Data Sheet
March 27, 2008
QBW018A0B Series Power Modules, DC - DC Converters:
36 – 75Vdc Input; 12Vdc Output; 18A Output Current
10 LINEAGE POWER
Feature Description (continued)
Forced Load Sharing (Parallel Operation with
– P option)
For additional power requirements, the power module
can be configured for parallel operation with active
load current sharing. Good layout techniques should
be observed for noise immunity when using multiple
modules in parallel. To implement active load
sharing, the following recommendations must be
followed:
• The parallel pins of all units in parallel must be
connected together. The path of these
connections should be as direct as possible, but
should not pass beneath the perimeter of the
module body, except immediately adjacent to the
parallel pin location.
• Parallel modules must use the same 48V source.
The VIN (-) input pin is the return path for the
active current share signal of the parallel pin.
Separate 48V sources will prevent the active
current share return signal from being connected
to other modules.
• The VIN (-) input connection should never be
disconnected from any of the parallel modules,
while another of the parallel modules is
operating, unless the VIN (+) pin, or the parallel
pin is also disconnected. The VIN (-) input
provides the internal logic ground and for the
module’s primary circuits, including the active
current share circuit; and there are sneak paths
through the module’s internal control ICs, when
the VIN (-) pin is disconnected (allowing the
internal logic circuit to float), while the parallel pin
and VIN (+) pin are connected to other operating
modules. These sneak paths do not cause
permanent damage, but do create false
conditions that can affect the module’s internal
logic configuration.
• The on/off pins of all modules should also be tied
together to the same external control circuitry, so
that the modules are turned on and off at the
same time, unless all parallel modules’ on/off
pins are tied to the input pins for automatic start
upon application of input voltage.
• When modules in parallel applications contain the
auto-restart (4) option, it is required that the total
maximum load current value be less than 90% of
[n-1] times the individual module output current
rating, where n is the number of modules in
parallel. For example, if the application is using
three modules rated at 18A, then the maximum
total load shall be less than 0.9 x (3-1) x 18A =
0.9 x 2 x 18A = 32.4A. This insures that a single
module can shutdown without causing the total
load to exceed the capability of the remaining
operating module(s). The shutdown module can
then automatically restart, and assume its share
of the total load.
• In all parallel applications (including applications
meeting the [n-1] sizing criteria discussed
earlier), if it is expected that a protective
shutdown event could cause more than one
parallel module to shutdown (for example, over
temperature due to a common fan failure, or
gross over current affecting two or more modules
simultaneously), then the use of the auto-restart
(4) option is not recommended. The auto-restart
interval of these modules is not synchronized to
other modules, nor is it precise. There will not be
a successful restart following multiple module
shutdowns, because the individual module’s
restart timings will be different. There will not be
sufficient module capacity to prevent the first
module which restarts from experiencing an over
current, and then again shutting down before the
slowest module has restarted. Meanwhile, the
slowest module will then restart, and then
shutdown during the interval the fastest module is
waiting for its next restart. And so on and so on.
In these cases, only latching shutdown modules
should be used; and either toggling the Vin source
or the on/off pin to simultaneously restart the
modules, following a shutdown, is advised.
When not using the parallel feature, leave the share
pin open.