Trade Study on Panasonic ELF-15N002A Common Mode Choke Inductor
Motivation:
The common mode choke inductor, a Panasonic ELF-15N002A, in the 96V
power circuit, between the twisted pair connection and the DC-DC
converter, is rated to only -25°C, according to the data
sheet. This minimum temperature does not meet the industrial
temperature range imposed on all other components in the DOM MB.
Analysis:
The ELF-15N002A was placed in the circuit to suppress common-mode noise
generated by the DC-DC converter that can otherwise modulate the
communications waveforms. Spice simulations suggest
that the resistance of the ELF-15N002A helps to tame resonances in
the filter circuit.
Objective:
Obtain, or perform qualification for the ELF-15N002A at -40°C, or
locate a substitute part with similar performance at -40°C, or
justify the removal of the problematic part from the circuit.
Action:
Panasonic Product Support department was contacted regarding the
temperature rating of the ELF-15N002A, by Ed Kujawski. No one
would commit to the suitability of the part below the published rated
temperature. The path was assumed to be a dead-end.
Several alternate products were obtained from Coil Craft. They
were physically much larger than the Panasonic part. They had lower
series resistance, and lower inductance. Tacking the smallest of
these into the circuit in place of the Panasonic part caused no
measurable degradation of the signal to noise ratio of the
communications front end.
The circuit was tested with the Panasonic part shorted out, but still
soldered in place. No measurable degradation in performance was
noted.
Removing the Panasonic part entirely from the circuit had no noticeable
affect on noise performance in the communications front end, however,
there was a hint that the noise level in the PMT front end.
Additional shielding studies, however, convinced us that the presence
of the Panasonic CM choke at that location in the PC board did not
"shield" the analog front end from the magnetic or electrostatic noise
generated by the DC-DC converter.
Five of the Panasonic Common Mode Chokes were temperature cycled ten
times between 77K and 0°C, by dunking in liquid nitrogen.
There was no evidence of degradation of the plastic. The wire and
Ferrite were assumed to be OK at -40°C.
Conclusion:
The Panasonic ELF-15N002A common mode choke may be safely dropped from
the design, and replaced by jumpers on the existing layout.
Alternately, the ELF-15N002A may be removed from the schematic,
and layout, provided the circuitry is corrected to maintain continuity
between the DC-DC converter and the connections to the main cable
twisted-pair circuit.
May 3, 2004, Gerald Przybylski