Design of High-efficiency RFID Handheld Power Supply

　　RFID handsets are widely used in transportation, access control, logistics, attendance, cargo management, and identification. RFID handheld devices have higher requirements on the efficiency, service life, reliability, volume, and cost of the power supply. Therefore, designing a power supply with good stability, high efficiency, and small spurs is of great significance for RFID handhelds.

　　1 RFID handset hardware structure

　　In the design of the RFID handset system based on the embedded system, the microprocessor LPC2142 is used as the main controller, and SRAM, Flash, SD card, keyboard, LCD display, and sound prompts are expanded according to the needs of the system for data processing and data storage. , Human-computer interaction and error alarm prompts, data communication can be carried out with the host through the USB interface, the backlight module can provide backlight for the LCD and keyboard, and the voltage detection module detects the battery voltage through the core processor’s A/D converter, thereby indirectly After detecting the remaining power of the battery, the RF module can transmit and receive radio frequency signals between the reader and the tag, and debug and download programs through the JTAG interface. The power supply part can provide power for each module that needs power in the system, which is the key content of the design of this article. The block diagram of the system hardware structure is shown in Figure 1.

　　2 Indicators of power demand

　　After design and calculation, the system needs two voltage power supplies, one of which is 3.3 V, which supplies power to the keyboard, LCD reset circuit, the externally expanded memory, and the RF module; the other is 5 V, which is the sound of the system The cue circuit and the backlight circuit of the keyboard and LCD provide power. For the convenience of carrying, the system adopts battery power supply, and the performance indicators to be achieved are as follows:

　　(1) Power conversion efficiency ≥80%;

　　(2) Output current requirements: 3.3 V output current 500 mA; 5 V output current 300 mA;

　　(3) The fluctuations of the two power supply voltages are controlled within ± 5%;

　　( 4) The battery can be charged via USB input.

　　3 Features of various power chips and precautions for selection

　　3.1 Comparison of the characteristics of various power chips

　　Table 1 is a comparison of 4 kinds of power chips.

　　Note: LDO is the abbreviation of Low DropOut, that is, low dropout linear regulator.

　　3. 2 Precautions for selection

　　First of all, the power chip type must be selected correctly. It is necessary to clarify the input voltage and the required output voltage, and then determine whether it is a boost, a buck, or a boost/buck. In particular, it should be noted that ordinary linear regulators, LDOs and Buck (or Step-down) DC-DCs can only step down but not boost, and Boost (or Step-up) DC-DCs can only boost but not Buck.

　　The reason for emphasizing this is that the manuals of some chips (LDO or step-down DC-DC) give a wide input voltage range and output voltage range, which can easily mislead inexperienced designers. Many of the output voltage ranges in the manual are for the given input voltage range. For a specific input voltage, in many cases, the actual output cannot reach the given output voltage. This point is very critical, which determines the success or failure of the system design and should arouse great attention.

　　Secondly, in the power supply design of handheld devices, pay attention to the quiescent current of the chip. This has a great impact on the standby time of the system. The quiescent current of a good power chip is in the μA level, and the quiescent current of a poor chip is in the mA level. The difference is thousands of times. , The smaller the quiescent current, the less power dissipation of the battery, and the longer the life span.

　　Once again, pay attention to the efficiency from the actual load. Power efficiency and output current are closely related. When the output current is very small or very large, the efficiency will become worse. The power supply chip needs to be selected according to the required current to maximize the efficiency.

　　4 Scheme selection and chip selection

　　4. 1 Scheme selection

　　Scheme 1: 3. The 3 V output adopts LDO, and the 5V output adopts charge pump.

　　Scheme 2: 3. 3 V output adopts Buck/Boost type DC-DC, 5V output adopts boost type DC-DC.

　　Since the voltage range of lithium-ion batteries varies widely, there should be a normal power output voltage between 2. 5V and 4.2 V (4.2 V is the voltage that can be reached at full charge). If 3. 3 V is used Since the output LDO needs to meet the requirements of the minimum voltage difference between input and output, when the battery voltage drops to about 3.4 V, the power supply may not reach the output voltage of 3.3 V. Using a charge pump to output 5 V, the efficiency of the charge pump will not be very high when the input and output voltages are relatively close. The second scheme can maximize the power conversion efficiency and extend the battery life.

　　Considering the above comparison, choose the second option.

　　4.2 Chip selection

　　Through inquiries, it was decided to use TI’s two chips, TPS63031 and TPS61240, as 3.3 V output and 5 V output voltage conversion chips, respectively. The TPS63031 can increase or decrease the input voltage in the range of 2.4 to 5. 5 V. The output current is up to 800 mA in the low-voltage working mode. In the energy-saving mode, when the output current changes between 100 and 500 mA, the efficiency is above 80%. TPS61240 is a step-up DC-DC that can work at 3.5 MHz, and the output current can reach 450mA. It has a PFM/PWM mode of operation. When the load current is about 200 mA, it can provide more than 80% of the battery voltage range. effectiveness.

　　Because the microprocessor has higher requirements for power ripple, an LDO is added behind the 3.3V output to filter out the larger ripple of the DC-DC output and improve the accuracy of the output voltage. In order to meet the requirements of differential pressure and reliable working voltage of the processor, the TPS78320 with an output voltage lower than 3.3V can be selected, which can output a voltage of 3.2 V, and can output a current of up to 150 mA. This voltage meets the reliable operation of the microprocessor LPC2142 Power supply voltage range (3.0 V to 3. 6 V) and current demand.

　　In addition, the quiescent current of the LDO is only 500 nA, which is in line with the energy-saving requirements of battery-powered handheld systems.

　　5 Power circuit design

　　Carefully read the chip manual, design and draw a schematic diagram of the power supply circuit as shown in Figure 2.

　　U2 and U3 in Figure 2 are respectively 3.3 V output and 5 V output DC-DC regulators, U4 is an LDO, and the 3.3 V output of the DC-DC is processed by the LDO after active filtering. The device provides a power supply of about 3.2 V. U1 is the lithium-ion battery charge management chip MAX1555 of Maxim, which can charge the lithium-ion battery via USB.

　　The capacitors C1, C5, C7, and C3 in the circuit are the input filter capacitors of the chip, which are used to improve the transient response and suppress noise and ripple. C4, C6, C8, and C2 are the output capacitors of the chip, which are used to keep the circuit stable and filter. Among them, C1 and C4 should use X7R ceramic capacitors with a rated voltage of not less than 6.3V, and other capacitors use X5R ceramic capacitors with a rated voltage of not less than 6.3V. Of course, X7R capacitors are better or better, but the price is more expensive. L1 and L2 should use inductors with a rated current not less than twice the output current and a small DC resistance, which can reduce the loss of the circuit.

　　The two Schottky diodes IN1 and IN2 in Figure 2 can protect the battery. IN1 is to prevent the reverse breakdown of the battery by the USB power supply. The function of IN2 is to prevent the battery and U1 from forming a self-charging loop. A diode is indispensable. The charger's pin /CHG right pull-up resistor R1 is used to indicate the charging state. The /CHG pin is connected to the GPIO pin of the microprocessor. When in the charging state, the pin outputs low level; when /CHG changes When it is in the high-impedance state, it means that the battery is fully charged.

　　6 Debug

　　6.1 Debugging steps

　　After soldering the components on the printed circuit board according to the parameters on the schematic diagram, carefully check whether the value of the component, the welding direction, and the polarity of the component are soldered correctly. Use a multimeter to carefully check whether there is a false solder in the soldering of the component. , Whether there should be short-circuit phenomenon in the closer components.

　　6.2 Notes on debugging

　　The debugging of the power supply system must first ensure that the power supply and the ground cannot be short-circuited, otherwise the battery will be burnt out.

　　Debug the modules separately, weld one to check and debug one, and then perform overall debugging when there is no problem with each module.

　　For more complex systems, the power supply of the system should be welded, checked, and debugged first, and then other modules should be debugged after successful debugging.

　　After powering up, first touch each chip to see if it is hot. If it is hot, in order to prevent the chip from being burnt for a long time, you must first cut off the power supply, and then power on and debug after finding the cause.

　　If you hear a sound from the chip after power-on, you should cut off the power, check whether there is a short circuit in the circuit where the problem occurs, and continue power-on debugging after the problem is found.

　　In order to find the problem easily, at least two boards must be welded to facilitate comparison during the test and find the problem.

　　7 Conclusion

　　After testing, the output voltage of the 3. 3 V power supply fluctuates within 0.097V, the output of 3.2 V fluctuates within 0.05 V, and the fluctuation of 5 V output fluctuates within 0.1 V, that is, the fluctuation of each channel voltage. All components are within ±3%. When the corresponding rated power resistance is connected externally, all components work normally, that is, the system can output a given current. Through the measurement of input current and voltage and output current and voltage, the calculated efficiency is above 83%. In short, the various indicators of the system have met the expected requirements.

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