Laser PCB Milling Machine

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Laser PCB Milling Machine

Group 18

Nathan Bodnar

David Dowdle

Ryan Maticka

Spring 2010

May 3, 2010

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7.1 – Previous Design... 25 7.1.1 – Rectification ... 25 7.1.2 – DC to DC Converter ... 26 7.1.3 – Switching ... 26 7.1.4 – LC filter ... 28 7.1.5 – Controller ... 30 7.2 – Current Design ... 31 7.2.1 – DC to DC Converter ... 31 7.2.2 – Switching ... 32 7.2.3 – Controller ... 33 7.3 – Prototype/Test ... 33 7.4 – Build ... 34 7.5 – Evaluation Plan ... 34

SECTION 8: TEC Power Supply Unit ... 35

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8.1.1 – Rectification and DC to DC Conversion ... 35 8.1.2 – Cooling Control ... 35 8.2 – Prototype/Test ... 36 8.3 – Build ... 36 8.4 – Evaluation Plan ... 37 SECTION 9: Q Switch... 38 9.1 – Types of Q Switches ... 38 9.2 – Previous Design... 40 9.2.1 – Q Switch High Voltage Power Supply ... 41 9.2.2 – Q switch Fast FETs ... 47 9.3 – Current Design ... 49 9.4 – Prototype/Test ... 49 9.5 – Build ... 51 9.6 – Evaluation Plan ... 51

SECTION 10: Motors and Motor Controller Unit ... 52

10.1 – Design ... 52 10.2 – Prototyping and Testing ... 56 10.3 – Building ... 56 10.4 – Evaluation Plan ... 56 SECTION 11: XY Table ... 58 11.1 – Design ... 58 11.2 – Prototype/Test ... 60 11.3 – Evaluation Plan ... 61 SECTION 12: Microcontroller ... 62 12.1 – Design ... 62 12.2 – Prototype/Test ... 66 12.3 – Build ... 68 SECTION 13: Software ... 69 13.1 – Design ... 69 13.2 – Prototype/Test ... 91 13.3 – Evaluation Plan ... 93 SECTION 14: Security ... 95 SECTION 15: Personnel ... 98 15.1 – Nathan Bodnar ... 98

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15.2 – David Dowdle ... 98

15.3 – Ryan Maticka ... 98

SECTION 16: Bibliography ... 99

SECTION 17: Facilities and Consultants ... 102

SECTION 19: Budget and Financing ... 103

SECTION 20: Milestone ... 104

SECTION 21: Project Summary ... 105

APPENDIX A: Copyright Notices ... 108

A.1 – jPicUsb License ... 108

A.2 – gerb2tiff License ... 108

A.3 – ImageMagick License ... 114

APPENDIX B: Example DRL File... 116

APPENDIX C: Password Generation Code ... 117

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SECTION 1: Executive Summary

This goal of this project was to replace the current milling machine used by senior design students. The main reasons that motivated us for replacing the current milling machine are the cost factor for the replacement of the milling bits and its safety. These cost factors, along with the amount of user errors need to be improved. Therefore, this project utilizes a new design using a high powered Nd:YAG laser. The laser light is used to mill the printed circuit boards allowing for tool-less machining. This reduced the cost load of maintaining a rapid printed circuit board prototyping machine. This process will also avoid some of the safety issues involved with the old milling machine, which included having fiberglass debris flying everywhere and the openness of the rotary blades cutting the copper clad boards.

This project consists of building the high powered Q Switched second harmonic Nd:YAG laser and the control units to power it. The project also includes the designing and building of a XY table to move the laser light to the target copper substrate. The designs for this venture also incorporate safety throughout the electronics. This will prevent the users from causing damage to the machine and to protect the user. The software is engineered to allow the user to engrave away the copper using either a Gerber file or a bitmap Portable Network Graphics (PNG) file. This will allow the senior design student to have the opportunity to utilize any design software that they choose and be able to mill their circuit board. This is something that the current milling machine cannot do. The framing of the machine incorporates a safety viewing window that will allow the senior design student to view their work as the laser mills the board out.

The cost of the parts for this project has been funded by the group. Attempts were made to try to acquire donations for the parts required to build this machine. The budget for this adventure was around three to eight thousand dollars. The budget was estimated from what the parts would cost if they were bought new from the manufacture. The team worked to acquire the materials through used and surpluses stores and through the online provider of eBay. These materials greatly reduced the cost of this project.

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SECTION 2: Motivation

The motivation for this laser PCB etcher is the constant down time of the TTech Quick Circuit 5000 PCB milling machine in the senior design lab. As two of our group are ARC members, and are tasked with keeping the machine repaired and running, we have found this to be an increasingly difficult task. In just the last year we have had to replace the lead screw, and are currently in the process of replacing the main spindle motor. These problems resulted from two areas: ignorance and fatigue. The lead screw only needed to be replaced because someone thought it would be a good idea to oil it. As the very large stickers warn, lead screws are not meant to be oiled. If the plastic shavings that were everywhere was not an indication that something was wrong, the horrible screeching noise would have. This case of ignorance on the part of a senior design student caused several months of down time as we needed to first search for the part, find funding for the part, and have that part replaced. The spindle motor has simply worn out because of fatigue. These things happen, but this particular part is very expensive and has been particularly difficult to find funding for. Our project intends to take care of both of these problems. It will be left with the amateur radio club when we are finished to be used by future senior design students. Ignorance has been solved through security and training. The current milling machine is in a locked room, inside another locked room, but this was not enough to keep it from being damaged. Our project will take this further in that our project is completely contained and monitored. There is no interaction with the machine without first having enrolled in senior design, and being certified by an ARC member. The certification will only come at the end of an intensive training session that we will develop as part of the project. At this time the user is granted a username and password that will determine their level of access to the machine. We are taking a hierarchical approach to security in this regard. No one will have access to a part of the machine that they do not need access to, and have not been trained in the proper use and maintenance of. For the standard senior design student this will equate to being able to open the main lid, and nothing else. The compartments storing the lasers, most of the optics and all of the electronics is sealed. Fatigue is solved through less moving parts. In fact, the only moving part of our project is the XY table to do the etching. The XY table, unlike the current system, is driven by stepper motors attached to a belt system. The motors were purchased from an online retailer for a reasonable price. In this project, the main thing that can break is the laser parts. Spare parts were purchased that will be required to keep our project in operation for many years.

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SECTION 3: Technical Objectives and Goals

Our goals for this project were that the finished machine is safe, reliable, and timely. This was accomplished through the extensive use of sensors for everything from an overheated diode to a user trying to force the machine open at the wrong time. Mediation schemes are in place for as many scenarios that we can think of. When higher level reasoning is required, the information is passed off to the computer based software portion of our project and the decision relayed. This is only used for the less immediate concerns, however. All of the lower level decisions are computed on chip in the project itself. Any part that goes out of specification or if a sensor requires immediate attention, the entire project will enter a programmed shutdown that will attempt to reduce the negative effects on the hardware. This is the default case if an unexpected error is reached. We attempted to meet the goal of reliability through the use of high quality components and through the reduction of moving parts. The use of a laser will further meet the goals of reliability and timeliness. By the very nature of our design, we do not have any need to change bits. This allows us to reach our reliability goal in that very few people will be able to access the laser portion of the project. As with any system, the less people that have access to it the less likely someone is going to try and tamper with it. In the current setup, everyone has access to the bits and often load them wrong, or chose the wrong one for the current job. While it is true that our project did take a little over double the amount of time as the current machine to remove all the copper from a standard sized board, we have the advantage of not requiring a user’s constant input. It will be entirely possible for a user to start the etch process in the morning, go to all of his or her classes and come back at a later time to find one entire side of their board finished. All they will need to do is flip the board over at the appropriate time. The function of this project is to replace the current milling machine used by senior design students. This is accomplished through the use of a focused high powered green laser. Furthermore, our project can not only be able to etch the copper off of the printed circuit boards, but can also cut the fiber glass substrate of the boards. This avoids the safety issues of using a shear or rotary saw to cut prototypes out of the master board and allow holes for through hole components to be made during the main etch process.

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SECTION 4: Specifications and Requirements

The specifications outlined below were defined in large part to satisfy the desires of our sponsor. The milling machine has features appropriate for use in an undergraduate setting for senior design students.

The software of this project is quite significant. It is able to handle communicating with microcontrollers, large input files, and running on a wide variety of hardware. It is also be responsible for providing users and technicians with an interface to work with or on the machine.

• Software

o XY Table

Allow positioning of board Import images

Import Gerber Store previous boards

Remember previous board cuts Checks safety measures

o GUI for Laser

Standard Users (Senior Design Students) • Allows the milling of FR-4 boards only • Allow only two boards to be milled • Cannot add boards

• Must follow all safety measures • Cannot add any user

Advanced Users (non-Senior Design Users)

• Users may have privileges from some but not all of advanced users settings

• Can be created from Administrators or Experienced Users

• Cannot add any users Experienced Users (Arc Trainers)

• Allow the addition of boards

• Allow the modification of unique sized boards • Allow control of laser power

• Allow the selection of different PCB materials • Must follow all safety measures

• Can add Standard and Advanced Users • Can remove any Standard or Advanced User

• Can change user access to all but Administrator and Experienced

Administrator (Laser Experienced Service Technicians) • All safety measures can be over written

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• Can create all users

• Can remove a user or change access level

The XY table is the only main moving part of this project. This table is responsible for moving the mirrors and lenses around, collectively called the milling head, and is able to reach the entire board.

• XY Table

o Belt driven

o Stepper motors with encoders for positions

o Capable of a maximum board size of 12 in by 12 in

The laser is the heart of this project. Copper is removed from the printed circuit boards through the power of this laser and the supporting structures alone. Therefore, a laser is used that is powerful, relatively cheap, and easy to find to allow for backups to be reasonably attainable.

• Laser

o 5 W @ 532 nm o 15 W @ 1064 nm o Q-switched

o Solid State Diode 808 nm, 40 W o Nd:YAG

o KTP (KD*P) • Optics

o Allow beam waste to be 1 mil o Allow Rz to be 0.25 in

o Directing mirrors to be 532 nm HR 45º, 1064 nm HR 45º o Automatic focusing

o Automatic board positioning and aligning o Automatic removal checker

o Pixel burning @ 1 kHz o 1 to 2 mm pixel size • Laser Power Supply

o Input: 120 V AC, 60 Hz o Output: 730 V DC o Current controlled o Monitoring current o Monitoring laser output • Q-switch Power Supply

o Pockel Cell 2.5 kV DC -2.5 kV DC

Pulsed with microcontroller • Stepper Motor Power Supply

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o H-bridge configuration • Power Management Circuit

o Turn on specific devices o Controlled by microcontroller o Emergency power shutdown

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SECTION 5: Research and Investigations

5.1 – Burn Test

Estimating the amount of power required to generate the plasma required to vaporize the copper substrate of a printed circuit board will help in designing the laser for this project. An equation was created to estimate the amount of energy to vaporize the copper substrate. This was created by calculating the volume of the copper target. The next step would be to multiply this value with the density. This value was then multiplied by the molar mass and the heat of vaporization constant for copper. This value was then used to estimate the total energy required to vaporize the copper. This value was then multiplied with the absorption spectrum and the graph below shows the amount of energy to vaporize the copper substrate depending on the wavelength (Figure 5.1).

Energy to Vaporize Copper

0 5 10 15 20 25 30 35 248 448 648 848 1048 W a ve le ngth (nm) Po w e r ( m J )

Figure 5.1: Graph of energy required to vaporize copper for given wavelength

A testing laser was used to verify the graph. The laser used was a second harmonic Nd:YAG laser. The laser was flash tube based with a pulse duration of around 10 ns. The pulse energy ranged from 0.7 mJ to around 2 mJ. The laser was the YG580 series Active Q switched Nd:YAG laser from Quantel. The pulse rate was 10 pulses per second.

The target that was used was a printed circuit board that was mounted in a stage to allow minimal movement. This would allow multiple shots to hit the same location. The focusing lens that was used was a convex lens with a focal point of around 100 millimeters. The laser diameter from leaving the laser cavity was about one centimeter in diameter. The target was placed at the focal point and

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the laser the laser pulses were counted until the laser light exited through the copper target. The number of pulses that were required to vaporize the copper target was about 40 pulses for the 0.7 millijoules and around 30 for the two millijoules. The following photo is the profile of the pulses of the laser hitting the copper target. The profile goes form a single pulse at the right of the image to five pulses, then to ten pulses and continuing this iteration to up to 25 pulses (Figure 5.2).

Figure 5.2: Burn testing with second harmonics of Nd:YAG

Looking at a close view of one of the shots, it seems that the laser pulse generated copper whiskers. The width of one of the widest whisker measured to be around one micron. The comparison of this size to the size of the hole is around 25 times its thickest width. This would be apparent that these whiskers should not be a problem for the milling process of the printed circuit boards. The following image is the crater that was generated from having the laser pulse hitting the printed circuit board (Figure 5.3). It can also be seen that for one of the firings that there was a misalignment and another pulse was shot below the original spot.

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5.2 – Laser Safety

The laser system that this project will employ is a class four laser. When the laser is enclosed in a sealed environment that has interlocks and other preventive measures, the laser class will drop to a class one. When the laser is being serviced the laser will change its class back to a four. This is what is

characterized by the Federal Laser Product Performance Standard and the ANSI Z136.1. There are numerous signs that will need to be placed along the project. The first sign will indicate that the project is a Class one laser. This is a yellow and black caution sign. The following image will show the layout of this sign (Figure 5.4).

CAUTION

LASER RADIATION DO NOT STARE INTO BEAM

CLASS 1 LASER Enclosed Nd:YAG, 532nm, 10mJ, 40ns

!

CAUTION

CLASS 1 LASER Enclosed Nd:YAG, 532nm, 10mJ, 40ns

!!

Figure 5.4: Laser Safety Label for exterior case of project

The next sign that is placed is in the material processing area. This will indicate that a class 2A laser is in progress. This is because when aligning up the printed circuit board a low powered 730 nanometer diode laser is present. This will help in indicating the header position. The following image will show the sign that is placed in the material processing area (Figure 5.5).

CAUTION

CLASS 2 LASER LASER DIODE, 730nm, 100mW

!

CAUTION

CLASS 2 LASER LASER DIODE, 730nm, 100mW

!!

Figure 5.5: Laser Safety Label for Material Processing area

The next signs that will need to be placed in the project are in the high powered laser section. Within this section the laser class is a class four. This section will

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need to have a sign that indicates the laser wavelength output and the power output. There will also need to be a laser sign for the output of the oscillator. The following images are the signs for this section of the laser (Figure 5.6, 5.7).

DANGER

LASER RADIATION WEAR PROTECTIVE EYEWEAR

CLASS 4 LASER Nd:YAG, 532nm, 10mJ, 40ns LASER DIODE, 808nm, 80W

!

DANGER

LASER RADIATION WEAR PROTECTIVE EYEWEAR

CLASS 4 LASER Nd:YAG, 532nm, 10mJ, 40ns LASER DIODE, 808nm, 80W

!!

Figure 5.6: Laser Safety Label for High Power Laser

Figure 5.7: Label at exit of High Powered Laser

There will have to be safety precautions that will need to be followed as given by the ANSI Z136.1. These safety precautions are necessary to allow the high powered laser to be run as a class one. The following procedures are to run the system as a class one laser. There has to be a protective housing, interlocks on the protective housing, a service access panel, and equipment labels. To run the laser as a class four, the following control measures must be followed. A

protective housing, interlocks, service access panel, key control, removable interlock connector, beam stop or attenuator, Activation warning system, laser controlled area, equipment labels, and laser area warning signs.

One of the precautions in working with the high powered laser is to wear laser safety glasses. The rating that the glasses that need to be worn are the following ratings that were generated from the ANSI Z136.1 Standard. The OD rating is five OD for 532 nanometers, five OD for 1064 nanometers and greater than four optical densities for 808 nanometers.

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Since there is high voltage for the Q switch there will have to be warning signs for the high voltage terminal of the Q switch and the power supply that drives the Q switch. The warning sign is as followed (Figure 5.8).

HIGH

VOLTAGE

DANGER

HIGH

VOLTAGE

DANGER

Figure 5.8: Warning Sign for the High Voltages need for the Q switch 5.3 – Air Scrubbing

Since the process in generating vaporized copper plasma creates particulates that can be considered as a hazard to the health of the user, the vapors from this process must be processed in a manor that will protect the user. There are two types of scrubbing schemes used to purify the pollutants from the system. The fist type of system is illustrated below; this is a wet scrubber (Figure 5.9).

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This process works by injecting the contaminated air with small water droplets. The droplets generate a static charge and pull debris out of the air. The biggest problem with this type of system is that the partials that can be collected range for 20 microns down to two microns [1]. This process will also generate a lot of waste water and give off a great deal of moisture. This process would work great for systems were the output of the system exits to the outside of the building. For this project this process would not work. The next option would be to use dry scrubbers. The main compound for collecting the pollutants generated through this system would be active carbon. The only downside of using active carbon is that the larger particles will clog the scrubber and reduce the efficiency. The best method would be to include a pre-filter that would collect the larger debris. For the initial design of this project there is no filtration during this stage. During the second revision then this system may be employed.

5.4 – Similar products

While researching this topic there were some manufactures that currently use lasers to mill printed circuit boards. The leading manufacture for this is LPKF [2]. There system uses a 1064 nanometer laser to etch the boards. The system that they use also using gravimeters to move the laser light to the target on the copper clad board. Another alternative to etching the copper substrate from the board is to coat the boards with an opaque polymer and using a CO2 laser to

remove the polymer. The newly prepared boards are then chemically etched. This process is something that this group currently employs in creating circuit boards. Another method of creating the printed circuit boards is to use mechanical methods to remove the copper substrate. This is the method that is currently employed for the senior design students. In previous senior design groups a similar project was created [3]. There project consisted of taking a one watt 808 nanometer laser diode and attaching it to an XY table. When etching occurred the XY table would move the laser and its optics to the location and the laser would turn on. This project compared to the previous project is that the pulse energy is much greater. The power of the laser diode that is being used for this projects laser system compared to that of the previous system is about 80 times greater. This will allow this laser system to be able to mill away the copper substrate.

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SECTION 6: Laser System

6.1 – Types of Lasers

The two common lasers that are used in industry for material processing are the CO2 laser and the Nd:YAG laser. The CO2 laser works by exciting a mixture of

gas that contains CO2 with a high voltage or with microwaves. The output of the

CO2 laser is around ten microns. For copper this wavelength is over 98 percent

reflective. Using a CO2 laser would require very high power design to allow the

two percent that is absorbed to vaporize the copper target. The Nd:YAG laser is created by exciting a neodymium doped rod of the crystal yttrium aluminum garnet. The main output frequency that this laser generates is 1064 nanometers. This too, is highly reflective for copper with a reflectivity of 96 percent. The big advantage to the Nd:YAG lasers is that there are nonlinear optics that can generate harmonics of the fundamental wavelength of this laser. For the second harmonic of 532 nanometers, the reflectivity of copper drops down to 45 percent. This will allow 55 percent of the laser pulse to be absorbed and will allow the laser to run at much lower powers than if just the fundamental wavelength would be used. The conversion to generate the second harmonic is not perfect and the best efficiency in generating the harmonic is 45 percent of the fundamental wavelength. If the third harmonic of the fundamental would be used the reflectivity drops down to 35 percent. The only problem with generating this third harmonic is that the conversion efficiency drops to at best 30 percent of the fundamental wavelength. For third harmonic generation, the process requires multiple nonlinear optics to generate it. The reason for multiple optics is that the first optic generates the second harmonic and the second nonlinear optic takes the second harmonic and the fundamental and generates the third harmonic. This process becomes even less efficient because it is almost imposable to generate in that laser cavity. So to first generate the fundamental the laser cavity will need to be pumped over 90 percent higher than if the third harmonic would be built in the laser cavity, this is because the standard output coupler is 90 percent reflective. Since the second harmonic can be created in the cavity, it is therefore more efficient to generate the second harmonic in the cavity at a higher power than to user the third harmonic. Nd:YAG lasers can also generate even higher order harmonics, but these conversions waist a numerous amount of the fundamental wavelength energy and are not used for material processing.

A new development is being implemented in generating high powered fiber lasers. This design consist of fiber that is doped with some rare earth metals that when excited by a pump laser and placed in a cavity they can generate high powered lasers. The most common fiber doped laser is the Ti:Sapphire. This laser has an output wavelength of around 790 nanometers. The biggest problem with Ti:Sapphire laser systems is that the fiber is hydroscopic and over a short time will need to be replaced regularly. Another type of fiber laser schemes is to use thulium doped fiber. This fiber system generates a wavelength of around two

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microns. The biggest drawback to this fiber system is that the cost of the fiber being used costs around a thousand dollars per foot. The second drawback is that the laser is in the infrared wavelength that cannot be detectable using standard IR viewers or standard IR fluorescents cards.

There is a new type of laser system that is called thin disk laser system. The leading developer in this laser system technology is Trumpf. Thin disk laser system works by generating the doping material to be a 100 micron thick disk. On one side of the disk is a very precise temperature controller. If used, this would generate a higher conversion efficiency. The reason for this higher conversion efficiency is that it reduces the thermal lensing and controls the crystal’s temperature to the ideal range for the conversion of the pump light to the laser’s output light. The doping material that is used for this system is Yb:YAG. This crystal structure generates a wavelength of 1060 nanometers. This is very close the Nd:YAG laser wavelength and this would allow many of the same optics to be used. This would also include the second harmonic crystals. The biggest downside of this system is that because this is a new process, the cost of the thin crystal is very high and the means of accurately controlling the temperature of the disk would be costly as well.

The last alternative in laser system design would be to use the laser diode as the fundamental laser and generating the second harmonic wavelength by using another second harmonic generation crystal. The downside of using this schema is that the energy output will reproduce the same as if the laser was running in continues wave mode. Comparing this design in continues wave configuration, the conversion efficiency is very high compared to Nd:YAG. The difference comes when the system is pulsed. Unlike Nd:YAG, this system cannot be Q switched. This will prevent the crystal from dumping all of its energy at once. So to compare the laser output energy, if 80 watts of laser diodes would be used the output energy is around 3.2 microjoules. Comparing this to a Nd:YAG laser the output would be about a thousand time greater.

6.1.1 – Previous Design

The laser design used for this project will consist of a second harmonic Nd:YAG Q-switched laser. The second harmonic generation is generated in the cavity. The following layout will show the design of the laser system (Figure 6.1).

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HT:1064 HR:532 HR:1064 HR:532 HR:1064 ¼ Wave Q-switch Polarizer HR:1064 HT:808 HR:1064 HT:808 Diode Diode KTP Nd:YAG HT:1064 HR:532 HR:1064 HR:532 HR:1064 ¼ Wave Q-switch Polarizer HR:1064 HT:808 HR:1064 HT:808 Diode Diode KTP Nd:YAG

Figure 6.1: Layout of Laser system

To generate the fundamental wavelength, a Nd:YAG rod is end pumped with one to two 808 nanometer laser diodes. The diodes chosen for this project are fiber coupled diodes. The output of each diode is rated for 40 watts of laser light output. The manufacture of the diodes is Coherent. The reason that these diodes were used was because they are commonly available on eBay. If the diodes were bought from the manufacture, they would have cost over ten thousand dollars each. Also the advantage of using fiber coupled diodes is that the optics required to generate a round beam is much easer than if a non fibered coupled diode is used. The reason is that in order to generate the high power output the diode must be multiple diodes stacked on top of one another. The shape of the output of the stacked diodes reproduces exactly how the diodes were stacked. This would require many optical elements to fix the shape of the beam and the addition of each optic would degrade the power output. The design is to first use only one diode, and if the power output is too low then a second diode is added to increase the pump power. When the two diodes are setup, it is important to prevent the diodes form having the pump laser light from one diode enter the other diode. So when the system is setup, the focal point of the diodes is off center of the crystal rod. This will prevent a large amount of the pumping light that exits the crystal from hitting the other diode.

The Nd:YAG rod that is used for this laser was collected from a old range finder that was used on a M1 tank. The rod was bought also from eBay. An alternative crystal that could have been used for this system is the Nd:YVO4. This crystal has the great advantage to have an acceptance pump wavelength that is much wider than Nd:YAG. The disadvantage to this crystal is the pricing for just a small crystal us about four times the cost for a larger Nd:YAG rod. Another disadvantage to this crystal is that the crystal is polarization dependent. The common practice for using Nd:YVO4 is to use it as a low power oscillator and to

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then use a Nd:YAG rod for an amplifier. This would increase the complexity of this projects design, therefore it is better to stick with using one Nd:YAG than the Nd:YVO4 crystal. The doping proportion of the neodymium is unknown, but comparing it to other rods, it looks to be around 0.7 percent. This is the standard doping used for diode pumping. The Nd:YAG rod will also need to be cooled when it is being pumped this is because the conversion efficiency from the pumping laser light to the laser’s fundamental light is around 45 percent. The crystal is mounted in an aluminum block that will water cooled. As a safety precaution the aluminum heat sink’s temperature will also be monitored. This aluminum heat sink will need to also be about a quarter of an inch longer than the Nd:YAG rod. The reason for the increase in length is that the crystal will start to generate thermal lensing at the ends of the rod. If the heat sink was not longer than the rod the crystal will destroy itself by fracturing at the ends.

To generate the second harmonic wavelength a nonlinear crystal is used. There are about four different crystals that are used for this generation. The easiest nonlinear crystal to grow is the Potassium Dideuterium Phosphate (KDP). This crystal is the cheapest second harmonic generation crystal. The downside of using this type of crystal is that it is hydroscopic and the conversion efficiency in generating the second harmonic is temperature dependent. The lasers that use this type of crystal normally need to be continually purged with desiccated nitrogen, or the crystal is placed in a small box that has anti-reflective windows and filled with expensive index matching fluid. The crystal would also need to have a peltier on it to regulate the temperature. If the humidity of the crystal is not low the crystal will start to fog over and the crystal could easily be destroyed by putting minimal pressure on it. This pressure would be the pressure of just mounting the crystal in a heat sink. The crystal also has the issue of aging. When the KDP crystal ages, it darkens and reduces the power output [4].

Another second harmonic generation crystal is the Lithium Triborate (LBO). It does not have the setback of KDP when it comes to being hydroscopic. The LBO crystal also has a conversion efficiency of about three times greater than KDP [5]. The Beta Barium Borate (BBO) second harmonic crystal also is not hydroscopic and has a conversion efficiency of about double that of LBO and six time that of the KDP [6]. BBO crystals also do not need to be temperature controlled like the LBO and KDP because it has a temperate bandwidth of 55 degrees centigrade.

Lastly the commonly used second harmonic crystal used today for the green laser pointers is the Potassium Titanyl Phosphate (KTP). The reason that this crystal is used is that it has a conversion efficiency of 15 times greater than the KDP [7]. It also is not hydroscopic and has a large working temperature range from 25 degrees centigrade to 80 degrees centigrade. The biggest problem with KTP crystals is that the cost of generating large crystals is much greater than KDP. So the general rule is that if a long crystal is needed than a KDP crystal is used, otherwise the more efficient KTP crystal is used. There was a more

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recently development method in generating Anti-gray Tracking KTP crystals. These crystals have about double the damage threshold than the standard KTP crystal [8]. Without the anti-gray tracking the aging of the crystal over time generates gray tracks within the crystal, the new crystals are engineered to reduce this problem and therefore increase the lifetime of the crystal.

For this project a KTP with the anti-gray tracking crystal is used. The crystal dimensions are three and a half millimeters by six millimeters. The crystal was also acquired from eBay. The KTP crystal was also coated with anti-reflective coatings for 1064 and 532 nanometers. To prevent overheating of the crystal it is placed in an aluminum heat sink. The heat sink will also be water cooled. The temperature of the heat sink will also be monitored. If the crystal is misaligned the temperature of the crystal will increase, measuring the temperature of the heat sink will help diagnose that there is a misalignment. This will also save the laser cavity because if the crystal heats up past a hundred degrees Fahrenheit the crystal will fracture.

To protect the diodes from the 1064 nanometer laser light the cavity is folded. The folding mirrors are coated to reflect the YAG fundamental light and to be transparent to the laser diodes wavelength. So what happens is that 99.8 percent of the fundamental light is reflected and the 0.2 percent hits the diode. The amount of light that hits the diode is a much less than 160 mircojoules. This amount of energy would not be enough to damage the diode.

The design of the cavity is to allow the second harmonic to be generated in the cavity. The cavity is folded to allow the pump diodes to be placed for end pumping the Nd:YAG rod. For the second harmonic generation crystal, it is placed as close to the second focal point in the cavity. The efficiency of the cavity is dependent to how much the majority of the YAG rod is in the cavity. To prevent the cavity from continuing to expand and preventing the second harmonic crystal from being completely in the beam, a lens was placed in the cavity. The lens is placed in a location that generates an image of the center of the YAG rod. At the focal point of the lens a highly reflective mirror is placed. The mirror will need to be designed for high power laser light for the fundamental wavelength of Nd:YAG and the second harmonic. The reason is that the cavity is converging to a very small spot size on the mirror. The mirror’s coating will need to be rated for a power of greater than 120 joules per centimeter.

The section that is in between the lens and the mirror is the location for the second harmonic crystal. This will allow the second harmonic crystal to be fully exposed without letting the edges of the crystal be exposed. The output coupler for this laser is a special coated mirror that allows the fundamental Nd:YAG laser light to pass through and reflect the second harmonic light. The second harmonic laser light that exits the cavity will need to be collimated. This is done with adding a convex lens on the output sections of the laser cavity. The lens reduces the beam from diverging. This will help in maintaining the same spot size when the

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laser is engraving. Since the output power of the laser pulse is very, high the lens will need to be coated with an antireflective coating. The focal point of the lens that is used for collimating the output of the laser will have to be determined after the laser is built. The measurements of the divergence will determine the lens that is used.

To generate pulsing of the laser a Q switch is implemented. The additional parts required to generate pulsing of the laser are the polarized beam splitter and a quarter wave plate. The pulse generation works by first having the Q switch change the polarity of the laser light by a quarter-wave. Then when the laser light passes through the quarter wave plate it rotates by another quarter wave. The light is then bounced back from the cavity mirror and passes back through the quarter wave plate and the Q switch then changes the polarity back to the original polarity and passes right through the beam splitter. When the Q switch is turned off and when the laser light is passed through the system the polarity is horizontal and when it goes through the beam splitter it is sent out of the cavity. This prevents the laser from oscillating and the laser is turned off. The two methods of preventing the polarization from passing are to use a cubic beam splitter or to use a polarizing filter. The reason that a cubic beam splitter was chosen was that polarizing filters absorb the entire laser light that is not the correct polarization. Since the design of this laser is for high power, this would burn and destroy the polarizing filter very quickly. Using a cubic beam splitter diverts the horizontally polarized light out of the cavity and allows the vertically polarized laser light to pass through. Once the light exits from the beam splitter it can be collected in a water-cooled beam dump. This will prevent damaging the polarizing optic. The polarizing beam cube that was chosen was designed for Nd:YAG fundamental wavelength. The manufacture of the polarizing beam splitter is CVI Lasers. The beam splitter was also bought on eBay at a heavily reduced price compared to buying it directly form the manufacture. The quarter wave plate that is used for the project is a Spectra Physics multi order wave plate with the center frequency of 1064 nanometers. This quarter wave plate was chosen because it was also available from eBay and the company is very reliable. The down side of using a multi order wave plate is that the acceptance wavelength is around one percent. If a zero order plate was used the acceptance bandwidth is around two percent. The cost of a zero order wave plate is about double the cost of a single wave plate, therefore a multi order wave plate was bought.

The cavity mirror that is going to be used for the laser is a highly reflective concave mirror. The focus point of the mirror is 500 millimeters. The lens that is used for relaying the center image of the Nd:YAG rod will have a focal point of 800 millimeters. These values were chosen because a model was created using a computer simulator named LASCAD. This software is designed to take in the crystal parameters and the different lenses and will generate the cavity stability. The software will also estimate the output power of the laser cavity. Using the

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Nd:YAG rod that was specified in the above test, a simulation of the entire cavity was designed. The following figure is the design of the cavity (Figure 6.2).

Figure 6.2: Cavity deigned in LASCAD

This cavity design was also simulated to be pumped with two end pumping lasers with a total pump power of 80 watts. The software design was not created for frequency doubled cavities, but it does follow the same rules for stability. The 11 elements in the software represent the objects of the cavity. The first element starting at zero is the highly reflective mirror. The second and third elements are the interfaces of the KTP crystal. The fourth element is the mirror that would separate the second harmonic from the fundamental wavelength. The fifth and sixth section is the Nd:YAG crystal. The seventh section is the beam splitter and the eighth element is the quarter wave plate. The ninth and tenth elements are the interfaces for the Q switch. The last element is the concave mirror. In simulating the laser cavity, the concave mirror was chosen to be the output coupler. The data collected from the simulations shows that this laser cavity will oscillate and that the output power is mostly exponential by the dependence of the input pumping power. This can be illustrated in the graph of the input pump power to the output lasing power (Figure 6.3). This data can be used to estimate the laser power for the second harmonic generation.

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The software will also simulate the ability to pulse the laser with a Q switch. The design criteria that were inputted were that the laser would be pulsed for 41 nanoseconds and that the laser pulsing would have a duty cycle at one kilohertz. The data collected showed that the output power for this laser cavity would generate pulses of over nine millijoules. If the efficiency of converting the pulse form the fundamental frequency to the second harmonic is 45 percent then the output power of the second harmonic pulse should be around four millijoules. This power output is about six times higher than the minimum amount of energy required in generating the plasma to vaporize the copper off the circuit boards. As previous experiments showed that the laser power output of the software is concretively lower than the experimental outputs. This indicated that this laser design will generate a laser system that should be able to generate the plasma needed to vaporize the copper substrate.

The control elements that are required to run the laser correctly need to be synced together in order to allow the proper operation of the laser. When some of the elements fail the entire system must act accordingly in order not to damage the laser system. The following is a flow chart that illustrates how these different elements interact with one another (Figure 6.4).

Main Computer Laser Power Supply USB USB AC Power Cooling Lines Thermal Electric Cooler

Laser Diode #1 Wire Cooling Flow Thermistor Cooling Lines Wire USB USB Laser Power Supply AC Power Cooling Lines Thermal Electric Cooler

Laser Diode #1 Wire Cooling Flow Thermistor Cooling Lines Wire Main Computer Laser Power Supply USB USB AC Power Cooling Lines Thermal Electric Cooler

Laser Diode #1 Wire Cooling Flow Thermistor Cooling Lines Wire USB USB Laser Power Supply AC Power Cooling Lines Thermal Electric Cooler

Laser Diode #1 Wire Cooling Flow Thermistor Cooling Lines Wire

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6.1.2 – Current Design

Unfortunately we were incapable of using the design outlined in the previous section. This is due to the fact that the laser 808 nm laser diode that we purchased from eBay was out of the listed specifications for it. The diode was running at 800 nm instead of the 808 nm that it was supposed to run at. As the Nd:YAG crystal that is part of our cavity design has an acceptance region of 0.6 nm, we would have only been capable of making this design work by actually heating the diode to a level that is dangerously out of the diodes safe operating range.

The current design of our laser is a flash tube based system. The main drawback of using this type of laser system instead of a diode system is that they are at best 2% efficient, have a low duty cycle, and have a shorter mean time to failure than a diode system. The flash tube system that we will be using will have a maximum duty cycle of 100 pulses per second, with a more realistic goal of 10 pulses per second. These pulses consist of containing four nanosecond pulse durations. The advantage of using a flash tube laser system is that we will have a higher output power from the oscillator. This will allow us to take fewer shots to achieve a burn through, but is mainly because we need the higher power because we are doing the frequency doubling outside of the cavity and our conversion efficiency is now much lower.

The theoretical power output of the current design will be 15 J per pulse. This will be obtained because we will have 100 mJ of 1064 nm light per pulse out of the flash tube cavity. This is our main laser, which will be referenced as the laser oscillator from now on. We will then use two amplifiers that are both flash tube based amplifiers. The amplifier’s régime will consist of a single pass amplifier meaning that the light will enter one port and exit from a different one by only making a single pass through the excited gain median. The light will then enter a focusing lens and near the focal point the laser light will enter a KTP crystal. At the focal point will be a highly reflective mirror that will reflect the beam back through the KTP for a second pass. After having the light pass through the nonlinear KTP crystal creating a double pass, the laser light is now converted into 532 nm green light which is the final light that we will be using for the mill operation. The light will hit a mirror that is highly reflective for 532 nm and transmissive for 1064 nm to separate the 532 nm light out of the laser amplifier system. The 1064 nm light will stay in the laser section and the 532 nm light will be transmitted to the XY stage.

6.2 – Prototype/Test

The first task in prototyping and testing this design is to first test the diodes. The diodes are designed to give out a wide range of wavelengths from 804 nanometers to 810 nanometers. The center frequency is determined by the temperature of the die. The first test would be to determine what temperatures

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are required to generate an output wavelength of 808 nanometers. The reason that this is important is because the efficiency for Nd:YAG conversion to the fundamental wavelength is best preformed with a pump wavelength of 808 nanometers. The test will also be used to determine what power outputs are generated for the current that is used by the diode. This data is helpful when building the laser power supply. Another measurement that was preformed is the calculation of the diodes laser power output to its internal power meter. This was used to determine if the diode is working correctly.

Once the laser diodes are fully tested, the next step is to build a simple laser cavity that would consist of the laser diode, the highly reflective concave mirror and a 99 percent output coupler. The next step would be to add in the imaging lens. Once the lens is placed back in, the cavity length will have to be changed to regenerate a stable cavity. Once the cavity is stable, the next element that would be added would be the mirror that is reflective for the second harmonic and transparent for the fundamental wavelength. The laser mirrors will then be tweaked to generate the highest power output. The next object that would be added would be the KTP crystal. The crystal rotation and position was adjusted until the highest output power was generated through the output of the reflective mirror for the second harmonic. The next step is to change out the 99 percent output coupler with a highly reflective mirror for the fundamental and second harmonic of Nd:YAG. The next element that would be placed in the cavity would be the beam splitter. Since the KTP crystal is polarity dependent, both the cubic beam splitter and the second harmonic generation crystal’s rotation will have to be adjusted until the highest output could be achieved. The next element that was added is the Q switch. The Q switch will need to be adjusted until the output power was the maximum. The last element that would be added would be the quarter wave plate. This was adjusted so that the cavity will not lase. The next step would be to test the Q switch to see if generates a pulse laser pulse. The power was measured and all components were adjusted to improve the output power of the laser. The next step would be to calibrate the power output compared to the input pump power. The next step would be to see if the power output can reach the minimum required energy to generate the plasma that vaporizes the copper substrate.

If the power output is below the required energy output then the second diode will have to be added to the setup. This will consist of first removing the Q switch, the quarter wave plate and the cubic beam splitter. The next step would be to add another highly reflective mirror for the fundamental Nd:YAG and transparent to the pump diode’s wavelength. Then the cavity was tweaked to produce the maximum energy with just one of the diodes. Once this is done the next step was to turn on both diodes at half power and adjust the cavity until maximum power is achieved in the output. Now that the maximum is found the next step will consist of placing back in the cubic beam splitter. After tweaking the beam splitter, the next step was to add the Q switch. As before, the next element that was added is

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the quarter wave plate. The final step was to adjust all the elements to generate the maximum output power.

After the adjustments, the laser system needs to be calibrated to understand what characteristics of diode power were needed to vaporize the printed circuit board material. The final test was to measure the divergence of the output beam. This data will help in choosing the correct focal point in the output lens to collimate the beam. Characteristic such as the mode and the M2 will also need to be measured to determine what the spot size was when the beam is being focused on the copper substrate. This data will also be used to determine what type of focusing lens was used. The next testing that will need to be done is to redo the burn testing that was done with the quantaray laser.

6.3 – Build

Once the burn tests were finished, the next step was to look at the layout of the prototype laser and redesign a new bread board that was the final placement of the elements. When the design of the bread board was designed, the next step was to prepare the stock aluminum by first flattening it and then drilling the holes to mount the elements. The next step was to attach the elements on the new bread board. Each element was placed on the bread board in the same order as when they were prototyped. The next step was to adjust the elements until the power outputs match the outputs that the prototype gave. When this process is finished the next step was to cover the laser. This will prevent dust form entering the cavity and degrading the power output. The exit portal for the laser is an antireflective coated window. This will allow the entire laser to be sealed.

For diagnostic and safety purposes temperature sensors and humidity sensors were added to the laser cavity. For the laser cavity mirror a photo diode will be placed in its location. This is used to measure the output of the laser cavity without using a high powered power meter. This can be used to detect faults and warn the computer that a fault has occurred and to shut down the system. The same could be said for the humidity sensors and the temperature sensors. The diodes are also equipped with photo detectors and with the calibrated data from the testing section this data could be used with the software to determine when a diode is going bad.

6.4 – Evaluation plan

The first step in evaluating the laser section of this project is to first test a simple cavity. The next step was to add additional elements to the laser cavity. This process allows the ability to diagnose problems before adding additional complexity. Once all the elements are added to the laser cavity. The next step was to test it to see if the power output is within specification. If the power is below specifications, then a second laser diode was added to the cavity. The next step was to determine the divergence of the laser beam and the modes. The

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beam will then be corrected to generate a near collimated divergence. The next step was to place the laser on a bread board that would be dedicated to this laser design. Lastly, the laser was sealed up and the laser beam was ready to be routed to the XY table.

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SECTION 7: Laser Power Supply Unit

7.1 – Previous Design

The previous power supply for the diode laser system requires a DC output with low voltage and high current. The power supply was to have an input of 120 V AC at 60 Hz from the AC mains and deliver an output of 0 to 5 V DC at 60 A to the laser diode. The method used to power the laser diode had to be reliable, precise, and have heat dissipation comparable to similar power supplies. The laser diode needed a constant source of power that had a precise value, which requires the power output of the conversion to have a small voltage and current ripple. If the change in input power is too large, it will not allow the laser diode to function and may damage the laser diode. The output voltage ripple must be less than 1 mV. It was current controlled, and it had current and temperature monitoring. The laser power supply was designed and simulated using LTspice. It was used because it is commonly used in engineering courses and uses equations that closely simulate actual circuit operation.

7.1.1 – Rectification

The AC mains was converted to a DC voltage before entering the converter. The first part of the rectifying stage filters out high frequency AC noise using a low-pass filter. A metal oxide varistor (MOV) has a high resistance at low voltages and a low resistance at high voltages, and it is used to protect the power supply circuit from power surges. After this, a 60 Hz transformer provides electrical isolation from the AC mains. The output of the transformer enters a rectifier bridge with a filtering capacitor on the output to change the single phase AC to a DC waveform. This DC waveform enters the DC to DC converter, which gives a regulated DC output that can be changed in magnitude and corrected by a control circuit if it differs from the desired output. There are different options to consider when creating the DC to DC conversion circuit. The following schematic is the rectifier used for converting AC to DC (Figure 7.1).

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7.1.2 – DC to DC Converter

A linear regulator is one option to convert the DC voltage. It can deliver a lower output voltage compared to the input, but it is not efficient enough. The regulating component is an adjustable voltage divider network that maintains a constant output voltage. The divided voltage that is not used as the desired voltage value is dissipated as heat. This can make the efficiency drop to as low as 40% [9]. They are more inefficient as the voltage drop from input to output increases, and they dissipate heat power equal to the product of the output current and the voltage drop [9].

A better option is the switched-mode DC to DC converter or switched-mode power supply. Instead of constantly dissipating unused power as heat, it uses power semiconductor devices to allow the input power to provide power at certain times, charging the power storing inductor and capacitor. The duty cycle, D, is the ratio of on-time of the semiconductor to the switching period. Changing the duty cycle allows for this converter to use the minimum input power necessary to achieve the desired output power. Today, switching devices have very high switching speeds and very high power handling capabilities, which allows switched-mode power supplies to operate with an efficiency greater than 90% with low cost and relatively small size and weight [9].

Another reason the switched-mode converter was used is because its complexity allows for the most control of the output voltage and current, compared to a linear regulator. It can be adjusted by changing the duty cycle of the PWM on the microcontroller. Changing the output power is more difficult on a linear regulator because in many cases of linear regulators more difficult hardware adjustments must be made.

The two switched-mode converter configurations that can be used in the design to decrease the voltage and increase the current are the buck, which can only have a voltage gain less than one, and the buck-boost, which can have a voltage gain less than, equal to, or greater than one. A gain below one is the only requirement for the laser power supply, so it was used instead of the buck-boost.

7.1.3 – Switching

There are several options for the semiconductor to be used for switching. The desired characteristics of the semiconductor are high forward current carrying capability and fast switching.

The BJT, IGBT, and the power MOSFET are the best options for achieving these requirements. The BJT has a high power rating with high current carrying capabilities in the on state, but if it becomes too hot, it is prone to malfunction

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from second breakdown [9]. Also, it requires more power to drive than the MOSFET, which gives the MOSFET an advantage. The IGBT is made of a BJT and a MOSFET so it has a high current, but its turn-off time is larger than a BJT, and can only operate up to 20 kHz in medium power applications, such as this power supply [9]. The laser power supply cannot be operated at this low of a frequency, because a much higher frequency must be used to obtain a voltage ripple smaller than 1 mV. The power MOSFET has the highest switching speeds compared to the other two, reaching more than 1 MHz, and a power rating of up to around 600 V with a current rating of 40 A [9]. This power and current rating is sufficient for the power supply, and the high switching speeds are good, so the MOSFET was used.

In Figure 7.2 the diode that blocks when the MOSFET is on is also an important part of the efficiency and successfully switching operation of the power supply. The diode dissipates power as heat equal to its forward voltage drop multiplied by the current through it. This heat requires the converter to be cooled to prevent heat from damaging the circuit, and the diode current increases exponentially with the voltage across it [9].

Figure 7.2: (a) A buck converter with a switch and diode (b) A buck converter

with two switches for synchronous switching.

A low power design uses a diode to block power from reaching the ground node of the circuit. The high power being delivered to the output puts a high stress on the diode. Diodes are not usually used for high power applications such as this. This is a major disadvantage of this circuit, because the diode can be prone to breakdown, and a power diode must be used to successfully block the high power being delivered to the output.

The switched-mode power supply was made more efficient by replacing the diode with a MOSFET, as seen in Figure 7.2.b. This is more efficient than using any type of diode. This method is called synchronous switching [10]. To replace the diode, M2 will turn off when M1 is on, and then M2 will turn on only for a short time when M1 is off, which will drain ground the negative voltage on the inductor. It is important that the two MOSFETs are not on at the same time, because some of the output power from M1 will drain to ground, and the desired amount of power will not reach the output of the converter.

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The three MOSFETs put in parallel in place of M1 in Figure 7.2 provide the 60 A of current needed for the output of the converter. Initial designs had current output lower than 60 A, so adjustments had to be made to the circuit. The requirements of a low voltage and high current output can be achieved by using more than one switching MOSFET in parallel instead of a single switching MOSFET. This will increase the output current while having little effect on the output voltage. Because the majority of the heat dissipation in the converter is proportional to the conduction time of the MOSFETs, having more MOSFETs allows the heat to be more spread out on the heatsink, compared to using a smaller number of switches.

7.1.4 – LC filter

The LC filter at the output of the buck converter serves to reduce the output voltage and current ripple. If the inductor is too large, it will require too much time to get to the desired steady state value. A larger inductor can decrease the voltage ripple by keeping the converter in continuous conduction mode (CCM), but because synchronous switching is used, the converter was operating in discontinuous conduction mode (DCM). Therefore, inductor was small to save space, have a lower resistance, and provide the desired inductor current in a reasonable time.

Increasing the output filtering capacitor was a method used to lower the voltage ripple. The capacitor was derived from

2 8 1 LCf D V V o C = − Δ

where ∆VC is the voltage ripple, Vo is the output voltage, D is the duty cycle

created with PWM for the three MOSFETs in parallel, L is the inductor value, and f is the switching frequency of the three MOSFETs in parallel, as can be seen below in Figure 7.3.

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Figure 7.3: Basic buck converter schematic using parallel MOSFETS.

The output voltage ripple must be smaller than 1 mV, which was achieved by adjusting the output filtering capacitor of the converter or changing the switching frequency of the switching MOSFETs. The ripple size decreases as the capacitor increases. The ripple can also be decreased when the switching frequency increases, but this also lowers the power delivered through the switching MOSFET due to the switching limitations of the MOSFET. The buck converter was simulated and the output values for voltage and current are given in the following figure (Figure 7.4).

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In this converter, a relatively moderate switching frequency is used, and a relatively large capacitor is used to achieve the desired output voltage ripple. The ripple measures approximately 0.7 mV, as seen in Figure 7.5 below.

Figure 7.5: Buck converter simulation measuring output voltage ripple. 7.1.5 – Controller

The first design used a microcontroller to control the output. A difference amplifier would measure the output current and provide a voltage that corresponded to the current. This voltage would be the signal sent to the microcontroller to adjust the duty cycle of switching MOSFETs. This design was not used because it would be simpler if one less microcontroller were used, and if a controlling chip is used, it could provide more features and the power supply can more separate from the rest of the systems.

The design was then changed to use the LT1339 to control the buck converter. It is a buck/boost controller for high power converters. Its capability of giving the converter a high efficiency is a major reason for using it. It also has anti-shoot-through circuitry for converters dealing with high power. The LT1339 controls the drivers of the two sets of power MOSFETs, which are BJTs that are powered by a 12 V source. The high currents involved generate substantial heat, and the high efficiency from the controller and the use of synchronous switching allows the heat dissipation to be at levels comparable to power supplies that have similar power conversions. The average current limit function is used to control the output current. An op-amp circuit controlled by a digital potentiometer sets the average current limit. The use of a digital potentiometer allows the laser to operate at different energy levels for testing or troubleshooting and be easily controlled using software. The output voltage is programmed with a resistor feedback network. When the input voltage goes above 30 V, the four MOSFETs in parallel exhibit phantom turn-on. This is solved by adding a negative 3 V offset

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to the gate driver for these MOSFETs. If the input voltage unexpectedly drops, it can cause the system to become locked in an undervoltage state. A shutdown function is used to shut down the system when this occurs. The complete buck converter with control circuit is depicted in Figure 7.5.

Figure 7.5: Buck converter with controller and current controlling circuit.

7.2 – Current Design

The previous laser design used laser diodes and required a converter to deliver power with low voltage and high current. The current laser design uses flash tubes and requires high voltage and a low current. This was achieved using a boost converter. The rectification for the current converter design is the same as the previous design. The flash tube system is not susceptible to moderate voltage and current ripple in the output, so the output voltage and current ripple do not need to be lowered much compared to the output for the laser diode design.

7.2.1 – DC to DC Converter

The best option for the DC to DC conversion is the switched-mode DC to DC converter to supply the laser flash tube system with 730 V. The boost converter

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was used to output a voltage gain larger than one. A double boost converter configuration was used to generate a faster current response to the output and to help spread out the heat dissipated by using more switches. A current transformer is used to lower the current through the main IGBTs. The boost converter is depicted in Figure 7.6.

Figure 7.6: Buck converter with controller and current controlling circuit. 7.2.2 – Switching

The BJT, IGBT, and the power MOSFET are the possible switching devices considered. The BJT is designed for high current, but the current used in this converter will be handled fine with the other two switches, and BJT requires a higher driving power than the other two devices. The power MOSFET has the highest switching speeds compared to the other two, reaching more than 1 MHz, and a power rating of up to around 600 V with a current rating of 40 A [9]. The boost converter requires a higher voltage than 600 V and does not require switching over approximately 100 kHz, so the MOSFET was not a good choice for the switching device.

The IGBT is made of a BJT and a MOSFET so it can handle high current and can be controlled easily because a voltage is used to control the gate instead of current, but its turn-off time is larger than a BJT, and can only operate up to 20 kHz in medium power applications, such as this power supply [9]. The

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disadvantage of the IGBT’s turn-off time is small compared to the disadvantages of the other two devices, and the converter will be operating at 17 kHz, so the maximum frequency limit of the IGBT is not a problem. Also, the IGBT is capable of high voltage blocking, which is needed for the high voltages in the boost converter. Compared to the MOSFET, the IGBT is generally more robust at maximum operating loads and are more efficient at high temperatures [25]. The IGBT is the best option for the boost converter.

7.2.3 – Controller

For the current design, the regulating pulse width modulator, UC3526, was used to control the boost converter. Its operating temperature range is 0° C to 70° C, allowing for proper function when the power supply is running at full power, which generates substantial heat. The related controllers from Unitrode did not have a high enough operating temperature range for this converter. Both of the IGBTs for the boost converter are controlled using this controller with added op-amps circuits. The soft-start feature protects transistors and diodes from high currents when the converter is turned on. If the input voltage unexpectedly drops below a certain voltage, it can cause the system to become locked in an undervoltage state. If this occurs, the output is disabled until the system comes out of the undervoltage state and can operate normally. Current limiting is controlled digitally, allowing the user to control the boost converter output via software.

7.3 – Prototype/Test

The parts were acquired via online distributors. All of the testing of the laser power supply was done in the UCF senior design lab. The control circuit and other circuits that do not have high voltages or high currents were constructed on a breadboard. They were constructed using the parts and wire that was acquired. The parts of the power supply using high voltages or high currents were soldered onto a printed circuit board (PCB), because a breadboard would have been damaged by the high currents, or high voltages would have caused a short between the small gaps in a breadboard’s circuits. This circuit was constructed on a PCB and tested with the circuits on the breadboards before all circuits were created on PCBs. The high voltage circuits must not have sharp points on the solder, because these can ionized the air and cause the power supply to fail. Oscilloscopes were used to probe the circuit and check to make sure there were no short circuits and the voltages and currents were at the designed values at all nodes in the circuit. The laser power supply was then connected to a GFCI outlet and the node values were checked. Then it was connected to a standard wall outlet and the node values were checked. A dummy load was the first load to be tested. This was a resistor rated for 50 W. The nodes were checked, especially the output current, which must not vary much from 730 V. Next, the power supply was connected to the flash tube laser system and the nodes were checked. It will

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also be turned on for continuous operation for a length of time to see if the heat is dissipated without damaging the power supply and the output stays constant.

7.4 – Build

All components were bought from online distributors, making sure they arrived on time and then the soldering and other building were done in the senior design lab. Some inductors were bought professionally wound, and other larger inductors were wound by hand in the senior design lab. The laser power supply is housed in a container to eliminate anyone from damaging it, or anyone being injured by the high power in the converter.

7.5 – Evaluation Plan

The laser power supply must meet the requirements of a voltage output of 730 V. Once all the circuits are combined on PCBs and tested, the final version is tested with all other hardware to see if the desired results had been achieved. If not, they would be redesigned either on breadboards or PCBs and tested again. The power supply is air cooled and it was made sure that it did not overheat and require more cooling. It is important that the design for the high power circuits were accurate, because they were not prototyped on a breadboard.

Laser PCB Milling Machine