Ultra-Compact Laser Diode Driver for the Control of Positioning Laser Units in Industrial Machinery

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ScienceDirect IFAC PapersOnLine 52-25 (2019) 435–440

Ultra-Compact Laser Diode Driver for the Control of Positioning Laser Units in Ultra-Compact Laser Diode Driver for the Control of Positioning Laser Units in Ultra-Compact for the Control of Positioning Laser Units in Industrial Machinery Ultra-Compact Laser Laser Diode Diode Driver Driver for the Control of Positioning Laser Units in Industrial Machinery Industrial Machinery Industrial Machinery

Svetozar Ilchev*, Rumen Andreev**, Zlatoliliya Ilcheva*** Svetozar Ilchev*, Rumen Andreev**, Zlatoliliya Ilcheva*** Svetozar Ilchev*, Rumen Andreev**, Zlatoliliya Ilcheva*** Svetozar Ilchev*, Rumen Andreev**, Zlatoliliya Ilcheva*** *Institute of Information and Communication Technologies, Bulgarian Academy of Sciences, *Institute of Information and Communication Technologies, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria (e-mail: [email protected]) *Institute of Information and Communication Technologies, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria (e-mail: [email protected]) *Institute of Information and Communication Technologies, Bulgarian Academy of Sciences, **,*** Institute of Information and Communication Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria (e-mail:Technologies, [email protected]) **,*** Institute of Information and Communication Technologies, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria (e-mail: [email protected]) 1113 Sofia, Bulgaria (e-mail: {rumen; zlat}@isdip.bas.bg) **,*** Institute of Information Communication Technologies, Bulgarian Academy of Sciences, 1113 Sofia,and Bulgaria (e-mail: {rumen; zlat}@isdip.bas.bg) **,*** Institute of Information and Communication Technologies, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria (e-mail: {rumen; zlat}@isdip.bas.bg) 1113 Sofia, Bulgaria (e-mail: {rumen; zlat}@isdip.bas.bg) Abstract: In this paper, we present the design and implementation of an ultra-compact laser diode driver Abstract: In thisforpaper, present the design of an ultra-compact laser diode driver that is intended use inwe positioning laser unitsand forimplementation industrial machinery and in battery-powered portable Abstract: In thisforpaper, we present the design and implementation of an ultra-compact laser diode driver that is intended use in positioning laser units for industrial machinery and in battery-powered portable Abstract: In this paper, we present the design and implementation of an ultra-compact laser diode driver laserisunits. Thefor driver provides enough current to industrial operate almost all laser diodes in a TO-18 portable housing that intended use in positioning laser units for machinery and in battery-powered laser Thefor driver provides enough to in operate almost all (diameter laser in a In TO-18 housing that isunits. intended use in positioning lasercurrent units for industrial machinery and diodes in battery-powered portable (diameter 5.6mm) and many of the bigger diodes a TO-5 housing 9mm). addition to a laser units.5.6mm) The driver provides enough current to inoperate all (diameter laser diodes in a In TO-18 housing (diameter and many of the bigger diodes aa thermal TO-5almost housing 9mm). addition to a laser units. The current driver provides enough current to operate almost all laserofdiodes in diode a TO-18 housing highly efficient regulation, the driver includes protection the laser via an NTC (diameter 5.6mm) and many of thethe bigger in aa thermal TO-5 housing (diameter 9mm). In addition to a highly efficient regulation, driverdiodes includes protection of laser diode via NTC (diameter 5.6mm) and many of the bigger diodes in TO-5 housing (diameter 9mm). In addition to a thermistor and acurrent current modulation input, which cana thermal be used to control thethe current from an an external highly efficient current regulation, the driver includes protection of the laser diode via an NTC thermistor and acurrent current modulation can be used protection to controlThe thethe current fromresults an an external highly efficient regulation, theinput, driverwhich includes aindustrial thermal of laser diode via NTC microcontroller, e.g. a microcontroller integrated in an machine. experimental show thermistor and ae.g. current modulation input, which can be used machine. to controlThe the experimental current fromresults an external microcontroller, aaccurately microcontroller integrated in an industrial show thermistor andworks a current modulation input, which can belaser useddiodes. to control the current from an external that the driver and reliably with various It can beexperimental integrated successfully in microcontroller, e.g. a microcontroller integrated in an industrial machine. The results show that the driver works accurately and reliably with various laser diodes. It can be integrated successfully in microcontroller, e.g. a microcontroller integrated in an industrial machine. The experimental results show positioning laser units and in portable laser units designed for short-time or continuous operation. that the driver works and reliably withdesigned various laser diodes. It or can be integrated successfully in positioning laser unitsaccurately and in portable laser units for short-time operation. that the driver works accurately and reliably with various laser diodes. It cancontinuous be integrated successfully in positioning laser units and in portable laser units designed for short-time or continuous operation. © 2019, IFAC (International Federation of Automatic Control) Hosting by Elsevier Ltd. All rights reserved. Keywords: laser Embedded systems; Laserlaser diode driver; Solidforstate lasers; or Current regulation; Positioning positioning units and in portable units designed short-time continuous operation. Keywords: Embedded systems; Laser diode driver; Solid state lasers; Current regulation; Positioning laser unit; Portable lasersystems; unit; Industrial machinery. Keywords: Embedded Laser diode driver; Solid state lasers; Current regulation; Positioning laser unit; Portable lasersystems; unit; Industrial machinery. Keywords: Embedded Laser diode driver; Solid state lasers; Current regulation; Positioning laser unit; Portable laser unit; Industrial machinery. laser unit; Portable laser unit; Industrial machinery. being typical for red diodes and higher voltages being typical 1. INTRODUCTION being typicaland for red diodes and diodes. higher voltages for green blue laser Like being many typical other 1. INTRODUCTION beinggreen typicaland for red diodes and diodes. higher voltages being typical for blue laser Like many other 1. INTRODUCTION being typical for red diodes and higher voltages being typical semiconductor elements, laser diodes are susceptible to The development 1.of new laser sources based on for green andelements, blue laser Like many other INTRODUCTION laserisdiodes. diodes are susceptible to for green discharges, and blue which laser diodes. manypractice other The development of new laserdiodes sources based on semiconductor electrostatic why it isLike a common semiconductor technologies – laser – keeps gaining semiconductor elements, laser diodes are susceptible to The development of new laser sources based on electrostatic discharges, is why it is are a common elements, diodes susceptible to semiconductor technologies – lasers laser – keeps gaining to embed a Zener diode which or laser another protection elementpractice in the The development of new laserdiodes sources based on semiconductor momentum. Unlike classical based on crystals (e.g. electrostatic discharges, which is why it is a common practice semiconductor technologies – laser diodes – keeps gaining to embed a Zener diode or another protection element in the electrostatic discharges, which is why it is a common practice momentum. Unlike classical lasers based on crystals (e.g. laser diode. semiconductor technologies laser diodeshave – keeps gaining to embed a Zener diode or another protection element in the ruby), or gases (e.g.classical CO2), –laser considerably momentum. Unlike lasersdiodes based on crystals (e.g. laser diode. embed a Zener diode or another protection element in the ruby), ordimensions gases (e.g.-classical CO2), laser considerably momentum. Unlike lasersdiodes basedhave onhousing crystals (e.g.a to smaller e.g. a TO-18 transistor with laser diode. ruby), or gases (e.g. CO2), laser diodes have considerably Unlike most LEDs and semiconductor diodes, laser diodes laser diode. smaller dimensions e.g. a TO-18 transistor housing with a ruby), or gases (e.g. CO2), laser diodes have considerably 5.6mm - and performance improves most LEDs and semiconductor diodes, diodes smaller diameter dimensions - e.g.their a TO-18 transistor housing rapidly with a Unlike are particularly susceptible to destruction by laser accidentally 5.6mm - and performance Unlike most LEDs and semiconductor diodes, laser diodes smaller dimensions - e.g.their a TO-18 transistor housingtorapidly with with thediameter technological Ratios ofimproves output inputa are particularly susceptible to destruction by accidentally 5.6mm diameter - and progress. their performance improves rapidly Unlike most andofsemiconductor diodes,voltage laser diodes reversing the LEDs polarity their power supply – the with the technological Ratios ofimproves output torapidly input are particularly susceptible to destruction by accidentally 5.6mm diameter -become and progress. their performance power of 1:3 have common occurrence and blue laser reversing the polarity of usually their supply voltage –Like the with the technological progress. Ratios of output to input are particularly susceptible to power destruction by accidentally maximum reverse voltage varies from 2V to 6V. power of 1:3 have become common occurrence and blue laser polarity of usually their power supply voltage –Like the with the technological progress. Ratios output replacing to input reversing the diodes dimensions have ofbegun reverse voltage varies from 2V to 6V. power ofof 1:3 compact have become common occurrence and blue laser maximum reversing the polarity of their power supply voltage – the LEDs, laser diodesvoltage are a usually light source (coherent laser light), diodes compact dimensions have begun replacing maximum reverse varies from 2V to 6V. Like power ofof 1:3 haveinbecome common occurrence and blue laser LEDs, traditional lamps applications such as projector systems. laser diodes are a light source (coherent laser light), diodes of compact dimensions have begun replacing maximum reverse voltage usually varies from 2V to 6V. Like whose intensity depends on thesource current passing through the traditional in applications suchhave as projector LEDs, laser diodes are a on light laser light), diodes oflamps compact dimensions begunsystems. replacing whose intensity depends the current(coherent passing or through the traditional lamps in applications such as projector systems. LEDs, laser diodes are a light source (coherent laser light), diode. Since power supplies (network adapters batteries) There are two main types of semiconductor laser diodes – whose intensity depends on the current passing through the traditional lamps in applications such as projector systems. Since supplies (network adapters or batteries) whose intensity on the current passing through There are twoand main types of semiconductor laser diodes – diode. typically havepower a depends constant or close to constant voltage but the do direct didoes DPSS (Diode-Pumped Solid-State) diodes. diode. Since power supplies (network adapters or batteries) There are two main types of semiconductor laser diodes – typically have a constant or close to constant voltage but do diode. Since power supplies (network adapters or batteries) direct didoes and DPSS (Diode-Pumped Solid-State) diodes. not have any current control functions, additional electronic There are two main types of semiconductor laser diodes – The consist a powerful infrared laser diode combined typically a constant or close to constant voltage but do directlatter didoes and of DPSS (Diode-Pumped Solid-State) diodes. not have have anyare current control functions, additional electronic typically have a needed constant close to constant voltage but do The latter consist of a powerful infrared laser diode combined fororthe current regulation. direct didoes and DPSS (Diode-Pumped Solid-State) diodes. components with a suitable crystal, e.g. YAG (Yttrium Aluminum Garnet) not have any current control functions, additional electronic The latter consist of a powerful infrared laser diode combined components are neededcontrol for thefunctions, current regulation. not have any current additional electronic with a suitable crystal, e.g. YAG (Yttrium Aluminum Garnet) The latter consist of a powerful infrared laser diode combined for green light of 532nm Depending on the laser components are needed for the current regulation. with a suitable crystal, e.g.wavelength. YAG (Yttrium Aluminum Garnet) The simplestare method regulation is by means of a components neededforforcurrent the current regulation. for green light Depending on the laser The with a the suitable crystal, e.g.wavelength. YAG (Yttrium Aluminum Garnet) diode, lightof can532nm be ultraviolet (below 400nm wavelength), simplest method for current regulation is by means for green light of 532nm wavelength. Depending on the laser resistor connected in series between the voltage source of andaa diode, the light can be ultraviolet (below 400nm wavelength), The simplest method for current regulation is by means of for green light of 532nm wavelength. Depending on thegreen laser resistor violet blue (below (450nm wavelength), connected in series between the voltage source and diode, (405nm the lightwavelength), can be ultraviolet 400nm wavelength), The simplest method for current regulation is by means of the laserconnected diode. Thisinmethod is inexpensive, but in source order toand bea violet (405nm blue (below (450nm wavelength), green the resistor series between the voltage diode, thewavelength), lightwavelength), can be ultraviolet 400nm wavelength), (520nm red (638nm wavelength) or infrared laser diode. This method is inexpensive, but in order to be violet (405nm wavelength), blue (450nm wavelength), green resistor connected in series between the voltage source and energy efficient, it ismethod necessary that the supply voltage is be as (520nm wavelength), red optical (638nm wavelength) orthe infrared the laserefficient, diode. This is inexpensive, but in order to violet (405nm wavelength), blue (450nm wavelength), green energy (808nm power emitted by diode itto isthe necessary that the supply voltage isthe as (520nm wavelength). wavelength), The red (638nm wavelength) or infrared the laser diode. This method is inexpensive, but in orderatto be close as possible voltage drop of the laser diode (808nm wavelength). The optical power emitted by the diode energy efficient, it is necessary that the supply voltage is as (520nm wavelength), red (638nm wavelength) or infrared is proportional to the The supply current, which mayby vary asefficient, possibleand voltage drop of the laser diode atisthe the (808nm wavelength). optical power emitted theover diodea close energy itto isthe necessary that constant the supply voltage as desired current that it remains throughout is proportional to the supply current, which may vary over a close as possible to the voltage drop of the laser diode at the (808nm wavelength). The optical power emitted by the diode wide range - from 10mA tocurrent, 5A. For optical powers overa desired current and that it remains constant throughout the close as possible to the voltage drop of the laser diode at the is proportional to the supply which may vary over operation of the laser diode. This is hardly achievable with wide range from tocurrent, 5A. For optical overa operation desired current that it remains throughoutwith the is proportional to the10mA supply which maypowers vary several dozen-- milliwatts, proper cooling in theover form of theand laser diode. This (e.g. isconstant hardly achievable wide range from 10mA to 5A. For measures optical powers over desired current and that itsupplies remains constant throughout the standard industrial power 12V DC or 24V with DC) several dozen milliwatts, proper cooling measures in the form operation of the laser diode. This is hardly achievable wide range from 10mA to 5A. For optical powers over of suitably sized heat sinks, fans,cooling TEC, etc. need to industrial power supplies (e.g. 12V DC or 24V with DC) several dozen milliwatts, proper measures in be the taken form standard operation of the laser diode. This is hardly achievable or batteries, e.g. three lithium-based batteries connected in of suitably sized heat sinks, fans,cooling TEC, etc. need to standard industrial power supplies (e.g.batteries 12V DCconnected or 24V DC) several dozen milliwatts, proper measures in be the form to dissipate the excessive generated in form of taken heat. or batteries, e.g. three lithium-based in of suitably sized heat sinks,energy fans, TEC, etc. need to be taken standard industrial power supplies (e.g. 12V DC ordepending 24V DC) series have a voltage ranging from 8.4V to 12.6V to dissipate the excessive energy generated in form of heat. threeranging lithium-based batteries connected in of suitably sized heat sinks, fans, laser TEC,diodes etc. need toproperties be taken or batteries, From the electrical point ofenergy view, a e.g. voltage todriver 12.6V depending to dissipate the excessive generated inhave form of heat. series or batteries, e.g. three lithium-based batteries connected in on the have charge level. The ideal from laser 8.4V diode should work From the electrical point of view, laser diodesinhave properties series have a voltage ranging from 8.4V to 12.6V depending to dissipate the of excessive energy generated form of heat. on similar to those LEDs and ordinary semiconductor diodes. the charge level. The ideal laser diode driver should work From the electrical point of view, laser diodes have properties series have a voltage ranging from 8.4V to 12.6V depending withtheacharge wide level. rangeThe of power supply voltages that may similar tofor those of LEDs and ordinary diodes. on laser diode voltages driver should From the electrical point of view, laser diodes have properties In order current to flow, mustsemiconductor be a voltage drop on with wide range ofIt ideal power supply that work may similar to those of LEDs andthere ordinary semiconductor diodes. on theacharge level. The ideal laser diodeas driver should work fluctuate during use. should waste little energy as In order for current to flow, there must be a voltage drop on with a wide range of power supply voltages that may similar to whose those of LEDs and ordinary semiconductor diodes. the diode value on both typedrop and the during use. ofIt the should waste as little energy asa In order for current to depends flow, there mustthe be diode a voltage on fluctuate with a wide range power supply voltages that may possible, which reduces temperature emissions and is the diodefor whose on both the typedrop and fluctuate which during reduces use. It the should waste as little energy In order to depends flow, must be diode a voltage on magnitude ofcurrent thevalue current. Forthere laser diodes, voltage dropthe is possible, temperature emissions and is as fluctuate during use. It the should as little energy as the diode whose value depends on both the the diode type and the prerequisite for reduces making driverwaste as small as possible. Aaa magnitude of the current. For laser diodes, the voltage drop is possible, which the temperature emissions and is A the diode whose value depends on both the diode type and the usually in the range of 1.5V to 6.5V with lower voltages prerequisite for making the driver as small as possible. magnitude of the current. For laser diodes, the voltage drop is possible, which reduces the temperature emissions and is a usually in of thetherange of 1.5V to 6.5V with magnitude current. For laser diodes, the lower voltagevoltages drop is prerequisite for making the driver as small as possible. A usually in the range of 1.5V to 6.5V with lower voltages prerequisite for making the driver as small as possible. A usually in the range of 1.5V to 6.5V with lower voltages 2405-8963 © 2019, IFAC (International Federation of Automatic Control) Hosting by Elsevier Ltd. All rights reserved. Peer review under responsibility of International Federation of Automatic Control. 10.1016/j.ifacol.2019.12.577

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compact driver fits where there is no space for large heat sinks or cooling fans and it is also suitable for use in portable units. Ideally, the driver should also have a modulation input, which provides a possibility for control from a microcontroller, and/or a mechanism for protection of the laser diode from reaching temperatures over 40-60°C depending on the specific diode type. In this paper will present an ultra-compact modern solution based on integrated circuits and optimized for precise regulation of the current through the laser diode. It is specifically designed for the control of positioning laser units in industrial machinery. An alternative application is its use in portable laser systems powered by lithium-based batteries, e.g. for use in the education (Savov et al., 2017). The driver has high energy efficiency throughout its voltage and current operating ranges, typically around 90%. It also offers a PWM modulation input and a temperature protection mechanism. The next section of the paper will describe briefly some related work. Then, we will describe the driver in detail on a conceptual and on an implementation level. Next, we will present some experimental results followed by a short summary and some directions for our future work. 2. RELATED WORK A fast laser diode driver for use with laser diode currents in the range of 80 mA is discussed in (Kabel, 2016). In (Thompson and Schlecht, 1997), the authors present a larger driver developed for use with more powerful diodes. It contains a power supply block based on a DC-DC converter and achieves good power efficiency. It can drive laser diodes with currents up to 2A. In (Stylogiannis et al., 2018), the authors discuss a laser diode driver with an innovative modulation mechanism that is used in the creation of a specialized imaging system. In (Fulkerson et al., 2014), a small-sized laser diode driver for the regulation of large currents is discussed. The authors take special care to achieve excellent dissipation of the excessive temperature. A highpower driver for currents up to 150A is presented in (Glaser, 2018a). It uses off-the-shelf FET transistors to deliver power up to 4 kW to the laser emitter. Some improvements of this driver are proposed in (Glaser, 2018b), so that it may power lasers in systems for 3D scanning that work with currents up to 35A. In (Lee and Moore, 2018), the authors use several reference voltages, each with a voltage level of 2.5V, for current regulation in their proposed laser driver. The integration of infrared laser diodes (800-1000nm wavelengths) on a single printed circuit board together with their corresponding drivers is presented in (Canal et al., 2017). In (Tanaka, 2015), the author presents a laser driver composed of a differential amplifier and multiple transistors whose main purpose is to maintain a constant optical output power of the laser diode. In (Ma et al., 2015), the authors illustrate the contemporary trend to miniaturization and integration of different discrete elements of a laser diode driver into a single system-on-chip (SOC), e.g. a microcontroller with flash-based permanent storage, a differential amplifier and a laser driver circuit. In (Barnes et al., 2019), a laser driver for the generation of short pulses is

presented. It consists of several FET transistors and multiple inductors, which enable the emission of a controlled series of very brief optical pulses used for distance measurements. 3. CONCEPTUAL MODEL OF THE LASER DIODE DRIVER On a conceptual level, the laser diode driver consists of several interconnected functional blocks, each of which solves a specific problem. On an implementation level, the functional blocks consist of electronic components and integrated circuits (ICs) soldered on a printed circuit board (PCB). We want the driver to be as compact as possible and on a conceptual level, this means minimizing the number of functional blocks and keeping their structure straightforward. On an implementation level, this means reducing the size of the PCB and the number and size of the electronic components and ICs. The driver must also handle efficiently a relatively wide input voltage range. Last but not least, laser units are built and maintained by humans and it is very useful – both from a technical and an economic point of view – to provide protection against common human mistakes and abnormal conditions as well as some possibilities for human operators to tweak and adjust the driver. The conceptual structure of the laser diode driver is shown in Fig. 1. The driver 100 consists of the following functional blocks: a polarity reversal protection 110, a switching regulator block 120, a current feedback control circuit 130, and a thermal protection and modulation block 140. The polarity reversal protection 110 includes an electronic element for protection against a polarity reversal of the power supply voltage. The electronic element may be either a Schottky diode or a MOSFET transistor. This protection is important because, with most battery-powered devices, the user can very easily put the batteries in reverse by mistake. The switching regulator block 120 and the thermal protection and modulation block 140 have very low tolerance to this kind of voltage reversal. In addition, the laser diode usually cannot tolerate a reverse voltage beyond 6V, which means that both the driver and the laser diode would be damaged beyond repair if such a situation arose.

POLARITY REVERSAL PROTECTION

OPERATIONAL AMPLIFIER BLOCK FOR TEMPERATURE ESTIMATION 141 MODULATION INPUT/ TEMPERATURE STATUS OUTPUT 142

SWITCHING REGULATOR 121 INPUT CAPACITOR 122 BOOT CAPACITOR AND OUTPUT CAPACITOR 123

RESISTOR GROUP FOR CURRENT MEASUREMENT 131 POTENTIOMETER AND RESISTOR FOR THE FEEDBACK LOOP 132

POWER INDUCTOR 124 RESISTOR FOR FREQUENCY SELECTION 125

Fig. 1. Laser diode driver: conceptual structure The switching regulator block 120 consists of a DC-DC switching regulator 121, an input multilayer ceramic capacitor 122, boot and output capacitors 123, a power inductor 124 and a frequency selection resistor 125. The



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switching regulator block is relatively complex and costly but it is indispensable in view of the need to cover a wide input voltage range with high efficiency and to utilize the whole capacity of the battery pack in case of In case of battery powered operation. The power conversion efficiency of the switching regulator remains high throughout the entire voltage range - typically around 90%. The 10% wasted energy make the switching regulator a far superior solution in comparison to a linear regulator or a discrete transistor control for this kind of devices. The more powerful laser diodes need from 5W to 10W of electrical input power PEL to generate 1.5W to 3.5W of optical output power POPT, so the laser diode driver is expected to dissipate from 500mW to 1W of energy in the form of heat. The switching regulator block converts the input battery power to match the laser diode current requirements. We target the use of an industrial power supply between 9V DC and 30V DC or 3-4 lithium batteries that may deliver a total input voltage ranging from 8.4V to 16.8V. The laser diode may have an operating voltage ranging from 1.5V to 6.5V depending on the model and current requirements ranging from several dozen milliamperes up to several amperes. These characteristics of the system result in the need for a step-down (buck) switching regulator 121, which measures and controls the output current that passes through the laser diode. In order to reduce the physical dimensions of the board, we select a switching regulator that is implemented as a specialized IC and contains an embedded FET transistor for current control. The input capacitor is needed even in the case of battery operation as an ultrafast intermediate storage for energy that is passed to the switching regulator at the moment the FET transistor turns on the current through the power inductor 124. The boot and output capacitors are two other capacitors required by the switching regulator. The boot capacitor is used to generate the necessary gate voltage VGS to drive the internal FET transistor if it is of the N-MOSFET type – the usual case because of the easier integration of NMOSFETs into ICs and their better drain-source resistance RDS values. The output capacitor reduces the ripple current through the laser diode. The power inductor 124 functions as the main energy storage. Its current handling capabilities must meet the laser diode requirements while maintaining acceptable operating temperature. Commonly used inductance values range from 3.3µH up to 33µH. Inductors that have larger inductance values provide better current regulation but they can handle smaller maximum currents and become hotter during operation. In addition, we have a choice between unshielded, semi-shielded and fully shielded inductors. The shielded inductors are more expensive and occupy more board space but they minimize the electromagnetic interference (EMI). The inductor choice also depends on the frequency selected by the resistor 125. Higher operating frequencies allow the use of smaller inductors but generally lower the efficiency and increase the temperature of the regulator. The switching regulator 121 needs a current feedback control circuit 130 to maintain the current regulation through the laser diode. This circuit comprises a group of resistors 131 and a voltage divider composed of a potentiometer and a resistor 132.

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The resistor group 131 consists of 3 to 5 resistors with values between 1-10 ohms. The exact number and values of the resistors are different for each use case and depend on the current needed by the laser diode. For currents above 500mA, we use values of 1 ohm and for currents above 1A, we use 5 resistors to ensure the cool operation of the resistor group. The voltage divider 132 is used in the current feedback loop to pass a fraction of the voltage measured by the resistor group 131 to the feedback pin of the switching regulator. The exact value of current through the laser diode depends on the adjustment of the potentiometer by the user. This enables the user to make some changes to the diode current without replacing any electronic components. The thermal protection and modulation block 140 is used to provide the driver with two additional features – the protection of the laser diode from high operating temperatures, which is important in industrial operating conditions and the possibility for controlling the diode current by means of an external device, e.g. a microcontroller (MCU) belonging to an industrial machine. The thermal protection is important for industrial use because the environmental temperature around the laser unit may be significant. In addition, if the laser unit is portable, its size is usually a major impediment to the integration of adequate cooling elements such as large heat sinks, fans, TEC, etc. that are needed for prolonged use of the laser diode. In their absence, the temperature of the laser diode rises with time. Most laser diodes should not be operated at temperatures beyond 55-60°C, so if the temperature reaches 45-50°C, the driver should turn off the current in order to protect the laser diode from thermal destruction. An NTC thermistor mounted in the housing near the laser diode is used to measure the operating temperature. The thermistor accuracy usually falls within the range 3-5%, which is usually sufficient. The operational amplifier block for temperature estimation 141 compares two voltages – one of them is a percentage of the power supply voltage that depends on the temperature and the other one is always equal to half the power supply voltage. The voltages are implemented through the use of voltage dividers. The first voltage divider is connected to the positive operational amplifier input. It is composed of a potentiometer adjustable by the user and the NTC thermistor. The adjustment of the potentiometer determines the temperature value at which the laser diode current is turned off. The second voltage divider is connected to the negative operational amplifier input. It consists of two equal fixedvalue resistors. When the diode temperature rises, the NTC thermistor resistance drops, which leads to a decrease of the voltage at the positive operational amplifier input. When this voltage becomes less than half the power supply voltage, the output of the operational amplifier becomes close to 0V. As the output is connected to the PWM (pulse-width modulation) input of the switching regulator 121, the current through the laser diode is turned off. If the thermal protection feature is used, the functional block 142 represents a binary temperature status output that may be used by an external MCU or an alarm block to alert the user about the high temperature. If the thermal protection feature is not used and if the operational amplifier is not soldered on the printed

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circuit board of the driver, the functional block 142 can be used as a modulation input to switch on or off the laser diode, e.g. by an external MCU. This can be used to implement strobe effects, send an SOS signal or to integrate the laser unit into a larger system.

area equal to 501 mm2. PCBs in this rectangular form are easy to panelize and cheap to produce and assemble both in Europe and in Asia. Due to the large currents, the copper thickness should be at least 35µm (1 oz.) and preferably 70µm (2 oz.).

200 MCU 270 LASER DIODE DRIVER 230

LASER DIODE + THERMISTOR 240

Fig. 2. Integration of the driver into a laser unit The building blocks of a laser unit 200 that uses the proposed laser diode driver 230 are shown in Fig. 2. The laser unit consists of a power supply block 210, wires 250 and 260, a power switch 220, a laser diode with or without an NTC thermistor 240 and an optional MCU 270. The power supply block 210 may consist of a 9V DC to 30V DC voltage adapter or 3-6 lithium-based batteries, e.g. 18650. It is connected through the mechanical switch 220 and the wires 250 to the laser diode driver 230. The laser diode 240 is connected to the driver 230 through the wires 260. If the thermal protection feature is used, a thermistor is mounted next to the laser diode in its copper or aluminum housing and connected to the corresponding terminals of the driver 230. The wires 250 and 260 are preferably stranded wires made of copper. They should be capable of withstanding at least 3A of current, so a cross-section of about 0.75 mm2 or more (AWG 20-22) is recommended. The optional MCU 270 makes use of the functional block 142 (Fig. 1) of the driver, either to observe the temperature status of the laser diode or to control the diode modulation and generate any desired pulse effects. 4. IMPLEMENTATION OF THE DRIVER The physical implementation of the laser diode driver is in the form of a printed circuit board (PCB). Several PCB parameters play an important role in the design – the number of PCB layers, the geometric form and dimensions and the copper thickness. We use a double-sided PCB (two copper layers) with the electronic components mounted on both sides of the board. This choice guarantees reliability, facilitates the prototype debugging and makes us independent of PCB manufacturers. As laser units usually have a cylindrical (tubular) shape (see Fig. 4), there are two suitable geometric forms for the PCB – a circle or a rectangle. A circular board can be mounted parallel to the cross-section of the laser unit. This restricts the diameter of the board and around 20% of the PCB area remains unused as most electronic components have rectangular shape. We chose a rectangular PCB that is mounted perpendicular to the cross-section of the laser unit. As we wanted the board to fit in devices powered by 18650 lithium-based batteries, we restricted the board width to be less than 18mm. After several optimization rounds, the board dimensions were fixed at 28.3x17.7mm resulting in a PCB

Fig. 3. KiCad 3D model and real PCB: front and back side In Fig. 3, the laser diode driver is shown as a 3D model and as a manufactured board populated with all necessary components and ready for use. The driver was created with the open-source system for PCB design KiCad (KiCad, 2019). KiCad supports the generation of the so-called Gerber files that are needed for the manufacturing of the board and the bill of materials (BOM) and the positioning files that are necessary for the board assembly by pick-and-place machines and reflow soldering. The polarity reversal protection is implemented by the diode D3 (e.g. SS34A or SS36A). We have chosen an implementation by means of a Schottky diode instead of a MOSFET transistor due to the lower price and the tolerance of higher voltages, which is important if we decide to use a 24V power supply adapter or more than 4 lithium batteries. The switching regulator U1 is A6211 from Allegro MicroSystems, which supports a power supply voltage reaching up to 48V and provides current regulation up to 3A. Due to the small board size, we use the driver only for currents up to 2.5A to be able to cope with the heat dissipation. We have chosen the version in a SOIC enclosure to facilitate the board assembly process and to make possible the attachment of a small heat sink on the regulator if necessary. The possible downside of the A6211 is its asynchronous buck topology, which requires the use of the Schottky freewheel diode D1. On the one hand, this reduces the converter efficiency, especially for laser diode voltage drops below 3V, but on the other hand, the dissipated heat is divided between the switching regulator and the Schottky diode, which makes it easier to handle. In addition, powerful laser diodes (POPT > 1W) for which the energy efficiency of the driver is critical, usually have voltage drops between 46V, which, for input voltages between 8-17V, makes the efficiency of the asynchronous A6211 similar to the efficiency of its synchronous counterparts. The A6211 switching regulator offers a PWM input, it has a variable working frequency adjustable by a resistor and it has protections from common fault conditions such as an overly large diode current (e.g. as a result from a misconfigured or faulty current feedback control circuit), incorrectly soldered pins or a missing Schottky freewheel diode. These protections make the regulator very robust and easy to work



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with because fault conditions can be detected and corrected without causing damage to the IC. The input capacitor is marked as C3. It must have as low ESR (equivalent series resistance) as possible, so a multilayer ceramic capacitor is chosen (size 1206, e.g. CL31A106KBHNNNE). The boot capacitor is marked as C1 and its recommended value is 100nF (e.g. CL10B104KBNC). The output capacitor is C2. Its recommended value is up to several microfarads (e.g. CL21B105KBFNNNF). The power inductor is marked as L1. We have chosen a shielded inductor to reduce the electromagnetic interference (EMI). It has a value of 15uH (e.g. SDI 104R-150M), which represents a balance between good current regulation and acceptable operating temperature. The resistor for frequency selection is R1 (e.g. 0603SAF0473T50 or CRCW060347K0FKTABC). It determines the frequency of the switching regulator, which can be from several hundred KHz up to 2MHz. We usually target frequencies of about 1MHz to be able to reduce the size of the inductor and maintain good efficiency. Above the switching regulator U1, we have positioned the resistor group for current measurement, in this case five resistors CRCW12061R00FKTABC with a value of 1 ohm. RV1 and R7 constitute the voltage divider for the feedback loop (respectively a potentiometer TC33X-2-502E and a resistor 0603SAF0332T50 or CRCW06033K30FKTABC). The output current of the driver in this configuration can be adjusted from 1A to 2.5A via the potentiometer. The resistors R8 to R12 (e.g. 0603SAF0473T50 or CRCW060347K0FKTABC), the potentiometer RV2 (e.g. TC33X-2-502E) and the operational amplifier (e.g. LM358) located next to RV2 are responsible for the temperature estimation and the protection of the laser diode from high temperatures. RV2 and the thermistor mounted in the housing of the laser diode form the voltage divider adjustable by the user that is connected to the positive input of the operational amplifier. RV2 selects a temperature value ranging from 42°C up to 150°C. R8 can be mounted instead of RV2 if no user adjustment of the temperature value is needed. Resistors R9 and R10 build the second fixed voltage divider connected to the negative input of the operational amplifier. The operational amplifier limits the maximum permissible power supply voltage of the driver to 30V. The connector P2 shows the connector pads for the thermistor (“Th” and “GND”). The “EN” connector pad can be used to check the thermal status or to turn on or off the laser diode by an external MCU. 5. EXPERIMENTAL RESULTS In order to evaluate the driver functionality and performance, we built prototypes of a positioning industrial laser unit (Fig. 4) and two portable laser units. We used power adapters of 12V DC and 24V DC and three 18650 lithium batteries as a power source but we also did tests with two 18650 batteries and four to five 18350 batteries. The positioning laser unit was equipped with a 30mW Sharp GH05035A2G green laser diode, and the portable laser units were equipped respectively with a 1.6W Osram PL TB450B blue laser diode and a 700mW Oclaro HL63193MG red laser diode in order to do a stress test of the board. The operating currents of the diodes

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are respectively 125mA, 1.2A and 820mA, their typical voltage drops are respectively 6.0V, 4.8V and 2.2V. For the Sharp diode, the resistor group for current measurement was changed to five resistors with values of 10 ohms.

Fig. 4. Positioning industrial laser unit During the assembly process and the tests, the A6211 proved to be a very robust switching regulator tolerant to various kinds of abnormal operating conditions, including accidentally shorted pins or pads and random disconnections of the battery power supply. We also tested the polarity reversal protection, which worked well and protected both the driver and the laser diode. The experiments showed that for proper operation the driver needs at least 6.2V of power supply voltage. A 9V DC power adapter proves to be fully sufficient for all diodes. A fully charged lithium-based battery has a voltage of 4.2V and the recommended lowvoltage cutoff is around 2.8V. If the power supply consists of two lithium-based batteries, it has a voltage range from 5.6V to 8.4V, so about 10% of the battery capacity remains unused. If a series of 3-5 lithium-based batteries is used, the whole battery capacity can be utilized. Among the possible combinations of 18650 and 18350 batteries, the largest power storage capacity is achieved by selecting a series of three 18650 batteries. As the power efficiency increases slightly when the difference between the output and the input voltage decreases, this is our recommended battery combination. One of the important working parameters we were interested in was the stability of the current regulation achieved by the driver. We measured the change of the voltage drop at the resistor group for current measurement through the oscilloscope and we observed only small deviations, which were within 1% of the target value. For our applications, this accuracy of the current regulation delivers excellent results. In addition, all three laser diodes are switched on and off without any noticeable overshooting of the current and the generated optical output power remains stable and close to the working parameters of the datasheet – about 34mW for the Sharp diode, 1.64W for the Osram diode and 732mW for the Oclaro diode. The adjustment of the current through the potentiometer RV1 also functions reasonably well. The main shortcoming we observed is due to the small size of the potentiometer, which provides only a single revolution for the selection of the current value. This makes it difficult to set an exact predefined value for the current but a resolution of 35% of the overall current scale is achievable. This is not perfect but it is acceptable and the current value remains stable once it has been set. An improvement of the resolution would require the use of a bigger potentiometer (e.g. one with 25 revolutions) leading to an increase of the PCB size. The operating temperature of the laser diode driver is good for a PCB of this small size. It is negligent at 125mA and at

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1.2A of diode current, the driver temperature is about 48°C. It rises to about 76°C at 2A. For currents over 2A, the attachment of a small heat sink to the A6211 IC is advisable in order to improve its heat dissipation capabilities. The thermal protection of the laser diode functions very well. The measured accuracy was within 3°C. In In our portable laser unit, the operating temperature of the Osram diode rises to about 62°C after about 10 min of continuous operation, which is acceptable as the maximum per datasheet is 85°C. After we set the thermal protection of the diode to 55°C, the current through the laser diode was turned off at about 58°C. We should also take into consideration that the thermistor is mounted near the diode and there is some thermal inertia of the housing, in which the diode is mounted, so the actual accuracy is a bit better than 3°C. The Oclaro diode reaches about 37°C during continuous operation and the Sharp diode – about 35°C, so the thermal protection is not applicable for them. For this reason, we equipped their laser drivers with a modulation input. We achieved a reasonably good square wave output up to about 5 KHz, which is more than enough for most strobe or SOS effects and would even make the driver usable for low quality laser animations. We also tested the laser driver with low-power laser diodes such as the GH05280E2K from Sharp working at 180mA. The current regulation remained within 1%, the operating temperature of the board did not exceed 30°C and the continuous operation remained well within the acceptable parameters. 6. CONCLUSIONS The proposed laser diode driver is designed to be versatile – suitable for different industrial applications and compatible with laser diodes from different manufacturers. Among its strengths are the accurate current regulation, the small size, the laser diode thermal protection, the modulation control input and the possibility for adjustments made by the end user. These features make it possible to integrate the driver in various kinds of devices such as visual guiding systems for large industrial machinery, e.g. for cutting wood or stone, engraving tools, communication equipment, etc. Our future work will encompass the integration of three laser diode drivers of this type into a single compact three-channel RGB driver for use in full color laser animation systems. We will also look into the integration of a small microcontroller into the laser driver to provide network connectivity and to control user-defined laser light sequences such as color gradients, strobe effects or messages in Morse code. ACKNOWLEDGMENT This research was supported by the Bulgarian FNI fund through the project “Conceptual Modeling and Simulation of Internet of Things Ecosystems (KoMEIN). REFERENCES Barnes, E., L. Singer and H. Weinberg (2019). Pulsed laser diode driver. US Patent: US10158211B2, URL: https://patents.google.com/patent/US10158211B2/en. Canal, C., A. Laugustin, A. Kohl and O. Rabot (2017). Disruptive laser diode source for embedded LIDAR

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