A Charge-Pump Type AC-DC Converter for Remote Power Feeding to a RFID Tag

Transcription

1 A Charge-Pump ype AC-DC Converter for Remote Power Feeding to a RFID ag 37 A Charge-Pump ype AC-DC Converter for Remote Power Feeding to a RFID ag Kei Eguchi 1, akahiro Inoue 2, Hongbing Zhu 3, and Fumio Ueno 4, Non-members ABSRAC In RFID (Radio Frequency Identification) tag systems, remote power feeding systems using RF electromagnetic induction are used. o convert an AC voltage provided by power receiving coils, AC-DC converters which supply the electric power to signal processing circuits are needed. In conventional AC- DC converters, however, the decrease in the power efficiency caused by the threshold voltage of diodeswitches is a serious problem. In this paper, a chargepump type AC-DC converter for RFID tags is proposed. By using novel bootstrap circuits, the proposed converter can avoid the threshold voltage drop caused by diode-switches. Hence the power efficiency of the converter is improved effectively. he validity of the proposed converter is confirmed through SPICE simulations. he SPICE simulations showed that the power efficiency of the proposed converter is more than 85 % and the electric power is about 2mW when an output load is 500 Ω. Furthermore, the properties concerning power efficiency and ripple voltage are analyzed theoretically. Keywords: AC-DC Converters, Switched-Capacitor Circuits, Charge-Pump Circuits, RFID ags, RF Electromagnetic Induction 1. INRODUCION Recently, RFID (Radio Frequency Identification) tag systems [1-4] have been receiving much attention in the field of transportation, traffic control, and so on. In the RFID tag systems, remote power feeding systems using RF electro-magnetic induction are usually used in order to supply the electric power to signal processing circuits in IC tags. hus, to convert an AC voltage provided by power receiving coils, AC-DC converters [5-10] are needed. For example, step-up type AC-DC converters are used in the smart RFID tag systems proposed in [5-8]. In the design Manuscript received on January 16, 2007 ; revised on April 15, he author is with Dept. of echnology Education, Shizuoka University, Shizuoka, , Japan, s:ekeguch@ipc.shizuoka.a.c.jp 2 he author is with Dept. of Computer Science and Electrical Engineering, Kumamoto University, Kumamoto, , Japan 3 he author is with Dept. of Computer Science, Hiroshima Kokusai Gakuin University, Hiroshima, , Japan 4 he author is with Dept. of Software Science, Sojo University, Ikeda, Kumamoto, , Japan Power receiving coils 1 =D Fig.1: C1+ 2 =(1-D) D2+ D1+ (or ) Converter Block-1 (Charge-pump type converter) C2 Converter Block-2 (Charge-pump type converter) 1... D1+ (or D1-) is on D2+ (or D2-) is on. Conventional AC-DC converter. Fig.2: t D2- D1- C1- Rin- Power receiving coils. of the AC-DC converters for RFID tags, to suppress adverse effects caused by heat, the converters which can realize high power efficiency as well as small chip area are desirable. However, in the conventional converters [5-8], threshold voltage drop caused by diodeswitches decreases the power efficiency. In this paper, a charge-pump type [5-8,11-12] AC- DC converter for RF ID tags is proposed. Since the proposed converter employing novel bootstrap circuits can avoid the threshold voltage drop caused by diode-switches, the power efficiency of the converter is improved effectively. o confirm the validity of the circuit design, SPICE simulations are performed concerning the proposed converter and the conventional converter [6-8]. Furthermore, the properties concerning power efficiency and ripple voltage are analyzed theoretically. 2. CIRCUI SRUCURE 2. 1 Conventional Circuit Figure 1 shows a charge-pump type AC-DC converter proposed in [6-8]. In RFID tag systems, remote power feeding systems using RF electromagnetic induction are used. In Fig.1, a pair of power receiving

2 38 ECI RANSACIONS ON ELECRICAL ENG., ELECRONICS, AND COMMUNICAIONS VOL.5, NO.2 August =D Cb+ Vup+ 2 =(1-D) Selector Block-1 C1+ Selector Block-2 t M2+ Vo+ M1+ M2- M1- Converter Block-1 C2 (=C2++C2-) Bootstrap M1+ (or M1-) is on M2+ (or M2-) is on. q 1,Vin q 2,Vin Rδ1 C1+ Rδ1 Ron2 (a) (b) Ron1 Rδ2 Rδ2 q 1, C2+ q 2, C2+ Fig.5: Instantaneous equivalent circuits of converter block-1. (a) State-1. (b) State-2. Ms3 Ms1 Fig.4: 2. (or ) Fig.3: Vup+ (a) Proposed AC-DC converter. Ms4 Vo+ Ms2 Ms3 Ms1 Ms4 Vo- Vup- (b) C1- Mb+ Bootstrap-1 Power receiving coils Rin- Converter Block-2 Mb- Cb- Vup- Vo- Ms2 Selector blocks. (a) Selector-1. (b) Selector- coils is modeled by a pair of AC voltage sources with opposite polarities. Figure 2 shows a model of the power receiving coils. he conventional converter of Fig.1 consists of 2 charge-pump type AC-DC converters with opposite polarities. he converter shown in Fig.1 can supply a stepped-up DC voltage. After the AC-DC conversion, the output DC voltage is regulated by a series regulator. For easy understanding of the circuit operation, let us consider the converter block-1. When the input voltage V in+ is negative, the diode D 2+ is turned on. hen the voltage of C 1+ becomes about V m V th, where V m and V th denote the amplitude of AC input and the threshold voltage of the diode, respectively. Next, when the input voltage V in+ is positive, the diode D 1+ is turned on. hen the output voltage of the converter becomes about 2(V m V th ). Hence, the threshold voltage drop caused by diodes has an influence on the power efficiency of the conventional converter. In this paper, we solve this problem by using novel bootstrap circuits Proposed Circuit Figure 3 shows the proposed AC-DC converter. he converter is designed to receive power by elec- tromagnetic induction in the dozens MHz range. In Fig.3, the transistors with bootstrap circuits are used instead of the diodes in Fig.1. he selector blocks in Fig.3 are shown in Fig.4. he operation of the proposed converter is as follows. When the input voltage V in+ is negative, the transistor M 2+ is turned on, because the gate voltage of M 2+ is connected to another input terminal with opposite polarity, V in. herefore, the voltage of C 1+ becomes V m. At the same time, the capacitor C b+ is charged up to 2V m V thb, where V thb denotes the threshold voltage of the diode connected transistor M b+. And the transistor M 1+ is turned off, because the gate terminal of M 1+ is grounded via M S2 in the selector block-1. Next, when the input voltage V in+ is positive, the transistor M 2+ is turned off. At the same time, 3V m V thb is given as the output of the selector block-1. herefore, the transistor M 1+ is turned on, because the gate voltage of M 1+ is larger than the output voltage of the converter block. Hence, the output voltage of the converter becomes about 2V m. 3. HEOREICAL ANALYSIS 3. 1 A. Equivalent Circuit and Power Efficiency he equivalent circuit and the power efficiency of the proposed converter are analyzed theoretically. o simplify the theoretical analyses, we assume that the time constant is quite larger than and parasitic elements are not effective. Firstly, the equivalent circuit of the converter block-1 is analyzed. he instantaneous equivalent circuits of the converter block can be expressed by the circuit shown in Fig.5. In Fig.5, R δk (k = 1, 2) denotes a resistor to model a dielectric loss. In the steady state, the differential values of the electric charges in C 1+ and C 2+ satisfy q k 1 + q k 2 = 0 (k = 1, 2), (1)

3 A Charge-Pump ype AC-DC Converter for Remote Power Feeding to a RFID ag 39 where q k 1 and qk 2 denote the electric charges when 1 and 2, respectively. he intervals of State 1 and State 2, 1 and 2, satisfy the following conditions: Vin I in RSC I out = 1 + 2, 1 = D, and 2 = (1 D), (2) where is a period of the input voltage and D denotes a duty factor (see in Fig.3). In the case of 1, the currents which flow C k+ and R δk are given by q k 1/ 1 and (q k 1/ 1 ) tan δ k, respectively, where δ k denotes a dielectric loss angle. hus the differential values of the electric charges in the input and the output terminals, q 1,Vin and q 1,, are given by q 1,Vin = q 1 1(1 + tan δ 1 ) and q 1, = q 2 1(1 + tan δ 2 ) q 1 1(1 + tan δ 1 ), (3) respectively. On the other hand, in the case of 2, the currents which flow C k+ and R δk are given by q k 2/ 2 and (q k 2/ 2 ) tan δ k, respectively. hus q 2,Vin and q 2,, are given by q 2,Vin = q 1 2(1 + tan δ 1 ) and q 2, = q 2 2(1 + tan δ 2 ), (4) respectively. Here, the electric charges in the input and the output, q Vin and q, are given by q Vin = q 1,Vin + ( q 2,Vin ), and q = q 1, + q 2,, (5) respectively. By substituting Eqs.(1), (3), and (4) into Eq.(5), the following equations are derived: q Vin = 2 q 1 1(1 + tan δ 1 ), q = q 1 1(1 + tan δ 1 ), q Vin = 2 q, and I in = 2I out, (6) where I in and I out denote an averaged input current and an averaged output current, respectively. In Fig.5, the energy consumed by resistors in one period, W, can be expressed by W = W 1 + W 2, (7) Fig.6: where Equivalent circuit of step-up SC converter. W 1 = (R in + R on1 ) { q 1 1 (1 + tan δ 1)} R δ1 ( q 1 1 tan δ R δ2 ( q 2 1 tan δ 2 1 and W 2 = (R in + R on2 ) { q 1 2 (1 + tan δ 1)} R δ2 ( q 2 2 tan δ 2. 2 In Fig.5 (a) and (b), the following equations can be obtained by Kirchhoff s law: V in 1 = R δ2 q 2 1 tan δ 2 R δ1 q 1 1 tan δ 1 (R in + R on1 )(1 + tan δ 1 ) q 1 1 and V in 2 = R δ1 q 1 2 tan δ 1 +(R in + R on2 )(1 + tan δ 1 ) q 1 2.(8) By substituting (2) into (8), the following equation can be obtained: q 2 1 tan δ 2 = q 1 1 +(1 + tan δ 1 )( R in + R on1 D D R δ2 {(R δ1 tan δ 1 )( 1 D D ) + R in + R on2 )}. (9) 1 D When the proposed circuit satisfies the following conditions: D = 1/2, R on R on1 = R on2, R δ R δ1 = R δ2, and tan δ tan δ 1 = tan δ 2, (10) Eq.(9) can be rewritten as q tan δ = q {R in + R on + (R in + R on + R δ ) tan δ}.(11) Under the conditions of Eq.(10), we derive the following equations by substituting Eqs.(1), (6) and (11)

4 40 ECI RANSACIONS ON ELECRICAL ENG., ELECRONICS, AND COMMUNICAIONS VOL.5, NO.2 August 2007 into Eq.(7): W 1 = 2(R in + R on ) ( q V out tan δ + 2R δ ( ( q 1 + tan δ )2 V out and + 2R δ [ 2{R in + R on + (R in + R on + R δ ) tan δ} ] 2 W 2 = 2(R in + R on ) ( q V out ( q V out + 2R δ [ 2{R in + R on + (R in + R on + R δ ) tan δ} ] 2 ( q V out. (12) Here, it is known that a general equivalent circuit of SC power converters can be expressed by the circuit of Fig.6 [13-14], where V in denotes an averaged voltage of V in+ and V out is an averaged voltage of V out. he consumed energy W in Fig.6 is defined by W = W 1 + W 2 ( q V out R SC. (13) From Eqs.(12) and (13), the resistance R SC in Fig.6 is expressed by tan δ R SC = 4(R in + R on ) + 2R δ ( 1 + tan δ )2 +16R δ { R in + R on + (R in + R on + R δ ) tan δ } 2.(14) Here, the dielectric loss tangent, tan δ, is given by tan δ = Hence, Eq.(14) can be rewritten as R SC = 4(R in + R on ) + 1 2πfCR δ. (15) 2R δ (1 + 2πfCR δ + 16 R δ { 2πfCR δ(r in + R on ) + (R in + R on + R δ ) 1 + 2πfCR δ } 2. (16) he equivalent circuit of Fig.6 is expressed by the determinant using Kettenmatrix. herefore, by using Eqs.(6) and (16), the equivalent circuit of the converter block can be given by the following determinant: [ Vin I in ] = [ 1/ ] [ 1 RSC 0 1 ] [ I out ]. (17) Hence, from Eq.(17), the equivalent circuit of the proposed converter can be expressed by the circuit shown Vin Fig.7: Fig.8: 1. I in 1 : 2 1 : 2 Rsc Rsc I out Equivalent circuit of proposed converter. Vm C1+ Ron1 Vm-(Ron2)IL Q C2+ Instantaneous equivalent circuit in State in Fig.7. From Fig.7, the power efficiency η can be given by η = = (I out R L (I out /2 R SC + (I out /2 R SC + (I out R L R L. (18) R SC /2 + R L 3. 2 Ripple Voltage In this subsection, the ripple voltage of the converter block-1 is analyzed by assuming that R δ1 = R δ2 =. In State 2, we define that the electric charge 2 Q consumed by the output load R L is charged in C 1+. hen, 2 Q can be expressed by IL R L 2 Q = I L (1 D), (19) where I L denotes an averaged charge current of C 1+. In the steady state, the instantaneous equivalent circuit of Fig.5 (a) can be rewritten as the circuit shown in Fig.8, where V m and V out denote a positive maximum value of the input voltage and an averaged output voltage, respectively. From Fig.8, by assuming the steady state, the following equation can be obtained by Kirchhoff s voltage law: 2V m (R in + R on2 )I L = V out + (R in + R on1 )(I L + 2 Q/D ) + Q/C 2+ + ( Q + I L D/2)/C 1+ and V out = I L R L. (20) From Eqs.(19) and (20), the averaged output voltage V out can be given by V out = 2DR LV m Dm, (21)

5 A Charge-Pump ype AC-DC Converter for Remote Power Feeding to a RFID ag 41 where Power Efficiency (%) Conventional Proposed Output Load (Ω) Fig.9: Ripple Voltage (V) Simulated power efficiency. Proposed Conventional Output Load (Ω) Fig.10: Simulated ripple voltage. Figure 9 shows the simulated power efficiency. he SPICE simulations of Fig.9 were performed under the conditions that V in+ = V in = 1V@20MHz, C 1+ = C 1 = C 2+ = C 2 = 1nF, C p = 1 pf 1, and R in = 0.2 Ω. As Fig.9 shows, the proposed converter can improve the power efficiency at high load. When the output load is 500 Ω, the power efficiency of the proposed converter is more than 85 % and the electric power is about 2 mw. Figure 10 shows the simulated ripple voltage. As Fig.10 shows, the ripple voltage of the proposed converter and the conventional converter is almost the same. When the output load is 500 Ω, the ripple voltage is about 0.03 V. 5. CONCLUSION In this paper, a charge-pump type AC-DC converter for RF ID tags has been proposed. Since the proposed converter using bootstrap circuits can avoid the threshold voltage drop caused by diode-switches, the power efficiency of the converter is improved effectively. he SPICE simulations showed that the power efficiency of the proposed converter is more than 85 % and the electric power is about 2 mw when the output load is 500 Ω. Furthermore, the properties concerning power efficiency and ripple voltage were analyzed theoretically. he IC implementation of the proposed converter is left to the future study. Dm = {(D + 1)R in + DR L + R on1 + DR on2 } + D 2 (1 D + 1 ). C 2+ C 1+ When the converter block satisfies the conditions of Eq.(10), Eq.(21) can be rewritten as V out = 8CR L V m 4C{3(R in + R on ) + R L } + 3. (22) Here, from Fig.8 and Eqs.(19) and (20), the ripple voltage of the converter block-1, V rip 1, can be expressed by V rip 1 = Q C 4. SIMULAION = I L(1 D) 2C = 4CR L V out. (23) o confirm the validity of circuit design, SPICE simulations were performed concerning the circuit shown in Figs.1 and 3. o be compatible with process limitations on the maximum allowable voltage on a chip, we adopted a 2-metal 2-poly 1.2 µm CMOS process provided by VDEC (VLSI Design and Education Center, the University of okyo). 6. ACKNOWLEDGEMENS his work is supported by the Ministry of Education, Culture, Sports, Science and echnology, Grantin-Aid for Scientific Research (B), , And the CAD tools used in this work is supported by VLSI Design and Education Center (VDEC), the University of okyo in collaboration with On- Semiconductor, Nippon Motorola LD., HOYA Corporation, and KYOCERA Corporation. References [1] S.Shepard, RFID: Radio Frequency Identification, New York: McGraw-Hill, [2] K.Rongsawat and A.hanachayanont, Ultra low power analog front-end for UHF RFID transponder, Proc. of the International Symposium on Communications and Information echnologies 2006, F4D-1 (CD-ROM), Oct [3] N.Rueangsri and A.hanachayanont, Coil design for optimum operating range of magnetically-coupled RFID system, Proc. of the International Symposium on Communications and Information echnologies 2006, F4D-2 (CD-ROM), Oct As Figs.1, 2, 4 show, the proposed circuit is inferior to the conventional circuit in the point of circuit size. However, the size of bootstrap circuit is small such as C p = 1 pf.

6 42 ECI RANSACIONS ON ELECRICAL ENG., ELECRONICS, AND COMMUNICAIONS VOL.5, NO.2 August 2007 [4] J.H.Choi, D.Lee, Y.Youn, H.Jeon, and H.Lee, Scanning-based pre-processing for enhanced RDIF ag anti-collision protocols, Proc. of the International Symposium on Communications and Information echnologies 2006, F4D-4 (CD- ROM), Oct [5].Yamakawa,.Inoue, S.Hino, E.Ichihara, Y.akamune, S.Eto,.akenaka, J.Chiyonaga, and A. suneda, A circuit design of a smart RF ID tag for heartbeat signal extraction, 47th Mid west Symposium on Circuits and Systems Proceedings, pp.iii , July [6].Yamakawa,.Inoue, S.Eto,.akenaka, J.Chiyonaga, and A.suneda, A smart RF ID tag circuit for mouse s heartbeat signal extraction, 2004 IEEJ International Analog VLSI Workshop Proceedings, pp , October [7].Yamakawa,.Inoue, S.Eto, J.Chiyonaga,.akenaka,.Umeda, and A.suneda, An advanced design of a smart RF ID tag circuit for heartbeat signal extraction, 2005 IEEJ International Analog VLSI Workshop Proceedings, CD-ROM, Oct [8] C.Song,.Inoue, S.Eto,.Yamakawa, and A.suneda, Design of an integrated CMOS power supply for wireless power feeding to a smart RFID tag, 2006 IEEJ International Analog VLSI Workshop Proceedings, CD-ROM, Oct [9] I.Oota, F.Ueno,.Inoue, and H.B.Lian, Realization and analysis of new switched-capacitor AC-DC converters,. IEICE, Vol.E72, No.12, pp , Dec [10] N.Hara, I.Oota, F.Ueno, and I.Harada, A programmable ring type switched-capacitor AC-DC converter, Proc. of the International Symposium on Nonlinear heory and its Applications, Vol.1, pp , Dec [11].anzawa and.anaka, A dynamic analysis of the Dickson charge pump circuit,.ieee, Solid-State Circuits, Vol.32, No.8, pp , Aug [12].Myono, A.Uemoto, S.Kawai, E.Nishibe, S.Kikuchi,.Iijima, and H.Kobayashi, Highefficiency charge-pump circuits with large current output for mobile equipment applications,.ieice, Electron., Vol.E84-C, No.10, pp , Oct [13] N.Hara, I.Oota, F.Ueno, and.inoue, A new ring type set-up switched-capacitor DC-DC converter with low inrush current at start-up and low current ripple in steady state,. IEEJ, Vol.J81-C-II, No.7, pp , July [14] N.Hara, I.Oota, I.Harada, and F.Ueno, Programmable ring type switched-capacitor DC-DC converters,. IEEJ, Vol.J82-C-II, No.2, pp Feb Kei Eguchi received the B.E., the M.E., and the D.E. degrees from Kumamoto University, Kumamoto, Japan in 1994, 1996, and 1999, respectively. From 1999 to 2006, he was an Associate Professor and a Lecturer in Kumamoto National College of echnology. In 2006, he joined the faculty of Shizuoka University, where he is now an Associate Professor. His research interests include nonlinear dynamical systems, intelligent circuits and systems, and low-voltage analog integrated circuits. He is a member of IEICE, IEEJ, INASS, and JSE. akahiro Inoue received the B.E. and the M.E. degree from Kumamoto University, Kumamoto, Japan in 1969 and 1971, respectively, and the D.E. degree from Kyushu University, Fukuoka, Japan in From 1971 to 1974, he worked as a Research Staff at Hitachi, Ltd., Yokohama, Japan. In 1975, he joined the faculty of Kumamoto University, where he is now a Professor. Dr. Inoue s current research interests include switched-capacitor/switched-current filters, continuoustime IC filters, low-power/low-voltage analog integrated circuits, and analog/digital intelligent circuits and systems. He is a member of the Institute of Electrical and Electronics Engineers and he served as an Associate Editor of ransactions on Fuzzy Systems during He is also a member of the Japanese Neural Network Society, and the Physical Society of Japan. Hongbing ZHU has the B.S., the M.S. and the the Ph.D. degrees. He was an Assistant Professor and Lecture of Information Science & Engineering faculty, Wuhan University of Science & echnology, and Information Processing Center, Kumamoto University. And he worked as a visiting scholar at Kyushu okai University and Kumamoto University. Now, he is a Professor of Hiroshima Kokusai Gakuin University. His current research interests include neural networks, non Neumann computer and high-speed processing, etc.. He is also a member of IEEE, IEICE, JNNS, IPSJ. Fumio Ueno received the B.E. degree in electrical engineering from Kumamoto University, Japan, in 1955, and M.E. degree and D.E. degree from Kyushu University Fukuoka, Japan, in 1964, 1968 respectively. He worked for the faculty of Kumamoto University, where He was a Professor, Dean in From 1994 to 2001, he was the President at Kumamoto National College of echnology. Now, he works for Sojo University as Dean at Faculty of Computer and Information Sciences, and IEICE Kyushu Branch Chair in His main interest lies in the field of active networks. Dr. Ueno is a member of IEICE, IEEJ, JSOF, and IEEE.