GaN Systems Powered the Winning Google Little Box Entry

After months of excitement and speculation, the results of the Google Little Box Challenge have been announced, and GaN Systems has revealed that their gallium-nitride power transistors were instrumental in achieving the winning design. Google and the IEEE Power Electronics Society awarded the $1 Million prize to CE+T's Red Electrical Devils for designing, building and demonstrating an inverter with the highest power density and smallest volume. The competition included more than 2,000 registered teams, from which 18 finalists were selected. After 4 months of testing the finalists' designs, the $1M prize winner was announced at the ARPA-E Energy Innovation Summit in Washington, DC.

In their 'Technical Approach Document' the Red Electrical Devils stated: "GaN transistors have many very interesting electrical characteristics (low Rds_on, low Qgate and Cds, ultra low Qrr); these create technological advantages over current MOSFET and IGBT devices (small size and low production costs). Unfortunately, they also have serious drawbacks due to their very fast switching characteristics: they are challenging to drive and require sensitive electromagnetic noise management. Another pitfall is the high voltage drop due to the reverse current when the GaN is turned off. The solution selected to overcome these difficulties is to control all the GaN transistors using soft switching for the entire operation range.

"In order to combine a continuous current at the 450 V input stage with an alternating 240 V output voltage, we have chosen a five legs topology because it minimizes energy transfer within the inverter. Two half bridges (HB) generate the neutral voltage, two further half bridges generate the line voltage and the last is used as an active filter.

Â	Google's request	CE+T Power
Maximum Power Tested	2000 VA	2062 VA
Volume of the rectangular enclosure	0.655 liter / 40 in³	0.226 lier / 13.77 in³
Resulting power density at 2 kW load	3050 W per liter 50W/in³	8850 W per liter 145.24 W/in³
DC voltage range	399.5 Vdc	300 to 450 Vdc
AC voltage (RMS) output	230 or 240 Vac	240 Vac split phase
DC to AC efficiency (CEC Method)	min 95%	95.4%
Voltage total harmonic distortion + noise	1.1%	1.1%
Current total harmonic distortion + noise	1.1%	1.1%
Input ripple current	7.8%	7.8%
Input ripple voltage	1.05%	1.05%
Max Temperature of Enclosure	60°C	48°C
Ambient Temperature of Test	30°C	29°C
Electromagnetic compliance	FCC part 15 B	FCC part 15 B

"The high density and the high efficiency of this inverter come from optimized control of the five legs. For any type of load, this control achieves soft switching operation of all GaN devices while minimizing reverse currents during the dead times. The algorithm ensures that the module is naturally protected against overcurrents. During the debug phase, we encountered problems due to the high processing load demanded by the control algorithms. We have finally upgraded the processor, using a 40 % faster pin to pin compatible model.

"The objectives of the control are achieved with the help of the following principles: digital control based on a fast microcontroller combined with a CPLD; fast measurement of input/output currents and voltages; efficient feedback on the switching events of the HBs; a learning algorithm for the active filter; optimization of the switching frequency between 35 and 240 kHz depending on the output current; a variable phase shift between the HBs (0° or 90°) and a dead time modulation of the five HBs (50 ns to 3 μs). The switching losses are almost canceled and the frequency increase helps to optimize the size of the passive components.

“The robustness of the GaN control is critical. Indeed, GaNs switch extremely fast so that they generate high dv/dt across the control isolation (far beyond the allowed values for most of the drivers on the market). Furthermore, the gate voltage threshold is very low. We have designed very compact, low cost and extremely robust driver circuitry that can drive GaN transistors well within their specifications.

"Selecting the right GaN package is also very important. An SMD model with 2 source accesses: one for the power, one for the command, is the best choice for this design. It allows safe control of the transistor. Moreover, a small package reduces the parasitic inductances and consequently the functional overvoltage. The PCB layout and the positioning of the decoupling capacitors are crucial to operating the GaN properly.

"The methodology applied comprises: precise dimensioning with analytical calculations and finite element modeling; the use of SPICE simulations for power and control; 3D mechanical modeling; and the use of thermal simulations. This allowed us to create an inverter meeting all the requirements in only 1 run."

Why did Google launch a power electronics contest and award $1,000,000? Because the world’s unrelenting demand for more power is unsustainable without using fewer manufacturing materials and consuming less energy to operate the burgeoning number of electronic devices. And why did they focus on inverters? Because inverters used to power homes, businesses, motors and cars, consume vast amounts of materials and energy. The Little Box Challenge organizers dared the world’s engineers, “Figure out how to shrink an inverter down to something smaller than a small laptop (a reduction of > 10× in volume) and smaller than everyone else, and you’ll win a million dollars (and help revolutionize electricity for the next century).”

The key goal of the challenge was to reach an inverter power density in excess of 50 W/cubic inch in a volume of under 40 cubic inches – a feat which had never been done before. The Red Electrical Devils presented their entry at the National Renewable Energy Laboratory (NREL) in Golden, Colorado, and successfully passed exhaustive testing. Their winning inverter design produced a power density of 143 W/cubic inch in 14 cubic inches, outperforming the Little Box Challenge power density goal by nearly a factor of 3, which, according to Google, “is 10 times more compact than commercially available inverters.”

Central to the team’s success were the GS66508P gallium nitride power transistors from GaN Systems, the leading manufacturer of GaN power devices. Commenting on the role that GaN transistors played in their design, team member Olivier Bomboir, VP of Product Management and New Business at CE+T Power, explained, “The use of GaN technology enabled our team to reach a power density of about 145 W/in³ for the 2 kVA inverter designed for this project. The reduced gate drive and switching losses of GaN Systems’ GS66508P were critical to our thermal and power density goals. Additionally, we were highly impressed at how reliably the devices performed over the months of rigorous, real-world testing by the NREL team.”

GaN Systems’ CEO, Jim Witham, said, “We congratulate the winners for their groundbreaking accomplishment, and are very excited that GaN Systems’ products played a key role in helping the Red Electrical Devils win The Little Box Challenge. This achievement is added confirmation that gallium nitride semiconductors are instrumental in helping power design engineers respond to the ever increasing need to develop more efficient power conversion solutions. GaN technology clearly paves the way toward more powerful, compact and efficient inverter designs.”