Development of power supply for smart transmitter - Power Circuit - Circuit Diagram

MOS power IC full range

Photocoupler

With the maturity of microcontroller MCU (or microcontroller) technology, the original analog transmitter was gradually replaced by a smart transmitter with a microcontroller as the data processing and control core. Smart transmitters extend the capabilities of analog transmitters to improve measurement accuracy and operational reliability, as well as linearization, temperature compensation, automatic zero and span adjustment, and digital communication. When developing a low-power intelligent two-wire transmitter, the internal micropower power supply design is critical. First of all, the smart transmitter with microprocessor must meet the power supply of the microcontroller, A/D, D/A and communication circuit. It needs more power than the original analog transmitter, and requires higher power supply from the internal power supply. effectiveness. In addition, for capacitive sensors and thermocouples, the grounding or the sensor may be in contact with the casing (grounding). The designed transmitter circuit must be isolated from the input and output to ensure the normal operation of the subsequent control system. And anti-common mode interference capability. Since the external circuit provides a working current of only 4 mA for a two-wire transmitter system, these specific requirements pose great difficulties and challenges for the design of the system power supply. The isolated two-wire transmitter power supply with micro input power is designed for use in RF admittance level transmitters. 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, and the buck converter mode is used to output two sets of isolated 5V power supplies. Input voltage 24VDC, a set of maximum 10mA load capacity not isolated from the input, a set of maximum 4mA load capacity isolated from the input, 24VDC bus current less than 3.5mA, efficiency can reach more than 85%, fully meet the input and output isolation type The two-wire smart transmitter requires power.

The overall design is multiplexed by the smart transmitter power line and signal line. When the RF admittance level transmitter is working normally, the current signal of 4~20mA is output according to the level of the level, and the circuit power consumption current cannot exceed 4mA loop current. There is also a need for a fault alarm function. The bus current requirement is 3.6 mA. In order to produce, a certain margin is required, that is, the power consumption current of the RF admittance level transmitter itself must be less than 3.5 mA. Now, simply estimate the maximum power consumption of this transmitter. The voltage from the control room to the transmitter is calculated according to 24V. The 4~20mA DC signal is sent to the distributor through the transmitter. (typically 250Ω) is converted into a DC voltage signal of 1~5V and sent to the control room. In theory, 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 portion and the like. Figure 1 shows the composition of the smart transmitter and the requirements for the power supply.

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