Power supply protection for aviation and defense applications

Linear Technology has supplied high-performance analog integrated circuits to the defense industry for nearly 30 years and continues to dedicate resources to support the specialized needs of the market. The company offers a wide range of product grades including industrial, automotive and military temperature ranges with industry-leading quality and a “best in class” reliability level of 0.1 FITS.

Introduction
LRUs (Line Replaceable Units) are modular subsystems that share standard physical and electrical interfaces. LRUs are widely used by the military and within civil aviation where maintenance operations in remote locations can be simplified through simple exchange of units. Each LRU must be designed to comply with rigorous specifications and standards to ensure compatibility and reliable operation in harsh environments.

This article will discuss the LRU power interface, specifically the requirements related to protection of downstream DC/DC converters and linear regulators from power supply abnormalities in the form of voltage spikes, surges and ripple.

Spikes, surges and ripple
The definitions of spikes and surges vary somewhat between standards. Individual countries have developed their own national government standards for the military, while aircraft manufacturers cite their own specifications and the RTCA (Radio Technical Commission for Aeronautics) develop international standards to bridge the two worlds. Table 1 shows three of the key references and summarizes their transient requirements.


Table 1 – Summary of transient requirements
(Click on image to enlarge)

The space available necessitates that this is a summary of requirements; each specification contains detailed performance envelope diagrams and test conditions. The ripple specification for MIL-STD-1275D generator only mode is shown as the worst case example, normal operating mode is specified at ±2V from steady state conditions. Numerous other national specifications exist such as the United Kingdom’s DEFSTAN 61-5 (part 6) for military vehicles which has broadly similar requirements to MIL-STD-1275D but for 12V and 24V systems.

The system must be designed to survive the worst case conditions specified but usually the objective is to develop a circuit that continues to operate through the specified transient excursions until a predetermined safety shutdown point is reached. In some cases full compliance to the specification cannot be achieved within the permitted LRU box dimensions due to the physical size of suppression components needed, this leads to amendments to the specifications being agreed between customer and supplier.

Such diversity makes it difficult to develop a single solution that fits all needs and in addition there are many variations on the standard requirements from project to project, with adjustment to compliance requirements reflecting the operational circumstances for the LRU in development. As a consequence, designers tend to adopt a case by case approach to protection circuits.