With the maturity of the microcontroller MCU (or single chip microcomputer) technology, the original analog transmitter is gradually replaced by the intelligent transmitter with the microcontroller as the core of data processing and control. The intelligent transmitter expands the functions of the analog transmitter, not only improves the measurement accuracy and working reliability, but also can easily realize the functions of linearization processing, temperature compensation, automatic zero point and range adjustment, and digital communication. When developing low-power smart two-wire transmitters, the design of the micropower power supply inside the instrument is critical. First of all, a smart transmitter with a microprocessor to meet the power supply of the microcontroller, A / D, D / A, and communication circuits requires more power than the original analog transmitter and requires a higher power supply from the internal power supply effectiveness. In addition, for capacitive sensors and thermocouples, it is also necessary to consider the situation of grounding or the sensor may touch the shell (grounding). The designed transmitter circuit must be isolated from the input and output, so as to ensure the normal operation of the subsequent control system And anti-common mode interference. Since the maximum operating current provided by the external circuit for the two-wire transmitter system is only 4mA, these specific requirements bring great difficulties and challenges to the design of the system power supply. The micro-power isolated two-wire transmitter power supply designed by me is applied to the radio frequency admittance level transmitter. It is designed with full integrated circuit and has the characteristics of simple structure, stable performance and low cost. . The input voltage range is 16 ~ 32VDC, using a buck converter, output two sets of 5V power supply isolated from each other. The input voltage is 24VDC, the group that is not isolated from the input has a maximum load capacity of 10mA, and the group isolated from the input has a maximum load capacity of 4mA. The 24VDC bus current is less than 3.5mA, and the efficiency can reach more than 85%, fully satisfying the input and output isolation type The requirements of the two-wire smart transmitter for power supply. Overall design Due to the multiplexing of the power line and signal line of the intelligent transmitter, when the RF admittance level transmitter is working normally, it outputs a current signal of 4-20mA according to the level, and the circuit power consumption current cannot exceed the loop current of 4mA. There is also a need for a fault alarm function, the bus current requirement is 3.6mA, and a certain margin is required for production, that is, the power consumption current of the RF admittance level transmitter itself must be less than 3.5mA. Now, simply estimate the maximum power consumption of this transmitter. The voltage sent from the control room to the transmitter is calculated as 24V. The 4-20mA DC signal passes through the transmitter and is first sent to the power distribution unit through the load resistance. (Generally 250Ω) Converted into a DC voltage signal of 1 ~ 5V, and then sent to the control room. Theoretically, the maximum power that can be consumed inside the transmitter should not exceed (24-1) × 3.5 = 80.15mW. This does not include the voltage loss of the input circuit. Figure 1 shows the composition of the smart transmitter and its power requirements. Figure 1 Smart transmitter composition and power requirements Figure 2 S-8251 basic circuit Figure 3 24V step-down to 16V circuit Figure 4 Basic circuit of LT-1934 Figure 5 Circuit design with isolated power supply We offer rgb led bar dmx lighting, similiar with american dj lighting or RC lighting, the LED Pixel Bar is Color Changing Light Bar. Led Rgb Bar , led rgbw bar is available. pixel bar have 1 led light bar, 2 led light bar, 3 led light bar, 6 Led Light Bar. It's dmx512 programming, the address have two type, one is auto address, another is manual address. That's to say we have led bar auto and led bar manual. Auto led bar, you don't need assign dmx address, it's convenience for event show and led light rental. Manual led bar, you need assign dmx address by Address Writer before light up, for DJ club, architecture light, it's ok. Maybe you want to know how to install a led light bar, don't worry, we have bracket, so that you can fix the bar easily. Regarding color changing led light bar, you can control by Led Controller. Led Pixel Bar Led Pixel Bar,Led Pixel Bar,Pixels Bar,Color Changing Light Bar Shenzhen Iseeled Technology Co., Ltd. , https://www.iseeledlight.com
At present, there are two main low-voltage power supply chips on the market: linear power converters and switching power converters. Linear power converters basically do not require external components, low cost, are not susceptible to electromagnetic interference, and the ripple voltage is small, but the main disadvantage is low power supply efficiency (generally less than 40%), especially used in low output voltage regulation, efficiency Lower. According to the working principle of the linear power supply, the output current is close to the input current, and the output current of 3.5mA can not meet the needs of the single-chip circuit and the detection circuit at all, and only a switching power supply converter can be used.
At present, there are not many types of micro-power switching power converter chips that can be applied to two-wire smart transmitters on the market, and they are expensive. To this end, we have proposed two schemes: one is a low-cost scheme, using the S-8251B40 chip produced by Seiko, adopting the method of stepping down the input bus voltage to 16V; the other is using the LT1934 chip produced by Linear Corporation.
Solution One This solution uses the S-8251B40 chip produced by Seiko Instruments, which has a lower cost and lower overall efficiency, and requires higher design of the electrical load circuit. Its input is 24VDC, a group of outputs not isolated from the input output 4.6V, 8mA current, a group isolated from the input output 3.6V, 3mA current, 24VDC bus current is less than 3.5mA, the efficiency can reach 57%. If the input voltage is calculated at 16VDC, the efficiency can reach more than 85%.
The S-8520 / 8521 series is a PWM control (S-8520 series), PWM / PFM switching control (S-8521 series) CMOS step-down DC / DC control consisting of a reference voltage source, an oscillation circuit, and an error amplifier. Device. The S-8520 series achieves low ripple, high efficiency, and good transient response characteristics by changing the duty cycle of the PWM control circuit and error amplification circuit in a linear manner from 0 to 100%. In addition, it has a built-in soft-start circuit to prevent overshoot during startup rise. The S-8521 series uses PWM / PFM switching control. Normally, it operates with a PWM control with a duty factor of 25% to 100%. At light loads, it automatically switches to PFM control with a duty factor of 25%. From the standby time of the device, high efficiency is obtained in a wide range up to the time of operation. By connecting external P-channel power MOSFET or PNP transistors, coils, capacitors and diodes, a step-down DC / DC controller can be constructed, which is suitable for power supply of mobile devices. Main indicators: input voltage is 2.5 ~ 16V, output voltage is 1.5 ~ 6.0V, can be set in 0.1V as an advanced unit; low current consumption operation: 60μA maximum (A and B type products); during sleep: 0.5μA maximum. The typical value of the oscillation frequency is 180kHz (type A and B products); the typical value of the soft start function is 8ms (type A and B products); the on / off control function is available. Figure 2 shows the basic circuit of S-8251.
When designing the step-down circuit with the S-8251 chip, pay attention to the design of the inductor. The inductance value (L) has a great influence on the maximum output current (IOUT) and efficiency (η). The smaller the L value, the larger the peak current circuit (IPK), which improves circuit stability and increases Iout. If the value of L is made smaller, the efficiency will be reduced and the current driving capability of the switching transistor will be insufficient, which will cause Iout to gradually decrease. When the L value gradually increases, the power consumption caused by the peak current (Ipk) of the switching transistor also decreases, and the efficiency becomes maximum when a certain L value is reached. Then, if the value of L is made larger, the power consumption due to the series resistance of the coil becomes larger, which leads to a decrease in operating efficiency, and Iout also decreases. In the process of S-8520 / 8521 series products, when the L value gradually increases, the output voltage may become unstable due to the different input voltage, output voltage, and load current conditions. When actually debugging the circuit, it is necessary to conduct a sufficient test before deciding the selected L value. Diodes need to use fast recovery or Schottky diodes. In order to ensure the stability of the circuit, the S-8251 chip has high requirements on the output capacitance. The most important point is that its equivalent series resistance ESR must be small enough, and at the same time have sufficient capacity. The circuit design uses a 10μF tantalum electrolytic capacitor with excellent performance, which can ensure a stable output. The S-8251 chip is the core of the circuit. The actual circuit layout has a great influence on the performance of the circuit, especially the output ripple. The unreasonable circuit board layout design will cause additional parasitic oscillations in the output. Must pay attention to.
Because the input voltage range of the S-8251 chip is 2.5 to 16V, and the bus input voltage range is 24V, it must first go through the step-down link, which will greatly reduce the conversion efficiency. The static power consumption of the buck circuit should be less than tens of μA, otherwise the bus current is difficult to be less than 3.5mA. I use the MOSFET series step-down method. The reference source does not use a zener diode, but the LM385, which achieves a quiescent current of 36μA. Figure 3 shows the 24V step-down to 16V circuit.
Solution 2 This solution uses the LT1934 chip produced by Linear Company, which has a high cost and high overall efficiency. It does not have high requirements for the design of the electrical load circuit and has a large adjustment margin. When inputting 24VDC, a group that is not isolated from the input outputs 5V, 9mA current, and a group that is isolated from the input outputs 5V, 4mA current. The 24VDC bus current can easily be less than 3.5mA, and the efficiency can reach more than 85%.
The LT1934 series chip is a PWM-controlled CMOS step-down DC / DC controller composed of a reference voltage source, an oscillation circuit, and an error amplifier. Main indicators: input voltage is 3.3 ~ 34V, output voltage is 1.5 ~ 6.0V, can be set in 0.1V as an advanced unit; low quiescent current 12μA maximum, maximum output current 300mA. Figure 4 shows the basic circuit of LT1934.
When designing the circuit, the device selection is basically the same as the S-8251. When designing the PCB layout, pay attention to the distance between the capacitor C2 and the chip LT1934. Try to use a thick line. It is best to use a ground plane, otherwise it will cause self-oscillation. Inductor L1 plays a decisive role in the conversion efficiency of DC / DC. If L1 is too small, the conversion efficiency of the circuit will decrease, the starting current will increase, and it will not even start. If L1 is too large, it will cause the output capacity to decrease, and at the same time, the DC / DC circuit will likely oscillate.
The isolated power winding RF admittance level transmitter also needs a set of isolated power for the sensor circuit. Ensure the safe operation of the transmitter and high resistance to common mode interference. The power circuit I designed provides an isolated secondary winding on the inductance of the buck converter, which uses the method of "stealing" electricity on the DC / DC output energy storage inductor L1A. Figure 5 shows the circuit design with isolated power supply.
L1B is the power supply coil of this isolated power supply. Because this set of isolated power is a secondary coil loaded on the DC / DC energy storage coil, the structure is open loop, so its output stability is relatively poor. The change of the primary load directly affects the stability of the secondary side. Therefore, when the circuit is actually used, the primary side circuit system needs to ensure the stability of power consumption as much as possible during operation.
Conclusion The two-wire transmitter isolated power supply has the characteristics of wide operating temperature range, wide input voltage range, high output efficiency, high integration, good isolation performance, small size, low cost, etc. It is a stable and reliable two-wire system The transmitter power supply can meet the use of various two-wire transmitters with complex requirements. Taking into account factors such as size and installation, we adopt the second solution for the RF admittance level transmitter. At present, the power supply has been applied to the RF admittance level transmitter. After a long time of field application test, the performance is excellent, and it fully meets the requirements of the isolated two-wire transmitter.