Micropower isolated power supply design-EEWORLD

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Regarding the design of micro-power isolated power supply for two-wire transmitters, let's first look at what a transmitter is. Sensors are a general term for devices or devices that can be measured and converted into usable output signals according to certain rules. They are usually composed of sensitive elements and conversion elements. When the output of the sensor is a specified standard signal, it is called a transmitter. Common types include power transmitters, current and voltage transmitters, etc. When developing low-power intelligent two-wire transmitters, the design of micro-power power supply inside the instrument is very critical. First of all, in general, intelligent transmitters with microprocessors need to meet the power supply of microcontrollers, A/D, D/A and communication circuits, which requires more power than ordinary 4~20mA transmitters, and requires the internal power supply to have 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 4mA, these specific requirements bring great difficulties and challenges to the design of the system power supply. The designed micro-input power supply for the isolated two-wire transmitter adopts a fully integrated circuit design, which has the characteristics of simple structure, stable performance and low cost. It uses the 12~35VDC dropped on the two-wire transmitter as the input power supply, designs a simple constant current voltage regulator front-end input circuit, consumes a fixed current of 315mA, and provides two sets of mutually isolated 3V power supplies. The group that is not isolated from the input has a maximum load capacity of 5mA, and the group that is isolated from the input has a maximum load capacity of 3mA, which fully meets the requirements of the input and output isolated two-wire transmitter for power supply. 1. Overall design Figure 1 is the power supply schematic. It consists of 3 main parts: a 315mA/812V constant current voltage regulator circuit composed of U1, R1 and Z1; a DC/DC conversion circuit composed of U2 as the core; and a group of isolated power supplies composed of L2 and U3. The system design strives to be simple and highly integrated, and all components are selected to be able to work in the extended industrial temperature range of -40~85℃, which can ensure that the power supply can be reliably applied to field transmitters. Figure 1 Power supply schematic 1.1 Constant current voltage regulator circuit As the power supply for the two-wire transmitter, it must be ensured that the maximum working current does not exceed 4mA. Considering that the transmitter needs to have a certain low zero output indication, the general system power supply standard is below 315mA. At the same time, this type of power supply must have a constant current characteristic to ensure the working characteristics of the two-wire transmitter. There are many ways to design a constant current source. This design uses a three-terminal adjustable voltage regulator LM317L to design a constant current source. LM317L is a three-terminal adjustable voltage regulator, and Figure 2 is its typical application. Figure 2 (a) is its basic application as a standard voltage regulator. At this time, a stable voltage difference is fixed between its output end and the adjustment end, and the typical value is 1125V. Therefore, its output voltage VO=1125 (1+Ra/Rb). Taking advantage of the stable voltage difference between the output and adjustment ends of LM317, it is often used to design constant current sources. Figure 2 (b) is a typical application circuit, which generates a current of I=1125/R. See Figure 1. In the design, R1 is 360Ω, so a constant current of about 315mA can be obtained. Considering that the operating voltage range of the subsequent DC/DC chip is 4~11V, and considering the output power of the actual power supply, an 812V voltage regulator Z1 is used to complete the parallel voltage regulation function and provide a stable input voltage for U2. The condition is that the total current consumption of U2 is less than 314mA. Z1 must be selected as a high-quality voltage regulator that can be stable when the breakdown current is less than 011mA (Philips products can be selected, and the minimum static stable current is only tens of μA). Figure 2 LM317L typical application diagram D1 at the front end of the circuit is an anti-reverse diode, generally 1N4148 can be used. The fuse is a PTC device resettable fuse with an index of 100mA/60V to ensure that the external power supply is not affected when the power fails. Since the final application of the power supply is a field transmitter, the ambient temperature range in which it is located is relatively large, so the temperature drift factor must be considered. The main temperature drift of the power supply is the drift of the constant current, which is caused by the temperature drift of the reference pressure difference of LM317L and the temperature drift of the constant current resistor R1. In the actual circuit, R1 selects a product with a temperature coefficient lower than 5×10-6/℃, and the temperature drift can be ignored. The relationship curve between the reference pressure difference and temperature of LM317L is shown in Figure 3. In the temperature range of -40~85℃, the influence of temperature is more obvious, and compensation must be made in high-precision applications. Considering that the actual application of power supply is for intelligent transmitters, in the intelligent transmitter system, for the purpose of sensor calibration and line compensation, temperature sensors such as LM75 or TC77 digital temperature measurement chips will be designed in the transmitter circuit. Therefore, this circuit does not design a special hardware compensation, but provides a software compensation algorithm, which users can use to compensate for the temperature drift of the power supply when applying the power supply. Figure 3 LM317L reference temperature characteristic curve As shown in Figure 3, the reference pressure difference and temperature relationship curve of LM317L is similar to a simple cubic polynomial function relationship. It only needs to design a compensation function in the reverse direction of the Y axis. The system is calibrated with 20℃ as the compensation base point. The specific compensation formula is ΔI=A(t-20)2+B(t-20)3, where t is the ambient temperature. The coefficients A and B can be derived based on the reference voltage temperature curve provided in the manual of the LM317L chip actually used. The simplest way is to take two points of -20℃ and 60℃ and obtain two binary linear equations to solve A and B. In this way, it is easy to obtain an approximate function of the compensation curve with a relatively good fitting degree, and the influence of the temperature drift after compensation can be basically ignored. 1.2 DC/DC conversion circuit Since the biggest design difficulty of the power supply is the extremely small input power, the isolation feedback mode with relatively large power consumption cannot be used for the design of the isolation end. The actual circuit adopts the secondary side open loop method. Specifically, MAX639 is used to design the DC/DC core circuit, which achieves a high power efficiency conversion. When the power input is 315mA, it can provide a current far greater than 315mA to power the circuit, thus solving the high current demand of the intelligent system. According to the requirements of the system, the core chip must have the advantages of micro power consumption, high efficiency, wide input voltage range, and simple peripheral devices. The DC/DC chip in Figure 1 is MAXIM's MAX639[3]. It is a step-down DC/DC conversion chip. Its main features are: wide input voltage range (4~1115V); high conversion efficiency (up to more than 90%); low quiescent current (10μA); fixed output or adjustable output. The circuit is designed for adjustable output, and the output is set to 3V. Output current: Io=（Vi Iiη）/Vom Where: Vi is the input voltage; Ii is the input current; η is the conversion efficiency; Vo is the output voltage. In the circuit, Vi=812V, Ii=315mA, η=90%, Vo=3V. When the isolated secondary output is not considered, the available Io is about 816mA. This output current is already a relatively large supply capacity in the micro-power system. The above calculation of Io is only theoretical. In order to reliably start the circuit under the condition of micro-input power of 315mA/812V and obtain a conversion efficiency of more than 90%, the circuit needs to be designed very carefully.

The reliable startup of DC/DC is subject to many conditions. The necessary condition is that a sufficiently large startup pulse current must be provided. A 10μF tantalum electrolytic capacitor C2 is connected in parallel next to Z1 to provide startup guarantee. At the same time, it can also effectively prevent the operation of DC/DC from interfering with the constant current characteristics of LM317. Inductor L1 plays a decisive role in the conversion efficiency of DC/DC. The algorithm provided by the MAXIM manual is L1=50/IO. The unit of L1 is μH and the unit of IO is A. The value of L1 in the actual circuit is 4mH, which can ensure that the circuit works stably at the maximum output power while ensuring a sufficiently high conversion efficiency. It should be emphasized that if L1 is too small, the conversion efficiency of the circuit will be reduced, the startup current will increase, and it may even fail to start. If L1 is too large, the output capacity will be reduced, and the DC/DC circuit may oscillate.

In order to ensure the stability of the circuit, the DC/DC chip has very high requirements for the output capacitor C3. The most important point is that its equivalent series resistance ESR must be small enough [4] and have sufficient capacity. The circuit design uses 10μF tantalum electrolytic capacitors with excellent performance to ensure stable output.

The DC/DC 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 may even bring additional parasitic oscillations to the output, so it must be paid attention to during design. The most important principle is that the C2 and LI lead ends should be as close to the MAX639 pins as possible, and the ground pins of C2, D2, MAX639, R3 and C3 should be as close as possible. Try to use thick wires, and it is best to use a ground plane. The input voltage of the DC/DC is set to 812V, which is guaranteed by Z1. If the actual transmitter requires a relatively small power supply, Z1 can choose a lower voltage regulation value, which can make the entire power supply have lower requirements for the input voltage. The designed input voltage limit is 12V. If Z1 chooses 612V, the input voltage limit can be reduced to 10V.
1.3 The main feature of the isolated power supply winding

circuit is that it provides an isolated power supply winding, which uses the method of "stealing" electricity on the DC/DC output energy storage inductor. As shown in Figure 1, L2 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, and the structure is an open loop, its output stability is relatively poor. In the overall design, it must be considered from multiple angles to obtain satisfactory results.

First, its output power must be determined. Since the method of "stealing" electricity from the energy storage coil is adopted, its output power is limited and can only be less than the primary output power. This set of isolated power supply output is mainly used to power the sensor conversion circuit, the front-end A/D converter and the isolation circuit in the specific transmitter application. The analog measurement circuit of sensors such as differential capacitance sensors, thermocouple sensors and thermal resistors consumes power at the μA level. The front-end A/D generally adopts a multi-integral type or a Σ-Δ type A/D, which consumes less than 1mA, and the overall low-power optoelectronic isolation can also be less than 1mA. Therefore, the isolation winding can meet the actual needs as long as it can provide 3mA of current. It has been calculated that the maximum output of the circuit is 816mA without a secondary winding. Obviously, with a secondary winding, it can provide 3mA of current.

Secondly, since the isolation winding adopts an open-loop structure, the change of the primary load directly affects the stability of the secondary. Therefore, when the circuit is actually used, it is required that the circuit system of the primary side needs to ensure the stability of power consumption as much as possible during operation, and try to avoid using the working/sleeping rotation method for devices with relatively large power consumption. The circuit can provide a maximum current of 5mA for the primary side, which can fully meet the work of the commonly used micro-power MCU control system. There is no need to use the sleep mode, and this can also achieve the maximum system operation speed.

Finally, since the isolated power supply winding is mainly used to power the front-end small signal analog circuit, the quality of the power supply is relatively high. Therefore, a low-dropout linear regulator and a DC/DC converter are used in combination during design. The output low voltage after DC/DC conversion is subjected to noise reduction and voltage stabilization by a low voltage dropout linear regulator (LDO). This can complement each other and improve the power supply efficiency while meeting the requirement of low ripple voltage. The specific LDO uses the MAX1726 chip [5], which has an operating current of only 2μA and an output of 313V. The output amplitude before voltage stabilization depends on the output power of the primary side and the inductance of L2. According to the test, L2 is 3mH. When the primary current varies between 3~5mA and the secondary current is 2mA, the voltage before voltage stabilization fluctuates between 318~418V, which meets the input requirements of LDO voltage stabilization.

2. 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 and low cost. It is a stable and reliable two-wire transmitter power supply that can meet the use of various two-wire transmitters with complex requirements. The power supply has been applied to the integrated intelligent temperature transmitter. After a long period of field application test, it has excellent performance and fully meets the use requirements of isolated two-wire transmitters.

Keywords：Micropower Reference address：Micropower isolated power supply design

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