Guide to success in MIL/Aero power supply arena

In this article, we will explore the military/aerospace grade power supply and identify its key differences as compared to commercial off-the-shelf or COTS designs. We will also discuss some design techniques used to help ensure high reliability which is so critical in these types of designs.

Reliability
Military supplies are typically used in aircraft, sea-going vessels and transportation equipment. These obviously need to have a more robust construction for the extreme environments in which they will operate than a typical commercial power supply. Power supplies need a far better mean time between failure (MTBF) rating in military usage because reliable operation without failure can save lives.

Such Hi-Rel power supplies need to anticipate and correct failures in the system. A high level of predictability will make the user aware of specific signs something is changing in the performance of the supply and that might indicate a possible failure coming soon. Designers of military supplies have strict adherence to the selection of parts, the design process and specifications for use in varying environments.

The defence market, in many cases, may build systems using COTS power supplies, but there may be risks involved. Things like obsolescence, process changes, environmental vulnerability, faulty electrical performance or EMI issues need to be addressed to ensure reliable performance in combat or critical situations. There is a balance that must be struck in the design between the high cost of a full military class power supply and the manpower spent on designing in modifications in a COTS supply to bring the failure rate to the level needed in a particular scenario of use for the supply. The designer must choose wisely.

In designing a power supply that will be used in a military application, many different and varied design rules must be adhered to.

Obsolescence
Obsolescence is out of the question on programs that are mission critical and need to be in field operation for 20-30 years. Ultimately form, fit, and function replacement for field supplies need to ensure no or acceptable minimal documentation and specifications differences from the original power supply.

Redundancy
Redundancy in design ensures that no single failure will disable the supply or hinder its proper operation. Over and above redundancy the system must not be compromised via any effect on other equipment with fire, smoke, noise or any other hindrance to other equipment.

Design margins
The use of commercial components may be considered as long as a comfortable and generous design margin is chosen between the component worst case operating point and its rated data sheet specifications. It is a fact that as temperature goes up, reliability will begin to degrade. Using the Arrhenius Model can be an accurate prediction of a semiconductor or other device's reliability. Here is the equation:

R(t) = Axe^-(EA/KT)

In the equation A is a constant, EA is the activation energy of the reaction, κ is Boltzman's Constant and T is temperature in degrees Kelvin. This model predicts that as temperature increases, the rate to failure increases (figure 1).

Hardware circuit design techniques

FET ORing²
Redundancy can be achieved in Fault-tolerant Power supplies by using the diode ORing technique of the outputs on multiple power supplies. Since diodes can be inefficient due to their large forward voltage drop, especially in low voltage designs, more efficient FETs are typically used. Since a FET is able to conduct in both directions, unlike a uni-directional diode, the added controller must be used to turn off the reverse path.