This Tutorial Text discusses the competent design and skilled use of laser diode drivers (LDDs) and power supplies (PSs) for the electrical components of laser diode systems. It is intended to help power-electronic design engineers during the initial design stages: the choice of the best PS topology, the calculation of parameters and components of the PS circuit, and the computer simulation of the circuit. Readers who use laser diode systems for research, production, and other purposes will also benefit. The book will help readers avoid errors when creating laser systems from ready-made blocks, as well as understand the nature of the "mystical failures" of laser diodes (and possibly prevent them).
In February 2015, SPIE Press offered Dr. Ilya Bystryak and me the opportunity to write a book based on our professional development course “Powering and Integration of Laser Diode Systems,” presented by both of us at the Photonics West conference. The course material was the result of our lengthy collaboration, initially in the Soviet Union, where we worked for the same research laboratory for an extended period of time, and later in the U.S.
Professional collaboration and the sharing of ideas are important factors of our successful careers in the field of power electronics. Such communication multiplies the engineering effectiveness of each other. The best-known examples of similar fruitful cooperation are the joint work of Steve Jobs and Steve Wozniak, Larry Page and Sergey Brin, and Bill Gates and Paul Allen. Constant, critical face-to-face or telephonic discussions around ongoing projects and emerging problems enables the fast and efficient resolution of these challenges. This form of cooperation is rarely achieved by project formal discussions at scheduled company meetings, design reviews, etc. Therefore, I recommend to my young colleagues to find a partner and a formidable opponent to cooperate with and to maintain this kind of professional relationship throughout their career.
Because the book is based on an instructional course, I decided to present it to a certain extent as a textbook; however, because strict technicalities are often not very useful and boring to read, the material is not written in the traditional textbook style. This Tutorial Text discusses the competent design and skilled use of laser diode drivers (LDDs) and power supplies (PSs) for the electrical components of laser diode systems. It is intended to help power-electronic design engineers during the initial design stages: the choice of the best PS topology, the calculation of parameters and components of the PS circuit, and the computer simulation of the circuit. Readers who use laser diode systems for research, production, and other purposes will also benefit. The book will help readers avoid errors when creating laser systems from ready-made blocks, as well as understand the nature of the “mystical failures” of laser diodes (and possibly prevent them).
The following questions guided the compilation of this book:
• What is the basis of choosing one or another technical solution?
• Which converter topologies are better suited than others for powering certain LD system parts?
• Why should soft-switching topologies be used when developing switchmode PSs?
• Why must one contend with the parasitic elements of the circuit components in some topologies, whereas other topologies can use parasitic elements beneficially?
• How does the choice of topology influence LD and LDD reliability?
• How can the right topology protect against overload and avoid the premature death of the LD (which usually costs more than the LDD)?
• Which topologies generate less electromagnetic interference (EMI) noise and therefore require fewer measures of protection against penetration EMI in the AC mains and the environment?
I tried not to overload the book with complex mathematical formulas. In my experience, most converter calculations are straightforward, and well-known mathematical formulations of the electrical circuit laws are sufficient. Excessive math does not promote but rather hinders understanding the essence of things.
The electrical characteristics of laser diodes and the specifics of the sources for their powering are discussed herein. It has been shown that laser diodes require electrical sources with the characteristic of either a constant current or a constant power source. Because the primary sources of electrical energy are voltage sources, the methods of converting voltage sources into a constant current or a constant power source are the main subject of the book.
Detailed attention is paid to the passive methods of conversion (Chapter 5). Passive conversion of the voltage to the current has several advantages when designing LDDs:
• Because there is no need for a feedback loop in passive converters, their circuits are less complicated than those of active converters.
• An inherent current limit prevents an overcurrent that could destroy the laser diode.
• It is simple to create powerful sources with the direct parallel connection of conversion modules.
• Multiple laser diode strings can be driven by a single converter.
• Pulse modulation of the laser diode current in the pulsed laser system is simple.
Modern power supplies are based on switching converters. The main problems that afflict these converters are switching losses and EMI noise generation, which can be eliminated or reduced by soft-switching. Soft-switching is the hallmark of professional PS design, so Chapter 6 addresses the design of quasi-resonant circuits and the calculation of lossless snubbers, which allows one to organize soft-switching.
Another problem facing the power-electronics designer involves the parasitic elements of the converter circuit. Section 6.7 demonstrates the proper topology that derives a benefit from parasitic elements instead of fighting them.
The initial stages of PS design are reflected in the book via examples. MathCad is used for calculations, and LTspice is used for electrical schematic plots and the computer simulations. In some complex cases, the purely mathematical calculation of circuits is impossible, so a combination of calculation and simulation is necessary.
Readers should understand the basic concepts and definitions used in power electronics. Sufficient context is provided to explain the topics at hand, but other publications should be consulted for an overview of laser-diode power design. Furthermore, some common design issues are not covered here, e.g., the design of synchronous rectifiers in PSs with a low voltage output, EMI filters, gate drivers for MOSFETs, and how to protect PSs from instabilities in the AC line. A list of references is provided for further reading regarding these matters.
Grigoriy A. Trestman