SMPS SWITCHING POWER SUPPLY DESIGN BASICS



THEORY, SCHEMATICS, TOPOLOGIES
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The industry drive towards smaller, lighter and more efficient electronics has led to the development of switching-mode power conversion technology. By definition, switching power supplies (SMPS) are devices that include power handling electronic components which are continuously commutating on and off with high frequency. These electronic switches effectively connect and disconnect energy storage inductor(s) and capacitor(s) to and from the input source or the output.
By varying duty cycle, frequency or phase shift of these commutations, an output parameter (such as voltage) is controlled. Output filters are then "averaging" energy transfer rate and provide continuous current flow into the load.

Of course, you can't learn SMPS design from a single webpage-- this guide is intended to provide just a starting point.
The typical frequency ("F") range of an offline SMPS circuit is from 50 kHz to 500 kHz. DC-DC converters with low-voltage input operate up to several MHz. The high operating "F" results in the smaller size of switch-mode power supplies since generally, the size of power transformers, inductors and filter capacitors is inversely proportional to "F". Switch mode operation also reduces energy losses and increases efficiency- when a switch is "off", its current is near zero; when it is "on", the voltage across it is low. On the other hand, this mode of operation introduces the switching losses due to overlap of current and voltage during each transition, which increase with operating frequency. It also introduces additional electrical noise (EMI).

Buck converter
Volt-second balance for L in continuous mode:
(Vin-VQ-Vout)×ton=
(Vout+VD)×toff

Solving for Vout:
Vout=(Vin-VQ)D-VD(1-D). Neglecting VQ and VD: Vout≈VinD, where D=ton/(ton+toff)- duty cycle
Its acceptable limits depends on the application. In most applications the EMI limits begin at 150 kHz. Therefore, the engineers often select F<150 kHz in order to place the fundamental harmonic below the requirement range. In general, the switcher's optimization is based primarily on size, efficiency, cost and EMI considerations. Realizing the switcher's advantages also requires the selection of the right components that can operate efficiently at the necessary frequencies.

One of the most important things to understand is the output-to-input transfer function of a converter. The DC gain is calculated based on the fact that in steady state, the net volt-seconds across any inductor over one switching cycle must be zero. This diagram illustrates this concept for a buck converter.

There are about a dozen basic topologies used in practical SMPS design. The best configuration for a given application is selected based on the specific requirements for the PSU (including cost and time factors). Since a great deal of overlap exists in the topology usages, this selection is normally influenced by a personal experience of the designer. If this is your first project, you may start with this guide to topology selection. Then see TI seminar books, which contain application notes and detailed reference designs with practical circuits and calculation procedures.
Below you will find theory, SMPS schematics and design guides, power electronics tutorials and other useful online resources for the engineers and hobbyists. If you don’t like to randomly surf the web, this list will save you hours of research.

SWITCHING POWER SUPPLY DESIGN: TUTORIALS,
TEXTBOOKS


TOPOLOGIES: THEORY and
APPLICATION NOTES


SAMPLES of PSU SCHEMATICS FOR ENGINEERS AND HOBBYISTS


TI (formerly Unitrode) seminars: all manuals from 1983 to 2010

Linear and switching voltage regulator basics

SMPS design- SWITCHMODE™ reference manual

Power factor correction (PFC) online handbook

Electronics and SMPS design books

POWER ELECTRONICS TECHNOLOGY- HANDBOOKS,
LECTURES, GUIDES
Fundamentals of power electronics (instructor's slides- see their terms of use)

Interactive textbook for beginners - rectifier circuits, basic converters, PSPICE and mathcad simulation

Electrical conversion reference textbook online- drivers, applications, and components

Mathematics of electric energy, power factor and THD- definitions and equations


Selection of converter topology

Switch mode voltage regulator schematics and formulas

Single transistor forward converter

Off-line forward converter with active clamp and reset

Flyback - the simplest SMPS

Phase shifted ZVT (soft switched) full bridge

Current-fed push-pull circuit

Current-doubler rectifier

Half bridge converter

Push-pull converter design procedure

SEPIC converter

Topologies and optimization of power factor correction circuits



48VDC to DC reference circuits

12 Volt AC-DC flyback switching power supply: schematic, PCB, theory of operation

ATX power supply with PFC

Switchmode PSU for car audio

Transformerless power supplies basics: resistive and capacitive circuits

SYNCHRONOUS
RECTIFIER CIRCUITS
SYNC THEREFORE I AM
Synchronous rectification for low voltage outputs

Synchronous rectification review and analysis

Multi-output flyback with synchronous rectifiers

POWER ELECTRONICS WEBSITES AND PORTALS
SMPS Technology knowledge base - tips, resources, solutions for problems

Ben Gurion power electronics group - tutorials, publications, models

How2Power - articles, guides and practical answers to design questions