1. Introduction
In recent years, with the rapid advancement of wireless communication and microelectronics technologies, non-contact IC card (RF card) technology has experienced significant growth and is now widely applied in various fields such as public transportation ticketing systems, resident ID cards, mobile phone cards, and more. A key component of RF card technology is the passive power supply system, which primarily relies on electromagnetic induction and integrated voltage regulation circuits. When an RF card enters the magnetic field of a reader, it draws energy through electromagnetic induction, inducing an alternating current in the coil, which is then rectified and regulated to produce a direct current voltage. This paper presents a self-feedback switching regulator circuit designed for RF cards using a 0.35µm CMOS process, aiming to enhance efficiency and stability under varying operating conditions.
2. Structure Design and Working Principle of Voltage Regulator Circuit
An integrated voltage regulator circuit, also known as a voltage regulator, ensures that the output voltage remains stable despite variations in input voltage or load current. It has become a standard component in many electronic devices, replacing traditional discrete-component power supplies.
2.1 Circuit Structure Design
The integrated voltage stabilizing circuit typically consists of three main components: a reference source circuit, a voltage regulation circuit, and a power switch circuit.
The reference source circuit is built using a bandgap reference composed of a two-stage CMOS differential amplifier and a transistor-based biasing network. Its structure is illustrated in Figure 1. The active resistor P0 and polycrystalline resistor R7 form a biasing network that provides a stable current to the circuit. The two inputs of the differential amplifier are connected to transistors Q1 and Q2. According to the principle of the reference source, the output maintains good performance when the amplifier’s input offset voltage is minimal and unaffected by temperature. Based on the operation of the amplifier and the bandgap reference mechanism:
I1R6 = I2R4 (1)
From equation (1), the input offset voltage of the amplifier is nearly zero, ensuring a stable reference voltage. The stabilized reference voltage VREF is given by:
VREF = VQ1 + VR6 = VQ1 + R6I1 = VQ1 + R4I2 (2)
Since the base and collector of the PNP transistor are connected, VQ1 corresponds to the forward voltage drop VBE of the BE junction, typically around 0.6–0.8 V. The temperature coefficient of the BE junction is negative, while that of the resistor is positive. These opposing temperature effects in equation (2) ensure that the reference voltage VREF remains relatively stable with temperature changes.
The voltage regulation circuit, shown in Figure 2, includes two stages of CMOS differential amplifiers (COMP) and feedback control circuits. The input to the differential amplifiers is obtained through a voltage divider. After comparison and amplification, the feedback and current limiting circuits determine the switching state of the power transistor in the switching circuit.
The power switch circuit comprises a storage capacitor, a rectifier made from an NMOS transistor, and a switching network. P1 and P2 are connected directly to the ends of the coil L0, where an AC voltage is induced through electromagnetic coupling. After rectification, a DC voltage VDD is generated at the C0 end of the storage capacitor. When the N2 transistor turns on, the voltage regulation capacitor C5 forms a discharge path, allowing current from P1 and P2 to charge C5 instead of C0, stabilizing the voltage across C0 and providing a consistent power supply for the load circuit.
2.2 Working Principle
When the RF card enters the reader's magnetic field, an AC current is induced on P1 and P2 via electromagnetic coupling. This current is converted into DC through the rectifier and simultaneously charges both the storage capacitor C0 and the voltage adjustment capacitor C5. Since C5 is small, it can be charged quickly. Initially, the N2 transistor is off, so no discharge path exists for C5, and the current from P1 and P2 only charges C0, generating a VDD voltage. As the charging process continues, the active resistor and diode in the rectifier cause the voltage across P1 and P2 to rise, increasing the potential at point a. At the same time, the voltage sampling circuit outputs a value that rises with VDD.
When VDD reaches a threshold V0 (as shown in Figure 4), the sampled voltage exceeds the reference voltage VREF. This triggers the voltage regulation circuit to turn on the N1 and N2 transistors sequentially. Once N2 is on, the voltage at point a begins to decrease, causing P1 and P2 to recharge C5. With N2 continuously on, C5 discharges simultaneously, creating a continuous charge-discharge cycle. This keeps the peak voltage on P1 and P2 at a fixed level, preventing further charging of C0 and maintaining a stable VDD.
If the load consumes power, VDD drops below V0, turning off N2 and allowing C5 to stop discharging. P1 and P2 then resume charging C0, raising VDD again. This self-regulating mechanism ensures a stable power supply for the RF card.
3. Simulation Results
In typical operating environments, the coupling coefficient between the RF card and the reader is generally between 0.1 and 0.35, with a reader signal voltage of approximately 12V. During simulation, a 12V excitation at 13.56MHz was applied to simulate the inductive current in the coil L0. Using a 0.35µm SPICE model with a coupling coefficient of 0.25, the VDD was set to stabilize at 3.35V. The HSPICE simulation results are shown in Figure 4.
4. Conclusion
Through design and simulation analysis, it has been demonstrated that the voltage regulator circuit can rapidly achieve stable output and automatically adjust to changing conditions. The multi-target flow test results align closely with the simulation data, meeting the design specifications. This circuit shows strong practicality and potential for real-world applications in RF card systems.
All Accessories Of Laser Level
Green Laser Protective Glasses,Laser Protection Glasses,Laser Protection Eyewear,Laser protection
Guangdong Tumtec Communication Technology Co., Ltd , https://www.gdtumtec.com