EMC & Circuit Protection


Managing power supply abnormalities


Steve Munns looks at a solution for securing power supply protection for aviation and defence applications


L


RUs (Line Replaceable Units) are modular subsystems that share standard physical and electrical interfaces. They are widely used by the military and within civil aviation where maintenance operations in remote locations can be simplified through the simple exchange of units. Each one must be designed to comply with rigorous specifications and standards to ensure compatibility and reliable operation in harsh environments and protect equipment from power supply abnormalities in the form of voltage 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 summarises their transient 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, and 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.


Challenges


Starting with voltage spikes, these tend to be characterised as several hundred volts for tens of microseconds, originating from lightening strikes or inductive coupling of


load steps. Present solutions using a transient voltage suppressor in the LRU connector assembly, combined with a PI filter and ferrite bead arrangement are effective and space efficient. A more challenging area is preventing


propagation of voltage surges of typically less than 100V for periods of tens or hundreds of milliseconds resulting from load dump. This occurs when the disconnection of one load circuit induces a short and rapid increase in voltage across the alternator and therefore in other loads sharing the same supply. One solution is to use a passive network comprising a series- inductor and high value electrolytic bypass capacitor, combined with a transient voltage suppressor and fuse. Such solutions tend to be bulky and some transmission of higher voltages can still occur, requiring the downstream components to be tolerant of higher input voltages than would otherwise be necessary. Around the industry, designers have independently developed active solutions based around discrete components using a MOSFET pass element but these typically require significant bench time to optimise the sensing, control loop and pass transistor circuitry. Keeping the MOSFET pass element from overheating and within its safe operating area is often cited as the most challenging part of the design.


Sometimes a fuse is still required to protect the MOSFET from an output short circuit fault condition. Naturally the replacement of blown fuses could present unwelcome logistics complications for civil aviation, or could put important military equipment temporarily out of action during critical operations. One solution to the voltage surge problem is the LT4356 surge stopper IC, a device well suited to the task, whose operation will be described in more detail below.


Finally voltage ripple on the incoming LRU supply can present further design challenges, in particular the MIL-STD- 1275D specification for military vehicles in generator mode is quite extreme. Various approaches are employed, including allowing the protection circuit to pass through the ripple to the voltage regulation stage or where voltage ripple is present at more modest amplitudes, smoothing it within the protection circuit itself. In the latter case the protection circuit must be optimised to cope with the dissimilar characteristics of large voltage surges and small amplitudes of slowly varying ripple.


Trends Pressure on costs, space, and weight, combined with the increasing requirement for multiple low- voltage, high-current supply rails to power complex FPGAs and


Table 1: Summary of transient requirements 20 September 2011 Components in Electronics


Table 2: Product options www.cieonline.co.uk


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