GE Industrial Solutions 40A Analog MegaDLynx - Manual (Page 12)

GE
Data Sheet
40A Analog MegaDLynx TM : Non-Isolated DC-DC Power Modules
4.5Vdc –14.4Vdc input; 0.6Vdc to 2.0Vdc output; 40A Output Current
April 24, 2013 ©2012 General Electric Company. All rights reserved. Page 12
down if the overtemperature threshold of 145°C (typ) is
exceeded at the thermal reference point T
ref
.Once the unit
goes into thermal shutdown it will then wait to cool before
attempting to restart.
Input Undervoltage Lockout
At input voltages below the input undervoltage lockout limit,
the module operation is disabled. The module will begin to
operate at an input voltage above the undervoltage lockout
turn-on threshold.
Synchronization
The module switching frequency can be synchronized to a
signal with an external frequency within a specified range.
Synchronization can be done by using the external signal
applied to the SYNC pin of the module as shown in Fig. 27,
with the converter being synchronized by the rising edge of
the external signal. The Electrical Specifications table
specifies the requirements of the external SYNC signal. If the
SYNC pin is not used, the module should free run at the
default switching frequency. If synchronization is not being
used, connect the SYNC pin to GND.
MODULE
SYNC
GND
+
─
Figure 27. External source connections to synchronize
switching frequency of the module.
Active Load Sharing (-P Option)
For additional power requirements, the Mega DLynx TM
power module is also equipped with paralleling capability.
Up to five modules can be configured in parallel, with active
load sharing.
To implement paralleling, the following conditions must be
satisfied.
 All modules connected in parallel must be frequency
synchronized where they are switching at the same
frequency. This is done by using the SYNC function of
the module and connecting to an external frequency
source. Modules can be interleaved to reduce input
ripple/filtering requirements.
 The share pins of all units in parallel must be connected
together. The path of these connections should be as
direct as possible.
 The remote sense connections to all modules should be
made that to the same points for the output, i.e. all VS+
and VS- terminals for all modules are connected to the
power bus at the same points.
Some special considerations apply for design of converters
in parallel operation:
 When sizing the number of modules required for
parallel operation, take note of the fact that current
sharing has some tolerance. In addition, under
transient conditions such as a dynamic load change
and during startup, all converter output currents will
not be equal. To allow for such variation and avoid the
likelihood of a converter shutting off due to a current
overload, the total capacity of the paralleled system
should be no more than 90% of the sum of the
individual converters. As an example, for a system of
four MegaDLynx
TM converters in parallel, the total
current drawn should be less that 90% of (3 x 40A), i.e.
less than 108 A.
 All modules should be turned ON and OFF together. This
is so that all modules come up at the same time
avoiding the problem of one converter sourcing current
into the other leading to an overcurrent trip condition.
To ensure that all modules come up simultaneously, the
on/off pins of all paralleled converters should be tied
together and the converters enabled and disabled
using the on/off pin. Note that this means that
converters in parallel cannot be digitally turned ON as
that does not ensure that all modules being paralleled
turn on at the same time.
 If digital trimming is used to adjust the overall output
voltage, the adjustments need to be made in a series of
small steps to avoid shutting down the output. Each
step should be no more than 20mV for each module.
For example, to adjust the overall output voltage in a
setup with two modules (A and B) in parallel from 1V to
1.1V, module A would be adjusted from 1.0 to 1.02V
followed by module B from 1.0 to 1.02V, then each
module in sequence from 1.02 to 1.04V and so on until
the final output voltage of 1.1V is reached.
 If the Sequencing function is being used to start-up and
shut down modules and the module is being held to 0V
by the tracking signal then there may be small
deviations on the module output. This is due to
controller duty cycle limitations encountered in trying
to hold the voltage down near 0V.
 The share bus is not designed for redundant operation
and the system will be non-functional upon failure of
one of the units when multiple units are in parallel. In
particular, if one of the converters shuts down during
operation, the other converters may also shut down
due to their outputs hitting current limit. In such a
situation, unless a coordinated restart is ensured, the
system may never properly restart since different
converters will try to restart at different times causing
an overload condition and subsequent shutdown. This
situation can be avoided by having an external output
voltage monitor circuit that detects a shutdown
condition and forces all converters to shut down and
restart together.
When not using the active load share feature, share pins
should be left unconnected.