Tuesday, September 8, 2015

DC Power Conversion

With so much interest in mobile electronics these days, Makers are tossing aside their long despised AC adapters and turning to DC power. Unfortunately, DC sources tend to fluctuate or be comprised of cells that produce voltages not directly usable by most electronics. It's relatively easy to reduce voltage by converting electrical energy to heat, but it's hard to find applications where efficiency isn't a factor. Efficient switched power conversion is gradually replacing inefficient linear voltage regulation.

Terminology


It's useful to know a few terms to understand the type of voltage conversion performed by devices.
  • Linear voltage regulation / conversion. A continuously regulating voltage divider (inefficiently) down-regulates voltage.
  • Switching voltage conversion. Current is rapidly switched to step voltage up or down.
  • Buck converter. This is a switching converter that reduces voltage to a desired level. In some converters the output voltage may be configurable but it will always be less than the input by some margin.
  • Boost converter. This is a switching converter that raises voltage to a desired level. A common use would be running 5V electronics from a 3.7V Li-Ion or Li-Po battery.
  • Buck-boost converter. This flexible switching converter can either raise or lower voltage to match the desired output.
A sampling of fixed and variable voltage converters.

Real World Examples


So let's say we want to power a tiny computer like the Raspberry Pi or a microcontroller board like Arduino from a 12V source.


Raspberry Pi


The Pi requires regulated 5V input through its Micro USB power connector. This allows the use of ubiquitous AC to Micro USB charging adapters and eliminates the need for a 5V regulator on the Pi. In hindsight, these were very questionable choices. Many existing chargers could not supply sufficient current for the Pi, and no onboard 5V regulation makes the Pi harder to run directly from a DC source. This has all been a big enough problem that there are now quite a few DC to 5V power boards designed to meet the specific requirements of the Raspberry Pi.

With the USB connector unplugged there seems to be no problem supplying regulated 5V power directly through the Pi's 5V header pin.

This Pi 2 works great off a tiny buck converter.

I'm happily running my Pi 2 using the tiny Adafruit "UBEC" buck converter pictured above. It's almost like the connector was specifically intended to provide power directly to the 5V rail of the Pi. (It almost certainly was.)

Arduino


Most Arduinos and clones are pleasantly flexible regarding DC input. The USB connector can be used to supply regulated 5V. The 2.1mm barrel jack is specified to accept 7-12V which is passed through a linear regulator to provide 5V. Considering what we know about linear regulators, it's better to stick toward the lower side of that range. A small number of 'duinos, like the Freaduino Mega2560 pictured below, feature an onboard switching regulator that can handle more than 20V. Finally, a regulated 5V supply may be tied to the internal 5V rail via the header pin.

Exotic "Freaduino" with onboard switching regulator.

I specifically sought out and bought my Freaduino so I could pass 12V (automotive) power into the barrel jack with minimal losses. Given a more typical 'duino, a variable buck converter set to ~7V and passed into the barrel jack would be a fine approach.

Odroid


Although not as popular as the Pi and 'duinos, the Odroid is a very capable yet inexpensive tiny computing platform. Please refer to my writeup on the new ODROID-XU4. This Odroid specifies power requirements as a beefy 4A of current at 5V. A fixed 5V 5A buck converter did not result in a stable system. The AC adapter output measures 5.15V with no load. The fixed 5V converter either droops enough under load or 5V isn't quite enough to stably run the Odroid. Fortunately I had a DROK 5A adjustable buck converter that worked flawlessly when set to 5.2V. I may actually write a review of this converter, as it has some interesting features. These include limited rate battery charging and voltage and current monitoring over an asynchronous serial interface.