Radio frequency tags (RFID) are a great technology, but noisy power supplies and other factors may reduce their performance. RFID readers using low-frequency signals (eg, 130 kHz), such as TIRIS RFM-007B, which is commonly used in the industry, are very sensitive to noise in this frequency range. Switching power supplies often produce noise in this frequency range, so for maximum sensitivity, it is often necessary to use heavier, expensive linear power supplies.
However, the RFID reader's duty cycle includes multiple phases, and only one phase is sensitive to noise. These phases are: high power phase (emission, power 10W, duration 50ms); low power phase (reception, 18ms); idle phase (up to 33ms, depending on the firmware). Noise is just a problem that occurs at a very short reception period.
This design example turns off the switching power supply during the receive and idle phases, allowing the module to continue to operate using the energy stored in C2.
Figure 1: RFID with selective power off.
A buck converter such as the LM2576 has a shutdown input pin, making the above operation easy to implement.
The MCU's RFID module transmit control line (/TXCT) can also control the shutdown line through U3 and Q1. If necessary, an independent MCU output can also be used to control the shutdown line.
Figure 2: Typical power cycle timing.
Forklifts often use RFID for position detection to implement safety functions such as automatic logistics and speed limiting.
The noise of the environment where the electric forklift is located is particularly large, and the high-frequency modulation and high current generated in the motor drive makes it very difficult for the RFID to work. If the control technique described in this article is used to keep the RFID cycle in sync with the modified motor drive cycle, the problem of sensitivity reduction caused by motor drive interference can be greatly improved.
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