16 Ways To Design A Switch-Mode Power Supply
16 Ways To Design A Switch-Mode Power Supply
16 Ways To Design A Switch-Mode Power Supply
Simply put, designing a power supply is a major undertaking. After making your
build vs. buy decision, you face a myriad of circuit choices—more than you probably
realize. Building a power supply used to be relatively straightforward, but with
switch-mode methods dominating these days, it has become a complex specialty. If
you’re not a power-supply expert and/or this is one of your first designs, you may
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need some guidance. The information presented here should help you identify your
options and zero in on one to follow through with.
It all begins with a good specification. It’s critical to take the time to research your
needs and write a detailed specification. As a starting point, list the following key
features:
• Output-current requirements
• Ripple maximum
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With these specifications, you should be able to make your first big choice: linear vs.
switch-mode design. Yes, linear supplies are still an option even in the current
switch-mode dominant environment. If your design can accept the lower efficiency
of a linear supply, you may appreciate its benefits. The major advantages of a linear
supply are simplicity of design, lower cost, abundant relevant components, proven
techniques, and low EMI emissions.
On the other hand, switch-mode designs are inherently noisy, and the circuits you’re
powering may be susceptible to that noise. For example, an oscillator, clock,
synthesizer, or other critical circuit may require low phase noise or jitter. A linear
supply with a low-dropout (LDO) regulator would provide a clean dc to meet that
need. At least keep the linear option in mind, as it may still be your best choice in
some designs.
Most new designs are of the switch-mode variety. The advantages of a switch-mode
power supply (SMPS) are just too great to ignore. Efficiency is the primary benefit,
with efficiencies over 90% for many designs. Small size and reasonable cost are
other benefits. The downside is complex and tricky design with many alternative
approaches. You can make a more informed design choice, though, if you expand
your specifications list.
In addition to the basic specifications compiled earlier, these should also be defined
for your design:
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• Switching frequency
• In addition to EMI needs, include the need for power factor correction (PFC), Underwriters
Laboratories (UL), or other certifications
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The most popular dc-dc SMPS topologies are buck (a), boost (b), inverting buck-
boost (c), SEPIC (d), and Zeta (e). The MOSFET does the switching, the inductors
and capacitors store energy, and the diode controls the direction of current.
You may already know that several different switch-mode circuit designs are
available. But did you know there are actually 16 topologies that you should be
aware of? One of these is sure to fit your needs:
• Buck
• Synchronous buck
• Boost
• Inverting buck-boost
• SEPIC
• Cuk
• Zeta
• Fly-buck
• Flyback
• Two-switch flyback
• Active-clamp forward
• Single-switch forward
• Two-switch forward
• Half-bridge
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• Full bridge
Lack of space here prevents full coverage. However, there are two great sources you
can explore to evaluate your topology choices:
• Power Topolgies Quick Reference Guide : These nine pages provide a quick refresher on the
most common switching power-supply topologies. It’s filled with relevant waveforms and
equations.
• More details are available in the 200-page Power Topologies Handbook . Circuit explanations
and design recommendations are based upon requirements.
If you need some isolation, transformers can be employed. Topologies that will
incorporate them in the design are flyback, clamp forward, push-pull, half bridge, or
full bridge.
As for switching frequency, it usually comes down to the best estimate based on your
application. Today, typical switching frequencies range from about 100 kHz to
several megahertz. Low frequencies are generally better for higher-power
applications requiring best efficiency. Higher frequencies make filtering easier with
smaller capacitors and inductors, and can lead to reductions in both size and cost.
Also be sure to consider the impact of the fundamental and harmonics on other
equipment nearby. One potential solution to limit switching EMI is dithering. That
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involves the random variation of the switching frequency to reduce any EMI by
spreading out the spectrum over a wider range.
Diodes and integrated circuits are at the heart of your design. Keep in mind that
most semiconductor companies have support products or services, such as online
design software like Texas Instruments’ WEBENCH. Don’t forget the possibility of
reference designs and evaluation kits to further speed up and simplify your design.
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