A brief discussion on the design and implementation of digital controlled DC current source

DC current sources with good stability, high precision and preset output are often used in electronic equipment . The digital control DC current source designed in this paper can effectively reduce the output error caused by aging of components, temperature drift and other reasons. The output current is adjustable from 20mA to 2000mA, the output current can be preset, with "+" and "-" step adjustment, the output current signal can be directly displayed and voice prompts. Hardware circuit uses Lingyang's single-chip microcomputer SPCE061A as the control core, uses the closed-loop control principle, and adds a feedback circuit to make the entire circuit form a closed loop. In terms of software, the PID algorithm is mainly used to achieve precise control of the output current. The system has high reliability, small size, simple and convenient operation, and a friendly human-machine interface.

System hardware implementation solution

This design uses a single-chip microcomputer as the main control component, presets the output current value through the keyboard and uses a liquid crystal module for real-time display. The entire system hardware consists of a microcontroller, voltage -current conversion, keyboard, display, DC voltage regulator power supply and voice prompt modules. The system composition block diagram is shown in Figure 1.

Figure 1 Basic module block diagram of digital controlled DC current source

The microcontroller is the core of the whole system and is responsible for the operation of the whole system. In order to simplify the hardware circuit, ensure the system performance is stable and reliable, and facilitate the coordination of functions such as voice broadcast, keyboard setting and real-time display of information, after multiple schemes were demonstrated, the microcontroller selected was SPCE061A from Lingyang Company. The microcontroller integrates ADC, DAC, PLL, AGC, DTMF, LCD-Driver and other circuits (related to the IC model). It uses a reduced instruction set (RISC), and the instruction cycle is in units of CPU clocks. In addition, it also has DSP functions, built-in 16-bit hardware multipliers and adders, and is equipped with special instructions owned by DSP, which greatly accelerates the running speed of various algorithms. At the same time, Lingyang microcontroller application development tools can be used in the Windows environment. The tool supports standard C language and Lingyang microcontroller assembly language, integrates assembly, programming, simulation and other functions, and greatly accelerates the software development process. It is more appropriate to use this microcontroller as a controller. Under the premise of simple hardware circuits, it is easy to realize A/D and D/A conversion, voice prompts, PID operations and other functions.

The main function of the display module is to display the set current output value and other human-computer interaction information. This part can use a seven-segment digital LED display to display information such as numbers, simple letters and decimal points, but because its display information is single and the human-computer interaction is not friendly, this article uses the character LCD display LCDSMC1602A module. This module has the advantages of being light, thin, short, low-voltage, low-power, small size, no radiation, flat right-angle display, and stable image without flickering. It also has a large visible area, good surface effect, high resolution, and strong anti-interference ability. It is suitable for displaying letters, numbers, symbols and other information, and does not require the expansion of too many peripheral circuits. It can be directly controlled by the single-chip microcomputer for output display.

The voltage-current conversion module is composed of a precision op amp and a Darlington circuit composed of three transistors. The conversion circuit uses the flat output characteristics of the transistor and the deep negative feedback circuit to stabilize the output current. As shown in Figure 2, this V/I conversion circuit has a strong load capacity and a current output range of 0"3A. The output current Io is fed through the feedback resistor RF to obtain a feedback voltage Vf, Vf= V11-V12, which is added to the two input terminals of the operational amplifier through R5 and R6. Assume that the voltages at both ends of the op amp are V1 and V2, and Vi is output by the microcontroller DAC. Because the input current of the ideal op amp is approximately zero, and V1=V2, then:

V12[1-R6/(R2+R6)]+ViR6/(R2+R6)=V11R1(R1+R6)

Since V12 = V11-Vf, then:

V11R2/(R2+R6)+(ViR6-VfR2)/(R2+R6)=V11R1/(R1+R5)

Let R1=R2=10kW, R5=R6=1kW, then Vf=ViR6/R2=Vi/10

If feedback is not considered for the time being, Io=Vi/(10Rf). It can be seen that the calibration of the output current is determined by the output voltage Vi obtained by D/A conversion and the resistance value of Rf, which is a linear transformation. Rf is made of large-diameter copper wire, and its temperature coefficient is very small (5ppm/℃). The large wire diameter can minimize its temperature influence. The 3 transistors should use high-power tubes TIP122, and use heat sinks to ensure that the tubes work in the linear region.

Figure 2 V/I conversion Circuit Figure

There is another solution for the voltage-current conversion module, which is to use three operational amplifiers to form a current source with variable output current, as shown in Figure 3. The output current I=Vi/R1. In order to keep the voltage across R1 constant, the differential amplifier IC1b monitors the potential across R1 through the emitter follower IC1c. This potential is added to the inverting input of the comparator ICa through the 7th pin of IC1b and compared with Vref. According to the comparison result, the output of the comparator changes until it is balanced, that is, Vr1=Vi. The capacitor in the circuit is used to compensate the frequency of ICa and reduce the delay of the control loop. As long as R1=R2=R3=R4=R5, the performance of this circuit is good, but the load capacity is not strong, and the loop delay compensation has a greater impact on the stability of the circuit.

Figure 3 Three op amp V/I conversion circuit

The system keyboard module can be connected in an independent or row-column (matrix) manner. This module mainly completes the setting of output current and other information. The DC voltage regulator power supply module supplies power to the entire system, and the voice module implements voice prompts, making the system design more humane and the system has a friendly working interface. Lingyang microcontroller internal integration of ADC, DAC, PLL, AGC, DTMF and other modules, the voice function can be realized by software programming, no external circuit is required, and system resources are effectively utilized.

[page]System operation and software process

Through the above analysis of each module, the system will convert the preset current by SPCE061A MCU through D/A conversion during operation, drive the V/I conversion circuit with output voltage, and input the voltage corresponding to the current into the MCU system after A/D conversion through the closed loop and A/D conversion by the signal processing circuit, and adjust the current output through the PID algorithm. The whole system workflow is shown in Figure 4.

Figure 4 System workflow diagram

The system software design is carried out in the Lingyang 16-chip microcomputer application development tool unSPIDE1.16.1. The Lingyang MCU assembly language and standard C language are used to program the MCU to realize various functions. It mainly includes: (1) system initialization, including the initialization of various peripheral interface devices; (2) keyboard input; (3) D/A and A/D conversion; (4) PID algorithm for current adjustment; (5) voice prompt and current display. The main program flow is shown in Figure 5.