Leakage Power – The Big Picture

Why the adapters you plug into the wall are evil and other strange things you’ll find around your house.


By Cary Chin
We’ve all been working on power engineering for a while now, and leakage power is always one of those generally confusing topics. Sub-threshold leakage, field-effect transistors, MOSFET weak-inversion regions—these are all things that cause even experienced engineers’ and computer scientists’ eyes to glaze over. It all falls under the general topic of static power, or “standby power” in more macro terms.

We all know of standby power as the power your television and computer (or other appliances) consume when they are in standby mode, waiting to be turned on by you. Generally, it’s energy that is consumed that doesn’t serve any useful purpose other than standing by for input. To stretch the definition, it might even include all of those blinking “12:00” clocks on our VCRs over the years. Thankfully we finally got rid of those.

The recommended practice for wasting as little energy as possible on your home electronics is to unplug them, or switch off the power strip when they are not in use. But did you know that it makes a difference which end you unplug? Even with the latest EPA Energy Star guidelines, power adapters are limited to 0.5 watts under NO LOAD. That’s right. Just the adapter plugged in without the appliance draws some energy. I don’t know about you, but we must have at least 40 to 50 adapters plugged in 24/7 in our house. Those things are going to add up.

In low power engineering terms, this is exactly the idea of “power gating” or “power shutdown.” Unplug those transistors when they’re not needed and you lower the leakage power to zero. Of course, unplugging your computer and plugging it back in every time you need it is a little inconvenient, so in our real life example we must make the same tradeoffs that we make when engineering our advanced low power chips—how long will it be off versus how long it takes to start back up.

And finally, I heard this morning that with the current heat wave on the East Coast, energy demand was soaring and the power grid was strained, but holding its own. I immediately thought of power-rail analysis. And when the news story added that some utility companies were lowering their output voltage to save energy, I could only think of three letters, “DVS”! Maybe I’ve been at this for too long…

-–Cary Chin is director of technical marketing for low-power solutions at Synopsys.