Design of intelligent soft-start high-power constant current source based on single-chip microcomputer control

　　The instantaneous current impact during the power supply startup process is very large, which has a serious impact on the service life of the power supply and devices. Using a good control method to control the startup current to reduce its harm, so that there is no instantaneous impact during the startup process and it can change continuously, is a key step in the power supply startup control. The power supply soft start method is to control the output voltage and current so that the voltage and current of the load gradually increase. For the controlled object of the linear time-invariant model, properly adjusting the PID parameters can obtain a more satisfactory control effect, which can well solve the problem of excessive current. PID control can well solve the problems of oscillation and overshoot during the startup process, can better protect the power supply, and start reliably and stably. Using a single-chip microcomputer as a controller is flexible in programming, high in cost performance, and easy to implement human-machine interface management. Use software to adjust the nonlinearity of the system to reduce the deviation between the measured value and the set value. Factors such as fluctuations in power supply voltage or current, aging of circuit components, and ambient temperature will affect the stability of the power supply. In order to stably control the power supply, this solution adopts a high-speed AD and DA data acquisition system based on a single-chip microcomputer, and uses a PID algorithm to achieve soft start of a high-power power supply. The system uses PID voltage sampling feedback to control the output current to remain constant, with high accuracy, fast response speed, good flexibility and high stability.

　　1 Realization of high-power precision constant current source

　　1.1 Power System Design

　　With the microcontroller as the core, the following functions are completed: Processing keyboard input values, including circuit preset values ​​and "+" and "-" steps; Controlling digital LCD to display preset values ​​and actual values; Controlling ADC and DAC; Compensating for deviation values ​​through program control algorithms based on the feedback signals obtained. Since one path of the op amp OPA549 is controlled by the D/A converter, the voltage at the input of the op amp OPA549 is adjusted, and one path is a proportional amplifier circuit. When the DAC outputs a preset value or a step value, the output of the current source varies within the range of 0 ~ 8 A. After the output voltage is sampled by a small resistor in series with the load, it is sent to the ADC. The sampled value and the preset value are calculated and compared inside the microcontroller to output the control signal to compensate for the deviation value. The nonlinearity of the system is adjusted by software to reduce the deviation between the measured value and the set value.

　　1.2 Power Circuit Design

　　(1) CNC core

　　A single C8051F is used to control the keyboard and display of the digital control DC source, and to control the output current with the D/A converter and A/D converter. The reference voltage of the A/D converter is powered by a dedicated ±9 V power supply, and the reference voltage of the D/A converter is powered by a +20 V power supply. The data sent by the microcontroller is converted by the DAC to output the control voltage.

　　(2) Operational amplifier OPA549 current source amplifier circuit.

　　OPA549 is a high voltage and high current power operational amplifier newly launched by BB. It can provide excellent low-level signals, output high voltage and high current, and can drive various loads. The main features of this device are: large output current, continuous output current can reach 8 A, peak current can reach 10 A; wide operating voltage range, single power supply is +8 V~+60 V, dual power supply is ±4 V~±60 V; large output voltage swing; overheat shutdown function, current limit is adjustable; enable and disable functions; overheat shutdown indication; conversion efficiency (slew rate) is up to 9 V/μs; operating temperature range is -40℃~+85℃. This device is mainly used to drive high current loads such as industrial equipment, test equipment, power supplies, audio power amplifiers, etc.

　　In this power supply system, a large current is mainly provided to the load, and the PID control algorithm is used to control the luminous intensity of the load. The input is the control voltage output by the microcontroller through the DAC, and one path is a proportional amplifier circuit, as shown in Figure 1, with a gain of G=1+R3/R2. The current-type DAC is converted into a voltage through R1 to control OPA549. The output current is converted into a sampling voltage through a sampling resistor, sent to the A/D converter and fed back to the microcontroller for deviation compensation.

　　Figure 1 OPA549 constitutes an adjustable high current constant current source

　　(3) Heat dissipation and anti-interference.

　　When the OPA549 high-power tube is working, it generates a constant large current, consumes a lot of power, and generates a lot of heat. Heat dissipation has become an urgent problem for the power supply. The motor of a general axial fan is equipped with a pulse drive circuit. When driven, the pulse component can easily "overflow" along the motor power line directly, interfering with other electrical equipment. Interference on video equipment is manifested as horizontal diagonal lines, and noise is generated on audio equipment. For this reason, a large area of ​​copper heat sink is installed, and a fan is used to force the electronic components in the equipment to dissipate heat. When installing the fan, a high-frequency magnetic ring needs to be wound in series on the fan motor power line to resist interference. The series magnetic ring effectively filters out these interference components, and generally only 1 to 3 turns are needed.

　　2 PID Control Algorithm

　　The control function of the system soft start is realized by the proportional integral differential controller. It is a mature, widely used control method that compares the deviation between the given signal and the feedback signal and performs proportional, integral, differential and other operations. It has a simple structure, strong flexibility, and convenient system parameter adjustment, and does not require a model.

　　The PID control principle is shown in Figure 2. PID control is a linear regulator that subtracts the set value W from the actual output value to obtain the control deviation e. The deviation value e is linearly combined to form the control quantity U after proportional, integral, and differential, and controls the object. Among them, the proportional regulator plays a basic regulatory role, mainly affecting the sensitivity and control speed of the control system. The integral regulator can automatically adjust the control quantity, eliminate steady-state errors, and make the system stable. The differential regulator can reduce overshoot, overcome oscillation, and at the same time speed up the system's stabilization speed and shorten the adjustment time, thereby improving the system's dynamic performance.

　　Figure 2 PID control principle diagram

　　The relationship between the output and input of the PID controller can be expressed as:

　　Where: Ti is the integral time constant; Td is the differential time constant; Kp is the proportional coefficient; Ki is the integral constant, Ki=Kp/Ti; Kd is the differential constant, Kd=Kp/Td.

　　The system startup time is short, the startup voltage and current are large, and the impact on the load is also large, resulting in the dynamic load peak value of the load in the startup stage being much greater than the load during normal operation, which is easy to cause damage to the load. To solve this problem, a new PID control soft start power supply system is designed, which is mainly composed of a power supply, a large current constant current source, and a sampling and control system for the output large current end, and the test in the laboratory has been completed. When the power supply starts, the single-chip microcomputer system first gives the set voltage, current or power. PID soft start controls the output according to the law of linear load increase. In the process of linear increase of load voltage, if the current exceeds the specified range, the voltage closed loop is immediately put into use, so that the current value is limited within the set range, and then the voltage is gradually increased linearly to the rated value, and the light intensity of the system gradually increases from zero to complete the startup process.

　　The soft start effect diagram of the PID control system is shown in Figure 3. The communication is done through the serial communication port com1, the voltage unit is mV, the current unit is mA, the power unit is mW, and the time unit is s.

　　From the soft start effect diagram in Figure 3, it can be seen that when working in the constant voltage, current and power mode, the overshoot during the system startup process is very small, which effectively controls the startup process, prevents the startup process from generating excessive disturbance voltage and excessive power, and effectively protects the load. [page]

　　3 Experimental Results

　　Since the output current reaches 8 A, the power requirement for the power supply is high, and it is easy to generate noise. This random noise will also have a certain impact on the output current. In order to reduce this noise, each module is powered separately to reduce cross interference. At the same time, more decoupling filter capacitors are installed on the circuit board to reduce the impact of interference. At the same time, OPA549 can effectively suppress ripple. There are many factors that affect the stability of the power supply, such as changes in load, changes in sampling resistance, the influence of A/D and D/A, etc. As shown in Figure 4, the power supply error is different under different loads. For a 10 W load, due to its low power, the error change is also small when the voltage and current increase. For a 35 W load, due to its high power, the range of variation of the working current is relatively large, the power consumption is large, and the error change of the power supply is correspondingly large. As shown in Figure 5, when the load is 10 W, 20 W and 35 W, the working state is stable and can meet the needs of large current and high power.

　　The system uses PID algorithm for control and adopts high-power op amp OPA549. The output current is adjustable in the range of 0~8 A, and the maximum peak value can reach 10 A. It can effectively suppress ripple current and overcome the shortcoming of small output current range of traditional current source. The output voltage, current and power measured values ​​can be set and displayed in real time. It has "+" and "-" step adjustment functions. The output can be displayed on LCD12864. At the same time, it communicates synchronously with the host computer through RS232, directly displays and saves experimental data. Through the analysis of the test results, the system has a small overshoot in the process of soft start, and the start effect is very good, avoiding the impact on the load. Because the current of the high-power adjustment tube changes in a large range, the problem of poor linearity and deviation between the measured value and the set value is solved by software compensation, amplification circuit adjustment and other methods. This power supply is suitable for high-power occasions and has good practicality.