Guide to success in MIL/Aero power supply arena

Designers have tried Sinusoidal pulse width modulation (SPWM) controlled 3-Leg, 4-Phase inverter architectures, but its DC voltage utility is too low to meet the higher voltage bus levels of 270VDC as compared to SVPWM methods and this technique would need higher DC input voltages than desired.

In Reference 4 we introduce a new saddle pulse width modulation (SAPWM) scheme which is in turn applied to a 3-Phase, 4-Leg inverter that creates a very high reliability inversion power supply. This architecture seems to be quite suitable to MIL/Aero applications.

Military power designs

I will use the Military Vehicle as a prime example of how power designs can become robust enough to meet the tough environment and reliability needs of the military market. Most of the following architectures and techniques can be applied to other military products needed in this sector.

Next-gen MIL vehicle power⁶
Next generation MIL vehicles will use higher voltage energy storage plants such as Nickel-Metal Hydride or Lithium-Ion Batteries, or Super Capacitors. These will typically operate at around 300VDC to optimise hybrid motor operation. These higher voltage power units have the ability to provide significant amounts of power with drastically reduced audible and thermal signatures, the availability of this higher voltage power presents some significant advantages to the export power conversion system. Distribution currents are reduced by 90% (for example from 500A to 50A) from the traditional 28V case, and components within the power converter can be reduced in size due to reduced operating currents. Lower currents also mean less copper which leads to lower cost and weight (figure 5).

Figure 5: Export power in an Electric Hybrid Military Vehicle (Image courtesy of TDI Power in Reference 6)

Most times export power converters will be located outside the crew compartment and will be exposed to a very harsh and wet environment. Operating temperatures typically will range from –46 to +54^o C. Mechanical vibration and shock requirements are characterized in MIL-STD-810F, Methods 514.5 and 516.5.

Power conversion densities in a vehicle can range from 3.4W/in3 to as high as 10.3W/in3 . Most cooling requirements will necessitate either forced air cooling or liquid cooling. The designer must choose the best method to get the heat out of the power supply depending upon the particular system being designed and where it is likely to be deployed in the worst case scenario.

Forced air cooling
Forced air and Liquid cooling are two of the top choices for both environmentally sealed and non-sealed power supplies. In a forced air system, a relatively protected and clean air environment will need to be available in the expected deployment of the vehicle otherwise dust, dirt and moisture will lead to system power failures. This type of a cooling system should allow for a good high power conversion density in the supply.

Figure 6: An Environmentally Hardened Air Cooled Assembly (Image courtesy of TDI Power in Reference 6)

An indirect system which has no direct air blowing onto internal components is known as a "wind tunnel" design. This system uses fans to cool heat sinks and allows the electronic components to be protected from the environment by locating them on the other side of the aluminium heat exchanger. A drawback with this technique is a larger package so the designer needs to decide upon a compromise in their system. Figure 7 depicts a typical wind tunnel design that has been used in airborne and ground based applications by TDI Power.

Figure 7: A 10kW DC/AC inverter Wind Tunnel design for MIL systems. The size of this unit is 20" x 8.17" x 12", delivering approximately 5W/in3 at 10kW from an input voltage of 360VDC. If this unit was redesigned for a 28VDC input, the power conversion density would be lowered to around 3W/in3. (Image courtesy of TDI Power in Reference 6)

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