Three-phase AC-DC conversion circuit (Question B)
[Undergraduate Group]
One task is
to design and produce the three-phase AC-DC conversion circuit shown in Figure 1. The DC output voltage Uo of this circuit should be stable at 36V, and the DC output current Io is rated The value is 2A.
Figure 1 Three-phase AC-DC conversion circuit principle block diagram
2 requirements
1. Basic requirements
(1) AC input line voltage Ui=28V, when Io=2A, Uo=36V±0.1V.
(2) When Ui=28V and Io changes within the range of 0.1A ~ 2.0A, the load regulation rate SI ≤ 0.3%.
(3) When Io=2A and Ui changes in the range of 23V~33V, the voltage regulation rate SU ≤ 0.3%.
(4) Under the conditions of Ui=28V, Io=2A, Uo=36V, the efficiency η of the AC-DC conversion circuit is not less than 85%.
2. Performance part
(1) Under the conditions of Ui=28V, Io=2A, Uo=36V, the power factor of the input side of the AC-DC conversion circuit is not less than 0.99.
(2) Under the conditions of Ui=28V, Io=2A, Uo=36V, the efficiency η of the AC-DC conversion circuit is not less than 95%.
(3) The three-phase AC-DC conversion circuit can automatically adjust the power factor according to digital settings. The power factor adjustment range is 0.90~1.00, and the absolute value of the error is not greater than 0.02.
(4) Others.
Abstract
According to the requirements of the competition question, a three-phase AC-DC conversion circuit with a stable output voltage of 36V and a rated DC output current of 2A was designed and produced. A three-phase bridge controllable PWM rectifier circuit and a DC chopper circuit are used to achieve high-precision voltage control and high-efficiency power changes. The STM32F103 microcontroller is used as the main controller, and the PWM wave generated by it is used to realize the on-off control of the power switch tube through the IR2110 drive circuit. The sampling circuit consists of a voltage transformer, a current transformer, an amplifier circuit and an adder circuit. The voltage and current signals obtained from the sampling signal are input to the microcontroller. After three-phase decoupling DQ coordinate transformation processing, a voltage and current double closed-loop feedback is formed. By adjusting and setting the PI controller parameters, the input power factor and output voltage are ultimately adjustable and controllable. In addition, this work also designed a mobile phone APP to monitor various circuit parameters and intelligently adjust power factor, output voltage, current and other functions. The test voltage, load regulation rate, and power factor regulation accuracy of the work far exceed the requirements of the problem indicators, and the efficiency is as high as 95.5%.
Keywords: AC-DC conversion; PWM rectification; microcontroller; power factor
catalog
1 system plan
1.1 system main circuit topology plan demonstration
1.2 demonstration and selection of control method
2 system theory analysis and calculation
2.1 component selection analysis
2.2 phase locked loop principle
3 Circuit and program design
3.1 Circuit design
3.1.1 Overall system block diagram
3.1.2 Circuit design and principle analysis
3.2 Program design
3.2.1 Program function description and design ideas
3.2.2 Program design principle block diagram
4 Test plan and test results
4.1 Test plan
4.2 Hardware test and results
4.2.1 Basic part
4.2.2 Performance part
4.3 Software test and results
4.4 Test result analysis
5 Things to note when reproducing
5.1 Program part
5.2 Hardware part
Three-phase AC-DC conversion circuit (Question B)
Participating students : Wang Dongmin, Ma Qiang, Zhou Yuhua
Participating school: Changsha University of Science and Technology
1 System Solution
This system mainly consists of a three-phase bridge rectifier circuit, BUCK circuit, voltage detection circuit, current detection circuit, drive circuit and auxiliary power supply. These are demonstrated below. module selection.
1.1 System main circuit topology scheme demonstration
scheme 1: Use diode uncontrollable rectification + hardware PFC (power factor adjustment circuit). The advantage of this solution is that it is simple and easy to implement. The disadvantage is that the power factor cannot be adjusted and cannot meet the requirements of the play part (3).
Option 2: Three-phase full-bridge PWM controllable rectifier circuit. The advantage of this solution is that it can meet all the index requirements of the question, but the control is complicated, the software workload is relatively large, and algorithms such as dq transformation decoupling need to be implemented in the software.
Based on the above two options, choose option two.
1.2 Demonstration and selection of control method
Option 1: Use hardware current hysteresis control. The advantages of this solution are: simple circuit, fast current response, and strong adaptability to the load. Since no carrier is required, the output voltage does not contain harmonic components of specific frequencies. In addition, this control method can also improve the DC voltage utilization and increase the rectifier output capability. However, while this control method responds quickly, the current ripple is also large, and the hysteresis width is difficult to control. If the ring width is too large, although the switching frequency and switching losses can be reduced, the tracking error will increase. On the contrary, if the ring width is too narrow, although the tracking error will be reduced, the switching ratio and switching losses will be greatly increased, and the switching device will be affected. operating frequency limitations. If the ring width is fixed, although the current following error range can be guaranteed to be fixed, the switching frequency of the power device changes randomly, which places excessive requirements on the operating frequency of the power electronic device.
Option 2: Adopt a software control scheme and use the internal comparator of the microcontroller to generate the required SPWM waveform. SPWM rectification has the advantages of small input current harmonics and controllable power factor. The input current is very close to a sine wave, and the power factor adjustment is very convenient. If the input current reference is set to be in the same phase as the input voltage, the power factor is approximately 1. The control is flexible and the switching frequency is fixed. Feedback can be established through software to flexibly control the entire circuit. output.
Based on the above two options, choose option two.
2 System theoretical analysis and calculation
2.1 Component selection analysis
question requires a maximum input line voltage of 33V. Since a bridge three-phase rectifier is used, the converted input voltage needs to be higher than 50V, so the MOS tube chooses IRF540 with low on-resistance and its withstand voltage value For 100V, the on-resistance Rds=22mΩ.
The final output stability of this system depends on the BUCK circuit. According to the competition requirements, the maximum input line voltage is 33V, then the DC bus voltage is 50V, the current ripple ratio is r=0.5, and the voltage ripple is 0.1V
2.2 Lock Phase loop principle
: Three-phase PWM rectification. When designing a feedback loop, the input three-phase current needs to be sampled and controlled, but the feedback current at this time is a time-varying quantity. When we generally design a feedback loop, the feedback quantity is generally is a DC quantity, so directly using the three-phase input current as our feedback quantity is not conducive to our design of the feedback loop.
In order to solve the above problems, we introduced the concept of coordinate transformation, converting the three-phase electricity in the stationary coordinate system abc to the two-phase electricity in the stationary coordinate system, and then converting the two-phase electricity in the stationary coordinate system into the single-phase electricity in the rotating coordinate system dqo. Constant flow. After equal-amplitude CLARK transformation, it can be expressed as:
Convert the AC signal into a DC signal for easy calculation.
3 Circuit and program design
3.1 Circuit design
3.1.1 Overall system block diagram
Figure 1 Overall block diagram of the three-phase system
From the three-phase system block diagram, it can be seen that there are a total of 8 signals sampling three-phase grid voltage, three-phase AC current and DC output voltage. When the three-phase rectifier system is running in a steady state, the voltage and current of the main circuit are very large and the AC quantity in it can be positive or negative. However, the sampling port of the ARM, which is responsible for processing these signals in the control circuit, can only receive 0~ The voltage signal of 3.3V will damage the ARM chip when the input voltage signal is in other ranges. Therefore, a voltage and current sampling and conditioning circuit must be used between the main circuit and the ARM for electrical isolation and electrical signal conversion. The main circuit voltage, The current signal is converted into a voltage signal that meets the ARM input requirements and then transmitted to its sampling port.
3.1.2 Circuit design and principle analysis
As shown in Figure 3, the voltage sampling and conditioning circuit consists of a voltage sensor and a subsequent conditioning circuit. The voltage sensor isolates the high voltage of the main circuit and proportionally reduces the sampling voltage signal. The conditioning circuit is responsible for conditioning. The voltage signal amplified by the voltage sensor is converted into a voltage signal of 0~3.3V through the addition and subtraction circuit and inverter circuit of the subsequent stage.
As shown in Figure 4, the specific working process of the current sampling and conditioning circuit is: the AC current of the main circuit is converted into a small voltage signal of -3~+3V through the transformer, and the voltage signal passes through the addition and subtraction circuit and inverter circuit of the subsequent stage. Finally it becomes a voltage of 0~3V. The signal after sampling and conditioning can be input to the ARM through AD sampling to participate in program operations.
Figure 3 Voltage detection principle
Figure 4 Current detection circuit
Usually, there are two main requirements for the performance indicators of the rectifier system. The first is that the DC voltage is adjustable, and the AC component, that is, the harmonic voltage, is small, and is generally required to be less than 1%; the second is that the AC side power factor is high, and the harmonic current in the current is small, and the THD is required to be less than 5%. At the same time, for a specific project, power transmission efficiency, converter volume and electromagnetic compatibility are all issues that need to be considered.
3.2 Program design
3.2.1 Program function description and design ideas
1. The program function mainly realizes keyboard control, automatic measurement and measurement result display.
1) The keyboard realizes the function of adjusting the power factor, which can be added and subtracted step by step.
2) Display part: displays input voltage and power factor.
3) Measurement part: ADC sampling module
2. Programming idea:
First, enter the modification content through the keyboard or APP, then enter the corresponding mode, and finally display the measurement results.
3.2.2 Principle Block Diagram of Programming
The principle block diagram of programming the microcontroller is shown in Figure 5. Based on this logic, the microcontroller software is written to realize various functions of the circuit.
Figure 5 Program design principle block diagram
4 Test plan and test results
4.1 Test plan
Figure 6 Three-phase AC-DC principle block diagram
Test conditions: Check multiple times, the simulation circuit and hardware circuit must be exactly the same as the system schematic diagram, and the check is correct, the hardware circuit Ensure no false soldering. Test the final hardware and software through multimeters, oscilloscopes and other equipment. The specific test requirements are tested step by step according to the title.
4.2 Hardware test and results
4.2.1 Basic part
(1) AC input line voltage Ui=28V, Io= At 2A, Uo=36V±0.1V.
Actual values: Ui=28V, Io=2A, Uo =35.98V.
(2) When Ui=28V and Io changes within the range of 0.1A~2.0A, the load regulation rate SI ≤ 0.3%.
Actual values: Ui=28V, Io1=0.1A, Uo1=35.98V.
Ui=28V, Io2=2.0A, Uo2 =35.98V.
SI=0;
(3) When Io=2A and Ui changes in the range of 23V~33V, the voltage regulation rate SU ≤ 0.3%.
Actual values: Ui=23V, Io2=2.0A, Uo1=35.98V.
Ui=33V, Io2=2.0A, Uo1 =35.98V.
SI=0;
(4) Under the conditions of Ui=28V, Io=2A, Uo=36V, the efficiency η of the AC-DC conversion circuit is not less than 85%.
Actual values: Ui=28V, Io2=2.0A, Uo1=35.98V.
η≈95.5%;
4.2.2 Performance part
(1) Under the conditions of Ui=28V, Io=2A, Uo=36V, the power factor of the input side of the AC-DC conversion circuit is not less than 0.99.
Actual value: power factor = 0.997.
(2) Under the conditions of Ui=28V, Io=2A, Uo=36V, the efficiency η of the AC-DC conversion circuit is not less than 95%.
Actual values: Ui=28V, Io=2.0A, Uo=35.98V
η≈95.5%.
(3) The three-phase AC-DC conversion circuit can automatically adjust the power factor according to digital settings. The power factor adjustment range is 0.90~1.00, and the absolute value of the error is not greater than 0.02.
Actual value:
Set value
actual output value
0.96
0.965
0.93
0.932
0.91
0.912
(4) Others.
See Appendix 3 for the mobile APP operation page. The APP has functions such as real-time display of circuit waveforms and parameters; modification of output voltage amplitude and power factor.
4.3 Software test and results
The software test mainly includes the microcontroller output PWM wave test and the ADC sampling settings. Through the debugging program, it can be seen that the PWM wave output and ADC sampling accuracy can meet the design target requirements.
4.4 Analysis of test results
It can be seen from the hardware test results that the work can meet all the requirements of the basic part and the performance part of the question. Since the actual power factor is difficult to reach 1.0, the power factor cannot be reached when the final power factor is adjustable. When the factor is set to 1.0, the actual circuit also meets the same requirements.
5 Things to note when reproducing
5.1 Program part
1 First, pay attention to the size of the offset value subtracted in the timer-interrupt service function and the proportion of sampling data. Each board is different. You can use MATLAB for curve fitting.
2 Pay attention to adjusting the PID parameters during rectification, and The PID parameters of DQ conversion to DQ inverse conversion are mainly affected by the amplification ratio during AC sampling.
5.2 For the hardware part,
please note that RA7, RA13, RB7, RB13, RC7, RC13, R21, R23
can adjust the rectifier circuit first. Then adjust BUCK.
In order to facilitate debugging, I organized the circuit and made several of my previous boards into one complete board.
Video link: 2021 National College Student Electronic Design Competition Question B Demonstration_bilibili_bilibili
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