A dc-dc converter and a dc-ac inverter have been developed for More Electric Aircraft (MEA) use. They are both highly compact and are designed to operate from 270 Vdc input power and produce output power that meets requirements for fighter aircraft, including compliance with MIL-STD-704E and MIL-STD-461D guidelines. Both units are designed to be cooled with Poly Alpha Olefin (PAO) at 30°C maximum inlet temperature and to operate for 30s after a loss of coolant flow before shutting down. Both units incorporate under/over voltage, short circuit, and over temperature protection. The dc-ac inverter also has over/under frequency, dc content, waveform distortion, and zero voltage content protection.
The dc-dc converter provides 5.6 kW at 29±0.5 Vdc with an efficiency of 85%. It weighs 9.1 lb and is 6″ long by 5″ wide by 5.5″ high (165 in3). The dc-ac inverter provides 8 kVA of three phase power at (115±1.5)/200 Vac and 400 Hz with an efficiency of 75%. It weighs 24 lbs and is 21.2″ long by 5″ wide by 5.5″ high (583 in3). The small size and light weight of the units results from the use of a high frequency series resonant input inverter, and the use of a small transformer with thin planar copper windings to transfer the resonant current energy through a rectifier to a dc voltage output capacitor. Insulated Gate Bipolar Transistors (IGBT) in the input inverters, used for both the dc-dc converter and dc-ac inverter, switch at zero current at a frequency of 120 kHz. In the dc-dc converter, the transformer output current is rectified with a full bridge of Schottky diodes.
In the dc-ac inverter, there are four independent secondary windings per phase whose rectified voltages are summed by switching Field Effect Transistors (FET) to produce a 400 Hz sine wave output. A novel switching technique combines pulse width modulation with the multi-step approach to produce an output sine wave with less than 2% total harmonic distortion and minimal filtering requirements.
The paper describes the design of both units and the resulting performance.