Electronic Rule-Breakers That Crept into Everything We Use

Students in grade school are usually taught square roots before or during junior high, and with these lessons comes one immutable fact: It’s forbidden to take the square root of a negative number. Not too much longer after that, however, the students all learn that this is a big fat lie and that taking square roots of negative numbers is critically important in many fields of study.

There’s a similar “lie” in existence for anyone studying electricity, whether they’re physicists, engineers, or electronics enthusiasts: it’s only possible to raise and lower voltage levels on alternating current (AC) circuits using a transformer. If you generate direct current (DC) voltage through the use of a generator or a battery and need a different voltage level for your new power distribution system in New York or your battery-powered electronics, well, you’re out of luck.

Of course we all know that DC-DC conversion, like taking square roots of negative numbers, is not only possible but fundamental to most modern electronics. After all, there are certain integrated circuits that we can drop into our projects to magically transform one DC voltage to another DC voltage without thinking too much about the problem. And we’re not just talking about linear regulators, which can only drop the source voltage to a smaller level by dissipating energy. Using switch mode DC-DC converters, it’s possible to decrease or increase a DC voltage, and do it at around 95% efficiency or higher for some applications (compared to around 30% efficiency for any linear regulator).  But unraveling the mystery of how switch-mode power supplies (SMPS) and other DC-DC converters work, and how they’re different from AC transformers, involves diving a little deeper.

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A Primer on Buck (and Boost) Converters

We all know that the reason the electrical system uses alternating current is because it’s easy to step the voltage up and down using a transformer, a feature which just isn’t possible with a DC system… or is it? Perhaps you’ve heard of mysterious DC-DC transformers before but never really wanted to look at the wizardry that makes them possible. Now, SparkFun Director of Engineering [Pete Dokter] has a tutorial which explains how these mysterious devices work.

Known as buck converters if they step the input voltage down and boost converters if they step the voltage up, [Pete] explains how these circuits exploit the properties of an inductor to resist changes in current flow. He goes into exquisite detail to explain how components like transistors or MOSFETs are used to switch the current flow to the inductor very rapidly, and just exactly what happens to the magnetic field which makes these devices possible.

The video gives a good amount of background knowledge if you’ve always wanted to understand these devices a little bit better. There are also a few projects floating around that exploit these devices, such as one that uses an AVR microcontroller to perform the switching for a small circuit, or another that uses the interesting properties of these circuits to follow the I-V curve of a solar panel to help charge a bank of batteries. The possibilities are endless!

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Make switched-mode power supplies do your bidding

[Ken] needed to supply 3.3 volts of regulated power. He started by using a linear voltage regulator but after a few calculations he discovered that 72% of what he put in was lost to heat. The solution to this is a switched-mode power supply. Rather than burn off energy through a voltage divider, an SMPS turns the power on and off very quickly to achieve the desired voltage.

A car charger-type USB regulator was chosen as [Ken’s] donor device. He figured that making adjustments to the resistors inside would affect the output voltage and he was right. He adjusted the potential divider and ended up with a steady 3.295V.

We asked him to share the schematic that he put together from studying the board and he came through. See that and get the link to the DC-DC converter datasheet after the break. Continue reading “Make switched-mode power supplies do your bidding”