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The inclusion
of the Oring diodes in series with
the output is critical to the (N+1)
configuration, and they are not
optional if the objective of the
paralleling is redundancy. These
diodes add cost, voltage drop and
heat, but it is a necessary penalty.
The diodes guarantee that no matter
what type failure occurs in one
of the PM’s, it will not drag
the DC bus down. Such a failure
can be any component open or shorted
(on primary or secondary side),
including shorted output capacitors
of the PM. In the opinion of the
author no true redundant power system
can be without Oring diodes.
The use of shottky
diods for the Oring function is
advisable due to their relatively
lower voltage drop, however this
drop may too high if the output
is low voltage (5V, 3.3V) and high
current (above 10A). In that case
a Fet Transistor with low Rdson
should be used instead. A 5 mOhm
Fet will drop only 250mV for a 100A
output in contrast with 750mV for
a shottky diode.
The saving in power
is 50Watts – a full 10% of
the output power, and a monumental
problem to deal with effectively
in a high density package.
But the use of
an Oring Fet can be rather trickey
due to its ability to conduct in
both direction especially if the
PM employs synchronous rectification
to produce the output voltage. In
such a case, the functioning modules
in the N+1 system can pump current
into the failed power module as
long as the Oring Fet permits such
reverse current flow. The synchronous
rectifier in this mode acts as a
booster and pump energy inside the
failed PM from the common output
DC bus to the primary side. As a
result of the extra current drain
on the remaining PM’s of the
redundant system, they may sag or
overheat.
It is therefore
necessary to ensure that the Oring
Fet does not get any gate voltage
(through some direct or sneak path)
if the PM fails. So that it will
not conduct in reverse. Such an
arrangement may be to produce the
gate voltage of the Oring Fet from
a dedicated secondary of the power
transformer. If the PM fails or
shut down or is willfully inhibited,
then the gate cannot receive any
drive and the Fet will be turned-off.
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