Design of semiconductor laser power supply for intelligent control

led2015

Design of semiconductor laser power supply for intelligent control [Copy link]

Due to its small size and light weight, semiconductor lasers (LDs) are increasingly widely used in information, communication, medical and other fields, and are combined with electronic devices to achieve monolithic optoelectronic integration. However, LDs are easily damaged by overvoltage, current or static charge, and the research on their power supplies has attracted more and more attention. If the output voltage or current waveform of the power supply is not of high quality and lacks effective protection, the performance of the laser will be reduced or damaged. Therefore, it is necessary to design a power supply with excellent performance to ensure the safe and stable operation of the LD. This
　　paper takes digital integrated circuits as the core and designs a semiconductor laser power supply that can achieve intelligent control.

　　Factors affecting the operation of semiconductor laser LDs
　　The core of semiconductor lasers is that once the PN junction is broken down or the resonant cavity surface is partially damaged, non-equilibrium carriers and radiation recombination cannot be generated, and the laser output is reduced or fails depending on the degree of damage.
　　The main causes of LD damage are cavity surface contamination and surge breakdown. Cavity surface contamination can be solved by purifying the working environment, while more damage is caused by surge breakdown. Surges can cause damage or breakdown of the semiconductor laser PN junction. The reasons for this are many, including: ① instantaneous current of the power switch; ② start-up and shutdown of other electrical equipment in the power grid; ③ lightning; ④ strong electrostatic field, etc. High voltage, static electricity, surge impact and other factors in the actual working environment will cause damage to the LD or shorten its service life, so measures must be taken to protect it.
　　Traditional laser power supplies are implemented with pure hardware circuits and use analog control methods. Although they can also drive the laser well, they cannot achieve precise control. In many industrial applications, the accuracy and degree of automation are reduced, and the application of lasers is also limited. Using a single-chip microcomputer to control the laser power supply can simplify the hardware structure of the laser power supply and effectively solve the problems of accuracy, stability and reliability of semiconductor lasers. With the rapid development of large-scale integrated circuit technology, the use of chips suitable for LD can greatly improve the reliability of the power supply.

　　System design
　　The system block diagram is shown in Figure 1. It mainly consists of the following parts.

　　Power supply: realizes the conversion between the system power supply voltage (AC 220V) and the system working voltage. And the filtering technology is adopted to make the voltage ripple of the semiconductor laser very small, ensuring the normal operation of the semiconductor laser.
　　Intelligent control: mainly completed by the CPU. The LD power supply works in the constant current mode. After setting the current, the CPU sends the output voltage to the feedback pin of the laser driver chip through the operational amplifier circuit according to the current signal value sampled by the sensor after a certain algorithm, and automatically adjusts to achieve the set current output to realize the intelligence of the laser.
　　Protection circuit: The semiconductor laser drive system must be equipped with a protection circuit. The protection circuit will reduce the external influence on the actual use of LD and enhance the reliability of the system. This part mainly includes over-temperature protection, over-current protection, surge protection and other circuits.

　　Hardware circuit
　　The power supply is designed to output a continuously adjustable current of 0 to 1.5A in the continuous mode, with high current stability and small ripple coefficient, which meets the resolution, stability and noise performance required by small and medium power LD.

　　Constant current source circuit
　　The LD power supply circuit is a constant current source (see Figure 2). The stability of the power supply voltage VEE plays an important role in the stability of the output constant current signal. Therefore, multiple filtering technology is used to control the ripple of VEE below 1mV to ensure the stability of the signals at the output pins 12, 13, and 14 of the HY*0 chip. The voltage between pins 5 and 6 to VEE can be adjusted to set the overcurrent protection threshold and overtemperature protection value respectively. In the constant current working mode, the output current can be continuously adjusted between 0 and 1.5A by adjusting the output level of pin 21.


　　Processing unit
　　C8051F020 is selected as the digital processing unit. CH451 is used in the implementation of the scanning key function. The chip has built-in de-jitter function and keyboard interrupt function, which can save the internal running time of the microcontroller and ensure the accuracy of key reading.

　　Peripheral circuit
　　In order to realize the independent adjustment of the output voltage of the modulation signal, two-stage output amplifiers U14A and U14B are added at the output end. Considering the bandwidth requirements, the high-speed amplifier MAX4215 is selected as the amplifier. A high-speed operational amplifier is used to form a subtraction circuit, so that the output signal changes from the original 2Vp-p symmetrical to the ground potential to 2Vp-p centered on 2.5V voltage. When an external modulation circuit is required, the core unit control relay is started to achieve the conversion between the built-in modulation circuit and the external modulation source.

Software Design
　　The software is written in C51, including the main program and the interrupt response program.
　　The main program mainly realizes soft start, slow shutdown and control ignition. When the system starts, the human-computer dialogue interface is entered after the system is initialized to scan whether there is a key pressed. If so, the key processing program is called. The operator can set the output current and output voltage reference values through the keyboard and display them at the same time for confirmation. Start working, and realize the soft start of the system by slowly increasing the voltage to protect the LD. During normal operation, the current signal is sampled in the hardware circuit, and the signal from the digital-to-analog conversion circuit passes through the sampling resistor to obtain the corresponding voltage signal, which is transmitted to the single-chip microcomputer and sent out for display. If current fluctuation occurs, PID control is performed, in which the median and mean composite filtering method is used. The system adjusts the D/A output signal and then adjusts the output current. The loop part in the main program continuously detects the working current, working temperature and transmission power of the LD, and displays them for viewing. If a fault occurs, the interrupt signal is sent to the microcontroller port (corresponding to overvoltage, overcurrent, and sudden power failure), and the system calls the interrupt program to realize rapid protection of the system. The main control functions are all realized by interruption to ensure the real-time response of the system. Finally, when the operator presses the button to shut down the device, the system calls the slow shutdown program to stop working safely.

　　Digital filtering
　　The impact signal that interferes with the system often has a wide spectrum and is random. In this regard, the system uses a software method to perform digital smoothing filtering on the sampled signal, and reduces the effect of interference on useful components by processing the signal. Common digital filtering methods include median filtering, mean filtering, etc. Combining the median filtering with the mean filtering method, a composite filtering method is constructed. The specific method is: first sort the sample signal, remove the maximum and minimum values, and then calculate the mean of the sequence composed of the remaining data as the filtering result, so that the impact interference can be filtered out and the useful signal components can be retained.

　　Protection settings
　　Soft start and slow shutdown: The system startup or shutdown is controlled by the start/stop button. If it is judged to be turned on, the LD driver chip is commanded to preheat and then the working current is gradually increased to the set value to achieve soft start. If it is judged to be turned off, the working current is gradually reduced to zero to achieve slow shutdown.
　　Current overload protection: The upper limit of the current value is set by the program or determined by the keyboard. The CPU controls the digital potentiometer to adjust the voltage of the laser driver chip PIN21 and detect the current to ensure the stability of the current flowing through the LD and prevent overcurrent from damaging the LD. The current setting value and the sampling value are compared in real time. When the actual value is greater than the upper limit, the system starts the current limiting protection action.

　　Test results
　　According to the design, a digital power supply was made and connected to the existing laboratory semiconductor laser for performance testing.
　　After the laser is turned on, it is preheated for half an hour. The working current of the laser is adjusted to 1.5A through software setting. The laser starts the system and the working current rises steadily to 1.5A. The dynamic response time is between 1.5 and 2s. The system output current is 1.5A, and it works continuously for 4 hours. The current is recorded once every 10 minutes. The test order and the corresponding current value are arranged according to time. The test result data plot curve is shown in Figure 3. The results show that the control current of the system is stable and the error is small. After the test, the laser is turned off, and the system gradually reduces the output voltage signal. After the output power is reduced to zero, the laser stops working. The results show that the power supply using the digital control scheme meets the steady-state accuracy requirements of the laser.


　　Conclusion
　　The designed digital semiconductor laser power supply uses the integrated circuit C8051F020 as the core, and is programmed to realize intelligent functions such as digital filtering and surge protection. The circuit uses the constant current source driver chip HY*0 and the high-speed integrated amplifier MAX4215, which simplifies the circuit and improves the control accuracy. Practical tests were conducted on semiconductor laser power supplies, and the results showed that the output current was 0-1.5A, the operating current was stable, and the power supply could also achieve soft start, slow shutdown, and surge protection functions. After testing, the digital power supply met the steady-state accuracy requirements of the laser power supply, improved the dynamic performance of the system, and simplified the hardware circuit.