October 11, 2018

Power in small packages

One proxy for Moore's Law these days is how small a package an HD video decoder can fit into. 4K media streaming devices from Amazon, Google, and Roku are not much bigger than an older USB flash drive. Boggles the mind.

The Roku Express (maxes out at a mere 1080p) isn't quite that small, about half the size of a proverbial pack of playing cards. But I am equally wowed by the power supply it comes with. It's one of those now ubiquitous 5V/2A wall units commonly used as cell phone chargers.

The DC output must be electrically isolated, else iPhone and Android users would be electrocuting themselves right and left. This normally requires a bulky transformer. Instead, the 120 VAC is rectified and an oscillator chip kicks the current frequency from 60 Hz to 100 KHz.

At those frequencies, the flyback transformer can be smaller than a dime.

With a high-voltage MOSFET driving the primary coil, the result is a switching power supply that draws almost no power on standby (the Roku doesn't have a physical switch or even a shut-down sequence) and is the size of my thumb.

The image above is the reference design for a switching 5V/2A power supply with variable 85-265 VAC input from Texas Instruments. The circuit diagram is posted on the TI website.

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