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Since the seventies, switchmode power supply design has developed from a somewhat neglected "black art" to a precise engineering science. The rapid advances in electronic component miniaturization and space exploration have led to an ever-increasing need for small, efficient, power processing equipment. In recent years this need has caught and focused the attention of some of the world's most competent electronic engineers. As a result of intensive research and development, there have been many new innovations with a bewildering array of topologies.
As yet, there is no single "ideal" system that meets all needs. Each topology lays claim to various advantages and limitations, and the power supply designer's skill and experience is still needed to match the specification requirements to the most suitable topology to define the preferred technique for a particular application.
The modern switchmode power supply will often be a small part of a more complex processing system. Hence, as well as supplying the necessary voltages and currents for the user's equipment, it will often provide many other ancillary functions--for example, power good signals (showing when all outputs are within their specified limits), power failure warning signals (giving advanced warning of line failure), and overtemperature protection, which will shut the system down before damage can occur. Further, it may respond to an external signal demand for power on or power off. Power limit and current limit circuitry will protect the supply and load from fault conditions. Overvoltage protection is often provided to protect sensitive loads from overvoltage conditions, and in some special applications, synchronization of the switching frequency to an external clock will be provided.
Hence, the power supply designer must understand and meet many needs.
To utilize or specify a modern power processing system more effectively, the user should be familiar with the advantages and limitations of the many techniques available.
With this information, the system engineer can specify the power supply requirements so that the most cost-effective and reliable system may be designed to meet these needs. Very often a small change in specification or rearrangement of the power distribution system will allow the power supply designer to produce a much more reliable and cost-effective solution to the user's needs. Hence, to produce the most reliable and cost-effective design, the development of the specification should be an interactive exercise between the power supply designer and the user.
Very often, power supply specifications have inflexible and often artificial boundaries and limitations. These unrealistic specifications usually result in overspecified requirements and hence an overdesigned supply. This in turn can entail high cost, high complexity, and lower reliability. The power supply user who takes the trouble to understand the limitations and advantages of modern switchmode techniques will be in a far better position to specify and obtain reliable and cost-effective solutions to power supply requirements.
This guide is presented in four (4) parts:
Part 1, "Functional Requirements Common to Most Direct-Off-Line Switchmode Power Supplies," covers, in simple terms, the requirements which tend to be common to any supply intended for operation direct from the ac line supply. It gives details of the various techniques in common use, highlighting their major advantages and limitations, together with typical applications. In this new edition, Section 23 has been expanded to include a current-fed, self-oscillating, resonant sine wave inverter adapted to providing multiple distributed independently isolated auxiliary supplies for a large system.
The need for semi-stabilized outputs with very low noise are addressed by a linear pre regulator that also affords current limiting and the use of sine wave power distribution for low system noise.
Part 2, "Design, Theory and Practice," considers the selection of power components and transformer designs for many well-known converter circuits. It is primarily intended to assist practicing power supply engineers in developing conservatively rated prototypes with more speed and minimum effort. It provides examples, information, and design theory sufficient for a general understanding and the initial design of the more practical switchmode power supplies. However, to produce fully optimized designs, the reader will need to become conversant with the more specialized information presented in Part 3 and the many references.
Part 3, "Applied Design," deals with many of the more general engineering requirements of switchmode systems, such as transformer design, choke design, input filters, RFI control, snubber circuits, thermal design, and much more.
Part 4, "Supplementary," looks at a number of selected topics that may be of more interest to power supply professionals.
The first topic covers the design of an active power factor correction system. The power distribution industry is becoming more concerned with the increasing level of harmonic content caused by non-corrected electronic equipment and in particular electronic ballasts for fluorescent lighting. Active power factor correction is still a relatively new addition to the power supply designer's tasks. It is difficult to display waveforms and design power inductors, due to the dynamic behavior of the boost topology, with its low- and high-frequency requirements. This part should help remove some of the mystery regarding this subject.
In most switchmode power supplies, it is the wound components that mainly control the efficiency and performance. Switching devices will work efficiently only if leakage inductances are small and good coupling is provided between input and output windings.
The designer has considerable control over the wound components, but it requires considerable knowledge and skill to overcome the many practical and engineering problems encountered in their design. The author has therefore concentrated on the wound components, and provided many worked examples. To develop a full working knowledge of this critical area, the reader should refer to the more rigorous transformer design information given in Part 3, and the many references.
The advances in resonant and semi-resonant converters have focused much attention on these promising techniques. An examination of the pros and cons of a fully resonant technique is demonstrated by the design of a resonant fluorescent ballast. The principles demonstrated are applicable to many other fully resonant systems.
A quasi-resonant system is demonstrated by the design of a high-power, full bridge converter that uses both semi-resonant techniques and phase shift modulation to achieve very high efficiency and low noise. This section includes a step-by-step analysis of each stage of operation of the circuit during the progress of the switching cycle.
In Part 4 Sections 4 and 5, the co-author shows a current fed, self-oscillating, fully resonant inverter using power MOSFETs. This version has the advantage of near ideal zero voltage switching transitions that result in harmonic free waveforms of high purity. He also shows a variable frequency sine wave oscillator, implemented with operational transconductance amplifiers. In this design the frequency can be adjusted with a single manual control, or electronically swept over a wide range from milli-Hertz to hundreds of kiloHertz.
No single work can do full justice to this vast and rapidly developing subject. The reader's attention is directed to the Reference section where many related web sites, books and papers will be found that extend the range of knowledge well beyond the scope of this guide. It is hoped that this guide will at least partly fill the need for a more general book or web site on the subject.
PART 1: Functions and Requirements Common to Most Direct-Off-Line Switchmode Power Supplies
PART 2: Design: Theory and Practice
PART 3: Applied Design (under construction--coming soon!!)
PART 4: SUPPLEMENTARY (under construction--coming soon!!)
Also see: Our other Switching Power Supply Guide