This relates generally to harvesting power from radio frequency signals.
A number of radio frequency devices may be operated at remote locations. In addition, some of these devices may be mobile. Therefore, a readily available, continuous source of power may not be possible. One way to power these devices is to power them from the radio frequency signal they receive using a technique called radio frequency power harvesting.
One application for radio frequency power harvesting is radio frequency identification (RFID) technology which may be used in public transportation, logistics, airline baggage tracking, asset tracking, inventory control and tracking, tracking goods in supply chains, tracking parts, security, access control and authentication, to mention just a few examples. Another application for radio frequency power harvesting is in connection with wirelessly powered embedded microprocessors and sensors.
One reason radio frequency identification tags are a good application for a radio frequency power harvesting is that their power requirements are relatively modest. However, radio frequency power harvesting may be used in a variety of other applications as well.
A simple radio frequency identification system may use a reader and passive tags that work with shorter range and lower frequency, while longer distance applications may use active tags. A radio frequency identification tag may be an integrated circuit with a tag insert or an inlay including an integrated circuit attached to an antenna. A reader/writer sends out electromagnetic waves to the tag that induce a current in the tags' antenna.
The reader/writer may be a fixed or portable device. The tag modulates the wave and may send information back to the reader/writer. Additional information about the items the tag is attached to can be stored on the tag.
Passive tags typically have no power source and rely on the energy delivered by the interrogation signal to transmit a stream of information. Active tags may have a power source such as a direct current battery. Semi-passive tags may have a battery that is used for only part of the tag's power needs.
Information may be exchanged between the tag and the reader/writer through either inductive coupling or back-scatter. Many different frequencies may be utilized for these systems, but the most common current frequencies are around 165 KHz, 13.56 MHz, 902 to 928 MHz, and microwave.
Referring to
The device 106 receives and processes a radio frequency signal 110 from the reader/writer 102. The device 106 may include power harvesting and voltage processing circuitry 112, a processor or state machine 114, a storage 116, and a modulator 118. The power harvesting and voltage processing circuit 112 may include circuitry for harvesting power to operate the device 106 from the radio frequency signal 110.
The storage 116 may contain a key for decryption, a device identification for signal authentication, or other information. The modulator 118 may control the switch 122 and may be used for upstream communications in some embodiments.
To access the device 106, an interrogation signal may be transmitted by the reader/writer 102 in the vicinity of the device 106. Upon receipt of the interrogation signal, the device 106 may respond by dynamically modulating the impedance of its antenna 108 to encode response information. The antenna 108 may be tuned for whatever impedance is convenient from an antenna design perspective.
Referring to
The signal output from the network 143, Vin, may be coupled to each of three capacitors 126. Each capacitor 126 may be coupled to a diode 134. The diodes 134 may be implemented as diode connected transistors in some embodiments. The diodes 134 may be coupled in parallel to an active, gate-controlled transistor switch 138. The transistor switch 138 may be controlled by a gate signal P2 or P1. The generation of signals P1 and P2 by the generator circuit 145 in
A reset switch 140 may be provided in some embodiments. The load resistor 142 illustrates the load to which power is being supplied, for example, a microcontroller or RFID tag.
A number of other transistors 138 can receive the signal P1 from the P1 and P2 generator circuit 145. The signals P1 and P2 are out of phase with one another. As shown in
In the embodiment shown in
The voltage doubler 130 generally includes a first paired diode 134 and a capacitor 132 to rectify a positive cycle of the applied radio frequency signal and then a second paired diode 124 and capacitor 126 to rectify that signal in the negative cycle. During the positive cycle, the voltage stored on the capacitor 126 in the negative cycle is transferred to the capacitor 132 used in the positive cycle. Thus, the voltage on the capacitor 132 used in the positive cycle is ideally doubled. The voltage multiplication may be increased by cascading a series of such inverter multipliers. In some embodiments, complementary metal oxide semiconductor (CMOS) diode-connected transistors are used instead of diodes.
By using dynamic switching transistors 138 in parallel with or instead of the diodes 124 and 134, the diodes 134 may be used in a first mode (with the transistors set to a high impedance state), to provide a static power source to supply power subsequently to the transistors 138 that provide more effective dynamic switching during a second mode.
The startup circuit 136, powered by the static mode operation of the harvester using the diodes 134, then enables generation of the two thresholded signals P1 and P2, which are 180° out of phase, to power selected ones of the transistor switches 138 during the second mode. Thus, the startup circuit 126 includes a voltage monitor 146 and a controller 144 that supplies a voltage to the phase generator 145 after the voltage across the startup circuit 136 reaches a predetermined level. Until that point, NMOS transistors 138 receive 0 volts, setting them to a high impedance state.
Referring to
This enhancement is further illustrated in
Thus, the same capacitors 126 and 132 may be used in both the static and dynamic operation, while at different times and in different phases. This sharing of capacitors may reduce cost and circuit footprint in some embodiments. In one embodiment, a battery may be used to supply power for the dynamic mode.
References throughout this specification to “one embodiment” or “an embodiment” mean that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one implementation encompassed within the present invention. Thus, appearances of the phrase “one embodiment” or “in an embodiment” are not necessarily referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be instituted in other suitable forms other than the particular embodiment illustrated and all such forms may be encompassed within the claims of the present application.
While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.