Control system of semiconductor laser driving power supply

　　The programmable control of the laser driving power supply by using a single chip microcomputer can not only effectively realize the above functions, but also improve the automation level of the whole machine. At the same time, it provides favorable conditions for improving and expanding the performance of the laser driving power supply.

　　1 Overall structure diagram

　　The principle of this system is shown in Figure 1. It mainly realizes the functions of current source driving and protection, optical power feedback control, constant temperature control, error alarm and keyboard display. The whole system is controlled by a single-chip microcomputer. The C8051F single-chip microcomputer is selected in this system. The C8051F single-chip microcomputer is a fully integrated mixed-signal system-on-chip (SOC). It integrates almost all analog and digital peripherals and other functional components required to form a single-chip microcomputer data acquisition or control system in one chip, such as the ADC and DAC used in this system. The high integration of these peripheral components provides convenience for designing small-volume, low-power, high-reliability, and high-performance single-chip microcomputer application systems, and also greatly reduces the cost of the system. The optical power and temperature sampling analog signals are amplified and converted into digital signals by the internal A/D of the single-chip microcomputer for calculation and processing. The feedback control signal is converted by the internal D/A and then sent to the laser current source circuit and the temperature control circuit respectively, forming a closed-loop control of optical power and temperature. The optical power setting is input from the keyboard, and the laser power and current data are displayed by the LED digital tube.

　　2 Design of semiconductor laser power supply control system

　　At present, most of the high-precision constant current sources use integrated operational amplifiers. The basic principle is to make the voltages added to the two input terminals of the comparison amplifier equal through negative reaction, so as to keep the output current constant. And the factors affecting the stability of the output current of the constant current source can be summarized into two parts: one is the internal factors that constitute the constant current source, including: reference voltage, sampling resistor, amplifier gain (including adjustment link), zero drift and noise voltage; the other is the external factors of the constant current source, including: changes in input power supply voltage, load resistance and ambient temperature.

　　2.1 Slow start circuit

　　Semiconductor lasers are often damaged by the sudden opening or closing of multiple electrical appliances connected to the same power grid. This is mainly because a large impact current will be generated at the moment of closing and opening the switch. It is this current that causes the semiconductor laser to be damaged. In this case, it must be overcome. Therefore, the input of the driving power supply should be designed as a slow start circuit to prevent damage, as shown in Figure 2: the left input terminal is connected to the DC voltage after voltage regulation, and the right is the output terminal. The structure of the entire circuit can be regarded as adding two type II filter networks to the emitter follower, which are composed of L1, C1, C2 and L2, C6, C7 respectively. The C-type filter network composed of capacitor C5 and a time delay network. The slow start input voltage V generates a large amount of high-frequency components at the moment of switching and closing. Most of the high-frequency components are filtered out through the two type II networks in the figure, and the DC and low-frequency components can pass smoothly. Arriving at the time delay network composed of resistors R and C, C2 and C4 are connected in parallel to reduce the inductance effect of the electrolytic capacitor on the high-frequency components.

　　2.2 Design of constant current source circuit

　　In order to make the semiconductor laser work stably, the current flowing through the laser is very strict. The power supply circuit must be driven by a low-noise stable constant current source. The specific circuit is shown in Figure 3. [page]

　　As shown in Figure 3, the constant current source is composed of an operational amplifier U1, a transistor T1, and a Darlington tube Q2 for current amplification, and then amplified and fed back through U2 to achieve constant current output. TQ2 uses a high-power Darlington tube as an adjustment tube, which is connected in the form of an emitter output. The semiconductor laser (LD) is connected in series with the emitter of the Darlington tube as a load, and the laser current is controlled by controlling the base of the Darlington tube. This design requires that the circuit can output a maximum working current of 3 A, which requires that the base current of the Darlington tube is also relatively large. However, since the integrated operational amplifier generally works in a small current state, it cannot directly drive the Darlington tube to work normally. Even if it is barely driven to work, it will cause the integrated operational amplifier itself to consume too much power and the temperature rise to be too high, affecting the output accuracy of the circuit. Therefore, a low-power transistor T1 is used to drive the high-power Darlington tube to work. The sampling resistor is connected to the lower end of the laser, and the sampling signal is amplified by the in-phase proportional amplifier link composed of U2 and then connected back to the inverting input end of U1, forming a current negative feedback circuit to achieve the purpose of outputting constant current.

　　2.3 Stable control of laser power

　　The optical power feedback uses the output photocurrent of the external monitoring photodiode, which is then sent to the CPU for processing after A/D conversion by the amplifier to obtain the control quantity and adjust the operating current of the laser, thereby performing closed-loop control of the laser power.

　　Temperature control in this system is achieved by using semiconductor refrigeration, which is a thermoelectric cooler. By controlling the magnitude and direction of the current flowing through the thermostat, the laser can be cooled or heated, thereby controlling the operating temperature of the laser.

　　2.4 Protection Circuit

　　Although the slow start circuit eliminates the harm of high-frequency impact current, it cannot effectively prevent the harm of DC or low-frequency current overload to semiconductor lasers. Therefore, an overload protection circuit should be set up. Generally, a current limiting protection circuit can be used. If it works in a short-circuit state for a long time, overheating will still cause damage to the adjustment tube. At this time, a current cutoff protection circuit can be used. The accuracy of overvoltage protection mainly depends on the voltage regulator diode, and its operating point changes with the current flowing through the voltage regulator diode and the ambient temperature. Therefore, the design must use a voltage regulator diode with very small temperature drift to stabilize the voltage.

　　3. Software Design

　　The system software adopts modular structure design, which is top-down and gradually refined, and uses subroutines to form each module, such as initialization module, keyboard module, display module, etc. The main program flow chart is shown in Figure 4.

　　In the main program flow, after the system is powered on and reset, it starts to initialize each module, then calls the display subroutine to display data, and then calls the key scanning subroutine. If a key is pressed, the corresponding key function program is called. If no key is pressed, the display program is called in a loop.

　　4 Conclusion

　　The control system of the semiconductor laser driving power supply designed in this paper solves the problems of constant current and unstable output power within the operating temperature range through slow start circuit, constant current source circuit and optical power feedback circuit, and has high stability.