Specifying a Capacitor to meet MIL-STD-704/DO-160 Power Hold-Up Requirement White Paper

Today’s aircrafts and airborne vehicles have evolved significantly from their origins. They contain a suite of advanced navigation, communication, and display systems. This is particularly true on military aircrafts which use cutting edge technologies that add capabilities and improve effectiveness, performance, and safety. As these advanced systems are increasingly incorporated into modern avionics systems, airborne electric power systems have evolved to support them. In avionics, these technologies are referred to as utilization equipment. Utilization equipment is defined as any equipment which receives power from the aircraft’s electric power system.1 Some types of utilization equipment will not properly function if their supply of electric power is interrupted. In these cases, power hold-up functionality is required to ensure that certain critical systems remain in operation during certain power interruption events. MIL-STD-704 and DO-160 are standards that designers use to determine the required power hold-up capability of a system. Power hold-up is achieved by using capacitors. This whitepaper will help designers of power supplies and/or airborne systems determine how much capacitance is needed to support an application’s unique power hold-up needs and how to choose the right aerospace grade SWaP (Space, Weight, and Power) optimized capacitor to ensure compliance with MIL-STD-704 and/or DO-160. Understanding Airborne Power Needs Aircrafts and airborne vehicles have multiple electric power sources. Engines drive onboard generators to provide the main power. Other onboard power sources include batteries, independent auxiliary power units, and air or hydraulically driven generators.2 While on the ground, an external power source such as a Ground Power Unit (GPU) can be used. A transfer operation occurs when there is a switch from one electric power source to another.3 During a transfer operation, there is a brief lapse in electric power that is supplied to the utilization equipment. This is known as a power interruption. Even a 20ms power interruption event can disrupt some types of utilization equipment to the point that they may not function for a period of time, which can create performance and safety issues. There are military and FAA standards (MIL STD-704 and DO-160) that specify that some types of utilization equipment must have the ability to continue to operate during a power interruption. This ability is referred to as “power hold-up”.
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