The well-established procedure for checking compliance of a
HEMP filter with the Mil-Std-188-125 specification is to inject a
very high current pulse into the front end of the filter and monitor
the residual current flowing through. Such pulse current injection
(PCI) testing is conducted independently of the filter manufacturer,
by trusted test houses utilising specialist and often proprietary
equipment. No test house in Europe offers such commercial PCI
testing, and filters are most commonly tested in the USA.
This PCI testing can only be carried out at the final stage of
development and requires that a prototype filter be made to the
same standard as a finished production filter would be. Hence it is
vital that any HEMP filter design submitted for PCI testing passes
first time, with no requirement for design modifications – which
can add both significant cost and further time delays.
When designing new or custom HEMP filter variants, MPE’s
philosophy centres upon achieving the most efficient and rapid
design, with the clear objective of minimising any changes
necessary to achieve the required filter specification and
Software modelling is at the core of this philosophy. Based upon
the OrCAD PSpice electrical design platform, MPE has built up a
library of parametric component designs which accurately replicate
real world conditions. In developing these libraries, particular
attention has been given to inductor core and capacitor winding
materials and their voltage, current and frequency performance.
Cabling and in particular insulation properties and characteristics
are modelled to accurately reproduce filter performance.
Whilst not exactly replicating Mil-Std pulse waveforms, MPE
has developed, in conjunction with the University of Liverpool,
an in-house pulse test capability which provides an accurate
representation of the Mil-Std E1 pulse for testing and optimising
new and variant designs.
Accordingly the Marx pulse generator comprises an 8/20μs pulse
tester for bulk current handling tests with a pulse current of up to
5kA, and a 5/200ns pulse tester for rise time checks. The energy
content of the 8/20μs pulse is higher than the Mil-Std E1 pulse,
but its rise time is much slower. The 5/200ns pulse demonstrates
the speed of response of the system. The 8/20μs pulse is used
with a view to meeting the E1 residual current limit of 10A.
These PSpice software modelling and in-house pulse testing
capabilities provide MPE with a high degree of confidence that,
during subsequent external PCI testing, any new or custom
HEMP fi lter designs will meet the requirements of Mil-Std-188-125
without the need for design modification work.
These in-house design and test capabilities are proving invaluable
for the successful supply of MPE’s new and variant, commercial
and modular, HEMP filter ranges for high-profile programs around