HEMP filter design to meet MIL-STD-188-125 PCI test requirements

Philosophies for the best method of EMP protection have been around for a long time. MIL-STD-188-125 -1 and -2 are more recent specifications for defining requirements for EMP protection. They specify PCI (pulse current injection) testing techniques to demonstrate the integrity of the protection system, but require specialist test equipment capable of injecting high voltage high current pulses. The residual current let-through of the protection system plus other parameters are monitored as a measure of the effectiveness and compliance of the protection system. Because the demand for EMP protection systems is specialised and not high volume, the practical method of meeting the MIL-STD-188-125-1 and -2 specifications has normally been based on adapting certain types of standard catalogue EMI facility filters. However, most catalogue EMI filters are designed for CW insertion loss performance, so are usually not ideal for pulse applications. The approach described here is to design filters to meet a pulse attenuation performance rather than to meet an insertion loss specification. This enables a filter circuit and components to be used which are optimised for pulse applications to achieve a more efficient design with a good pulse performance. This paper describes the design process and validation of the design by testing using the MIL-STD-188-125 specified pulses.