Design of the driving power supply system for the

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Abstract: This paper introduces the design of a high-precision semiconductor laser drive power system with dsptms320f2812 control module as the core. The system takes the high-power Darlington tube as the adjusting tube and adds the current negative feedback circuit to realize the constant current output. The output current is sampled by the analog-to-digital converter integrated in the DS, and the PWM output is controlled after being processed by the PI algorithm to realize the dynamic error adjustment and eliminate the static error in the circuit. In order to improve the stability of the system, functions such as over-current, over-voltage protection and delayed soft start protection are added to the system. The results show that the output current range is 10 ~ 2500ma, and the absolute value of the output current change is less than 0.1% of the output current value + 1mA, thus ensuring the reliability of the semiconductor laser

Keywords: DSP; Semiconductor laser; PI algorithm; PWM

0 Introduction

semiconductor laser (LD) is a solid-state light source, which has been widely used because of its good monochromaticity, small size, light weight, low price, low power consumption and a series of advantages. LD is a direct electron photon converter of egg dream, which has high quantum efficiency. Small current and temperature changes will lead to great changes in its output optical power. Therefore, the driving current requirements of LD are very high. It must be a constant current source with low noise and high stability. It is difficult for general power supply to meet the requirements []. In addition, transient current or voltage spikes, as well as overcurrent and overvoltage, can damage semiconductor lasers. Here, the DSP chip TMS320F2812 of TI company will be used as the control core to realize a double closed-loop high-precision semiconductor laser driving power supply system with multiple protections

1 overall design of the system

constant current source is generally realized by the method of voltage controlled constant current source composed of integrated operational amplifier, some discrete components and single chip microcomputer. Compared with the constant current source composed of pure analog components, this method has significantly improved the accuracy and linearity of constant current. However, in this method, the single chip microcomputer is used as the setting of display and control voltage, and does not detect and control the output current in real time. It belongs to an open-loop control system, which affects the stability and accuracy of the constant current source. The system is composed of voltage control constant current circuit, signal sampling and conditioning circuit, protection circuit, keyboard, LCD display, RS232 communication interface and DSP processor. The system structure block diagram is shown in Figure 1

input the given value through the keyboard and display it on the LCD. At the same time, the PWM signal with corresponding duty cycle is output after F2812 operation and processing. For more information, PWM realizes d/a conversion after low-pass filter, amplification and conditioning, and realizes voltage controlled constant current as the control voltage of v-iconstantcurrent module. F2812 samples the output current in real time, compares the sampled data with the given current value after digital filtering, analysis and processing, and obtains the difference value as the input in the PI adjustment algorithm expression. The control value u is obtained through PI operation to adjust the PWM output to achieve constant current

2 system hardware design

2.1 DC power module implementation

DC power module is mainly composed of voltage transformation, rectification, filtering, voltage stabilization and current expansion circuits. The DC power module is shown in Figure 2+ 15V is used for power supply of voltage control constant current module and operational amplifier- 15V is used for negative power supply of operational amplifier+ 5V is used for power supply of CNC module. The +5v voltage is stabilized by a high-precision and high-stability voltage stabilizing chip before supplying power to the DSP processor

the current expansion circuit is composed of a resistor RP3 and a high-power Darlington TIP147. Adjusting RP3 can make the +15v current get a high current output of more than 2A. In order to reduce ripple in DC current, RC type active filter method is adopted. Variable resistors RP1, Q1, C3, RP2, Q2, C4 form two RC filter circuits to filter +15v and -15v power supplies efficiently respectively. It is an NPN type transistor, which can indirectly increase the capacitance of the filter capacitor by using the current amplification effect of the transistor

suppose Q1 and Q2. If the amplification factor is 1 and 2, the base capacitance C3 and C4 of Q1 and Q2 are equivalent to the emitter, which are (1+ 1) C3 respectively. (1+ 2) C4, which realizes large resistance and large capacitance filtering and reduces the volume of the circuit. D5 and D6 in the figure are power failure displays, and D7 and D8 protect voltage regulators lm7815 and lm7915. When there is load at the output end, if the input end of lm7915 voltage regulator is open circuit, then lm7915 has no output, +15v is added to the output end of lm7915 through load, resulting in damage to lm7915. The protection principle of lm7815 is the same

2.2 constant current source module cash purchase

voltage controlled constant current controls the output current by controlling the change of input voltage. The principle of constant current source circuit is shown in Figure 3. The closed-loop negative feedback is realized through the hardware circuit, that is, the inner closed loop. In Figure 3, resistors RS, R4, R5, RF and op amp U5 form a feedback network. If the operational amplifier U4 is ideal, set the input voltage as V5 and the output voltage as uo. It can be obtained from the virtual short of the operational amplifier:

rs, R5, RF are constant, the input voltage is constant, and the output current is constant

the stability of operational amplifiers U4, U5, resistors RS, R5, RF plays a decisive role in the stability of constant current source. Therefore, for U4 and U5, high-precision op-27 is selected, and its drift is only 0.2 v/℃, and its maximum noise voltage is 0.25 v. R5 and RF are resistors with low temperature drift coefficient and high precision. The sampling resistance Rs is a high-power manganese copper wire resistance with a resistance value of 0.01, and its accuracy is 1%. Q5 is a high-power Darlington tube 2sd1559. Since the integrated operational amplifier generally works in a low current state, a low-power transistor Q4 (9014) is used to drive Q5. C15, C16, D9, L1 constitute low-pass filtering to reduce the impact of higher-order harmonics in the power supply on LD. D5 plays a choking role when Q5 is cut off

2.3a/d and d/a modules realize

f2812 chip has built-in 12 bit ADC (analog-to-digital converter) input, the voltage is 0 ~ 3V, and the resolution of 12 bit ADC sampling is (3.0V ~ 0V)/212=0.73mv. F2812 sets the a1a0 pin of pga103 according to the preset current value (a1a0=00, a1a0=01, a1a0=10, the corresponding amplification factor is 1,10100 respectively). The signal conditioning is shown in Figure 3. There is no DAC module configured in F2812. To realize d/a function, an external d/a conversion chip is required. The conversion accuracy is directly proportional to the price of the chip, which undoubtedly increases the hardware cost

the PWM signal provided by F2812 chip is a pulse width modulation (PWM) signal with variable cycle and duty cycle, high level VH = 3.3V, low level vl=0v. According to the Fourier transform, for the unipolar PWM signal with the origin of the time axis as the symmetrical point, the expression can be written:

where: t is the signal period; n= 1, 2, 3 ; An and BN are independent Fourier coefficients

according to the formula (futures: Coke 1609 contract oscillation operation 3), as long as the high-frequency DC component an is filtered and the PWM signal duty cycle Q (q=0 ~ 1) is changed, the output voltage of 0 ~ 3.3V can be obtained. Because the third-order low-pass filter has better performance than the first-order and second-order low-pass filters [5]. A Butterworth third-order feedback active low-pass filter is designed using the normalization method, as shown in Figure 4. The transfer function of the low-pass filter is expressed as:

, where: G, bn-1, B0 are appropriately selected constants. Figure 4 the low-pass filter must meet the following conditions in equation (4), where:

is obtained by the normalization method, and the cut-off frequency fc (Hz) and capacitance C21 are normalized, so the resistance coefficient is k=100/fcc ˊ, C ˊ It is the C21 value in F. to make the gain g=2, it can be seen from the table in literature [6]. When k = 1, the coefficients of the corresponding resistance R6 ~ R10 capacitance C22 ~ C23 (f) are 2.491,2.339,0.692,11.043, 11.043, C21, C21 respectively. Select fc=1000, C ˊ= At 0.01, R6 ~ RL0 and C21 ~ C23 in Figure 4 are 24.491, 23.39, 6.92110.43110.43,0.01,0.01 and 0.01 respectively

the EWB simulation software shows that the third-order filter circuit has good filtering performance. Butterworth filter has no ripple in the passband, which ensures the accuracy of PWM to d/a conversion. The simulation results are shown in Figure 5

2.4 keyboard and display implementation

the function of the keyboard is to input the preset current value and can be modified in real time. The keyboard adopts 16 keys, 0 ~ 9 and keys are used for digital input; The enter and cancel keys indicate confirmation and cancellation, Key indicates the increase or decrease of the step amount; The num key indicates the step selection. The preset current step is divided into 10mA and 1mA, and the current value within the range of 10 ~ 2500ma can be input. The preset current input can be displayed on the LCD after pressing the Enter key. The commonly used liquid crystal display lcd1602a is selected for data display, and the preset output current value and real-time sampling current value are divided into two lines for display

2.5ld protection circuit

the PN junction of semiconductor laser LD is very fragile and easy to be damaged. Transient current mutation is easy to damage the cavity mirrors at both ends of the semiconductor laser, causing permanent damage to the laser []. Slow start (also known as soft start) means that after the driving power is turned on, the control voltage vs is not suddenly added to the whole constant current circuit, but gradually rises from zero to vs within the set time. Connecting several forward guiding diodes in series with the laser l can effectively prolong the service life of the LD tube, because when a large forward voltage occurs, these diodes are connected, and the current will not pass through the laser tube LCD, so as to avoid damaging the LD tube [9,10]. A small capacitor is connected in parallel at both ends of the LD, and a reverse diode is connected in parallel to prevent LD from being damaged by excessive reverse voltage. In order to prevent overcurrent, software and hardware protection can be adopted, that is, the sampled current value is processed and compared with the current limit value. When it is greater than the current limit value, switch Q6 is turned on, V4 is clamped to 0V, and the regulating tube is cut off to achieve the purpose of current limit

3 system software design

the software is written in assembly language, which can modify the given value of current in real time through the keyboard, and LCD can display the given value and measured value at the same time. In order to facilitate debugging, the system software adopts modular design, mainly including the main program, given, LCD display, PI adjustment and other subroutines

initialization of the system includes initialization of DSP peripheral interface chip and current setting, and keyboard scanning includes setting and step adjustment. The flow chart of the system main program and outer ring adjustment program is shown in Figure 6

4 conclusion

in this design, the method of combining hardware closed-loop negative feedback with digital closed-loop is adopted to form a double closed-loop constant current power supply. The hardware closed-loop negative feedback has strong constant current characteristics and reduces the workload of digital closed-loop. The digital closed loop is mainly used for fine adjustment to improve the constant current accuracy of the system. In addition, making full use of F2812's built-in resources simplifies the complexity of peripheral chip design. At the same time, 16 ADC channels and PWM outputs can measure and control multiple constant current power supplies. Therefore, the system can be widely used in optical fiber sensing, optical fiber communication, laser power supply of active current transformers, etc. in China, it has a good application prospect in the convenience of receiving waste plastic modified plastic materials


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