is passed easily while the other half is blocked, depending on the polarity of the rectifier. This configuration is adopted because of simplicity of circuits.
Full-wave Rectifiers Full-wave rectification converts both polarities of the input waveform to
CMOS RF Power Rectifier Design
10
DC, and is more efficient. However, more diodes are needed in this configuration. A full wave rectifier (Fig.11) converts the whole of the input waveform to one of constant polarity (positive or negative) at its output by reversing the negative (or positive) portions of the alternating current waveform. The positive (negative) portions thus combine with the reversed negative (positive) portions to produce an entirely positive (negative) voltage/current waveform.
Schottky Rectifiers Schottky rectifiers have been used for over 25 years in the power supply industry. The primary advantages are very low forward voltage drop and switching speeds that approach zero time making them ideal for output stages of switching power supplies. This latter feature has also stimulated their additional use in very high frequency applications including very low power involving signal and switching diode requirements of less than 100 picoseconds. These require small Schottky devices with low capacitance.
CMOS RF Power Rectifier Design
11
What little reverse recovery time they may exhibit is primarily dictated by their capacitance rather than minority carrier recombination as in conventional pn junction rectifiers. This characteristic provides very little reverse current overshoot when switching the Schottky from the forward conducting mode to the reverse blocking state. These make schottky rectifiers a very attractive choice for low parasitic switching losses.
Design considerations with Schottky devices are limited in some applications compared to pn junction rectifiers because their reverse leakage currents are many times higher. Also Schottky rectifiers have maximum rated junction temperatures typically in the range of 125°C to 175°C, compared to the typical 200°C for conventional pn junctions which further influences leakage current behavior. For some applications, Schottky devices are limited in available reverse blocking voltage ratings compared to conventional pn junction rectifiers.
The Schottky rectifier properties described above are primarily
CMOS RF Power Rectifier Design
12
determined by the metal energy barrier height of material deposited on the silicon by the manufacturer. A metal with a low energy barrier height will minimize forward voltage, but will also be restricted in its high temperature operating capability and have very high reverse leakage currents. A high barrier metal height selection will minimize temperature and leakage current sensitivity but will increase the forward voltage. [3]
B. Voltage Multiplier i.
Traditional Voltage Multipliers Traditional voltage multipliers make use of schottky diodes that have low series resistance and allow for a high conversion efficiency of the received RF input signal energy to dc supply voltage.
CMOS RF Power Rectifier Design
13
Fig. 1 Schematic of the voltage multiplier converting RF input signal to dc supply voltage
Schottky diodes have advantages of fast switching speed and low forward voltage drop. Due to these excellent high frequency performances, they have been widely used in power detection and microwave network circuit. Schottky diodes are often fabricated by depositing metals on n-type or p-type semiconductor materials such as GaAs and SiC. The properties of forward-biased Schottky diodes are determined by majority carrier phenomena, while minority carriers primarily determine those properties for p-n diodes. In order to increase high frequency performance and decrease the
CMOS RF Power Rectifier Design
14
supply voltage of IC, integrating the Schottky diode into modern IC is very important. But the processes can integrate Schottky diode are often not commercially available and don’t have the capability of integrating CMOS circuits monolithically with them.
In [5], a voltage multiplier using a self-designed schottky diode model with a multiplier stage of 5 obtains a conversion efficiency of 9.6% in the operating frequency of 915MHz. This is not a fair nor complete comparison to the design listed in this thesis but provides, to some extend, a reference. A simulation of the schottky diode voltage multiplier is not available in this thesis because there is no dedicated industry standard compact Model to be used for circuit simulation.
The design parameters of the voltage multiplier are a tradeoff between power efficiency, useful impedance, and operating point (load). Optimization parameters include the number of stages, the size of Schottky diodes, and the size of coupling capacitors.
CMOS RF Power Rectifier Design
15
To get a good efficiency, it is very important to have Schottky diodes with large saturation current (resulting in low forward voltage drop) and a low junction capacitance, as well as a small series resistance and small parasitic capacitance to substrate. Larger Schottky diodes have a larger saturation current and a smaller series resistance, but also larger junction and substrate capacitances, which then may dominate the power losses, so that an optimum size of the Schottky diode has to be found. Similarly, for the coupling capacitors, it is also important to have small series resistance and parasitic capacitance to substrate.
ii.
CMOS Voltage Multipliers Schottky diodes are generally used for its low conduction resistance and low junction capacitance. However, the particularity of manufacturing processes for Schottky diodes and the inconsistency in quality between different product batches often make the integration of Schottky voltage multiplier incompatible with
CMOS RF Power Rectifier Design
16
standard CMOS circuits and thus limit its applications. [6] Therefore, instead of using expensive Schottky diodes, voltage multipliers are replaced by MOS-connected diodes with standard CMOS technologies as shown in Fig. 2 below.
Fig. 2 Schematic of the simplified voltage multiplier
iii.
Parameter Analysis In a voltage multiplier, there are several parameters that can be optimized and designed for a tradeoff of high conversion efficiency and output voltage: •
Number of multiplier stage
CMOS RF Power Rectifier Design
17
By analyzing a single unit voltage multiplying cell, as shown in Fig. 3 below, multiplying capacitor Cn-1 and Cn can look like a pair of DC voltage sources; Cc is a coupling capacitor that combines input voltage Vi and Vn-1, voltage drop on Cn-1, to provide voltage for the next multiplier. Suppose Cdn-1 is the voltage drop on NMOS FET Mn-1, Vdn for Mn and Vc is the DC voltage at point C, under steady-state condition, Vc = Vn-1 – Vdn-1, Vc = Vn + Vdn
(1)
If W/L of 2 MOSFETs is identical, we have Vdn = Vdn-1, ∴Vc = (Vn + Vn-1)/2
(2)
The actual input signal for Mn is Vc + Vi. Assume ΔV is the unit voltage increment, then ΔV = Vi – Vd, ∴(Vn + Vn-1)/2 +ΔV = Vn, Vn = Vn-1 + 2ΔV
(3)
If redefining a pair of MOSFET and capacitor as new unit voltage multiplying cell, the stage number becomes 2, such that ∴Vn = Vn-2 + 2ΔV
(4)
where n = 2k+1, k is the ordinal number of initial unit voltage multiplying cells and equal to 1,2,3… With the same aspect ratio for
CMOS RF Power Rectifier Design
18
all the MOSFETs in the charge pump, everyΔV would be identical. Further iterating the formula, we get Vn = Vn-4 + 4ΔV = Vn-6 + 6ΔV
(5)
Finally, V = nΔV = n(Vi - Vd) where n is the number of multiplying stages. In RFID applications, due to the lack of input power, output voltage and conversion efficiency of the voltage multiplier are hence 2 primary performance parameters. [6]
Fig. 3 Single unit voltage multiplying cell
•
Coupling capacitor In order ensure a small ripple in the output voltage Vout, the coupling capacitors have to be dimensioned so that their time constant is much larger than the period of the input signal,
CMOS RF Power Rectifier Design
19
that is, Iout/(2πCVout)
Author:
Law Carlos
Student I.D.:
03614511
Supervisor:
Professor K.K.Cheng
Associate Examiner:
Professor H.P.Ho
A project report presented to the Chinese University of Hong Kong In partial fulfilment of the Degree of Bachelor of Engineering
Department of Electronic Engineering The Chinese University of Hong Kong
April, 2006
i
Abstract A set of design criteria for the radio frequency section of a passive RF identification (RFID) transponders operating at 900MHz industrial, scientific and medical (ISM) range is derived in this thesis report. Particular focus is put on the analysis, verification and IC implementation of the voltage multiplier and the power matching sections. A reader-to-tag read range of around 4m is achieved with a 4W Effective Isotropic Radiated Power (EIRP) from the base station. A voltage multiplier sensitivity of –14dBm is achieved with proper power matching to give at the output of the voltage multiplier an approximate 1.5V and 1.5A to drive the digital section of the RFID passive transponder with a 10% conversion efficiency. Two IC designs are submitted to Austriamicrosystems for manufacturing. The first design includes the complete set of the voltage multiplier circuit. The second has the input capacitors omitted to see the effect of such an omission to conversion efficiency and size reduction.
ii
Acknowledgements The author would like to thank his supervisor, K.K. Cheng, for his patience and kind suggestions during the course of the project undertaking, and his insights in assigning an experienced tutor to assist in the guidance and troubleshooting of the CADENCE design tool. He would also like to thank F.L. Wong, W.F. Chung, and C.F. Au Yeung for their invaluable technical support in the project. H.Y. Yim, C.P. Kong and K.K. Tse have also been very helpful in the microwave laboratory.
iii
Contents Abstract
i
Acknowledgements Contents 1.
iii
Introduction A.
1
Radio Frequency Identification (RFID) i.
Brief History
1
ii.
Key Features
2
B.
Types of RFID Tag i.
Passive
ii.
Semi-passive
iii.
Active
C.
3 5
9
Rectifiers
B.
Voltage Multiplier (VM)
9 12
Traditional Voltage Multipliers
ii.
CMOS Voltage Multipliers
iii.
Parameter Analysis Matching
D. Antenna E.
16
32
Limiter and Regulator
Rectifiers
34 40
40
12 15
28
Simulation Results A.
6
7
A.
i.
3.
3
Radio Frequency Identification Application
Theory
C.
1
5
D. Overview 2.
ii
iv
i.
Comparison
40
ii.
Conclusion
45
B.
C.
Voltage Multiplier i.
Schematic
46
ii.
Layout
51
Matching
57
D. Proposed Layout 4.
Discussions
46
60 61
A.
Comparison of performance
B.
Design Limitations
C.
Further Improvement Work
5.
Conclusions
64
6.
References
65
61 62 63
1
1. Introduction A. Radio Frequency Identification (RFID) Radio Frequency Identification (RFID) is an automatic identification method, relying on storing and remotely retrieving data using devices called RFID tags or transponders. An RFID tag is a small object that can be attached to or incorporated into a product, animal, or person. RFID tags contain silicon chips and antennas to enable them to receive and respond to radio-frequency queries from an RFID transceiver. Passive tags require no internal power source, whereas active tags require a power source. [1]
i.
Brief History In 1945 Léon Theremin invented an espionage tool for the Soviet government which retransmitted incident radio waves with audio information. Even though this device was a passive covert listening device, not an identification tag, it has been attributed as the first known device and a predecessor to RFID technology. The technology used in RFID has been around since the early 1920s according to one source. [1]
CMOS RF Power Rectifier Design
2
ii.
Key Features [2] •
Not line of sight WID tags do need to be visible to be read / Written.
•
Robust Because they don't need to be visible, they can be encased within rugged materials protecting them from the environment they are being used in. This means they can be used in harsh fluid and chemical environments and rough handling situations.
•
Read speed Tags can be read from significant distances (especially the active variety) and can also be read very quickly. This is especially useful when the items needing to be identified are moving quickly for example on a conveyor.
•
Reading multiple items A number of tagged items can be read at the same time within a RF field. This cannot be done as easily with "visual" identifiers.
•
Security Because tags cm be enclosed, they are much more difficult to tarnper
CMOS RF Power Rectifier Design
3
with, A number of tag types now also come programmed with a unique identifier (Serial Identification) which is guaranteed to be unique throughout the world. •
Programmability Many tags are read / write capable, rather than read only. This means that information can be written to the tag, perhaps to show that the item being tagged has gone through a particular process, or that it's condition or status has changed somehow. Or in some instances to store information about the tagged items e.g. the results of a test that it has undergone.
B. Types of RFID Tag RFID tags can be classified as passive, semi-passive, or active. i.
Passive tag Passive RFID tags have no internal power supply. The minute electrical current induced in the antenna by the incoming radio frequency signal provides just enough power for the CMOS integrated circuit (IC) in the tag to power up and transmit a
CMOS RF Power Rectifier Design
4
response. Most passive tags signal by backscattering the carrier signal from the reader. This means that the aerial (antenna) has to be designed to both collect power from the incoming signal and also to transmit the outbound backscatter signal. The response of a passive RFID tag is not just an ID number (GUID); the tag chip can contain nonvolatile EEPROM for storing data. Lack of an onboard power supply means that the device can be quite small: commercially available products exist that can be embedded under the skin. Passive tags have practical read distances ranging from about 2 mm (ISO 14443) up to a few meters (EPC and ISO 18000-6) depending on the chosen radio frequency and antenna design/size. Due to their simplicity in design they are also suitable for manufacture with a printing process for the antennae. Passive RFID tags do not require batteries, and can be much smaller and have an unlimited life span. Because passive tags are cheaper to manufacture and have no battery, the majority of RFID tags in existence are of the passive variety. [1]
CMOS RF Power Rectifier Design
5
ii.
Semi-passive Tag Semi-passive RFID tags are very similar to passive tags except for the addition of a small battery. This battery allows the tag IC to be constantly powered, which removes the need for the aerial to be designed to collect power from the incoming signal. Aerials can therefore be optimized for the backscattering signal. Semi-passive RFID tags are faster in response and therefore stronger in reading ratio compared to passive tags.
iii.
Active Tag Unlike passive and semi-passive RFID tags, active RFID tags have their own internal power source which is used to power any ICs and generate the outgoing signal. They may have longer range and larger memories than passive tags, as well as the ability to store additional information sent by the transceiver. At present, the smallest active tags are about the size of a coin. Many active tags have practical ranges of tens of meters, and a battery life of up to 5 years.
CMOS RF Power Rectifier Design
6
C. Radio Frequency Identification Application •
RFID are used as replacement of barcode tags
•
High-frequency RFID tags are used in library book or bookstore tracking, pallet tracking, building access control, airline baggage tracking, and apparel and pharmaceutical item tracking. High-frequency tags are widely used in identification badges, replacing earlier magnetic stripe cards. These badges need only be held within a certain distance of the reader to authenticate the holder.
•
Systems for prepaying for unlimited public transport have been devised, making use of RFID technology. The design is embedded in a credit-card-like pass, that when scanned reveals details of whether the pass is valid, and for how long the pass will remain valid.
•
UHF RFID tags are commonly used commercially in case, pallet, and shipping container tracking, and truck and trailer tracking in shipping yards.
•
Microwave RFID tags are used in long-range access control for vehicles.
•
RFID tags are used for electronic toll collection at toll booths. The tags are read remotely as vehicles pass through the booths, and tag
CMOS RF Power Rectifier Design
7
information is used to debit the toll from a prepaid account. The system helps to speed traffic through toll plazas as it records the date, time, and billing data for the RFID vehicle tag. •
During the 2006 NASCAR racing season, the Goodyear Tire and Rubber Company began testing RFID tags provided by Advanced ID Corporation embedded in racing tires. It is expected that these tags will be commercially available by the end of 2006.
•
A number of ski resorts, particularly in the French Alps, have adopted RFID tags to provide skiers hands-free access to the lift system.
E. Overview The architecture of a passive microwave RFID transponder is shown in Fig. 0. The coupling element is an antenna, which typically is a dipole or a patch antenna. A voltage multiplier converts the input alternating voltage into a dc voltage, which is used by a series voltage regulator to provide the regulated voltage required for the correct operation of the transponder. The voltage multiplier is matched with the antenna in order to ensure the maximum power transfer from the transponder’s antenna to the input of the voltage
CMOS RF Power Rectifier Design
8
multiplier. A backscatter modulator is used to modulate the impedance seen by the transponder’s antenna, when transmitting. The RF section is then connected to the digital section, which typically is a very simple microprocessor or a finite-state machine able to manage the communication protocol.
Fig. 0 A Passive transponder architecture [7] The objective of this thesis is to design a RF power-rectifying unit that suits in the application of a RFID passive transponder and to maximize the operating read range. Special attention is given to the voltage multiplier design and the matching network. The rectifier is expected to give 1.5V and 1.5uA to the digital section with reasonable efficiency as shown in Fig. 0.
CMOS RF Power Rectifier Design
9
2. Theory A. Rectifier Rectification is a process whereby alternating current (AC) is converted into direct current (DC). Rectifiers are devices that perform this duty. Almost all rectifiers comprise a number of diodes in a specific arrangement for more efficiently converting AC to DC than is possible with just a single diode. Rectification is commonly performed by semiconductor diodes. Rectifiers can have various configurations and are chosen depending on applications.
Half-wave Rectifiers In a half-wave rectifier (Fig. 9), either the positive or negative half of the AC wave is passed easily while the other half is blocked, depending on the polarity of the rectifier. This configuration is adopted because of simplicity of circuits.
Full-wave Rectifiers Full-wave rectification converts both polarities of the input waveform to
CMOS RF Power Rectifier Design
10
DC, and is more efficient. However, more diodes are needed in this configuration. A full wave rectifier (Fig.11) converts the whole of the input waveform to one of constant polarity (positive or negative) at its output by reversing the negative (or positive) portions of the alternating current waveform. The positive (negative) portions thus combine with the reversed negative (positive) portions to produce an entirely positive (negative) voltage/current waveform.
Schottky Rectifiers Schottky rectifiers have been used for over 25 years in the power supply industry. The primary advantages are very low forward voltage drop and switching speeds that approach zero time making them ideal for output stages of switching power supplies. This latter feature has also stimulated their additional use in very high frequency applications including very low power involving signal and switching diode requirements of less than 100 picoseconds. These require small Schottky devices with low capacitance.
CMOS RF Power Rectifier Design
11
What little reverse recovery time they may exhibit is primarily dictated by their capacitance rather than minority carrier recombination as in conventional pn junction rectifiers. This characteristic provides very little reverse current overshoot when switching the Schottky from the forward conducting mode to the reverse blocking state. These make schottky rectifiers a very attractive choice for low parasitic switching losses.
Design considerations with Schottky devices are limited in some applications compared to pn junction rectifiers because their reverse leakage currents are many times higher. Also Schottky rectifiers have maximum rated junction temperatures typically in the range of 125°C to 175°C, compared to the typical 200°C for conventional pn junctions which further influences leakage current behavior. For some applications, Schottky devices are limited in available reverse blocking voltage ratings compared to conventional pn junction rectifiers.
The Schottky rectifier properties described above are primarily
CMOS RF Power Rectifier Design
12
determined by the metal energy barrier height of material deposited on the silicon by the manufacturer. A metal with a low energy barrier height will minimize forward voltage, but will also be restricted in its high temperature operating capability and have very high reverse leakage currents. A high barrier metal height selection will minimize temperature and leakage current sensitivity but will increase the forward voltage. [3]
B. Voltage Multiplier i.
Traditional Voltage Multipliers Traditional voltage multipliers make use of schottky diodes that have low series resistance and allow for a high conversion efficiency of the received RF input signal energy to dc supply voltage.
CMOS RF Power Rectifier Design
13
Fig. 1 Schematic of the voltage multiplier converting RF input signal to dc supply voltage
Schottky diodes have advantages of fast switching speed and low forward voltage drop. Due to these excellent high frequency performances, they have been widely used in power detection and microwave network circuit. Schottky diodes are often fabricated by depositing metals on n-type or p-type semiconductor materials such as GaAs and SiC. The properties of forward-biased Schottky diodes are determined by majority carrier phenomena, while minority carriers primarily determine those properties for p-n diodes. In order to increase high frequency performance and decrease the
CMOS RF Power Rectifier Design
14
supply voltage of IC, integrating the Schottky diode into modern IC is very important. But the processes can integrate Schottky diode are often not commercially available and don’t have the capability of integrating CMOS circuits monolithically with them.
In [5], a voltage multiplier using a self-designed schottky diode model with a multiplier stage of 5 obtains a conversion efficiency of 9.6% in the operating frequency of 915MHz. This is not a fair nor complete comparison to the design listed in this thesis but provides, to some extend, a reference. A simulation of the schottky diode voltage multiplier is not available in this thesis because there is no dedicated industry standard compact Model to be used for circuit simulation.
The design parameters of the voltage multiplier are a tradeoff between power efficiency, useful impedance, and operating point (load). Optimization parameters include the number of stages, the size of Schottky diodes, and the size of coupling capacitors.
CMOS RF Power Rectifier Design
15
To get a good efficiency, it is very important to have Schottky diodes with large saturation current (resulting in low forward voltage drop) and a low junction capacitance, as well as a small series resistance and small parasitic capacitance to substrate. Larger Schottky diodes have a larger saturation current and a smaller series resistance, but also larger junction and substrate capacitances, which then may dominate the power losses, so that an optimum size of the Schottky diode has to be found. Similarly, for the coupling capacitors, it is also important to have small series resistance and parasitic capacitance to substrate.
ii.
CMOS Voltage Multipliers Schottky diodes are generally used for its low conduction resistance and low junction capacitance. However, the particularity of manufacturing processes for Schottky diodes and the inconsistency in quality between different product batches often make the integration of Schottky voltage multiplier incompatible with
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standard CMOS circuits and thus limit its applications. [6] Therefore, instead of using expensive Schottky diodes, voltage multipliers are replaced by MOS-connected diodes with standard CMOS technologies as shown in Fig. 2 below.
Fig. 2 Schematic of the simplified voltage multiplier
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Parameter Analysis In a voltage multiplier, there are several parameters that can be optimized and designed for a tradeoff of high conversion efficiency and output voltage: •
Number of multiplier stage
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By analyzing a single unit voltage multiplying cell, as shown in Fig. 3 below, multiplying capacitor Cn-1 and Cn can look like a pair of DC voltage sources; Cc is a coupling capacitor that combines input voltage Vi and Vn-1, voltage drop on Cn-1, to provide voltage for the next multiplier. Suppose Cdn-1 is the voltage drop on NMOS FET Mn-1, Vdn for Mn and Vc is the DC voltage at point C, under steady-state condition, Vc = Vn-1 – Vdn-1, Vc = Vn + Vdn
(1)
If W/L of 2 MOSFETs is identical, we have Vdn = Vdn-1, ∴Vc = (Vn + Vn-1)/2
(2)
The actual input signal for Mn is Vc + Vi. Assume ΔV is the unit voltage increment, then ΔV = Vi – Vd, ∴(Vn + Vn-1)/2 +ΔV = Vn, Vn = Vn-1 + 2ΔV
(3)
If redefining a pair of MOSFET and capacitor as new unit voltage multiplying cell, the stage number becomes 2, such that ∴Vn = Vn-2 + 2ΔV
(4)
where n = 2k+1, k is the ordinal number of initial unit voltage multiplying cells and equal to 1,2,3… With the same aspect ratio for
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all the MOSFETs in the charge pump, everyΔV would be identical. Further iterating the formula, we get Vn = Vn-4 + 4ΔV = Vn-6 + 6ΔV
(5)
Finally, V = nΔV = n(Vi - Vd) where n is the number of multiplying stages. In RFID applications, due to the lack of input power, output voltage and conversion efficiency of the voltage multiplier are hence 2 primary performance parameters. [6]
Fig. 3 Single unit voltage multiplying cell
•
Coupling capacitor In order ensure a small ripple in the output voltage Vout, the coupling capacitors have to be dimensioned so that their time constant is much larger than the period of the input signal,
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that is, Iout/(2πCVout)
