As a highly professional technology, RFID may also recruit professional RFID engineers in some companies. This article only deals with the basic type selection design and circuit framework, as well as the RFID antenna debugging and low-power card inspection debugging. Other chapters will continue to be updated in the future!

NFC (near field communication) chip selection:

Main considerations:

The protocol supported by the chip, whether it supports low-power card inspection, whether it can pass the financial certification, and the chip price

Chip support agreement:

Iso14443a/b, iso15693, iso18092, iso21481, etc

Iso14443a card: Mifare series, ultralight series, plus series, CPU card series, etc.

ISO14443B card: ID card, sr176, sri512, etc.

Iso15693: icode series of NXP, tag of Ti_ It HF-I, St LRI, etc.

Iso18092: including card reading mode, card mode and point-to-point communication mode.

Iso21481: compatible with iso15693 protocol on the basis of iso18092.

Lpcd function: chip low-power detection card function. When there is no card approaching, the chip is in a low-power state, and only 10uA current is needed to complete the card detection. When the card approaches, the chip detects the card and wakes up the MCU to read the card.

Financial certification: pboc2.0/3.0 standard, EMV standard

Circuit architecture:

The external circuit of NFC chip is usually composed of the following parts: power supply circuit, communication interface circuit, antenna circuit and oscillation circuit;

Power supply circuit:It mainly includes analog power supply avdd, digital power supply dvdd, transmitter power supply tvdd, pin power supply PvdD, test pin power supply pvdd2;

a.If the transmission power needs to be increased, the voltage of tvdd can be increased. For example, the transmission power formed by 5V powered tvdd will be stronger than 3V;

b.The power supply current of the chip is usually tens to hundreds of Ma, and the main energy is consumed in the circuit of the transmitter. For example, the antenna emission current of fm175xx is 100mA, while that of rc663 can reach 250mA. Therefore, when selecting power supply chips and inductors, pay attention to leaving enough margin;

c.The 13.56MHz sine wave signal of the card reading chip antenna will interfere with the power supply. In order to reduce conducted interference, a π filter can be added to the power supply end, but in order to reduce the redundancy of circuit design, it is generally not added.

Communication interface:

Generally, spi/i2c/uart is supported, which is generally selected through external pin configuration. In order to facilitate upgrading, compatible design can be done;

Antenna design:

The antenna circuit is mainly composed of four parts: EMC filter, matching circuit, antenna and receiving circuit. Take fm17550 as an example, as follows:

pYYBAGLeX-6AMr8qAADbEvfoacg501.png

Filter circuit:

The low-pass filter composed of L1 and C1 is used to filter the derived harmonics of 13.56MHz, and the cut-off frequency of the filter should be designed above 14mhz. L1 inductors should not be placed close to each other to avoid mutual interference (mutual inductance effect). Matching formula of filter circuit elements: f=1/ (2 π √ LC)

 

Matching circuit:

It is used to adjust the emission load and resonance frequency. The power of RF circuit is affected by the internal resistance and external impedance of the chip. When the internal resistance and external impedance of the chip are consistent, the transmission power efficiency is the highest. C2 is the load capacitance. The larger the antenna inductance is, the smaller the value of C2 is. C3 is the resonant capacitance, and its value is directly related to the antenna inductance, so that the resonant frequency is 13.56MHz.

Receiving circuit:

C4 filters the DC signal, R2 and R3 form a voltage dividing circuit, so that the amplitude of the sine wave signal at the RX receiving end is between 1.5-3V.

Antenna:

It consists of R1 resistor (usually 1ohm or 0ohm) and printed PCB.

The larger the antenna is, the farther the card reading distance is. When the antenna area reaches 5cm x 5cm, then increase the antenna, and the card reading distance does not increase significantly.

It is recommended to choose 0.5mm – 1mm antenna line width. Antenna larger than 5cm x 5cm cannot be more than 3 turns, and smaller than 3cm x 3cm cannot be less than 4 turns

In order to reduce EMC radiation interference, it is necessary to draw an arc at the PCB wiring corner.

It is forbidden to draw the signal, power supply and ground wire into a circle or semicircle in the antenna area and at the edge of the antenna. Large areas of metal objects and metal coatings are not allowed in the antenna circle to avoid serious loss of capacity caused by the eddy current effect of the magnetic field.

The winding method of antenna PCB is relative, not in the same direction.

The accuracy of antenna circuit design elements should be controlled within 2%, otherwise it is easy to cause the deviation of antenna resonance frequency point, resulting in serious degradation of card reading performance, and product consistency is difficult to ensure

 

Relationship between antenna size and card reading distance

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Reviewed by: Liu Qing

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