LD Drive Current Source Circuit Model

The drive current source circuit model is shown in Figure 2. In the figure, H1 (s) and H2 (s) are the transfer functions of the control circuit, and H3 (s) is the transfer function of the DC / DC conversion circuit. The DC / DC conversion circuit consists of a voltage-controlled PWM-type high-efficiency switching power supply. The output voltage is the output voltage of the drive current source of the LD module and the relationship between the control voltage Vc is approximately: V0 = 5Vc + 6, (1) where 0 ≤ Vc ≤2.

Laser diodes are non-linear devices. Their volt-ampere characteristics are similar to those of diodes. If they are equivalent to linear resistors (RD = VD / ID) near the operating point, VD and ID are the operating voltage and current of the LD module , Then the transfer function of the linearized model of the current source circuit shown in Figure 2 can be expressed as: I (s) = H0 (s) Vs (s) / [RD + Rf + RfH0 (s)], (2) where H0 (s ) = H1 (s) H2 (s) H3 (s).

If the circuit design satisfies | RfH0 (s) |>= RD + Rf, the steady-state current output by the laser power supply is: I (s) = Vs (s) / Rf.  (3)

(3) When the value of H0 (s) is large enough, in the linear region of the DC / DC conversion circuit, the output steady-state current is independent of the equivalent resistance of the LED module, that is, the DC / DC module has a constant current output. characteristic. Rf generally takes a resistance value of 0.01 ~ 0.10Ω as a precision resistor with good temperature stability as the current detection sampling resistor. As long as Vs (s) is changed, the steady-state operating current of the LED module can be adjusted.

3.1 Frequency response characteristics of the current source of the LED module

Avoiding inrush current is one of the key technologies of laser power supply. The main reasons for generating current surge are: 1) there is a power-on shock in the voltage control DC / DC conversion circuit; Current surge. For the DC / DC conversion circuit, technologies such as electromagnetic compatibility design and power-on soft start can be used to overcome power-on shocks. For the impact of the control system under-damping, it needs to be solved from the aspect of circuit system design. From the perspective of suppressing the current surge, it is desirable that the bandwidth of the DC / DC conversion circuit is as narrow as possible, but from the perspective of the dynamic performance of the current source, the larger the bandwidth is, the faster the dynamic response is; through actual measurement, DC / DC conversion in the linear region The transfer function of the circuit is approximately: H3 (s) = 5 / [1+ (s / ω3)],  (4)

The first-order approximate corner frequency ω3 = 6π × 103 rad / s. In Figure 2, H1 (s) and H2 (s) are composed of low-drift op amps, and their closed-loop transfer functions are:

Ｈ１（ｓ）＝ Ｋ１／［１＋ （ｓ／ω１）］， （５）

Ｈ２（ｓ）＝ Ｋ２／［１＋ （ｓ／ω２）］． （６）

From (4) to (6), the closed-loop transfer function of the drive current source of the LD module is:

H (s) = H1 (s) H2 (s) H3 (s) / [1 + KfH1 (s) H2 (s) H3 (s)].  (7)

For the convenience of analysis, the system design satisfies ω1 <10ω3 and ω2 <10ω3. A third-order system can be used as a second-order approximation, ignoring the influence of ω3, and substituting (4) to (6) into (7) and sorting out:

In the formula, ωn = (ω1 + ω2) / 2, ω2n = 5K1K2ω1ω2, in order to avoid the transient response of the circuit system, the engineering design needs to take the damping coefficient ξ> 0.707, ξ = 1 in the text, and the system works in the over-threshold state. Determine ω1 and ω2 and the DC gains K1 and K2 of the circuit according to formula (8). Take K1K2 = 1000, let ω1 = aω2, and set ω1 = 10, then ω2 = 2 × 105. The transfer function of the drive current source of the LED module is：

3.2 Current stability characteristics of the current source of the LED module

The current relative stability is defined as the ratio of the maximum change of the output current within a specified time to the average value of the current, that is, ΔI / I. Use dI (s) to approximate ΔI. From formula (2), if the forward path circuit parameter changes cause the forward path transfer function to produce dH0 (s) change, the relative change in the drive current source current is: dI (s) / I (s) = dH0 (s) / ｛[1 + KfH0 (s)] H0 (s)｝, (10)  where Kf = Rf / (Rf + RD).

If the laser diode module is affected by temperature and its equivalent resistance causes dRD (dRD = ΔVD / ΔID) instantaneous change, the relative instantaneous current change generated by the laser power supply is: dI '(s) / I (s) = -dRD / [ RD + Rf + RfH0 (s)]. (11)

The coordinates of dH0 = 0 point correspond to the DC gain of H0 (s) H0 = K1K2K3 = 5000, and K3 = 5 is the DC voltage gain of the DC / DC conversion circuit (voltage control current source) (the output voltage of the DC / DC conversion circuit and control Ratio of input voltage value at the terminals), set dH0 / H0 = ± 4%, and when the measured operating current ID of the LD is 4 A, the equivalent DC resistance of the LD is 2.6 Ω, and its dynamic resistance can be obtained from the operating characteristic curve of the LD. dRD is very small, generally less than 0.1 Ω. In the case of a change of ± 0.2 Ω, the relative stability characteristic curve of the LD drive current source obtained according to equations (10) and (11) is shown in Fig. 3, where The absolute value of the ordinate indicates the relative stability of the current. The relative stability of current in actual measurement is better than 0.3%.

For the cases where H1 (s) and H2 (s) are realized by using analog technology design, H1 (s) and H2 (s) are synthesized by analog operational amplifiers with deep negative feedback, which can obtain a very high gain stability, that is, dH0 is small. For the case of using all-digital technology, since H1 (s) and H2 (s) are converted into H (ｚ) and controlled by the program, which is equivalent to dH0 = 0, a higher current can be obtained. Relatively stable (9) Degree.

3.3 Digital Control LD Module Drive Current Source Implementation Technology Based on ADUC831

Although the H1 (s) and H2 (s) system functions obtained by analog operational amplifiers have good stability, they also have the disadvantages of temperature stability, interference resistance, and poor reliability of traditional analog circuits. In Figure 2, the control signal of the DC / DC converter is set to Vc (s). Under the action of the input Vs (s), the transmission function of the control circuit of the drive current source of the LD module is:

Hd (s) = Vc (s) / Vs (s) = H1 (s) H2 (s) / [1+ KH1 (s) H2 (s)] = A / (s2 + Bs + C), (12)

Where A = K1K2ω1ω2, B = ω1 + ω2, C = (1 + K1K2K3Kf) ω1ω2, and K = K3Kf.

Figure (4) shows the digital controller signal flow diagram.

In terms of hardware circuit design, an on-chip ADuC831 single-chip microcomputer with eight sampling rates of 247kS / s12bit ADC and two 12bit + D / A chips is selected as the controller, and the all-digital design is implemented.

、

The ADC0 channel of the one-chip computer is used as the input terminal of the reference voltage V (s). By adjusting V (s), the output current of the fiber-coupled laser current source can be changed, and the output current of the laser power can also be adjusted by the keyboard. If the reference voltage VR of ADC0 is 1.5V, and the current sampling resistor Rf is 0.1Ω, the current setting error is ± 3 mA. Similarly, the current detection error of ADC2 channel is ± 3 mA. For applications that do not need to adjust the working current of the laser, the output current of the current source can also be set directly by programming. The digital method is used to design the current source without the problem of the system function changing with temperature, which can further improve the stability of the current source.

The ADC2 channel of the one-chip computer is used as the current sampling input terminal, and the ADC1 is connected to the PIN photoelectric sensor as the working state detection input terminal of the LD module. DAC0 output control voltage is used to stabilize the output current of DC / DC circuit. DAC1 output control voltage is used as the circuit fault protection control signal. According to Figure 4, the digital control driven by the LD module is realized through programming. Figure 6 shows the physical diagram of the fiber-coupled LED module drive current source and temperature control module. Figure 7 is a sampling waveform diagram of the semiconductor laser power supply current at the moment of system power-up under the environment of -40°C. It can be seen that the semiconductor laser driving circuit has no power-on impact.