Power supply design for erbium glass laser rangefinder

1.Preface

With the development of laser ranging technology, the erbium glass laser ranging technology with a wavelength of 1.54 μm is becoming more and more mature, which has the advantages of human eye safety, strong smoke penetration ability, and long range, and is the key direction of laser ranging technology development［^1－5］. The pulsed constant-current drive power supply is the core component of the erbium glass laser rangefinder, and its performance directly affects the output laser energy, peak power, beam quality, and product life［^6－9］. Currently, with the further increase in the integration of optoelectronic systems, the drive power supply needs to meet the technical specifications while facing increasingly stringent requirements in terms of size, weight, and protection functions. For this reason, it is important to design a pulsed constant-current driver power supply with small size, light weight, and multiple protection functions for erbium glass laser rangefinders.

In recent years, research on laser pulsed constant-current drive power supplies has made some achievements: a high-power semiconductor laser pulsed power supply［¹⁰］was designed using a combination of a front-stage LCC charging unit and a back-stage pulse current generation unit, with a pulse current of 80 A, a pulse voltage of 320 V, a pulse width of 200 μs, and a pulse signal frequency of 100 Hz. The LCC resonant circuit used has soft switching characteristics and resistance to load short-circuit and open-circuit, and low electromagnetic radiation. The LCC resonant circuit with soft switching characteristics and resistance to load short circuit, open circuit, low electromagnetic radiation, but its control circuit is complex, the circuit size is large for high-voltage output, and the drive power supply is not considered for the design of the protection circuit. Another fiber laser drive power supply with two modes of operation, continuous drive and pulse drive［¹¹］, pulse drive mode, the output pulse current of 10 A, pulse frequency up to 100 kHz, pulse rise/fall time of less than 50 ns. where the power conversion circuit uses a half-bridge topology, high output power, anti-unbalance ability; pulse output circuit using modulated bus voltage generation The pulse output circuit uses modulated bus voltage to generate pulses, and the circuit structure is simple. However, the half-bridge topology circuit is large in size and weight, which is mostly used in medium and high power applications, and the modulated bus voltage pulse generation method has poor current stability, and the protection circuit design is not considered in the drive power supply.

2 .Overall solution design

According to the actual use of demand, the specific indicators of the drive power supply are as follows: drive current 12 A ~ 14 A, drive voltage adaptive (≤ 2. 2 V), pulse width 2. 3 ms ~ 2. 5 ms, repetition frequency ≤ 10 Hz.

There are three main technical solutions for the common pulsed constant-current drive power supply for lasers: (1) avalanche tube amplifier circuit; (2) constant-current source circuit with high-speed switch; (3) energy storage network with switching circuit［^12－13］. The pulse current generated by the avalanche amplifier circuit can be up to the ns level, but the leading edge, pulse width, frequency, and current magnitude of the pulse current signal are limited by the parameter characteristics of the avalanche transistor itself, which cannot be adjusted arbitrarily［¹⁴］. The constant current source circuit with high-speed switches can achieve lower current ripple and higher output current stability, but its circuit is relatively complex and difficult to implement. Energy storage network with the switch circuit to achieve the pulse constant current source pulse current up to several 10 A or higher, and the circuit structure is simple, high efficiency power supply.

Comparing the above three technical solutions, the drive power supply adopts the energy storage network with the switching circuit, and the system block diagram of the drive power supply is shown in Figure 1.

As can be seen from the figure, the drive power supply consists of a charging circuit, an energy storage network, a discharge circuit, a signal level conversion unit and a protection circuit. The charging circuit converts the DC power supply into the charging voltage at both ends of the energy storage network to realize the energy transfer from the DC power supply to the energy storage network. The energy storage network is a large-capacity capacitor or capacitor bank, which accumulates energy for high-power pulse discharge. The laser load is connected in series between the energy storage network and the discharge circuit. The discharge circuit determines the characteristics of the pulse drive current flowing through the laser load based on the input control signal. To improve the immunity to interference, a differential signal of RS-422 standard is used for the external trigger signal. The signal level conversion unit is used to convert the differential external trigger signal to a single-ended external trigger signal. The protection circuit includes protection circuit 1 and protection circuit 2. protection circuit 1 is used to provide anti-static and surge voltage protection for the energy storage network and the laser load. Protection circuit 2 is used to provide output overcurrent protection for the laser load. The discharge circuit includes a discharge switch circuit, a current sampling circuit and a feedback control circuit. The discharge switch circuit is used to control the on/off and magnitude of the drive current of the laser load, the current sampling circuit is used to provide real-time feedback on the drive current of the laser load, and the feedback control circuit generates the drive signal of the discharge switch circuit based on the feedback signal and the reference signal.

Figure 1 Block diagram of drive power system

3. Detailed circuit design

3. 1 Energy storage network selection

The energy storage network is composed of energy storage capacitors, and the selection of energy storage capacitors should satisfy the following relations:

U_CHARGE －U_Cres－ΔU≥U_Laser    ( 1)

In the equation ( 1), U_CHARGE indicates the charging voltage at both ends of the energy storage capacitor, UCres indicates the voltage drop across the equivalent series resistance (ESR) of the energy storage capacitor during the discharge process, ΔU indicates the voltage drop across the energy storage capacitor due to energy release during the discharge process, and U_Laser indicates the voltage across the laser load during the discharge process, which is not greater than 2. 2 V.

The main parameters of the energy storage network are rated voltage 35 V, class voltage 20 V, nominal capacity 4 000 μF and ESR 0. 2 Ω. The following analysis calculates whether the energy storage capacitor meets the design requirements, and the pulse drive current is calculated based on a maximum pulse width of 2. 5 ms and a maximum current of 14 A. During the discharge process, the voltage drop on ESR U_Cres = 14 A × 0. 2 Ω = 2. 8 V, and the voltage drop generated by energy release

In equation ( 2), ΔQ represents the charge change at both ends of the capacitor, C represents the nominal capacitance of the capacitor, I represents the pulse drive current, and Δt represents the pulse drive current pulse width. The calculation shows ΔU = 8. 75 V. Equation ( 1) can be transformed to

The energy storage capacitor derating voltage 20 V > 13. 75 V, so the energy storage network is selected to meet the design requirements.

3. 2 Charging Circuit

The charging circuit provides energy to the energy storage network in the form of charging voltage. In order to improve the efficiency of the power supply and reduce the size and weight, the charging circuit uses a synchronous rectifier type BUCK switching converter structure. Because the dynamic resistance of the laser is very small, small output voltage fluctuations will lead to large changes in the drive current. At this time, if the feedback control loop response speed is slow, the laser will be subjected to a longer period of high current, may be damaged.

To reduce the influence of switching noise on the driving current in the charging circuit, the charging and discharging of the energy storage network are operated in a time-sharing manner. When there is no pulse drive current, the charging circuit charges the energy storage network; when the pulse drive current is output, the charging circuit stops working and relies only on the energy storage network to provide the discharge energy. The maximum pulse width of the pulse-driven current is 2.5 ms, and the repetition frequency is 10 Hz. The charging circuit can operate 97.5 ms between discharges, and the duty cycle D is 90%, taking 90 ms as the margin.

The output power requirement of the charging circuit is calculated as follows: From Section 3.1, the charging voltage U_CHARGE at both ends of the energy storage network should be greater than 13. 75 V. The maximum power output of the charging circuit is taken here as 15 V.

In the equation ( 4),I_MAX indicates the maximum pulse drive current,Δt_MAX indicates the maximum pulse width of pulse drive current, and f indicates the pulse drive current repetition frequency. The calculation shows that P_MAX = 5. 83W, corresponding to the maximum output current of charging circuit I_{CHARGE－OUT－MAX} =P_MAX /( 15V×90%) = 0. 432 A.

The detailed circuit of the charging circuit is shown in Figure 2.

Figure 2 Detailed circuit of charging circuit

The LT8609 series from ADI is used as the main control chip, with integrated upper and lower arm switch tubes, simple peripheral circuits, a package size of only 3 mm × 3 mm, a maximum output voltage of more than 30 V, a maximum continuous output current of 2 A, and a power supply efficiency of more than 90%, and the function of charging and discharging the energy storage network can be realized through its enable pin 11. It is well suited to meet the design requirements.

3. 3 Discharge circuit

The discharge circuit includes three parts: discharge switch circuit, current sampling circuit and feedback control circuit, whose basic components are shown in Figure 3.

In Figure 3, C1 is the energy storage network, E1 is the laser load, Q1 is the discharge switch tube, R1 is the gate resistor of the discharge switch tube, R2 is the current sampling resistor, K1 is the error amplifier,U_CHARGE is the charging voltage at both ends of the energy storage network, USET is the drive current setting signal, and U_TRIG is the pulse trigger signal.

The discharge switch tube is the key device of the drive power supply, which has an important impact on the overall reliability, safety and service life of the drive power supply. The following is a detailed discussion of the reasonableness of the discharge switch tube selection. The selected discharge switch is an N-channel field effect transistor (hereinafter referred to as “NMOS tube”), whose model number is IRF3205, and the main parameters are as follows:V_DSS = 55 V，I_{D_MAX} = 110 A，P_{D_MAX} = 200 W。

When there is no pulse drive current, the NMOS tube is turned off and the drain source voltage UDS =U_CHARGE = 15 V＜V_DSS，which meets the requirements. When the output pulse drives the circuit, the NMOS tube is under the strongest electrical stress at the moment of conduction, and the voltage at both ends of the energy storage network has not started to discharge, i.e. ΔU = 0 V, and the drain-source voltage U_DS =U_CHARGE －U_Cres －U_Laser = 10 V＜V_DSS，The maximum drain current I_D = 14 A＜ I_{D_MAX}，at which point the NMOS tube power consumption P_D =U_DS×I_D = 140 W＜ P_{D_MAX}。According to the IRF3205 datasheet, its safe operating area is shown in Figure 4. According to the IRF3205 datasheet, the safe operating area is shown in Figure 4. For the on-time pulsed operating condition of 2. 5 ms, the ID is greater than 20 A for VDS of 10 V, and the V_DS is greater than 15 V for ID of 14 A. It can be seen that the IRF3205 is operating in the safe operating area with a certain safety margin.

In summary, the discharge switching tube IRF3205 can meet the actual needs of the drive power supply and has a certain safety margin.

3. 4 Protection Circuit

When the positive and negative electrodes of the erbium glass laser are subjected to transient surge voltage or static electricity, the optical power generated by the forward overcurrent will damage the solution surface, which will lead to laser failure［¹⁵］. In addition, laser current injection above a certain limit will lead to laser light-emitting surface damage or laser temperature overburning［^16－17］. In order to save the erbium glass laser from damage, the corresponding protection circuit is designed. Protection circuit 1 provides antistatic and surge voltage protection for the energy storage network and the laser load, and protection circuit 2 provides overcurrent protection for the laser load.

Figure 4 IRF3205 Safe Work Area

The detailed circuit of protection circuit 1 is shown in Figure 5.

Figure 5 Protection circuit 1 Detailed circuit diagram

In Figure 5, C1 is the energy storage network, E1 is the laser load, R3 is a transient suppression diode (TVS tube) of type SMAJ26A-Q from BOURNS, R4 is a voltage regulator diode of type MMSZ5225-G from VISHAY, and C2 is a filter capacitor of 10 μF/35 V.

The main circuit of protection circuit 2 is shown in Figure 6.

Figure 6 Protection circuit 2 Main circuit

In Figure 6, K3 is a voltage comparator, model LMV331, and the entire circuit forms a hysteresis voltage comparator. When the pulse drive current setting signal voltage U_{SET_IN} changes from low to high and is lower than the rising threshold U_{TH_Ｒ} of the hysteresis voltage comparator, the output logic signal U_{OUT_LOGIC} is high, and when it is higher than U_{TH_Ｒ}, the output logic signal U_{OUT_LOGIC} is low; when U_{SET_IN} changes from high to low and is higher than the falling threshold U_{TH_F} of the hysteresis voltage comparator, the output logic signal U_{OUT_LOGIC} is low, and when it is lower than U_{TH_F}, the output logic signal U_{OUT_LOGIC} is low. When U_{SET_IN} changes from high to low and is higher than the falling threshold of the hysteresis voltage comparator U_{TH_F}, the output logic signal U_{OUT_LOGIC} is low, and when it is lower than U_{TH_F}, the output logic signal U_{OUT_LOGIC} is high. In the circuit design, U_{SET_IN} is approximately 10 times related to the pulse drive current, so hereU_{TH_Ｒ}= 1. 5 V，U_{TH_F} = 1. 4 V.

The external trigger signal of the drive power supply contains information on the frequency and pulse width of the pulse drive current. In order to improve the anti-interference ability and transmission distance of the external trigger signal, the differential form of RS-422 level standard is used. The signal level conversion unit is used to convert the differential external trigger signal into a single-ended external trigger signal that can be recognized by the feedback control circuit, and its detailed circuit is shown in Figure 7. Among them, K4 is the level conversion master chip, the model is ADI ADM2587E.

Figure 7 Signal level conversion unit

4. Experiment and analysis of results

4.1 Specification Test

In order to test the technical specifications of the pulsed constant-current drive power supply, a test setup was built as shown in Figure 8.

Figure 8 Test setup

The laser load is an ERDI LASER LTD 200μJ Erbium glass laser. The level converter board converts the single-ended external trigger signal generated by the signal generator into a differential signal of RS422 standard.

In the test, the signal generator sets the external trigger signal pulse width to 2. 5 ms, the repetition frequency to 10 Hz, the DC power supply side sets the drive current to set the signal voltage to 1. 4 V (corresponding to the drive current of 14 A), set the supply voltage to +28 V. The pulse voltage waveform seen on the oscilloscope is shown in Figure 9.

Fig. 9 Pulse voltage waveform (right figure is enlarged)

The pulse voltage is 1.419 V (corresponding to a pulse current of 14.19 A), the pulse width is 2.5 ms, the rise time is 38.9 μs, the fall time is 38.5 μs, the waveform stability is good, there is no overshoot, and the repetition frequency is 10 Hz. The drive power supply has been tested to operate continuously for more than 30 min without failure, and is highly reliable.

4. 2 Comparison of indicators

To ensure objectivity, the pulsed constant-current drive power supplies configured in the two official Erbium glass laser rangefinder products were selected here for comparison tests, which are referred to as Product A and Product B for ease of description.

Comparison of the indicators of the three products

Comparison Items	Drive power	Product A	Product B
Size/mm	60×18×8. 5	65×35×20	60×50×8
Weight/g	11. 8	53. 4	15. 2
Protection function	There are	None	There are
Pulse overshoot	No	Yes	Yes
Auxiliary heat dissipation	No	Yes	No

In terms of size, weight and pulse current overshoot, this design is better than Product A and Product B. In terms of protection and auxiliary heat dissipation, this design and Product B are better than Product A. Overall, this design is the best drive power supply.

4. 3 Summary of the experiment

The above test shows that the drive power supply can be well applied to the erbium glass laser rangefinder because of its small size, light weight and full protection function while meeting the electrical indexes. At present, the driving power supply has been used in the erbium glass laser rangefinder to complete a number of tests, and the range is more than 30km, which has good application value.

5. concluding remarks

The overall scheme and detailed circuit design were completed by comparing three pulsed constant-current drive schemes and establishing the technical path of energy storage network with switching circuit for the Erbium glass laser rangefinder. The experimental results show that the drive power supply can output pulse current with pulse width from 2.3 ms to 2.5 ms, size from 12 A to 14 A, and repetition frequency from 10 Hz, with good current stability and no overshoot, and has the advantages of high reliability, small size, light weight and full protection function, which can meet the system requirements.

To further improve integration, the erbium glass laser rangefinder was designed to integrate the drive power supply and communication control with the range solver circuit board.