Magnetic devices continue to be a problem area for TCSS. Poor quality
magnetics can have a high fall-out rate from TCSS. The problem is the
magnetics are not effectively screened at the PWA level because the
magnetic’s temperature rate of change lag the rate of change of the rest of
the parts by an excessive amount. A piece of equipment that, excluding the
magnetics, can reach thermal stability in 15 minutes may take several hours
for the magnetics to reach thermal stability. The dwell time at the
temperature extremes is normally set for the smaller electronic parts and
not for the magnetics.
There is an alternative screen. Magnetics should be built to MIL-T-27 and
should be subjected to the optional MIL-STD-202 thermal shock screen on a
100 percent basis. Data supports that thermally shocked magnetics have
little or no additional fallout with subsequent equipment thermal cycling.
Thermally shocking magnetics has for sometime been a recommendation for
power supply reliability and is a specification requirement for the Navy’s
Standard Power Supply Program.
4.7.2 WATER COOLED POWER SUPPLIES
Power supply thermal cycling has presented some unique problems. There
have been several cases of power supplies being thermal cycled while mounted
on water cooled cold plates. This means that the unit cannot be cycled to
temperatures much below 0°C or the water in the cold plate will freeze.
Worse, the water stabilizes the internal temperature of the unit which means
the electronics are not actually experiencing the thermal cycling.
A better alternative is to perform most thermal cycling on power supplies
without the water cooled plates. In many cases, power can still be applied,
but care must be taken not to cause degradation by thermally overstressing
same guidelines should be followed for non power supply water cooled
There are many questions about the value of thermal cycling of potted
power supplies and modules. Most questions arise out of a lack of
understanding. Thermal cycling must be accomplished prior to potting (so
that repair is feasible and to insure adequate screening parameters). Power
must be off if the potting compound provides electrical insulation or a
better thermal path than convective