Design and implementation of parallel power supply with adjustable current ratio based on single chip microcomputer

In order to meet the requirements of large load power, the power supply system often needs to use several switching power supplies in parallel [1]. Moreover, in practical applications, there are often situations where the power of two parallel power supplies is different and the current cannot be evenly shared. This requires the power module with higher power to share a larger current. Therefore, it is necessary to adopt an effective shunt control scheme to ensure that the output current of the entire power supply system is shared according to the output capacity of each unit module, so that the output capacity of the unit power module can be fully utilized and the working reliability of each unit power supply can be guaranteed [2]. Based on the flexibility requirements, it is very necessary to apply the single-chip microcomputer to the parallel shunt control of the switching power supply. Based on the current sharing technology of the master-slave setting method of the parallel power supply system [3-6], this paper designs a semi-intelligent parallel power supply system based on a single-chip microcomputer. The single-chip microcomputer module can monitor the shunt situation of each module in real time, and realize the arbitrary adjustment of the shunt ratio of the parallel power supply system through the human-computer dialogue port, which greatly broadens the application occasions of the parallel shunt switching power supply system.

1 System Design

The overall design scheme of the system is shown in Figure 1. This design adopts the master and slave working modes to control the voltage and current ratio respectively and track them accurately. Among them, the master channel stabilizes the output voltage of the power supply; the slave channel ensures that the current output ratio is consistent with the system setting value; the single-chip microcomputer module and the display and input control port realize the semi-intelligent system, that is, the shunt ratio is adjustable and the current of each module can be monitored in real time. The proportion data is input through the display and input control port, and the single-chip microcomputer generates a current ratio adjustment signal to control the slave channel current feedback control circuit, thereby adjusting the two PWM signals to make the two DC/DC modules output the corresponding current value. The main channel voltage feedback control circuit realizes the voltage stabilization of the entire system by sampling the output voltage, and the single-chip microcomputer module realizes the monitoring of the total current of the system by sampling the total current of the system, and promptly starts the overcurrent protection circuit when the total current exceeds the set range.

1.1 Main channel module design

The main channel module design is shown in Figure 2. It is mainly composed of voltage sampling, comparison amplification, PWM modulation, drive and output circuit, low-pass filtering and other links. The master channel samples the load sample voltage through resistor voltage division, compares the load sample voltage with the reference voltage UR generated by the control system, and obtains the PWM modulation error signal. This signal is compared with the standard triangle wave signal to form a PWM modulation signal with a certain duty cycle. After the signal is split by 180°, it forms a pair of PWM signals and is sent to the drive circuit to drive the half-bridge output stage circuit. Then after low-pass filtering, the output voltage amplitude is a DC voltage with stable.

1.2 Slave channel module design

The slave channel module design is shown in Figure 3. In order to control the current output ratio of the master and slave channels, the output current of the master and slave channels is sampled through the Hall current sensor and converted into the corresponding current sample voltages UI1 and UI2. UI3 is multiplied by the master/slave channel current ratio adjustment voltage Uk sent from the single-chip microcomputer system to proportionally control the slave channel current, and after comparison and amplification with the master channel current sample voltage UI1, it is sent to the PWM control system.

The circuit structures of the driving and power output parts of the master and slave channels are exactly the same. The master channel is used to stabilize the load voltage, and the master/slave channel current output ratio is controlled by the slave channel, which simplifies the feedback loop structure, makes the system loop control stable, and has a high adjustment rate for both voltage and current, and the control accuracy is very high.

1.3 Single-chip microcomputer system design

The single-chip microcomputer system is mainly used to display the system working status and key parameter information, and respond to user control instructions. Its flow chart is shown in Figure 4.
First, the single-chip microcomputer system samples and analyzes the total current of the system to determine whether it meets the "sum of the master and slave currents is less than 4.5 A". If not, it will determine whether it meets the "sum of the master and slave currents is less than 6 A". If it meets the requirements, it will be forced to output according to the 1:1 split ratio to avoid excessive single-channel output power and damage to the power supply. Otherwise, the overcurrent protection will automatically shut down the drive circuit. If the "sum of the master and slave currents is less than 4.5 A" is satisfied, the output current ratio of the human-machine exchange platform input is read, and the system analyzes the current ratio to determine whether it satisfies the "master and slave currents are both between 0.4 A and 3 A". If it is satisfied, the system will output according to the specified shunt ratio; if it is not satisfied, the system will force the output according to the 1:1 shunt ratio. Secondly, this shunt ratio control based on the single-chip microcomputer can not only monitor and protect the power supply system in real time, but also its "4.5 A" and "0.4 A ~ 3 A" conditions can be set according to actual conditions, which has great flexibility, which is not available in the traditional parallel current-sharing switching power supply system.

1.4 Overcurrent protection circuit design

The overcurrent protection circuit is detected and controlled by the single-chip microcomputer. When the sum of the two currents is greater than the set current limit value (the default value is 6 A, which can be set independently), the control program automatically shuts down the drive circuit, and automatically restores the current detection control after a certain time delay. In addition, according to the performance indicators of this design, users can set the master-slave current ratio arbitrarily, but when the ratio is not set properly or the load changes, there is a single-channel current over-limit phenomenon (the upper limit default value is 3 A, the lower limit default value is 0.4 A, which can be set independently). In order to ensure the normal operation of the over-limit current module and at the same time ensure that the total power output of the parallel power supply system remains unchanged, the microcontroller will adopt a forced 1:1 output mode within the system total current output threshold (the default value is 4.5 A~6 A, which can be set independently), and the master-slave current ratio will automatically recover after it meets the requirements again.

2 Experimental test
2.1 Shunt ratio setting and shunt error test
The shunt ratios were set to 1.5:2.5 and 2.5:1.5 respectively, the load resistance was adjusted, the current values ​​were read, and the shunt relative error was calculated. The shunt current relative error was: δi=(Ii measured - Ii theoretical)/Ii theoretical. The results are shown in Table 1.

Adjust the load resistance to stabilize I0 at 4.008 A, adjust the shunt ratio, read each shunt value, and calculate the shunt relative error. The results are shown in Table 2.

2.2 Analysis of test results
The test results show that when the total current I0>4.5 A and the shunt outputs I1 and I2 are between 0.4 A and 3 A, the shunt ratio can be set arbitrarily, the shunt error is within 5 mA, and the shunt relative error is less than 0.5%, which has high accuracy; when the total current is 4.5 A6 A upper limit current, the system will shut down the drive and detect the system current value after a certain time delay to prevent damage to the power supply due to excessive current, thereby achieving the system protection function.
This paper designs a semi-intelligent parallel power supply system based on the current sharing technology of the master-slave setting method of the parallel power supply system. The single-chip microcomputer module can monitor the current shunt of each module in real time, and realize the arbitrary adjustment of the current shunt ratio of the parallel power supply system through the human-computer dialogue port, which greatly broadens the application occasions of the parallel shunt switching power supply system and has strong practicality.
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