900 mhz cmos rf to dc converter using a cross coupled charge pump for energy harvesting

8/9/2019 900 MHz CMOS RF to DC Converter Using a Cross Coupled Charge Pump for Energy Harvesting

http://slidepdf.com/reader/full/900-mhz-cmos-rf-to-dc-converter-using-a-cross-coupled-charge-pump-for-energy 1/4

900 MHz CMOS RF-to-DC Converter Using a Cross-Coupled

Charge Pump for Energy Harvesting

Kwangrok You1, Hyungchul Kim, Minsu Kim, Youngoo Yang2

1School of Information and Communication Engineering, SungKyunKwan University, Suwon,

440-746, Republic of [email protected], [email protected]

Abstract — This paper presents a CMOS RF-to-DCconverter with a cross-coupled charge pump for energyharvesting. The proposed RF-to-DC converter consists of across-coupled rectifier and a cross-coupled charge pump. Thecharge pump can operate with a non-overlapping clock witha switching frequency of 25 MHz using a pump capacitanceof 4 pF. The RF-to-DC converter IC was designed andfabrica of 2.05 V was achieved at an RF input power of -6 dBm withan input frequency of 900 MHz and the load of 100 K this condition, the charge pump exhibited a high efficiency ofabout 74 %.

Index Terms — CMOS RF-to-DC converter, charge pump,energy harvesting, rectifier, ultra-high frequency (UHF).

I. I NTRODUCTION

Batteries have a limited life time and hazardous

chemicals. Finite battery life drives many researchers to

propose new ideas for the wireless systems including

energy harvesting [1]. There are many interesting ways forenergy harvesting from the ambient sources, such as light,

heat, vibration, RF, and so on. Among all, RF signals are

available everywhere as well as every time [2]-[3].

An RF-to-DC converter is very important for the energy

harvesting devices using RF signals. It generates stable

DC power with an appropriate voltage level which is

required by the target devices [4]. Conventional RF-to-DC

converter uses a Dickson’s circuit [5]. An RF-to-DC

converter based on Dickson’s circuit suffers from

efficiency drop due to the turn-on voltage of the diode.

Especially for the integrated circuit, a MOSFET diode has

to be generally used to design the circuit, because anadditional processing step is required to provide a

Schottky diode which has very low turn-on voltage [6]-[7].

In this paper, a CMOS RF-to-DC converter integrated

circuit is presented for the 900 MHz band. The proposed

RF-to-DC converter has a cross-coupled rectifier based on

MOSFET to convert the RF input signal to a DC voltage.

It also has a high-efficiency cross-coupled charge pump to

boost the low input DC voltage. The cross-coupled

structure has advantages in low reverse leakage current

and low voltage drop across the MOSFET diode.

II. CIRCUIT DESIGN

Fig. 1 shows a block diagram of the proposed RF-to-DC

converter. It has a cross-coupled rectifier and a cross-

coupled charge pump. The rectifier operates at the 900

MHz band. The charge pump operates at a very lowfrequency of about 25 MHz. The integrated oscillator

generates a non-overlapped clock for the charge pump.

The rectifier is optimized for a load resistance of 10 k

A. Cross-coupled rectifier

Fig. 2 shows an N-stages cross-coupled rectifier [8]. A

voltage generated by a single-stage rectifier is not enough

to operate the clock generator and the charge pump.

Therefore, the rectifier should be designed more than two-stages. A differential RF signal is applied to the input of

Fig. 1. A block diagram of the proposed RF-to-DC converter

C1

C2

N1

N2

P1

P2

Cs

Vout

C1N

C2N

N1N

N2N

P1N

P2N

RF+

RF-

A

B

Fig. 2. A schematic diagram of an N-stage cross-coupledrectifier.

RFIT2011-IEEE International Symposium on Radio-Frequency Integration Technology, Nov. 30 - Dec. 2, 2011, Beijing, China

978-1-4577-0520-5/11/$26.00 ©2011 IEEE 149



the rectifier. The gates of the NMOSFETs and thePMOSFETs are connected each other. When the node A

has a positive signal swing, N1 and P2 are turned on,

while N2 and P1 are turned off. The voltage of the first

stage is nearly doubled by the peak voltage of the RF

signal [9]-[11].

To maximize the RF-to-DC conversion efficiency, the

aspect ratio between NMOS and PMOS transistors should

be optimized. Channel widths for the NMOS and PMOS

transistors were selected to be 30 and 42 ,

respectively. Channel length of 0.13 for both

transistors is selected. Flying capacitors of 0.7 pF were

selected for the optimized performance.Fig. 3 shows the measured output voltages of the two- and

four-stage rectifiers according to the RF input power with

a load resistance of 10 k . The output voltage of the two-

stage rectifier is larger than that of the four-stage rectifier

at an RF input power level less than -8 dBm. The four-

stage rectifier outperforms and generates more than 0.8 V

at the input power of higher than -6 dBm.

B. Charge pump

Fig. 4 shows the cross-coupled charge pump, which is

composed of clock generator and cross-coupled doubler.

Typical Dickson’s charge pump has limited performance

due to turn-on voltage of the MOSFET-based diode and

reverse charge sharing. However, the cross-coupled

charge pump has better efficiency due to a cross-

compensation of the threshold voltage. Fig. 4(a) shows a

two-stage gate cross-coupled doubler. Fig. 4(b) shows a

clock generator using an inverter chain. In the charge

pump design, switching frequency, load condition, flying

capacitor, channel width of the transistors, and dead time

of a non-overlapping clock should be carefully considered

[12].

Fig. 5 shows the simulated clock frequency according to

the bias voltage. The clock generates non-overlapping

clock signal with a short dead time. For the bias voltage of

0.8 V, the clock generator generates a non-overlapping

clock with a frequency of 25 MHz.

Fig. 6 shows the simulated and measured PCE

according to the input DC voltage. The measurement was

carried out with a load resistance of 100 k

Fig. 3. Measured output voltages of the two- and four-stagerectifiers.

Fig. 4. A schematic diagram of the charge pump circuit: (a)cross-coupled doubler, (b) clock generator .

Fig. 5. The simulated clock frequency according to the biasvoltage

150



integrated charge pump has a good efficiency which is

more than 60 % at input voltages of more than 0.6 V. The

implemented charge pump exhibited a very high peak

PCE of 74 % at an input voltage of 0.8 V.

III. EXPERIMENTAL R ESULT

Fig. 7 shows a photograph of the fabricated RF-to-DC

converter IC whose size is 400x512 2. The chip was

designed and fabricated using Dongbu’s 0.13

process. An RF input signal from the signal generator

having a frequency of 900 MHz is applied to the proposed

RF-to-DC converter IC via a transformer for differential

signal excitation. Then, the output voltage was measured

through an oscilloscope.

Fig. 8 shows the measured output voltages of the RF-to-

DC converter IC with two-stage and four-stage rectifiers

with a load resistance of 100 k The RF-to-DC converter

with a four-stage rectifier has better performance at an

input power level of more than -7 dBm.

Fig. 9 shows the measured output voltage as a

parameter of the load resistances for the RF-to-DC

converter with a four-stage rectifier. With a load

resistance of 100 k -6 dBm,

PCE’s of the RF-to-DC converter and the charge pump

are 24 and 74 %, respectively. The output voltage for this

condition was as high as 2.05 V.

IV. CONCLUSION

In this paper, an RF-to-DC converter using a charge

pump for RF energy harvesting is presented for the 900

MHz band. It consists of a cross-coupled rectifier and a

cross-coupled charge pump with a clock generator which

generates non-overlapping anti-phased clock signal. The

voltage drop and reverse leakage current of the transistor

is minimized using the cross-coupled circuit configuration.

A high output voltage is achieved using a charge pump

which operates with a non-overlapping clock whose

frequency is about 25 MHz. The implemented RF-to-DC

converter exhibited an output voltage of as high as 2.05 V

Fig. 6. The simulated and measured performances of theimplemented charge pump.

Fig. 7. A photograph of the fabricated RF-to-DC converter IC.

Fig. 8. The measured output voltages of the RF-to-DCconverters based on the two-stage and four-stage rectifiers.

Fig. 9. The measured output DC voltages as a parameter of theload resistances.

151



at an RF input power of -6 dBm with a load resistance of

100 k . This CMOS RF-to-DC converter can be applied

to an RF energy harvesting systems.

ACKNOWLEDGEMENT

The chips were design with Cadence and fabricated

Through the IC Design Education Center.

R EFERENCES

[1] D. Puccinelli and M. Haenggi, “Wireless sensor networks:applications and challenges of ubiquitous sensing,” IEEECircuits and Systems Magazine, vol. 3, no. 3, pp. 19-29,2005.

[2] J. B. Lee, Z. Chen, M. G. Allen, A. Rohatgi, and R. Ayra, “Ahigh voltage solar cell arrays as an electrostatic MEMS

power supply,” in Proc. IEEE Workshop on Micro ElectroMechanical Systems, pp. 331-336, Jan. 1994.

[3] N. Penglin, P. Chapman, “Design and Performance of LinearBiomechanical Energy Conversion Devices,” 37th IEEEPower Electronics Specialists Conference, pp. 1-6, June2006.

[4] Klaus Finkenzeller, RFID Handbook: Fundamentals andApplications in Contactless Smart Cards and Identification,2nd ed. New York: Wiley, pp. 85-96, 70-80, 55-63, 2003.

[5] U. Karthaus and M. Fischer, “Fully integrated passive UBFRFID transponder IC with 16.7- tw minimum RF input power,” IEEE J .Solid-State Circuits, vol. 38, no. 10, pp.1602-1608, Oct. 2003.

[6] J. Dickson, “On-chip High-Voltage Generation in NMOSIntegrated Circuits Using an Improved Voltage MultiplierTechnique,” IEEE J. Solid-State Circuits, vol. 1, no.6, pp.374-378, June 1976.

[7] R. Barnett, et al, “A passive UHF RFID transponder for EPCGen 2 with -14 dBm sensitivity in 0.13 urn CMOS,” inIEEE ISSCC Dig., pp. 582-583, Feb. 2007.

[8] A. Sasaki, et al, “Differential-drive CMOS rectifier for UHFRFIDs with 66% PCE at -12 dBm input,” in Proc. IEEEASSCC, pp.105-108, Nov. 2008.

[9] H. Nakamoto, et al, “A passive UHF RF identificationCMOS tag IC using ferroelectric RAM in 0.35-flmtechnology,” IEEE J. Solid-State Circuits, vol. 42, no. 1, pp.101-110, Jan. 2007.

[10] K. Kotani and T. Ito, “High efficiency CMOS rectifiercircuit with selfVth-cancellation and power regulationfunctions for UHF RFIDs,” in Proc. IEEE ASSCC, pp. 119-122, Nov. 2007.

[11] K. Kotani and T. Ito, “Self-Vth-cancellation high-efficiencyCMOS rectifier circuit for UHF RFIDs,” IEICE Trans.Electron., vol. E92-C, no.1, pp.153-160, Jan.2009.

[12] K.-S. Min, Y.-H. Kim, J.-H. Ahn, J.-Y. Chung, and T.Sakurai, “CMOS charge pumps using cross-coupled chargetransfer switches with improved voltage pumping gain andlow gate-oxide stress for low-voltage memory circuits,” inProc. IEEE Int. Symp. Circuits Syst., vol. 5, pp. 545–548,May 2002.

152

900 mhz cmos rf to dc converter using a cross coupled charge pump for energy harvesting

Documents