Thursday, September 11, 2014

A One Million Dollar Inverter

The old saying goes "Build a better mousetrap, and the world will beat a path to your door."  Well if you can build a better inverter Google will drive up to your door and hand you a check for one million dollars. It's called the Little Box Challenge.  So let's talk about how to make a million dollars. An inverter is an electronic component that changes DC current into AC current.  The output might be sine wave, pulsed sine wave,  modified sine wave or even square wave.. but the conversion is one direction DC to AC. Or in Google's words:
"Inverters are the essential boxes that take direct current from devices such as solar panels and batteries and turn it into alternating current for use in homes, businesses, and cars"
The problem is that inverters are big. Google compares them to a picnic cooler. I'd compare them to a 1980s VCR. Same difference. In an era of ubiquitous handheld smart electronics, inverters are nowhere to be seen. Google wants one smaller than a laptop. That's a 10x reduction. The Deadline is September 30th. That sounds like a tall order. The detailed requirements are even taller... 30 pages taller. Here are the high lights:
  • Must be able to handle up to 2 kVA loads
  • Must achieve a power density of equal to or greater than 50 W/in3
  • Must be able to handle loads with power factors from 0.7–1, leading and lagging in an islanded mode
  • Must be in a rectangular metal enclosure of no more than 40 in3
  • Will be taking in 450V DC power in series with a 10 resistor
  • Must output 240V, 60 Hz AC single phase power
  • Must have a total harmonic distortion + noise on both voltage and current of 5%
  • Must have an input ripple current of 20%
  • Must have an input ripple voltage of 3%
  • Must have a DC-AC efficiency of greater than 95%
  • Must maintain a temperature of no more than 60C during operation everywhere on the outside of the device that can be touched.
  • Must conform to Electromagnetic Compliance standards as set out in FCC Part 15 B
  • Can not use any external source of cooling (e.g. water) other than air
So my first thought that this is not for use in those ubiquitous handheld smart electronics. The 220Vrequrement is more in line with what your Washer, Hot Water heater, Dryer and Kitchen stove might require. But it's also single phase. In the US we use split-phase 220 to provide 110 power. So this is clearly intended for residential use. Most homes have only two 220 circuits in the breaker box. The rest are split-phase 110 adding up to 180 -200 Amps in the average house. The 450V DC input requirement is in line with something like a 7kW solar array. I'm assuming the 60 Hz requirement means they intend to feed this directly into a transformer. But I also notice that the input voltage is 450V DC so technically this is also a very fancy buck converter. (A buck converter steps down the voltage but step up the current.)

There are still a couple more qualifications: It has to be smaller than 40 cubic inches. That's only a 3.4" cube. It's bigger than a Rubiks Cube, but smaller than two Rubiks cubes. It also has to meet FCC Part 15B so it can't be blasting out interference. Assuming multiple teams manage to produce a working model that meets all the criteria, the million dollar prize goes to the one with the highest power density. Don't lose hope.. that device you plug into your car cigarette lighter is also an inverter.

So let's talk about early inverters.. before these solid state toys. The first of these were rotary converters. It was a motor-generator...sort of a mechanical rectifier.  It's both a dynamo and an alternator which is why old texts call it a "dynamotor." In this configuration, DC power can be taken from the commutator, or AC power taken from the collector rings. (There is yes another arrangement that allows both to be taken from the armature.) These rotary converters were replaced by mercury arc rectifiers in the 1930s, and then by semiconductor rectifiers in the 1960s. While inverters have gotten smaller since then, it's largely been a maturation of that 1960s technology.

Modern inverters use CMOS or MOSFET components in their design and I've seems claims of efficiency between 75% and 95%. (Most of that loss is into heat.) The two obvious paths to victory are to shrink or reconfigure components in within the constraints of modern inverter designs.... or to invent new materials that can replace or obviate their components. But the basic components, capacitors, inductors, transformers, resistors, MOSFETS, heat sinks.. these are very mature technologies. They are not getting much smaller without significant effort. So the latter is actually the better path for a 10x reduction in size. Good luck all.