Using a single-chip microcomputer to programmatically control the laser drive Power Supply can not only effectively realize the above functions, but also improve the automation of the whole machine. At the same time, it provides favorable conditions for the improvement and expansion of the performance of the laser drive power supply.
1 Overall structure frame
The principle of this system mainly realizes the functions of current source driving and protection, optical power feedback control, constant temperature control, error alarm and keyboard display. The entire system is controlled by a single chip microcomputer. In this system, C8051F single chip microcomputer is used. C8051F MCU is a fully integrated mixed-signal system-on-chip (SOC). He integrates almost all analog and digital peripherals and other functional components needed to form a single-chip data acquisition or control system in one chip, as used in this system ADC and DAC. The high integration of these peripheral components provides convenience for designing a single-chip application system with small size, low power consumption, high reliability, and high performance, and also greatly reduces the cost of the system. The optical power and temperature sampling analog signals are amplified by the internal A / D of the single-chip microcomputer to digital signals for arithmetic processing. After the internal D / A conversion, the feedback control signals are sent to the laser current source circuit and the temperature control circuit respectively to form light Closed-loop control of power and temperature. The optical power setting is input from the keyboard, and the LED digital tube displays data such as laser power and current.
2 Design of semiconductor laser power control system
At present, most high-precision constant current sources use integrated operational amplifiers. The basic principle is to make the voltage applied to the two input terminals of the comparison amplifier equal through negative reaction, thereby keeping the output current constant. And the factors that affect 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 resistance, amplifier gain (including adjustment links), zero drift and noise voltage; two It is an external factor where the constant current source is located, including: input power supply voltage, load resistance and ambient temperature changes.
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 due to the moment when the switch is closed and opened. A large inrush current is generated, which is the current that causes the semiconductor laser to be damaged. In between, this situation 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 a regulated DC voltage, and the right side is the output terminal. The structure of the entire circuit can be seen as the addition of two Type II filter networks to the emitter output, consisting of L1, C1, C2 and L2, C6, C7. C-type filter network and a time delay network formed by capacitor C5. 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. The time delay network composed of the arrival resistors R and C, C2 and C4 are connected in parallel is to reduce the inductive effect of electrolytic capacitors on high frequency components.
2.2 Design of constant current source circuit
In order to make the semiconductor laser work stably, the requirements for the current flowing through the laser are 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.
As shown in Figure 3, the constant current source is amplified by the op amp U1 and the transistor T1, and the Darlington tube Q2, and then the feedback is amplified by U2 to achieve a constant current output. TQ2 uses a high-power Darlington tube as an adjustment tube, and connects it to the form of an emitter output. A semiconductor laser (LD) is used as a load in series with the emitter of the Darlington tube. By controlling the base of the Darlington tube Laser current control. This design requires that the circuit can output a maximum operating current of 3 A, which requires that the base current of the Darlington tube is also relatively large, but because the integrated operational amplifier generally works in a small current state, it cannot directly promote the normal operation of the Darlington tube. Even the reluctance to promote its work will cause the power consumption of the integrated operational amplifier itself to be too large, and the temperature rise to be too high, which affects the output accuracy of the circuit. Therefore, the small power transistor T1 is used to promote the work of the high power Darlington tube. The sampling resistor is connected to the lower end of the laser, and the sampling signal is amplified by the phase-comparison amplification link composed of U2 and then connected back to the inverting input terminal of U1 to form a current negative feedback circuit to achieve the purpose of outputting a constant current.
2.3 Stable control of laser power
Optical power feedback uses the output photocurrent of the externally monitored photodiode, which is then A / D converted by the amplifier and sent to the CPU for processing to obtain the control amount, adjust the working current of the laser, and thus perform closed-loop control of the laser power.
Temperature control is implemented in this system using semiconductor refrigeration. This is a thermoelectric cooler. As long as the size and direction of the current flowing through the thermostat are controlled, the laser can be cooled or heated to control the operating temperature of the laser.
2.4 Protection circuit
Although the slow-start circuit eliminates the hazards of high-frequency surge currents, it cannot effectively prevent the damage of DC or low-frequency current overload to semiconductor lasers. Therefore, an overload protection circuit should be established. Generally, a current-limiting protection circuit can be used. If working in a short circuit for a long time, overheating will still cause damage to the adjusting tube. At this time, a shut-off protection circuit can be adopted. The accuracy of overvoltage protection mainly depends on the voltage stabilizing diode, and its operating point changes with the current flowing through the voltage stabilizing tube and the ambient temperature. Therefore, the design must use a voltage stabilizing tube with a very small temperature drift of stable voltage.
3 software design
The system software adopts a modular structure design, which is gradually refined from top to bottom, and subroutines are used to form each module, such as initialization module, keyboard module, display module and so on. 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 adjust the display subprogram, display data, and then adjust the key scan subprogram. If a key is pressed, the corresponding key function program is adjusted. Press the key, the display program will be called cyclically.
4 Conclusion
The control system of the semiconductor laser drive power supply designed in this paper solves the problem of instability of constant current and output power within the operating temperature range through a slow start circuit, a constant current source circuit, and an optical power feedback circuit.
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