Research on DC/DC Converter for Fuel Cell Bus Based on DSP Control

The problems of energy shortage and environmental pollution have become comprehensive problems that urgently need to be solved in the world today, and the traditional automobile industry is one of the biggest killers of energy and environment. The development of clean and efficient new power energy for automobiles has become a very urgent task. Fuel cells (Fuel Cel1) is such a green energy technology. In order to improve the relatively "soft" output characteristics of fuel cells, the voltage of the fuel cell is converted through a DC/DC converter to the main drive motor and its control system to meet their input characteristics. In this way, the output characteristics of the fuel cell become "hard" and match the output impedance of the converter. Therefore, the development of a reliable and stable DC/DC converter with good control characteristics and digital control, communication and protection has become an urgent need.

In recent years, the improvement of DSP technology has marked the rise of digital technology, which has brought about another major technological change in the control field. Therefore, research on the digital technology of high-power DC/DC converters for fuel cell buses and the development of domestically produced special digital high-power DC/DC converters will have very important theoretical significance and engineering application value for the development and popularization of electric vehicles in my country [1][2].

1 DC/DC converter main circuit composition

The main circuit of the converter is the foundation and directly affects the performance of the DC/DC converter. The main circuit structure of the DC/DC converter is simple; the working efficiency is high, which significantly improves the economy of the whole vehicle; and the operating frequency is high, with high response speed, it is easy to realize complex and variable input and output characteristics, and can meet the requirements of different control strategies. Therefore, the power hybrid conversion device for fuel cell buses abandons the main circuit topology of the full-bridge inverter and selects the Boost and Buck main circuit topology.

DC/DC converters can be divided into boost converters, buck converters and boost-buck converters according to their functions. Boost or buck converters are mainly used in fuel cell vehicles, and their schematic diagrams are shown in Figures 1 and 2.

Figure 1 Buck circuit schematic

Figure 2 Boost circuit schematic

Taking the Boost converter in Figure 2 as an example, its working principle is briefly analyzed: when the switch tube S is turned on, the current Iin flows through the inductor L, the current increases linearly, and the electric energy is stored in the inductor coil in the form of magnetic energy. At this time, the capacitor C discharges, the current Io flows through the load and forms an output voltage Vo at both ends, with a positive polarity at the top and a negative polarity at the bottom. Because the switch tube S is turned on, the anode of the diode D is connected to the cathode, and D is subjected to reverse pressure, so the capacitor cannot discharge through the switch tube. When the switch tube S changes from on to off, the magnetic field in the inductor L will change the voltage polarity at both ends of the coil L to keep Iin unchanged. In this way, the voltage VL converted from magnetic energy is connected in series with the power supply voltage VFC, and the capacitor C and the load are supplied with power at a voltage higher than Vo. When it is higher than Vo, the capacitor C has a charging current; when it is equal to Vo, the charging current is zero; when Vo has a decreasing trend, the capacitor discharges to the load to maintain Vo unchanged. When VL and VFC supply power to the load, Vo is higher than VFC, resulting in a voltage boost[3].

2 Control circuit hardware design based on TMS320LF2407A

Using a digital signal processor as the controller of a switching power supply can not only overcome the shortcomings of too many discrete components, poor circuit reliability, and complex circuits, but also solve the weakness of the inflexibility of monolithic integrated controllers; and the DSP digital processor has the advantages of high operating frequency, short instruction cycle, and improved bus structure, and has powerful digital processing capabilities.

TMS320LF2407A is a member of the 24X series DSP controller of Texas Instruments (TI). It has been widely used in the digitization of motors. Through programming and the cooperation of external circuits, it can fully realize the digitization of high-power DC/DC converters for fuel cells. Figure 3 is a functional block diagram of the control system. The control system is based on TMS320LF2407A and realizes various control functions required by the system through external additional circuits:

(1) The sensor input signal is processed by the filter circuit, and then digitally sampled by the ADC sampling circuit and sent to the central processing unit;

(2) The PWM control signal is directly generated by TMS320LF2407A, which is then amplified by the isolation driver to control the on and off of the power switch tube;

(3) Use the I/O port inside the processor to implement some peripheral additional control functions, such as indicator light display, circuit slow suction, contactor control, cooling fan switch control, etc.;

(4) Remote communication and control with the vehicle controller via CAN2.0 controller.

Figure 3 Control system functional block diagram

The purpose of the data sampling circuit of the fuel cell bus is to obtain the output voltage and current control feedback signals of the system; the temperature, current, and voltage protection feedback signals of the power device. The signal is transmitted to the control board through the 10-bit precision analog-to-digital conversion module (ADC) with built-in sampling/holding integrated in the TMS320LF2407A for data calculation and logic judgment. The 10-bit ADC is a high-speed ADC with a minimum conversion time of 500ns. For a 20~50kHz switching power supply, it can fully meet the control requirements of periodic sampling, thereby ensuring the high-speed response time of the DC/DC converter [4].

In DC/DC operation

During operation, some abnormal conditions may occur, such as DC/DC not working due to device damage, circuit short circuit, IGBT and power diode overcurrent, heat sink overheating, etc. For the above abnormal conditions, the hardware circuit is fully designed and corresponding protection measures are taken.

3. Control system software design

The control system is responsible for the control and communication of the entire conversion process, realizing the digitization of the process. The control software of the high-power DC/DC converter for fuel cell buses is compiled in a mixed way of C language and assembly language. While completing its control function, it strives to make the program structure reasonable and simple to meet the requirements of high-power switching power supply for the stability and reliability of the control system.

3.1 Overall structure of the software

The control software mainly includes the following parts: sampling processing link; calculating the output pulse width by the sampling value, and adjusting the output PWM pulse width according to this value; receiving control instructions through CAN communication and sending output current, voltage value, temperature, status code and other information; interrupt service program; fault handling and protection function program. The control system initialization program and main program flow chart are shown in Figure 4.

Figure 4 Control system main program flow chart

In order to improve the running efficiency of the software, the parts that do not need to be processed in time are placed in the main program, and some control processes that need to be processed in time are processed by interruption, such as the modulation of PWM waveforms, which require periodic processing and must be processed in time. In addition, the CAN communication program is also processed by the interrupt service program, which determines the specific working mode and working parameters according to the received information and adjusts the conversion process.

3.2 Brief Design of Digital PID Control

PID control has the advantages of simple structure and easy adjustment of parameters, so it is widely used in continuous system control technology. It is a control method based on the proportional, integral and differential deviation of the controlled quantity through linear combination. Its control law is:

Where: K is the proportionality coefficient;

e is the voltage deviation signal;

Ti is the integration time constant;

Td is the differential time constant;

uo is the initial value.

Since digital PID control is a sampling control, it calculates the control quantity according to the deviation value at the sampling time. The integral and differential terms in equation (1) cannot be directly and accurately calculated. Therefore, the incremental PID algorithm is used in this control system. The numerical formula of its control law is:

Where: T is the sampling period.

It can be seen from formula (2) that the incremental algorithm only needs to save the deviation values ​​of the previous three moments, which takes up little space, has little impact on the system when the calculation error or insufficient precision occurs, and the cumulative error is also relatively small. Moreover, each time the system is restarted, it can be controlled on the original basis, reducing the response time of the system. At the same time, it also avoids the controller output from making large and drastic changes due to accidental factors, greatly improving the reliability of the system.

For this system, the parameters of the PID controller are mainly determined through experiments. The sampling period of the system is the switching period of the DC/DC. The output pulse width of the next period is calculated based on the sampling value of the previous period. Each sampling interrupt must be calculated once. The PID algorithm is embedded in the interrupt handler of the ADC.

3.3 Programmable digital input and output characteristic control

The input voltage range of high-power DC/DC converters for fuel cell buses is about 100V. It is necessary to set input undervoltage protection to prevent the fuel cell voltage from being too low and causing failure. Based on the real-time high-speed sampling of the undervoltage protection program, power is limited after reaching the undervoltage point to ensure the normal operation of the fuel cell. At the same time, it can be digitally set according to the changes in the requirements of the fuel cell and the vehicle. The output characteristics of the DC/DC converter must match the normal operating range of the motor controller and the power battery, and must also cooperate with the complex control strategy of the vehicle controller (ECU). Therefore, the output characteristics are designed as two modes: constant voltage current limiting and constant current voltage limiting, as shown in Figure 5. The ECU sends a given value through CAN, and the two characteristics can be switched within a CAN control command cycle, realizing programmable output characteristic control.

Figure 5 Schematic diagram of constant voltage current limiting and constant current voltage limiting output characteristic curve

4 Test results and technical parameters

The whole experimental system consists of the developed fuel cell engine 90 kW Boost DC/DC converter, 100 kW fuel cell simulation device, motor and its controller, PC, digital oscilloscope and other test equipment. The test results of the converter's IGBT switch voltage waveform and PWM drive waveform are shown in Figure 6. It can be seen from the waveform that the switch voltage and drive waveform of the Boost converter switch tube are ideal, and the converter's switch power loss is small.

Figure 6 IGBT switch voltage and PWM drive waveform

The output response curve of the system is shown in Figure 7. It can be seen from Figure 7 that it only takes less than 200 ms for the output voltage of the system to drop from 380V to 340V. The response speed is fast, the overshoot is small, and the steady-state control accuracy is high.

Figure 7 Given response curve of the system

90kW Boost DC/D for fuel cell engine

C converter technical parameters are as follows:

(1) Input voltage: DC 350V;

(2) Output voltage: DC 350~450V;

(3) Output current: 200~250A;

(4) Efficiency at rated power point: 97%;

(5) Output ripple is less than 1%;

(6) Realize programmable external characteristic control through CAN communication, i.e. constant voltage and current limiting, constant current and voltage limiting;

(7) The system can switch between autonomous control and vehicle control unit (ECU) control at will.

5 Conclusion

The digital 90 kWBoost DC/DC converter for fuel cell buses developed by the company uses IGBT as the power switch tube, which has high efficiency and reliability; the control system uses digital processing chips and digital control technology, which has high real-time performance and good programmable control functions, meeting the complex control requirements of the whole vehicle; the whole machine adopts a modular approach, which can communicate reliably with the whole vehicle and has good human-computer interaction. The converter has been successfully used in the first fuel cell city bus in China, and all technical indicators meet the use requirements.

References [1] Quanshi Chen, Bin Qiu, Qicheng Xie
, et al. Fuel Cell Electrical Vehicle [M]. Tsinghua University Press. 2005. [2] Quanshi Chen, Bin Qiu, Qicheng Xie. Fuel Cell Electrical Vehicle [M]. Tsinghua University Press. 2005. [3] Chan C C. The Present Status and Future Trends of Electric Vehicles [A]. Proceedings of First Circular of China -Japan Electric Vehicle Joint Conference [C]. Beijing: Beijing Institute of Technology Press. 2001. [4] Zhang Zhansong, Cai Xuansan. Principle and Design of Switching Power Supply [M]. Beijing: Electronic Industry Press, 2005. [5] Liu Heping, Wang Weijun, Jiang Yu, Deng Li. TMS320LF240x DSP C Language Development and Application [M], Beijing: Beijing University of Aeronautics and Astronautics Press, 2003.