Development of isolated power supply for intelligent transmitter

     With the maturity of microcontroller MCU (or single-chip microcomputer) technology, the original analog transmitter has gradually been replaced by intelligent transmitters with microcontrollers as the core of data processing and control. Intelligent transmitters have expanded the functions of analog transmitters, not only improving the measurement accuracy and working reliability, but also easily realizing linearization processing, temperature compensation, automatic zero and range adjustment, and digital communication. When developing low-power intelligent two-wire transmitters, the design of the micro-power supply inside the instrument is very critical. First of all, the intelligent transmitter with a microprocessor needs to meet the power supply of the microcontroller, A/D, D/A and communication circuits, which requires more power than the original analog transmitter, and requires the internal power supply to have a higher power supply efficiency. In addition, for capacitive sensors and thermocouples, it is also necessary to consider the situation where 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 and anti-common-mode interference capability of the subsequent control system. Since the maximum working 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 isolated two-wire transmitter power supply with micro input power designed by me is applied to RF admittance level transmitter. It adopts full integrated circuit design, with the characteristics of simple structure, stable performance and low cost. The input voltage range is 16-32VDC, and it adopts step-down converter to output two sets of mutually isolated 5V power supplies. The input voltage is 24VDC, and the group that is not isolated from the input has a maximum load capacity of 10mA, and the group that is 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%, which fully meets the power supply requirements of the two-wire intelligent transmitter with input and output isolation.

　　Overall Design

　　Since the power line and signal line of the intelligent transmitter are reused, when the RF admittance level transmitter works normally, it outputs a current signal of 4 to 20 mA according to the level. The circuit power consumption current cannot exceed the loop current of 4 mA. It also needs to have a fault alarm function. The bus current requirement is 3.6 mA. For production, a certain margin needs to be reserved, that is, the power consumption current of the RF admittance level transmitter itself must be less than 3.5 mA. Now, let's simply estimate the maximum power consumption of this transmitter. The voltage sent from the control room to the transmitter is calculated as 24 V. The 4 to 20 mA DC signal is first sent to the distributor after passing through the transmitter, and then converted into a 1 to 5 V DC voltage signal through a load resistor (usually 250 Ω), 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 = 80.15 mW. This does not include the voltage loss of the input circuit part. Figure 1 shows the composition of the intelligent transmitter and the power supply requirements.

Figure 1 Smart transmitter components and power requirements

　　There are two main types of low-voltage power supply chips on the market: linear power converters and switching power converters. Linear power converters basically do not require peripheral components, are low in cost, are not susceptible to electromagnetic interference, and have low ripple voltage, but their main disadvantage is low power efficiency (generally less than 40%), especially when used in low output voltage regulation, where the efficiency is even lower. According to the working principle of a linear power supply, its output current is close to the input current, and the 3.5mA output current cannot meet the needs of the microcontroller circuit and the detection circuit at all, so a switching power converter can only be used.

　　There are not many types of micro-power switching power converter chips that can be used in two-wire smart transmitters on the market, and they are expensive. To this end, we proposed two solutions: one is a low-cost solution, using the S-8251B40 chip produced by Seiko Electronics, and taking the method of first reducing the input bus voltage to 16V; the other is using the LT1934 chip produced by Linear.

　　Solution 1

　　This solution uses the S-8251B40 chip produced by Seiko Electronics, which has a low cost and low overall efficiency, but has high requirements for the design of the power load circuit. Its input is 24VDC, and the output of a group that is not isolated from the input is 4.6V, 8mA current, and the output of a group that is isolated from the input is 3.6V, 3mA current. The 24VDC bus current is less than 3.5mA, and the efficiency can reach more than 57%. If calculated based on the input voltage of 16VDC, the efficiency can reach more than 85%.

　　The S-8520/8521 series is a CMOS step-down DC/DC controller that is composed of a reference voltage source, an oscillation circuit, and an error amplifier. The S-8520 series achieves low ripple, high efficiency, and good transient response characteristics by using a PWM control circuit and an error amplifier circuit that change the duty factor linearly in the range of 0 to 100%. In addition, it has a built-in soft start circuit to prevent overshoot during startup. The S-8521 series uses PWM/PFM switching control, and operates with a PWM control with a duty factor of 25% to 100% in normal conditions. When the load is light, it automatically switches to PFM control with a duty factor of 25%. High efficiency is achieved in a wide range from standby to operation. By externally connecting a P-channel power MOSFET or PNP transistor, a coil, a capacitor, and a diode, a step-down DC/DC controller can be constructed, and this product is suitable for power supplies for mobile devices. Main specifications: Input voltage is 2.5~16V, output voltage is 1.5~6.0V, and can be set in 0.1V steps; low current consumption operation: 60μA maximum (A, B type products); sleep: 0.5μA maximum. The typical oscillation frequency is 180kHz (A, B type products); the typical soft start function is 8ms (A, B type products); with on/off control function. Figure 2 shows the basic circuit of S-8251.

Figure 2 Basic circuit of S-8251

　　When designing a step-down circuit using 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 the circuit stability and increases Iout. If the L value is made smaller, the efficiency will be reduced, resulting in insufficient current driving capability of the switching transistor, causing Iout to gradually decrease. As 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 L value is made larger, the power consumption caused by the series resistance of the coil increases, resulting in a decrease in operating efficiency and Iout will also decrease. In the process of gradually increasing the L value of the S-8520/8521 series, the output voltage may become unstable due to different conditions of input voltage, output voltage and load current. When debugging the circuit in practice, it is necessary to conduct sufficient experiments before deciding on the selected L value. The diode needs to use a fast recovery or Schottky diode. To ensure the stability of the circuit, the S-8251 chip has very high requirements for the output capacitor. The most important point is that its equivalent series resistance (ESR) must be small enough and have sufficient capacity. The circuit design uses a 10μF tantalum electrolytic capacitor with excellent performance to ensure stable output. The S-8251 chip is the core of the circuit. The actual circuit layout has a great impact on the performance of the circuit, especially the output ripple. An unreasonable circuit board layout design will cause additional parasitic oscillations in the output, so attention must be paid during design.

　　Because the input voltage range of the S-8251 chip is 2.5~16V, 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 step-down circuit must be less than tens of μA, otherwise the bus current will be difficult to be less than 3.5mA. I use MOSFET series step-down method, and the reference source does not use a voltage regulator diode, but uses LM385 to achieve a static current of 36μA. Figure 3 is a 24V step-down to 16V circuit. [page]

　　Solution 2

　　This solution uses the LT1934 chip produced by Linear, which has a high cost, high overall efficiency, low requirements for the design of the power load circuit, and a large adjustment margin. When the input is 24VDC, the output of a group that is not isolated from the input is 5V, 9mA current, and the output of a group that is isolated from the input is 5V, 4mA current. The 24VDC bus current can be easily less than 3.5mA, and the efficiency can reach more than 85%.

Figure 3 24V to 16V step-down circuit

　　The LT1934 series chip is a PWM-controlled CMOS step-down DC/DC controller consisting of a reference voltage source, an oscillator circuit, and an error amplifier. Main specifications: input voltage is 3.3~34V, output voltage is 1.5~6.0V, and can be set in 0.1V increments; 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 S-8251. When designing the PCB layout, it should be noted that the distance between capacitor C2 and chip LT1934 should not be too far. Try to use thick wires and preferably 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 be reduced, the starting current will increase, and it may even fail to start. If L1 is too large, the output capacity will decrease, and the DC/DC circuit may oscillate.

　　Isolation of power windings

　　The RF admittance level transmitter also requires an isolated power supply for the sensor circuit to ensure the safe operation of the transmitter and high anti-common mode interference capability. The power supply circuit I designed provides an isolated secondary winding on the inductor of the buck converter, which uses the method of "stealing" power on the DC/DC output energy storage inductor L1A. Figure 5 shows the circuit design with isolated power supply.

Figure 4 Basic circuit of LT-1934

　　L1B is the power supply coil of this isolated power supply. Since this set of isolated power supply is a secondary coil loaded on the energy storage coil of the DC/DC, the structure is an open loop, so its output stability is relatively poor. The change of the primary side load directly affects the stability of the secondary side. Therefore, when the circuit is actually used, it is required that the primary side circuit system needs to ensure the stability of power consumption as much as possible during operation.

Figure 5 Circuit design with isolated power supply

　　in 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 transmitter power supply that can meet the use of various two-wire transmitters with complex requirements. Considering factors such as size and installation, we use the second solution on the RF admittance level transmitter. At present, this power supply has been applied to the RF admittance level transmitter. After a long period of field application test, it has excellent performance and fully meets the use requirements of isolated two-wire transmitters.