DC Generation for Inductively Coupled RFID Systems

Sammanfattning: The ability to store information electronically in small tags that can be read wirelesslyhas great potential. Radio frequency identification (RFID) technology is used today in anumber of different areas, such as logistics, supply chain management, access control andenvironmental monitoring. Recently, research on RFID technology has focused on sensortags, localization techniques, antennas and propagation, data security, communicationprotocols and circuit design for the tags and the readers.In a typical RFID system, a passive tag is powered up remotely by a radio frequencysignal sent from a reader unit. The RF signal received by the tag antenna is convertedto a DC-supply voltage by the rectifier in the analog front-end of the tag. To avoidpower loss in the rectifying operation, low-voltage Schottky diodes are often used in amulti-stage rectifier. However, the use of Schottky diodes is not cost efficient becausethese diodes must be designed in advanced semiconductor processes. Because one of thedemands on future RFID technology is to reduce the cost, efficient rectifiers that canbe 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 havethe 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 aninductive RFID system are also included. Furthermore this thesis presents techniquesfor inductive transponders that allow improved DC generation and reduced orientationsensitivity for transponders that trace moving objects.Part I of the thesis presents a theoretical analysis of the RF to DC generation usingsingle diode rectifiers. This analysis illustrates how different properties, such as the voltageand power conversion efficiency of the rectifier, the Q-factor of the resonance circuitand the coupling coefficient between coil antennas, affect the tag DC generation. Furthermore,Part I also discusses DC generation for inductive transponders using multiple coilantennas. In Part II, Paper A discusses the limitations with the CMOS cross-connectedbridge rectifier and proposes a modified bridge with active diodes to improve rectifierperformance. Paper B presents a theoretical model for diode-connected MOS transistorswith internal threshold cancellation (ITC), as well as a design procedure that describeshow to optimize a rectifier based on MOS ITC diodes. In Paper C a highly efficient activeMOS 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 voltagedrop of less than 100 mV and have a power consumption of a few μW. Paper D presentsa 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 pulsed RFID system. InPaper E, the modeling and design of multi-stage rectifiers for inductively coupled RFIDtags is presented. Finally, Paper F presents front-end circuit solutions for transpondersusing multiple coil antennas.The work presented in this thesis demonstrates that highly efficient RF to DC conversioncan be achieved using CMOS rectifiers for low-power applications. New techniquesin CMOS with the potential to replace Schottky diodes in RFID rectifiers are demonstrated.Furthermore, new design criteria for voltage multipliers to achieve maximizedDC generation in inductively coupled RFID tags and techniques for reduced orientationsensitivity are presented. These results are promising for improving and reducing cost ofinductively coupled RFID systems.