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Diode Protection For Redundant Power Supplies
A DC power supply is a critical piece of a control system. Power supply
failure can lead to a process shutting down resulting in considerable lost
revenue. To prevent unplanned shutdowns users often use redundant power
supplies, i.e. power supplies with their outputs connected in parallel, where if one
supply fails there are enough remaining power supplies to deliver the required
load current.
Redundant power supplies can be also be used for increased current
capacity – in this case the concern is less about unplanned shutdowns but rather
the fact one supply cannot deliver the load current required. For increased
current capacity power supplies with “active load sharing” are preferred. This
involves the power supplies communicating with each other and adjusting their
output voltages so that each supply delivers the same current. If two supplies
are used each one will deliver half the required current. If three supplies are
used each one delivers a third of the total current required. Without active load
sharing each power supply must be adjusted so that their output voltages are
identical, they much be maintained at the same ambient temperature and the
wiring between each power supply and the common point should be of equal
lengths – all difficult conditions to maintain.
Figure 1 – Redundant Power Supplies with “ORing” Diodes
Regardless of the reason for using redundant power supplies it is common
practice to connect a diode in series with the output of each power supply so a
failed power supply cannot draw current from any operating power supplies. If
there is a short circuit in the output of a power supply that unit will shutdown but it
could also short circuit any power supplies connected in parallel causing those to

shutdown as well. A diode on the output of each supply prevents this from
happening.
Figure 2 – Diode prevents P/S 1 from shutting off if P/S 2 short circuits
Using diodes to isolate power supplies, often referred to as “ORing
diodes”, has the disadvantage of increasing power dissipation within the control
cabinet. A typical diode drops the voltage 0.7V when conducting a current. This
means 0.7W per amp of load current. If two redundant power supplies are
delivering a total of 10A then the diodes will dissipate 7W in the cabinet. Using
Schottky diodes could lower the power dissipation to approximately 4W. Either
way, this power dissipation is unwanted and increases the ambient temperature
within the cabinet. Sometimes bridge rectifiers are used because they are
relatively inexpensive, are designed for high power dissipation, and can be
mounted directly on the wall of the cabinet so that the cabinet acts as a heat sink.
Reliability is a concern (or should be) when using ORing diodes. If a
diode fails in a short circuited state then there is no protection and the voltage
from that supply appears to increase by 0.7V. Unless active load sharing is used
that supply may try to deliver the total load current. If the power supplies are
connected in parallel for increased current capacity then the supply might go in to
an overcurrent condition and shutdown. If one of the ORing diodes fails in an
open circuit state then there is no longer any redundancy and a shutdown of thr
remaining power supply could occur.

Diode
No additional circuitry required
Low cost
High power dissipation
Large packaging due to heat sink
MOSFET
Control circuitry required
Higher cost
Very low power dissipation
Compact packaging
Although the circuitry used in an active diode circuit is more expensive
than a diode alone this is partially offset by the fact there is no need for a heat
sink (or only a small heat sink is required). Active diode circuits can be very
compact and easily packaged in DIN rail mounted housings.
The photo above shows two ORing modules with the same rating – two
20A inputs and a 40A output. The one on the right uses diodes and requires just
over 110mm of DIN rail and includes a cooling fan to allow it to operate at high
ambient temperatures. The unit on the left is an active diode module and

equires just 38mm of DIN rail. It can operate at the same elevated ambient
temperature but requires no cooling fan.
Shown above is an active diode module to be released by Emphatec in
early 2013. It is rated 15A and requires just 8mm of DIN rail. This unit features a
single input and output – it does not perform an ORing function on its own. One
of these modules would be installed per power supply.