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bitstream [soylentnews.org] writes:

In 2014 Google and IEEE announced a one million USD prize to build the most efficient and compact DC to AC converter (450 V DC to 240 V 60 Hz). It was called the Little Box Challenge, with the goal [littleboxchallenge.com] of a 2 kVA electrical converter with a 95% efficiency and a power density greater than 3,1 kW/dm³. A typical solar DC to AC converter have a power density of about 0.31 kW/dm³. The results are in [blogspot.com] from the competition. Where the winners from Belgium accomplished 8.7 kW/dm³ using GaN transistors [wikipedia.org]. Two other teams also meet Google's goals.

More than 2000 teams from across the world registered for the competition and 80 proposals qualified for review by IEEE and Google. The winning team the Red Electrical Devils exceeded the power density goal for the competition by a factor of 3, which is more than 28 times more compact than commercially available electrical power converters.

Rated for power conversion density:
CE+T Power Red Electric Devils: 8.7 [kW/dm³]
Schneider Electric: 5.9 [kW/dm³]
Virginia Tech's Future Energy Electronics Center: 4.2 [kW/dm³]
(Typical converter: 0.31 [kW/dm³])

Volume given power conversion density greater than 3.1 W/dm³ and minimum 2 kVA capacity:
CE+T Power Red Electric Devils: 0.23 [dm³]
Schneider Electric: 0.34 [dm³]
Virginia Tech's Future Energy Electronics Center: 0.48 [dm³]
(Typical converter: 6.6 [dm³])

The winner design [littleboxchallenge.com] uses GaN transistors operating in ZVS (Zero Voltage Switching) mode to accomplish low Rds_on, Q_gate, C_ds, and ultra low Qrr. This gets one ahead of MOSFET and IGBT designs but are more challenging to drive and manage electromagnetic interference from due to the fast switching. Another pitfall is the high voltage drop due to the reverse current when the GaN is turned off. The selected solution to handle this is to control all the GaN transistors using soft switching [samexent.com] for the entire operation range. The converter is controlled by a microcontroller combined with a CPLD, fast measurement of input/output currents and voltages, efficient feedback on the switching events of the half-bridges (HB [wikipedia.org]), a learning algorithm for the active filter, optimization of the switching frequency between 35 - 240 kHz depending on the output current, a variable phase shift between the HBs (0° or 90°) and a dead time of the five HBs (50 ns to 3 μs). This almost cancels the switching losses and the frequency increase helps to optimize the size of the passive components. Important is also the selection of a surface mounted device (SMD) GaN package with 2 source accesses: one for the power, one for the command.

The parallel active filter makes the ripple requirement on the DC input to be within set boundaries and by using ceramic capacitors whose capacitance rises with the voltage. The magnetic components are mainly composed of a ferrite and Litz wires directly wound onto the ferrite without a coil former. Cooling is done with a fan and aluminum oxide foil placed in the middle of the ferrite to create the requested air gap.

A soft switching using two inductors and one capacitor (“L-L-C [microsemi.com]”) resonant topology is used for the isolated 10 W auxiliary supply.

The electromagnetical interference is kept under control by the use of soft switching of the main switches and auxiliary supply independently of the load, variable frequency and a specific spread spectrum modulation, double shielding enclosure, last filter stage shielding, an AC out filter referenced to the negative input lead, and many small filters instead one large, suppression of all the resonant poles [wikipedia.org] at frequencies higher than 50 kHz, the use of ceramic capacitors to minimize the parasitic inductance. The use of multi layer capacitors (MLC) components for energy storage is also important.

(remove this line at publication - Please have a check at the grammar, I suspect some mistakes)

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