Design of high voltage constant current charging power supply monitoring system

With the continuous deepening of electromagnetic emission technology research, the role of high-voltage power supply in electromagnetic emission test is becoming more and more important, and higher requirements are also put forward for the performance of high-voltage power supply, especially in terms of charging power, safety performance and human-machine interface. In response to this requirement, this paper introduces the design of a 10 kV constant current charging power supply monitoring system based on a single-chip microcomputer. The system has the function of querying the voltage of the capacitor to be charged before charging, setting the charging voltage and charging current by the keyboard, and displaying the voltage value of the capacitor to be charged and possible faults during the charging process, making the entire charging process transparent and having good practicality.

1 System composition and working principle
The monitoring system is mainly composed of a keyboard input system, a display control system, a sampling circuit, etc. The keyboard input system consists of a 4×4 keyboard, an AT89C2051 single-chip microcomputer and corresponding peripheral circuits; the display control system consists of a JHD12864 liquid crystal display, an STC12C5410AD single-chip microcomputer and corresponding peripheral circuits; the sampling circuit includes a voltage sampling circuit and a current sampling circuit.
Before introducing the working principle of the system, let's first introduce SG3525A. SG3525A is a PWM integrated controller with functions such as external synchronization, soft start, error amplification, and shutting down output drive signals. It is a key device for controlling constant current charging. The working principle of the system is shown in Figure 1. The keyboard input system is responsible for providing key information to the display control system and setting the voltage and current of the display control system. After setting, STC12C5410AD will generate a PWM wave with a certain duty cycle, which will become a low level after filtering and amplification, and will be added to the shutdown end of SG3525A to start the power supply. At the same time, the 10-bit A/D converter integrated in STC12C5410AD converts the analog quantity provided by the voltage and current sampling circuit into a digital quantity representing the actual voltage and current, and then displays it through the LCD. As the charging progresses, when the actual voltage value is greater than the set value, a PWM wave with a certain duty cycle will also be generated, which will become a high level after filtering and amplification, and will be added to the shutdown end of SG3525A to stop the power supply. For current regulation, the microcontroller will output a PWM wave with a corresponding duty cycle according to the set current value, and then add it to the SG3525A non-inverting input after filtering. The inverting input is connected to the current sampling circuit to form a feedback loop, thereby controlling the size of the charging current and achieving constant current charging.

The specific process of power supply charging is shown in Figure 2. When the system is powered on, the LCD screen displays "Welcome to use this current source, press 1 for charging, press 2 for query". After the charging parameters are set, charging begins. During the charging process, if the voltage or current is detected to be too large or too small, the system will control the power supply to stop working, and the LCD screen will display the corresponding fault type.

2 System Hardware Design
2.1 Control System Circuit Design
Since the system requires more I/O ports, the control system uses two single-chip microcomputers, STC12C5410AD and AT89C2051. The control system circuit is shown in Figure 3. The keyboard uses a 4×4 matrix keyboard, which can save a lot of I/O resources. R0~R3 are connected to the lower 4 bits of the P1 port of the slave AT89C2051, and C0~C3 are connected to the upper 4 bits of the P1 port. The query method is used to scan the keyboard's pressing status; since there is no need for long-distance transmission, the LCD and the host STC12C5410AD are connected in parallel, with the P2 port as the data port, the control terminal RS, the read/write terminal RW, and the enable terminal E are connected to the P3.3, P3.4, and P3.5 pins respectively; P1.0 and P1.2 are used as A/D conversion ports, and the P3.5 and P3.7 pins are used as PWM wave output ports. PWM0 controls the charging current, and PWM1 controls the start and end of charging. In order to improve the reliability of the system in high voltage and strong electromagnetic interference environment, an external watchdog method is adopted. The P3.2 pin and RST pin of the microcontroller are connected to the watchdog detector input pin WDI and reset pin RST of MAX6304 respectively. At the same time, the two microcontrollers are connected through the serial port.

2.2 Sampling circuit design
The sampling circuit includes voltage sampling circuit and current sampling circuit. First, the current sampling circuit is introduced. Its circuit is shown in Figure 4. In the figure, the current outputs a voltage signal after passing through the current sensor. After the voltage signal is added to the integral operational amplifier circuit composed of R3, R4, LM358, and C2, the output signal is sent to the A/D converter and the inverting input terminal of SG3525A on the one hand, and on the other hand, it is added to the 3525 closing terminal after passing through the comparator composed of LM358, etc., to form an overcurrent protection circuit. R1 is the load resistor of the current sensor, C1 and C3 are filter capacitors, and D1 is a diode, and it is connected to C1 to absorb negative pulses to protect LM358.
The voltage sampling circuit is shown in Figure 5. After the high voltage passes through the current limiting resistor, it becomes a current signal less than 5 mA, and then the optocoupler P521 (when the current is below 5 mA, the optocoupler is linear) is used to convert the current signal into a voltage signal to achieve isolation between high voltage and low voltage and improve the anti-interference ability of the system. The voltage output by the optocoupler is sent to the A/D converter on the one hand, and on the other hand, it is added to the shutdown terminal of SG3525A after passing through the comparator composed of LM358, etc., to form overvoltage protection.

3 System software design
The system software design mainly implements 5 functions: keyboard input; LCD display; serial communication; A/D conversion and PWM wave output. The software design program mainly includes the host (STC12C5410 AD) program, the slave (AT89C2051) program, the LCD display program, the A/D conversion program and the PWM wave output program. The first two are mainly introduced below.
3.1 Host program flow chart
The host is mainly used to receive slave information, control LCD display, start A/D conversion, and output PWM wave. It is the core of the entire monitoring system. Its program flow chart is shown in Figure 6. The system is initialized after power-on. The initialization includes A/D initialization, LCD initialization, serial port initialization, and PWM output initialization. After initialization, the system is in standby state. When receiving information from the slave, the corresponding subroutine is called according to the key information, such as setting the charging voltage subroutine. After charging starts, the A/D conversion is started, and a PWM wave with a corresponding duty cycle is output according to the set current value. Then, the voltage setting value and the sampling value are continuously compared. When it is detected that the sampling value is greater than the set value, a power-off signal is issued.

3.2 Slave program flow chart
The slave is mainly responsible for scanning the keyboard pressing status and sending the key information to the host. Its program flow chart is shown in Figure 7. After the system is powered on, the baud rate of the serial communication is directly set, and then the keyboard key status is scanned. In order to eliminate jitter, after the key information is detected, a delay of 20 ms is made, and then the key information is detected. If it exists, it is determined that a key is pressed, and then the information representing the key is sent to the host. After the sending is completed, the key status is scanned.

4 System optimization design
4.1 Reliability design
The monitoring system can change the charging mode of the power supply from manual to program-controlled, realizing the automation of the charging process. However, due to working in a high-voltage environment, the system reliability is reduced. In order to improve the system reliability, the following three measures are taken:
(1) Add an external microcontroller monitoring circuit (watchdog circuit). The selected chip is the MAX6304 chip of Maxim. This chip has both reset and watchdog functions, and has the feature of adjustable watchdog timeout. In this way, the maximum program cycle can be tested, and then a slightly larger timeout can be set accordingly to ensure that the program is reset in the shortest time after running away.
(2) Optocouplers are used to isolate high voltage from low voltage to improve the system's anti-interference ability.
(3) When designing the software for the keyboard input system, the key functions are interlocked to avoid misoperation.
4.2 Precision design
The main technical indicators of high-voltage charging power supplies are generally voltage accuracy, charging speed, and control methods. In order to improve the charging voltage accuracy, the following two measures are taken:
(1) Software design methods are used to improve system accuracy. In order to improve the system's anti-interference ability, optocouplers are used for isolation. However, since optocouplers are not completely linear, the sampled voltage is prone to large errors. To this end, the linear curve of the optocoupler is first measured by an experimental method, and then fitted by a software design method. The following takes a 10 kV charging power supply monitoring system as an example. The experimental measurement shows that the optocoupler has good linearity in the three ranges of 0-750 V, 750-3 000 V, and 3 000-10 000 V. Therefore, when data processing is performed, it is also performed in intervals. Assuming that the proportional functions of the three intervals are f(1), f(2), and f(3), the software flow chart is shown in Figure 8.

(2) Use the reference voltage output by SG3525A as the input voltage of the optocoupler to ensure the stability of the input voltage.

5 Experiments
Given that the 10 kV constant current charging power supply is still in the testing stage, the system performance test adopts a simulation method. The test mainly tests the system reliability and sampling accuracy. The test circuit diagram is shown in Figure 9. The 12 V power supply voltage is adjustable, and its variation range is 0 to 12 V. By taking the point method, the actual voltage value is compared with the LCD display value after A/D conversion, and then Origin is used for fitting. The fitting curve is shown in Figure 10. Among them, B represents the actual voltage value, and C represents the voltage value displayed by the LCD. It can be seen from the figure that the system display voltage value is basically consistent with the actual voltage value, and the error is less than 1%.

6 Conclusion
In order to adapt to the development trend of high-voltage power supply, a monitoring system for high-voltage constant-current charging power supply is designed, which changes the traditional charging method from manual to digital. The simulation charging test shows that the system has the advantages of easy operation, high reliability and high precision. However, since it is only a simulation test and lacks a strong electromagnetic interference environment, its reliability needs to be further verified. However, based on its various advantages, its application prospects are still relatively broad.