Guide to success in MIL/Aero power supply arena(4)

Drawbacks of the Wind Tunnel architecture include a fan that is exposed to the environment. This is a mechanism that can wear out. Also, the air inlet or outlet is prone to possibly become blocked. A water-tight enclosure would need to be employed if water immersion is possible.

Liquid cooling
In the case where a fully environmentally sealed package is needed, a liquid cooling architecture is often used. In this type of a system the heat exchanger may be located in the front of the vehicle. The normal cooling liquid employed is either water or a mixture of water and ethylene glycol (a.k.a., antifreeze).

For best design practice heat producing components should be mounted to a flat cold plate, which has the liquid circulating through it. The liquid is then routed through an external heat exchanger to cool it (figure 8).

Figure 8: An immersion capable, vehicle mounted converter with an internal cold plate is shown here that produces 21kW of 28VDC and 110VAC power from a 300VDC input, while being cooled with 80^o C coolant delivered from the vehicle. (Image courtesy of TDI Power in Reference 6)

Figure 9: The internal view of this complex Cold Plate design. (Image courtesy of TDI Power in Reference 6)

The cold plate approach has its drawbacks though. This architecture necessitates a flat shapes for heat transfer. This will spread the layout over a larger area, which can make electromagnetic noise difficult to deal with. This design also uses an excess of wiring, leading to higher cost and possible reduced reliability because of an increased number of interconnects.

An alternative design architecture shown in figure 10 provides 7kW of DC output power and uses a central liquid cooled and customised extrusion that provides cooling for a number of both flat and irregularly shaped components.

Figure 10: A Power Converter with an Extruded Liquid Cooled Central Structure (Image courtesy of TDI Power in Reference 6)

Summary
The Military/Aerospace sector is not all that different than any other design using COTS products. There are more stringent temperature, shock, humidity and vibration expectations due to the harsh environment these designs might encounter as well as the increased reliability that is expected and needed. Designers in this arena use different or modified architectures than those in most markets in order to meet their stringent goals and expectations by the customer. This article gives some insights into how designers deal with MIL/Aero challenges and what is expected to develop a robust design to meet the specific needs of this market.

References
1 Power Conversion Reliability, Gary Mulcahy, CTO of TDI Power, TW0059
2 Active ORing Solutions in Redundant Power System Architectures, Vicor
3 New Materials and Technologies Solve Hermetic SMD Integration, Bussarakons, International Rectifier, 1999
4 More Electric Aircraft Power System Development Facility, Applied Dynamics International.
5 Design of a Highly Reliable Three-Phase Four-Leg, Inversion Power Supply for More Electric Aircraft, C. Jiawei, W. Changyun, C. Jayampathi, AK Gupta, IEEE 10^th Conference on Industrial Electronics and Applications (ICIEA) 2015
6 NEXT GENERATION MILITARY VEHICLE POWER CONVERSION MODULES, G. Mulcahy, J. Santini, TDI Power

About the author
Steve Taranovich is a senior technical editor at EDN with 41 years of experience in the electronics industry. Steve received his MSEE from Polytechnic University, Brooklyn, New York, and his BEEE from New York University, Bronx, New York. He is also chairman of the Educational Activities Committee for IEEE Long Island. His expertise is in analogue, RF and power management with a diverse embedded processing education as it relates to analogue design from his years at Burr-Brown and Texas Instruments. Steve was a circuit design engineer for his first 16 years in electronics. He then served as one of the first field application engineers with Burr-Brown Corp and also became one of their first global account managers, traveling to Europe, India and China.