Design of high-precision semiconductor laser drive power supply system

O Introduction

Semiconductor laser (LD) is a solid light source. It has been widely used due to its advantages such as good monochromaticity, small size, light weight, low price and low power consumption. LD is an ideal electron-photon direct conversion device with high quantum efficiency. Slight current and temperature changes will lead to large changes in its output optical power. Therefore, the driving current requirement of LD is very high. It must be a constant current source with low noise and high stability. It is difficult for general power supplies to meet the requirements. In addition, transient current or voltage spikes, as well as overcurrent and overvoltage will damage semiconductor lasers. Here, TI's DSP chip TMS320F2812 will be used as the control core to realize a dual closed-loop high-precision semiconductor laser drive power supply system with multiple protections.

1 Overall system design

The constant current source is generally realized by the "voltage-controlled constant current source" method composed of an integrated operational amplifier, some discrete components and a single-chip microcomputer. Compared with the constant current source composed of pure analog components, this method has obvious improvements in constant current accuracy and linearity. However, in this method, the single-chip microcomputer is used to display and control the given 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 consists of a "voltage-controlled constant current" circuit, a signal sampling and conditioning circuit, a protection circuit, a keyboard, an LCD display, an RS 232 communication interface, and a DSP processor. The system structure diagram is shown in Figure 1.

The given value is input through the keyboard and displayed on the LCD. At the same time, the PWM signal with the corresponding duty cycle is output after being processed by F2812. After low-pass filter, amplification and conditioning, PWM realizes D/A conversion and is used as the control voltage of the "voltage-controlled constant current" module (VI Constant Current) to realize "voltage-controlled constant current". F2812 samples the output current in real time. After digital filtering and analysis, the sampled data is compared with the given current value to obtain the difference as the input quantity in the PI adjustment algorithm expression. The control quantity Uk is obtained through PI operation to adjust the PWM output to realize constant current.

2 System Hardware Design

2.1 DC power module implementation

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

The "current expansion circuit" is composed of resistor Rp3 and high-power Darlington tube TIP147. By adjusting Rp3, the +15 V current can obtain a high current output of more than 2 A. In order to reduce the ripple in the DC current, the RC-π type active filtering method is adopted. The variable resistors Rp1, Q1, C3 and Rp2, Q2, C4 form two RC filter circuits to efficiently filter the +15 V and -15 V power supplies respectively. It is an NPN transistor, and the current amplification effect of the transistor can indirectly increase the capacitance of the filter capacitor.

If the amplification factors of Q1 and Q2 are β1 and β2, then the base capacitors C3 and C1 of Q1 and Q2 are equivalent to the emitters, which are (1+β1)C3 and (1+β2)C4, respectively, which realizes large resistance and large capacitance filtering and reduces the size of the circuit. In the figure, D5 and D6 are power failure displays, and D7 and D8 play the role of protecting the voltage regulators LM7815 and LM7915. When there is a load at the output end, if the input end of the LM7915 voltage regulator is open, the LM7915 has no output, and +15 V is added to the output end of the LM7915 through the load, causing damage to the LM7915. The protection principle of the LM7815 is the same.

2.2 Constant current source module implementation

"Voltage-controlled constant current" controls the output current by controlling the change of input voltage. The principle of the constant current source circuit is shown in Figure 3. Closed-loop negative feedback is achieved through hardware circuits, that is, inner closed loop. In Figure 3, resistors Rs, R4, R5, RF and op amp U5 form a feedback network. If op amp U4 is ideal, set the input voltage to Vs and the output voltage to Uo. From the "virtual short" of the op amp, we can get:

When Rs, R5, and Rf remain unchanged, the input voltage Vs is constant and the output current Io is constant. The stability of the operational amplifier U4, U5, resistors Rs, R5, and Rf themselves play a decisive role in the stability of the constant current source. Therefore, U4 and U5 use high-precision operational amplifiers OP-27, whose drift is only 0.2μV/℃ and the maximum noise voltage is 0.25μV. R5 and Rf use resistors with low temperature drift coefficient and high precision. The sampling resistor Rs uses a high-power manganese copper wire resistor with a resistance of 0.01 Ω and an accuracy of 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, and L1 form a low-pass filter to reduce the impact of high-order harmonics in the power supply on LD. D5 plays a role in throttling current when Q5 is cut off.

2.3 A/D and D/A module implementation

The F2812 chip has a built-in 12-bit ADC (analog/digital converter), the input voltage is 0~3 V, and the sampling resolution of the 12-bit ADC is (3.0 VO V)/212=0.73mV. The F2812 sets the A1A0 pin of the PGA103 according to the preset current value (A1A0=OO, A1A0=01, A1A0=10 correspond to the magnification of 1, 10, 100 respectively), and the signal conditioning is shown in Figure 3. There is no DAC module configured in the F2812. To realize the D/A function, an external D/A conversion chip is required. The conversion accuracy is proportional to the price of the chip, which undoubtedly increases the hardware cost. The PWM signal provided by the F2812 chip is a pulse width modulation (PWM) signal with a variable period and duty cycle, a high level VH=3.3 V, and a low level VL=0V. From the Fourier transform, it can be seen that the unipolar PWM signal with the origin of the time axis as the symmetric point can be written as: