DSP-based precision semiconductor laser drive power supply system

　　At present, semiconductor lasers (LD) have been widely used in many fields such as communications, information detection, medical treatment, precision processing and military. Laser power supply is an important part of the laser device, and its performance directly affects the technical indicators of the entire laser device. This design uses a constant current source controlled by DSP to provide current for the semiconductor laser. In the circuit, the negative feedback principle is used to control the output current of the composite power adjustment tube to achieve the purpose of stabilizing the output current. The system uses a combination of circuit design and program control algorithm design to detect and control the working status of the semiconductor laser in real time from many aspects, greatly improving the performance of the system and effectively solving the problem of accurate work of the semiconductor laser. , stability and reliability issues, further improving the output indicators of semiconductor lasers.

System principle

　　In order for the laser to output laser light with a stable wavelength, the current flowing through the laser is required to be very stable, so a low-noise, stable constant current source is selected for the power supply circuit. The constant current source current can be continuously adjusted between 0A and 3A to adapt to lasers of different specifications. At present, pure hardware circuit systems or microcontroller control are generally used in the secondary development of semiconductor laser power supplies. With the rapid development of embedded microprocessors, digital control based on DSP can more effectively solve the problems of stability, accuracy and reliability of semiconductor laser operation. The principle of DSP secondary development is shown in Figure 1.

　　Figure 1 System schematic diagram

　　The voltage control signal output by the DSP is output to the operational amplifier, which is amplified and output to control the composite adjustment tube composed of the transistor 8050 and the adjustment tube TIP122. The emitter of the adjustment tube is connected in series with a relay and a high-power sampling resistor. The voltage signal is taken from both ends of the sampling resistor and sent to the differential amplifier circuit U2, thereby obtaining the voltage on the sampling resistor. The voltage signal passes through a voltage follower and enters the analog signal input channel of the ADC controlled by the DSP. The ADC converts the input analog signal into a digital signal, and then the DSP performs data processing on the converted digital signal. The sampling resistor should be a high-power metal film resistor of 0.15Ω, which requires a good temperature coefficient. The amplification factor of the operational amplifier U1 determines the current control accuracy. The smaller the amplification factor, the higher the current output accuracy. At the same time, the amplification factor of the differential feedback circuit U2 will also affect the current control accuracy. The greater the amplification factor, the higher the stability of the current, but the output range of the current becomes smaller. When the control voltage is constant, accurately selecting the multiple of the operational amplifier U1 and the amplification factor of the differential feedback circuit U2 will become an important factor in determining the current output accuracy and current output range of the constant current source.

　　Figure 2 System workflow

TMS320F2812 control system

　　The design circuit takes the digital signal processor TMS320F2812 as the core. The power supply consists of several parts such as control circuit, protection circuit and main circuit, in which DSP plays a core role. Its control tasks are mainly: 1. Control the data acquisition system. The 12-bit ADC that comes with the DSP chip is used to control the sampling signal after PID operation and processing. The data conversion start command is controlled by the pin XF of the F2812, that is, the software sets the pin XF to high level to control the data conversion of the ADC. After the data conversion is completed, the signal BUSY will become low level, triggering the F2812 interrupt and reading the data immediately from the 16-bit data line D[15:0]. The data code of this system is two's complement. After F2812 processes the received data, it caches it and sends it to the LCD for real-time display.

　　2. Use a DAC7724 chip to interface with DSP. This chip is a 4-channel 12-channel double-buffered DAC, using 2 channels to set the output voltage reference and maximum current limit reference. 3. Human-machine interface circuit. LCD and 8279 are connected to DSP as external I/O devices. LCD is used to display current, voltage, power, as well as fault display and alarm. 4. Fault detection. The interrupt signal of the fault detection circuit is input to the XINT2 pin of the DSP. If a falling edge interrupt occurs, the overvoltage and overcurrent signals are detected through the GPIO port lines GPIO8 and GPIO9 respectively.

Digital filter and system software design

Digital filter design

　　In view of the shortcomings in the current filter design in the previous development process of this project, a digital filter based on TMS320F2812 is now introduced to filter the current sampling signal. In order to design the filter quickly and conveniently, directly use the filter library function library provided by TI to design. The design steps are as follows: determine the filter performance indicators according to the actual task requirements; in Matlab, call the ezfir function in the filter library to perform simulation; determine the values ​​of each parameter based on the simulation results; call filter.asm in the filter library DSP assembles the program module, copies the simulation parameter values ​​in Matlab into the program, and implements filtering on F2812.

　　Figure 3 Constant current source control curve diagram

System software design

　　The system workflow is shown in Figure 2. After power-on, the system starts self-test. After the self-test is completed, it enters system initialization, including DSP, DAC, LCD, and the interrupt controller and counter inside the DSP. After the system is ready, enter the startup screen. Turn on the keyboard interrupt and wait for the key to select the corresponding function. If "Parameter Setting" is selected, press the work key to enter the "Parameter Setting" interface, where you can set the voltage, current and power values. After the setting is completed, return to the startup screen and start the laser. After the system enters the running state, the user can still set new values ​​without terminating the laser's operation. After the settings are complete, the laser outputs laser according to the new requirements.

　　When an error occurs during the system self-test and control process or the system is over-current or over-voltage, the protection program will be automatically called. When the system shuts down or suddenly loses power, in order to prevent the voltage at both ends of the laser from plummeting to zero, the system adopts a full shutdown method. The principle is: the sampling value output is gradually reduced, and the shutdown is not allowed until it drops to zero.

Conclusion

　　The experiment in this article determined that the amplification factors of U1 and U2 are both 1, the output current is adjustable from 0A to 3A, and the laser output power is adjustable from 0W to 2W. The introduction of DSP control system has significantly improved compared with previous single-chip microcomputer control. The main manifestations are: due to the high integration and good performance of TMS320F2812, the system has the advantages of small size, fast speed, strong processing power, high reliability and low power consumption; the digital filtering method implemented in TMS320F2812 is simple and improves development efficiency. . After the driver and protection circuit of the semiconductor laser is designed, weld and debug. Table 1 shows the relationship between the control voltage and output current of the constant current source at 25°C. Figure 3 is a constant current source control curve drawn based on the data in Table 1. The output voltage range is 0V~5V, and the output current error rate is 0.1%. The output voltage has a linear relationship with the current, which meets the requirements.

references

　　1. ZOU Wen-dong, GAO YI-qing. Semiconductor laser power supply controlled by sing-chip microcomputer [J]. Laser Journal. 2002, 23(4): 70-71
　　2. Fu Yan-jun. ZOU Wen-dong. Optic power control of LD driver circuit[J]. Infrared and Laser Engineering.1007-2276(2005)05-0626-05TI DSP TOOLBOX[M/CD].2002.5