DC Generation for Inductively Coupled RFID Systems

Sammanfattning: The ability to store information electronically in small tags that can be read wirelessly has great potential. Radio frequency identification (RFID) technology is used today in a number of different areas, such as logistics, supply chain management, access control and environmental monitoring. Recently, research on RFID technology has focused on sensor tags, localization techniques, antennas and propagation, data security, communication protocols and circuit design for the tags and the readers. In a typical RFID system, a passive tag is powered up remotely by a radio frequency signal sent from a reader unit. The RF signal received by the tag antenna is converted to a DC-supply voltage by the rectifier in the analog front-end of the tag. To avoid power loss in the rectifying operation, low-voltage Schottky diodes are often used in a multi-stage rectifier. However, the use of Schottky diodes is not cost efficient because these diodes must be designed in advanced semiconductor processes. Because one of the demands on future RFID technology is to reduce the cost, efficient rectifiers that can be integrated in a low cost semiconductor process is highly desirable. For this reason, different rectifiers in standard CMOS have been proposed. This thesis discuss recent work and present new ideas on rectifiers in CMOS that have the potential to replace Schottky diodes in rectifiers for low-power RFID applications. The design and modelling of multi-stage rectifiers for maximized DC generation in an inductive RFID system are also included. Furthermore this thesis presents techniques for inductive transponders that allow improved DC generation and reduced orientation sensitivity for transponders that trace moving objects. Part I of the thesis presents a theoretical analysis of the RF to DC generation using single diode rectifiers. This analysis illustrates how different properties, such as the voltage and power conversion efficiency of the rectifier, the Q-factor of the resonance circuit and the couplingcoefficient between coil antennas, affect the tag DC generation. Furthermore, Part I also discusses DC generation for inductive transponders using multiple coil antennas. In Part II, Paper A discusses the limitations with the CMOS cross-connected bridge rectifier and proposes a modified bridge with active diodes to improve rectifier performance. Paper B presents a theoretical model for diode-connected MOS transistors with internal threshold cancellation (ITC), as well as a design procedure that describes how to optimize a rectifier based on MOS ITC diodes. In Paper C a highly efficient active MOS diode is presented that can be used in multi-stage low-power rectifiers. In addition, this study shows that active diodes in CMOS can be designed to have a diode voltage drop of less than 100 mV and have a power consumption of a few μW. Paper D presents a model for the DC charge-up phase in a rectifier driven by a coil antenna. This model was used to determine the available chip current in a typical pulsedRFID system. In Paper E, the modeling and design of multi-stage rectifiers for inductively coupled RFID tags is presented. Finally, Paper F presents front-end circuit solutions for transponders using multiple coil antennas. The work presented in this thesis demonstrates that highly efficient RF to DC conversion can be achieved using CMOS rectifiers for low-power applications. New techniques in CMOS with the potential to replace Schottky diodes in RFID rectifiers are demonstrated. Furthermore, new design criteria for voltage multipliers to achieve maximized DC generation in inductively coupled RFID tags and techniques for reduced orientation sensitivity are presented. These results are promising for improving and reducing cost of inductively coupled RFID systems.