Technical Field of the Invention
This invention relates generally to wireless communications more particularly to power harvesting.
Description of Related Art
Wireless communication systems are known to include wireless transceivers that communicate directly and/or over a wireless communication infrastructure. In direct wireless communications, a first wireless transceiver includes baseband processing circuitry and a transmitter to convert data into a wireless signal (e.g., radio frequency (RF), infrared (IR), ultrasound, near field communication (NFC), etc.). Via the transmitter, the first wireless transceiver transmits the wireless signal. When a second wireless transceiver is in range (e.g., is close enough to the first wireless transceiver to receive the wireless signal at a sufficient power level), it receives the wireless signal via a receiver and converts the signal into meaningful information (e.g., voice, data, video, audio, text, etc.) via baseband processing circuitry. The second wireless transceiver may wirelessly communicate back to the first wireless transceiver in a similar manner.
Examples of direct wireless communication (or point-to-point communication) include walkie-talkies, Bluetooth, ZigBee, Radio Frequency Identification (RFID), etc. As a more specific example, when the direct wireless communication is in accordance with RFID, the first wireless transceiver may be an RFID reader and the second wireless transceiver may be an RFID tag.
For wireless communication via a wireless communication infrastructure, a first wireless communication device transmits a wireless signal to a base station or access point, which conveys the signal to a wide area network (WAN) and/or to a local area network (LAN). The signal traverses the WAN and/or LAN to a second base station or access point that is connected to a second wireless communication device. The second base station or access point sends the signal to the second wireless communication device. Examples of wireless communication via an infrastructure include cellular telephone, IEEE 802.11, public safety systems, etc.
In many situations, direct wireless communication is used to gather information that is then communicated to a computer. For example, an RFID reader gathers information from RFID tags via direct wireless communication. At some later point in time (or substantially concurrently), the RFID reader downloads the gathered information to a computer via a direct wireless communication or via a wireless communication infrastructure.
In many RFID systems, the RFID tag is a passive component. As such, the RFID tag has to locally generate one or more supply voltages from the RF signals transmitted by the RFID reader. Accordingly, a passive RFID tag includes a power supply circuit that converts the RF signal (e.g., a continuous wave AC signal) into a DC power supply voltage. The power supply circuit includes one or more diodes and one or more capacitors. The diode(s) function to rectify the AC signal and the capacitor(s) filter the rectified signal to produce the DC power supply voltage.
The magnitude of the DC power supply voltage is a function of the peak voltage of the AC signal (Vpeak) and of the voltage drop of the diodes (Vdiode). For example, if one diode is used, the DC power supply voltage is approximately equal to Vpeak−Vdiode. The diode is a necessary component of the power supply circuit, but its voltage drop decreases the efficiency of the overall power supply circuit.
Techniques have developed to decrease the effective voltage drop of the diodes by using field effect transistors (FET) for the diodes and using voltage drop compensation circuits. For example, the compensation circuit includes a mirroring transistor of a smaller geometry coupled to the FET to reduce the “on” resistance and voltage of the FET, thereby effectively reducing its voltage drop and increasing the efficiency of the power supply circuit.
The voltage drop compensation circuits, however, only have a limited effect on increasing the efficiency of the power supply circuit since the compensation circuit requires a start up current to turn on the mirroring transistor. In addition, for the compensation circuit to start, the peak voltage of the AC signal needs to be at a high enough level to turn on the mirroring transistor. This requirement adds an element of delay in powering up a passive RFID tag.