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Guide to success in MIL/Aero power supply arena

Posted: 11 Feb 2016     Print Version  Bookmark and Share

Keywords:military/aerospace  power supply  mean time between failure  MTBF 

A typical and efficient active ORing solution will combine a Power MOSFET and a controller IC. The MOSFET has an on-state resistance characteristic, RDS(on), that when multiplied by the square of the current through the device, creates power loss in the MOSFET. This can be substantially lower than the power loss of a Schottky diode for the equivalent current. A good active ORing solution can typically show a 10x reduction in power loss versus diodes.

Figure 2 shows a diagram for this architecture.

To have a successful solution using this technology an active ORing solution2 designers must observe the following rules: (1) accuracy with respect to the reverse current threshold across the MOSFET, (2) fast response time from detection of any reverse current, and (3) high efficiency resulting in very low power dissipation and lack of dependence on thermal management overhead.

Figure 2: A redundant system using high-side active ORing (Image courtesy of Vicor, Reference 2)

Hi-Rel Power Elements

The power element, such as a power MOSFET, can carry high currents and voltages while driving demanding loads at high temperatures in a harsh environment scenario. The hermetic, surface-mount device (SMD) package must be used in high reliability applications. The package has three terminal pads, a ceramic housing, seal ring, and lid brazed together forming a hermetic, semiconductor die package. Figure 3 shows a SMD package cross section.

Figure 3: A SMD package cross-section not to scale (Image courtesy of IR, Reference 3)

The hermetic SMD has smaller size, lighter weight, great thermal performance, improved switching waveforms and better efficiency in the circuit driving the load than other types of packages.


More Electric Aircraft Power Systems (MEAPS)3
In the last ten years the aircraft industry, military and commercial, has been making a concerted effort to move away from traditional power presently used to a more electric approach in non-propulsive power systems as a first order of business. This secondary power has been in the form of hydraulic, pneumatic, electrical and mechanical power extracted from the engines of the aircraft.

Energy efficiency is a big driver of this effort and as a result power electronics development has brought such innovations and enhancements in electromechanical actuators (EMAs), Power converters, fault-tolerant electric motors/generators, and electro-hydrostatic actuators (EHAs) (figure 4).

Figure 4: An electromechanical actuator simulation that is executed on the active load Hyperfast real-time simulator. (Image courtesy of Reference 4)

The industry is looking into 270VDC systems as a high voltage DC power distribution main. This will mean lower currents at higher voltages, but lower currents give way to less copper and thus lower weight on the aircraft. This translates into better fuel economy.

A Model Based Systems Engineering Solution (MBSE) has been proposed which has evolved over the last 20 years from power system and power electronics concepts. Reference 4 is a most recent proposal of a Power-Hardware-in-the-Loop development and test facility combined with flight simulation and power system simulation.

A 3 Phase, 4-Leg Inversion Power Supply for MEA
Powering many legacy 115VAC/400Hz loads on an aircraft is facilitated by an inversion power supply. The most recent effort in this area has been with Space Vector Pulse Width Modulation (SVPWM) controlled 3-Phase, 3-Leg inverter architectures. Unfortunately, this architecture cannot operate in an unbalanced load condition, especially in the event of a short-circuit. This will not be acceptable in military or commercial power in an aircraft.

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