2021 Question B - Three-Phase AC-DC Conversion Circuit STI Capacitor Exploding Team

## 1. Preface

When participating in this e-tournament, three people in our group had already secured graduate degrees, so this was a competition that had nothing to do with personal interests and was about fighting for honor, so I was very impressed.

I would also like to thank the classmates, teaching assistants and teachers of the Power On team of the Electrical and Electronic Technology Innovation Center of Huazhong University of Science and Technology for their help. Without you, the brilliant achievements of the base this year would not have been possible!

## 2. Team introduction

Our team all comes from the Electrical and Electronic Technology Innovation Center of Huazhong University of Science and Technology (hereinafter referred to as the "base"). We joined the base from the beginning of our freshman year until our senior year this year. We spent a lot of time in the base. I spent a lot of time studying and participated in various competitions and trainings.

Our team is divided into three parts: two hardware and one software.

Bao Junyang is mainly responsible for the module production of the hardware; Zhou Qingyue is mainly responsible for the main circuit production of the hardware; Hu Qianjie is mainly responsible for writing and debugging the software code.

## 3. Project analysis

task: 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 rating is 2A.

![qJbG6LP5QiCGnnfyQ2KAiP2E9reIKCNHpogGSd1q.png]
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 in 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.

## 4. Abstract

This system consists of a three-phase PWM rectifier circuit, a Buck voltage reduction circuit, a measurement circuit, a control circuit, and an auxiliary power supply. The system uses STM32F407ZGT6 as the main controller, calculates the grid voltage phase through a phase-locked loop, and directly controls the amplitude and phase of the rectifier input current under dq coordinates. The rectifier outputs a 50V DC bus voltage, and after passing through the Buck step-down circuit, it outputs a stable DC voltage of 36V. Under rated operating conditions, the system input power factor is not less than 0.998, and can be set arbitrarily within the range of 0.90~1.00, with a step value of 0.01; the output DC voltage is stable, and the load adjustment rate and voltage adjustment rate are both less than 0.05%, and The overall efficiency of the machine can reach 96.2%. In addition, the system also has over-voltage and over-current protection functions and a good human-computer interaction interface.

## 5. System scheme details

The system consists of main circuit, measurement circuit, auxiliary power circuit and control circuit. The main circuit is composed of a cascaded rectifier circuit and a buck circuit. The front stage adopts a three-phase PWM rectifier topology to achieve AC-DC conversion. According to the input and output voltage relationship of the three-phase PWM rectifier, the input voltage range of the circuit is set in the title The internal circuit cannot constantly output 36V DC voltage, so the rear stage adopts Buck topology to achieve constant voltage output. The measurement circuit realizes the measurement of input voltage, current and output voltage and current. The overall plan is shown in the figure.

![Screenshot 2022-01-17 172955.jpg]
### 5.1 Main circuit and device selection

The front stage of the main circuit uses a three-phase PWM rectification topology, and the rear stage uses a Buck topology. The circuit diagram is shown in the figure.

![Screenshot 2022-01-17 173540.jpg]
![_$_3`Y7M%Q{SABR(X9305Q.png]
According to the relationship between SVPWM modulation ratio M and voltage, the maximum line voltage Ui is 33V, which can be calculated, The maximum value of the DC bus voltage Ubus is at least 46.7V. Select the maximum bus voltage of 50V to leave a margin.

#### Switch tube selection:

The maximum voltage stress of the switching tube in the circuit is 50V. Leave a margin and select 100V withstand resistance. Switch tube with high voltage. In order to reduce the conduction loss and switching loss of the circuit, the smaller the on-resistance and parasitic capacitance of the switch tube, the better. Choose a MOSFET from Texas Instruments, model CSD19533KCS, with an on-resistance of 8.7mΩ. , the gate charge is 27nC, which meets the above requirements.

#### Rectifier inductor and capacitor design:

Filter inductor design: Rated input three-phase line voltage Ul=28V, at this time, the DC bus voltage Ubus=42V, the modulation ratio M= can be calculated 0.94. The circuit bus rated output current Ibus=1.8A. The switching frequency fs=20kHz, and the maximum ripple rate of the inductor current is taken as r=0.3. According to equation (1), the parameters can be substituted to calculate Ls=1.9mH. Since each line voltage is filtered The inductor is the sum of the inductance values ​​of the two half-bridge arm inductors. The actual inductor is 950μH.

![1.jpg]
Output capacitor design: The output bus capacitor is used to support the bus voltage. Since the three-phase input power is constant, the bus voltage fluctuations are mainly It comes from the Buck circuit on the output side. According to formula (2), the output current Iout=2A, the switching frequency fs=20kHz, the duty cycle D=0.72, and the voltage ripple is 0.1V, it can be calculated that Cbus=720μF, the actual selection is 1000μF Electrolytic capacitor.

![2.jpg]
#### Buck converter inductor capacitor design

filter inductor design: output DC bus voltage Ubus=50V, rated output DC voltage Uo=36V, calculated duty cycle D=0.72. Circuit The rated output current of the bus Io=2A. The switching frequency fs=20kHz, and the inductor ripple rate r=0.3. According to equation (3), substituting the parameters into the calculation can get Lo=840μH. Therefore, the inductor of Lo=840μH is selected.

![3. jpg]
Filter capacitor design: A better filtering effect can be obtained when the filter cutoff frequency fc is less than 0.1 times the switching frequency. Taking an electrolytic capacitor with C=47μF, the calculated cutoff frequency is 800Hz, which meets the requirements.

### 5.2 Control The programming

system uses STM32F407ZGT6 as the controller. The control system is divided into two parts: rectifier input current control and BUCK output voltage stabilization control. The two parts run at the same time. The program flow chart is shown in Figure 4.

![Screenshot 2022-01-17 175533 .jpg]
#### Rectifier control strategy

According to the relevant principles of direct current control, the control loop is divided into phase-locked loop and rectifier control loop. The system calculates the frequency and phase of the grid voltage through a digital phase-locked loop, and establishes a synchronously rotating dq coordinate system based on the phase. The coordinates of each parameter in the rectification control loop are based on this dq coordinate system.

As shown in the figure, the rectification control loop is divided into a voltage outer loop and a current inner loop. The voltage outer loop stabilizes the DC bus voltage, and the output of the outer loop controller is used as the current d-axis command. The current inner loop adopts grid voltage feedforward and dq current decoupling control, which can realize independent control of d-axis and q-axis currents and has good dynamic performance. The current inner loop controls the input current, and the d-axis component of the current is the active component and the q-axis component is the reactive component. Therefore, according to the power factor relationship, setting the q-axis current command can achieve any setting of the power factor.

![Screenshot 2022-01-17 175711.jpg]
#### The Buck circuit control strategy

uses Buck topology in the rear stage, and uses a voltage PI controller to achieve voltage stabilization control of the output voltage. In addition, voltage feedforward decoupling control is used to decouple the DC bus voltage from the controller, so that the output voltage can remain stable when the bus voltage fluctuates, improving the stability of the system. The control block diagram is shown in Figure 6.

![image.png]
## 6. Summary

This system is a three-phase AC-DC conversion circuit. The system consists of a two-stage converter. The front stage uses PWM rectifier topology to achieve AC-DC conversion; the rear stage uses Buck topology to stabilize the DC output voltage. The system uses STM32F407ZGT6 as the main controller. When the system works under rated operating conditions, the overall efficiency is 96.2%, the input side power factor is 0.998, and the power factor is adjustable within the range of 0.90-1.00, with an error of less than 0.02. In addition, the load regulation rate and voltage regulation rate of the system are both less than 0.05%. At the same time, the system also has over-current and over-voltage protection functions and a good human-computer interaction interface.

This competition has better exercised the teamwork ability of the three of us, and also brought into play the accumulation and learning results of our power electronics technology in the past three years. Thank you again to everyone at the Electrical and Electronic Technology Innovation Center!

## 7. Video display

! [IMG_0308.JPG] Video link:
[https://www.bilibili.com/video/BV1bS4y1f7bD?share_source=copy_web](https://www.bilibili.com/video/BV1bS4y1f7bD?share_source =copy_web)