What does the chip of the UHF RFID passive tag rely on to supply power?

As the most basic part of passive Internet of Things technology, UHF RFID passive tags have been widely used in a large number of applications such as supermarket retail, logistics and warehousing, book archives, anti-counterfeiting traceability, etc. Only in 2021, global shipping amount is more than 20 billion. In practical applications, what exactly does the chip of the UHF RFID passive tag rely on to supply power?

The power supply characteristics of UHF RFID passive tag 

1. Powered by wireless power 

Wireless power transmission is making use of wireless electromagnetic radiation to transfer electrical energy from one place to another. The working process is to convert electrical energy into radio frequency energy through radio frequency oscillation, and the radio frequency energy is converted into radio electromagnetic field energy through the transmitting antenna. The radio electromagnetic field energy propagates through space and reaches the receiving antenna, then it is converted back to radio frequency energy by the receiving antenna, and the detection wave becomes DC energy.

In 1896, the Italian Guglielmo Marchese Marconi invented the radio, which realized the transmission of radio signals across space. In 1899, American Nikola Tesla proposed the idea of ​​using wireless power transmission, and established an antenna which is 60m-high,   inductance loaded in botton, capacitance loaded in top  in Colorado, using a frequency of 150kHz to input 300kW of power. It transmits over a distance of up to 42km, and obtains 10kW of wireless receiving power at the receiving end.

UHF RFID passive tag power supply follows this idea, and the reader supplies power to the tag through radio frequency. However, there is a huge difference between UHF RFID passive tag power supply and Tesla test: the frequency is nearly ten thousand times higher, and the antenna size is reduced by a thousand times. Since wireless transmission loss is proportional to the square of the frequency and proportional to the square of the distance, it is clear that the increase in transmission loss is huge. The simplest wireless propagation mode is free-space propagation. The propagation loss is inversely proportional to the square of the propagation wavelength and proportional to the square of the distance. The free-space propagation loss is LS=20lg(4πd/λ). If the unit of distance d is m and the unit of frequency f is MHz, then LS= -27.56+20lgd+20lgf.

The UHF RFID system is based on the wireless power transmission mechanism. The passive tag does not have its own power supply. It needs to receive the radio frequency energy emitted by the reader and establish a DC power supply through voltage doubling rectification, which means establish a DC power supply through  Dickson charge pump.

The applicable communication distance of the UHF RFID air interface is mainly determined by the transmission power of the reader and the basic propagation loss in space. UHF band RFID reader transmit power is usually limited to 33dBm. From the basic propagation loss formula, ignoring any other possible losses, the RF power reaching the tag through wireless power transmission can be calculated. The relationship between the communication distance of the UHF RFID air interface and the basic propagation loss and the RF power reaching the tag are shown in the table:

It can be seen from the table that UHF RFID wireless power transmission has the characteristics of large transmission loss. Since RFID complies with the national short-distance communication rules, the transmission power of the reader is limited, so the tag can supply low power. As the communication distance increases, the radio frequency energy received by the passive tag decreases according to the frequency, and the power supply capacity decreases rapidly.

2. Implement power supply by charging and discharging on-chip energy storage capacitors

(1) Capacitor charge and discharge characteristics

Passive tags use wireless power transmission to obtain energy, convert it into DC voltage, charge and store the on-chip capacitors, and then supply power to the load through discharge. Therefore, the power supply process of passive tags is the process of capacitor charging and discharging. The establishment process is a pure charging process, and the power supply process is a discharge and supplementary charging process. The supplementary charging must start before the discharge voltage reaches the minimum supply voltage of the chip.

(2) Capacitor charge and discharge parameters

1) Charging parameters

Charging time length: τC=RC×C

Charging voltage:

recharging current:

where RC is the charging resistor and C is the energy storage capacitor.

2) Discharge parameters

Discharge time length: τD=RD×C

Discharge voltage:

Discharge current:

In the formula, RD is the discharge resistance, and C is the energy storage capacitor.

The above shows the power supply characteristics of passive tags. It is neither a constant voltage source nor a constant current source, but the charging and discharging of the energy storage capacitor. When the on-chip energy storage capacitor is charged above the working voltage V0 of the chip circuit, it can supply power to the tag. When the energy storage capacitor starts to supply power, its power supply voltage begins to drop. When it falls below the chip operating voltage V0, the energy storage capacitor loses its power supply capability and the chip cannot continue to work. Therefore, the air interface tag should have sufficient capacity to recharge the tag.

It can be seen that the power supply mode of passive tags is suitable for the characteristics of burst communication, and the power supply of passive tags also needs the support of continuous charging.

3 Balance of supply and demand

Floating charging power supply is another power supply method, and the floating charging power supply capacity is adapted to the discharging capacity. But they all have a common problem, that is, the power supply of UHF RFID passive tags needs to balance supply and demand.

(1) Supply and demand balance power supply mode for burst communication

The current standard ISO/IEC18000-6 of UHF RFID passive tags belongs to the burst communication system. For passive tags, no signal is transmitted during the receiving period. Although the response period receives the carrier wave, it is equivalent to acquiring the oscillation source, so it can be considered as simplex work. Way. For this application, if the receiving period is used as the charging period of the energy storage capacitor, and the response period is the discharging period of the energy storage capacitor, the equal amount of charge and discharge to maintain the balance of supply and demand becomes a necessary condition to maintain the normal operation of the system. It can be known from the power supply mechanism of the above-mentioned UHF RFID passive tag that the power supply of the UHF RFID passive tag is neither a constant current source nor a constant voltage source. When the tag energy storage capacitor is charged to a voltage higher than the normal working voltage of the circuit, the power supply starts; when the tag energy storage capacitor is discharged to a voltage lower than the normal operating voltage of the circuit, the power supply is stopped.

For burst communication, such as passive tag UHF RFID air interface, the charge can be charged before the tag sends a response burst, enough to ensure that enough voltage can be maintained until the response is completed. Therefore, in addition to the strong enough radio frequency radiation that the tag can receive, the chip is also required to have a large enough on-chip capacitance and a long enough charging time. The tag response power consumption and response time must also be adapted. Due to the distance between the tag and the reader, the response time is different, the area of ​​the energy storage capacitor is limited and other factors, it may be difficult to balance the supply and demand in time division.

(2) Floating power supply mode for continuous communication

For continuous communication, in order to maintain the uninterrupted power supply of the energy storage capacitor, it must be discharged and charged at the same time, and the charging speed is similar to the discharging speed, that is, the power supply capacity is maintained before the communication is terminated.

Passive tag code division radio frequency identification and UHF RFID passive tag current standard ISO/IEC18000-6 have common characteristics. The tag receiving state needs to be demodulated and decoded, and the response state needs to be modulated and sent. Therefore, it should be designed according to continuous communication. Tag chip power supply system. In order for the charging rate to be similar to the discharging rate, most of the energy received by the tag must be used for charging.

Shared RF resources

1. RF front-end for passive tags

Passive tags are not only used as the power source of the tags and postcards to the radio frequency energy from the readers, but more importantly, the instruction signal transmission from the reader to the tag and the response signal transmission from the tag to the reader are realized through wireless data transmission. The radio frequency energy received by the tag should be divided into three parts, which are respectively used for the chip to establish the power supply, demodulate the signal (including the command signal and synchronization clock) and provide the response carrier.

The working mode of the current standard UHF RFID has the following characteristics: the downlink channel adopts the broadcast mode, and the uplink channel adopts the mode of multi-tag sharing single-channel sequence response. Therefore, in terms of information transmission, it belongs to the simplex mode of operation. However, since the tag itself cannot provide the transmission carrier, the tag response needs to provide the carrier with the help of the reader. Therefore, when the tag responds, as far as the sending state is concerned, both ends of the communication are in a duplex working state.

In different working states, the circuit units put into work by the tag are different, and the power required for different circuit units to work is also different. All the power comes from the radio frequency energy received by the tag. Therefore, it is necessary to control the RF energy distribution reasonably and when appropriate.

2. RF energy application in different working hours

When the tag enters the reader's RF field and starts to build power, no matter what signal the reader sends at this time, the tag will supply all the received RF energy to the voltage-doubling rectifier circuit to charge the on-chip energy storage capacitor, thereby establishing the chip's power supply.

When the reader transmits the command signal, the reader's transmission signal is a signal encoded by the command data and amplitude modulated by the spread spectrum sequence. There are carrier components and sideband components representing command data and spread spectrum sequences in the signal received by the tag. The total energy, carrier energy, and sideband components of the received signal are related to modulation. At this time, the modulation component is used to transmit the synchronization information of the command and the spread spectrum sequence, and the total energy is used to charge the on-chip energy storage capacitor, which simultaneously starts to supply power to the on-chip synchronization extraction circuit and the command signal demodulation circuit unit. Therefore, during the period when the reader sends an instruction, the radio frequency energy received by the tag is used for the tag to continue to charge, extract the synchronization signal, demodulate and identify the instruction signal. The tag energy storage capacitor is in a floating charge power supply state.

When the tag responds to the reader, the transmitted signal of the reader is a signal that is modulated by the amplitude of the spread spectrum spread spectrum chip rate sub-rate clock. In the signal received by the tag, there are carrier components and sideband components representing the spread spectrum chip rate sub-rate clock. At this time, the modulation component is used to transmit the chip rate and rate clock information of the spread spectrum sequence, and the total energy is used to charge the on-chip energy storage capacitor and modulate the received data and send a response to the reader. The chip synchronization extraction circuit and the response signal modulation circuit unit supply power. Therefore, during the period when the reader receives the response, the tag receives the radio frequency energy and is used for the tag to continue charging, the chip synchronization signal is extracted and the response data is modulated and the response is sent. The tag energy storage capacitor is in a floating charge power supply state.

In short, in addition to the tag entering the reader's RF field and starting to establish a power supply period, the tag will supply all the received RF energy to a voltage-doubling rectifier circuit to charge the on-chip energy storage capacitor, thereby establishing a chip power supply. Subsequently, the tag extracts synchronization from the received radio frequency signal, implements command demodulation, or modulates and transmits response data, all of which use the received radio frequency energy.

3. RF energy requirements for different applications

(1) RF energy requirements for wireless power transmission

Wireless power transfer establishes the power supply for the tag, so it requires both sufficient voltage to drive the chip circuit, and sufficient power and continuous power supply capability.

The power supply of wireless power transmission is to establish the power supply by receiving the RF field energy of the reader and voltage doubling rectification when the tag has no power supply. Therefore, its receiving sensitivity is limited by the voltage drop of the front-end detection diode tube. For CMOS chips, the receiving sensitivity of voltage doubling rectification is Between -11 and -0.7dBm, it is the bottleneck of passive tags.

(2) RF energy requirements for received signal detection

While the voltage doubling rectification establishes the chip power supply, the tag needs to divide a part of the received radio frequency energy to provide a signal detection circuit, including command signal detection and synchronous clock detection. Because the signal detection is performed under the condition that the power supply of the tag has been established, the demodulation sensitivity is not limited by the voltage drop of the front-end detection diode tube, so the receiving sensitivity is much higher than the wireless power transmission receiving sensitivity, and it belongs to the signal amplitude detection, and there is no power strength requirement.

(3) RF energy requirements for tag response

When the tag responds to sending, in addition to detecting the synchronous clock, it also needs to perform pseudo-PSK modulation on the received carrier (containing the clock modulation envelope) and realize reverse transmission. At this time, a certain power level is required, and its value depends on the distance of the reader to the tag and the sensitivity of the reader to receive. Since the working environment of the reader allows the use of more complex designs, the receiver can implement a low-noise front-end design, and the code division radio frequency identification uses spread spectrum modulation, as well as spread spectrum gain and PSK system gain, the sensitivity of the reader may be designed to be high enough. So that the requirements for the return signal of the label are reduced enough.

To sum up, the radio frequency power received by the tag is mainly allocated as the wireless power transmission voltage doubler rectification energy, and then the appropriate amount of tag signal detection level and the appropriate amount of return modulation energy are allocated to achieve a reasonable energy distribution and ensure the continuous charging of the energy storage capacitor. is a possible and reasonable design.

It can be seen that the radio frequency energy received by passive tags has various application requirements, so a radio frequency power distribution design is required; the application requirements of radio frequency energy in different working periods are different, so it is necessary to have a radio frequency power distribution design according to the needs of different working periods; Different applications have different requirements for RF energy, among which wireless power transmission requires the most power, so RF power allocation should focus on the needs of wireless power transmission.

UHF RFID passive tags use wireless power transmission to establish a tag power supply. Therefore, the power supply efficiency is extremely low and the power supply capability is very weak. The tag chip must be designed with low power consumption. The chip circuit is powered by charging and discharging the on-chip energy storage capacitor. Therefore, in order to ensure the continuous operation of the label, the energy storage capacitor must be continuously charged. The radio frequency energy received by the tag has three different applications: voltage-doubling rectification for power supply, command signal reception and demodulation, and response signal modulation and transmission. Among them, the receiving sensitivity of voltage-doubling rectification is restricted by the voltage drop of the rectifier diode, which becomes an air interface. bottleneck. For this reason, signal reception and demodulation and response signal modulation and transmission are the basic functions that the RFID system must ensure. The stronger the power supply capability of the voltage doubler rectifier tag, the more competitive the product. Therefore, the criterion for rationally distributing the received RF energy in the design of the tag system is to increase the RF energy supply by voltage doubler rectification as much as possible on the premise of ensuring the demodulation of the received signal and the transmission of the response signal.