Friday, May 16, 2014

DC-DC conversion

Trading volts for amps and vice-versa was what won The War of the Currents for Westinghouse. And for a century or more it's what's made AC our power distribution mechanism.

But at least on the small to medium scale, such shenanigans are possible with DC as well - they're just not quite as easy.

I've been using "canned" DC-DC converters from CUI for the Hydra so far with great success. But in putting together the GPS Pack project, I was introduced to the concept of the "Boost" converter, as well as it's polar opposite analog the "Buck" converter. Both are switching power supplies designed to output a different voltage than that which they are supplied. For the GPS pack, the NCP1401 is used with a couple of discrete external components to get 5 volts DC from one or two alkaline cells (so somewhere around 2-3 volts in).

For a boost converter, an inductor, a switch to ground and a diode from a "T". When the switch is closed, a circuit is formed from the input through the inductor to ground. The inductor is, effectively, "charged," in more or less the same way as a capacitor in parallel is (in fact, inductors are the opposite of capacitors in many ways, including how they behave in series or in parallel). When the switch is opened, the current through the inductor suddenly drops to a fraction of what it was. Inductors resist changes in current. In this case, it does so by creating voltage. That voltage passes through the diode (which, when the switch was closed prevented the driven circuit from shorting to ground through the switch), and powers the load, now at a higher voltage. As the voltage drops, the controller in charge of the switch closes it to start another cycle. Lather, rinse, repeat.

For a buck converter, the T is rearranged so that the switch is on the left, the inductor on the right, and the diode vertical. While the switch is closed, the supply will attempt
To raise the voltage supplied to the load through the inductor. When the voltage rises too high, the switch will open. The diode will "connect" the input lead of the inductor to ground, and the inductor - since it resists a change in current by supplying voltage - will power the circuit.

The CUI converters are certainly convenient. And I will still probably use the bipolar ones if for no other reason than to save space. But the next version of the Hydra may be powered by a discrete 5v buck converter from the 12v supply instead of a module.

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