2015 Electric Competition Question A: Bidirectional DC-DC Converter +812968A

1. Question content and requirements

    Design and produce a bidirectional DC-DC converter for battery energy storage devices to realize the charging and discharging functions of the battery. The functions can be set by buttons or automatically converted. The system structure is shown in Figure 1. Except for the DC stabilized power supply, all other components in the figure need to be prepared by yourself. The battery pack consists of five 18650 type lithium-ion batteries with a capacity of 2000~3000mAh connected in series. The absolute value of the resistance value error of the resistor used is not greater than 5%.

1. The basic requirements are
  to connect S1 and S3, disconnect S2, and set the device to charging mode.
  (1) Under the condition of U2=30V, constant current charging of the battery is achieved. The charging current I1 is step-adjustable within the range of 1~2A,
the step value is not greater than 0.1A, and the current control accuracy is not less than 5%.
  (2) Set I1=2A, adjust the output voltage of the DC regulated power supply, so that U2 When changing within the range of 24~36V,
the change rate of charging current I1 is required to be no more than 1%.
  (3) Set l1=2A, under the condition of U2=30V, the efficiency of the converter n1≥90%.
  (4) Measure and display the charging current l1, the measurement accuracy is not less than 2% in the range of l1=1~2A .
  (5) With overcharge protection function: set 1=2A, when U2 exceeds the threshold U1th=24±0.5V, charging will stop.

2. play part

  (1) Disconnect S1, connect S2, set the device to discharge mode, keep U2=30±0.5V, at this time, the converter efficiency n2≥95%.

  (2) Connect S1 and S2, disconnect S3, and adjust the output voltage of the DC regulated power supply so that when Us changes within the range of 32~38V
, the bidirectional DC-DC circuit can automatically switch the working mode and maintain U2=30±0.5V .
  (3) Simplify the structure and reduce weight while meeting the requirements, so that the total weight of the three parts of the bidirectional DC-DC converter, measurement and control circuit and
auxiliary power supply is no more than 500g.
  (4) Others.

2. Design ideas

  The general program block diagram of this design is as follows:

    Therefore, the main hardware circuit here needs to overcome the two levels of the IR2110 driver circuit and the bidirectional DC-DC main circuit.

The IR2110 circuit design is as follows: https://oshwhub.com/11idiotic45/ir2110-qu-dong-dian-lu_-gai

    The main function of this hardware circuit is to introduce the complementary PWM wave generated by the microcontroller through pins 10 and 12 of the IR2110 chip, so that the output of pins 1 and 7 of the IR2110 chip can drive the main For the PWM wave of the IRF540 MOS tube of the circuit, pin 6 of IR2110 should be connected to the S pole of the upper IRF540 MOS tube of the bidirectional DCDC circuit (that is, the D pole of the lower IRF540 MOS tube) to obtain the bias voltage. Effectively drives the upper side IRF540 MOS tube to turn on and off. In addition, a small resistor of 30 ohms is connected to the input end of each signal to reduce the interference of clutter signals and output a purer PWM wave to achieve higher converter efficiency.

    In order to meet the needs of specific occasions, two IR2110 circuits are placed here on a PCB board . Here, the input terminal is designed to be connected to a pin header for the input of the microcontroller signal, and the output terminal and power terminal are connected to terminal blocks to provide large current. The signal input and output are stable signals. The diode here uses FR307 diode, which has the characteristics of large current, high withstand voltage and fast recovery. In actual circuit work, its working condition is quite stable.

    For the design of the bidirectional DC-DC main circuit, many switches and resistor access terminals need to be added according to the requirements of the topic. The design is as follows https://oshwhub.com/11idiotic45/buck_boost-shuang-xiangdcdc-dia- lu

     The boost and step-down functions of the bidirectional DCDC circuit are mainly realized by connecting inductors and capacitors. The voltage is connected from left to right. When the circuit does not appear in a cut-off state, according to the freewheeling characteristics of the inductor, the voltage on the right side can be equal to the voltage on the left multiplied by the duty cycle of the MOS tube. In the same way, when the voltage is connected from right to left, and the circuit does not appear to be in a cut-off state, according to the freewheeling characteristics of the inductor, the voltage on the left side can be divided by the voltage on the right side (1-duty cycle). According to the question requirements, the bidirectional withstand voltage here does not exceed 100V, so IRF540 MOS tube is used here, which has the characteristics of high withstand voltage and low internal resistance. All inputs and outputs here are connected to terminal blocks for large current input. and output, the function of each terminal is marked in the schematic diagram, so it will not be explained too much.

    The two ends connected to the constantan wire in the schematic diagram are then connected to the INA282 chip. According to the internal structure of the INA282 chip, it is easy to see that it can differentially amplify the voltage on the constantan wire 50 times, thereby achieving the minimum loss of converter efficiency. The current used to charge the battery pack is collected to realize current display and constant current control.

3. Physical display

1. Select STM32F407ZGT6 microcontroller as the main control microcontroller, and its minimum system board can be purchased online:

2. IR2110 drive circuit display:

3. DC-DC main circuit display:

4. Demonstration of the external resistance of the DC-DC circuit. A resistance plate is used here to make the accuracy of the access resistance meet the question requirements as much as possible:

5. DC-DC circuit rechargeable battery pack display. Here, five 18650 batteries are connected in series to form a battery pack, and its access point is connected to a switch:

6. INA282 current sampling circuit, which is connected in series to the rechargeable battery line:

4. Overview of software code

  The code mainly sets three timed interrupts. The lowest priority interrupt is used to detect key presses and display OLED . The timer is 1 second . The other two timed interrupts are controlled according to the key functions. The control functions include opening and steady flow adjustment. . The functions of the remaining two timers are to sample the voltage ADC and adjust the PWM wave, and the ADC to sample and adjust the PWM wave after the current is converted to voltage . Otherwise, they are all declarations of library functions. For details, please see the folder uploaded together with the document." Battery Pack Charging Procedure". A rough overview is as follows:

Timer initialization:

Timer 4 service function:

Timer 5 service function

Timer 3 service function:

5.  Performance display required by the question

1. Introduction to the work. For details, please see the attached attachment " 2.0 Work Introduction Demonstration Video.mp4 " . A clear version can be found at: https://www.bilibili.com/video/BV12K4y1Q7gB

2. Performance test, for details of basic requirements 1-4 , please see the attached attachment " 3.0 Basic Requirements 1-4 Video Display.mp4 ". A clear version can be found at: https://www.bilibili.com/video/BV1a54y1Y764

3. Regarding basic requirement 5 , since the battery capacity we bought is too large (although it is cheap, the capacity of each battery reaches 4800mWh ), it takes at least more than two hours to charge at 2A , so it is difficult to demonstrate.

4. Demonstration of extended requirements 1-2 . For details, please see the attached attachment " 4.0 Partial Requirements 1-2.mp4 ". A clear version can be found at: https://www.bilibili.com/video/BV1Xo4y1974s

5. Demonstration of extended requirement 3. For details, please see the attached attachment " 5.0 Partial Requirements 3.mp4 ". A clear version can be found at: https://www.bilibili.com/video/BV1Sh411r7en