UG:017
vicorpower.com
Applications Engineering: 800 927.9474
Page 16
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Both the visible LED and the opco-coupler draw current from the FT node in a fault condition. The FT
pin on the DCM has limited drive-high capabilities, and so care must be taken to avoid excess loading
of the pin. To avoid overload, do not configure J10 to use both the LED and opto-coupler indicators
simultaneously. When connecting external circuitry or test equipment to the FT test point, ensure that
the maximum load on the FT node is within the DCM datasheet ratings.
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In a parallel setup using the J02 & J03 paralleling connectors, all boards besides the top one should
have the fault jumper select block at J10 open.
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When using the opto-coupler, the status of the FT node can be easily transferred to the secondary side
of the DCM(s) isolation boundary. To resolve the fault state on the secondary side, the collector side
of the opto requires a bias voltage. A 5V bench supply should be connected between the “+5V” and
“SEC_SG” testpoints. With no fault present, “FT_SEC” will be at 0V, and when a fault occurs and the
opto-coupler is active, “FT_SEC” will pull up to 5V, relative to SEC_SG.
Chassis Ground
The heatsink assembly of the DCM is connected to the CHASSIS_GND node of the board, as well as the
y-caps from each power connection of the DCM. A connection from the CHASSIS_GND lug to earth
ground is required.
Paralleling
The paralleling and sharing performance of multiple DCMs can be easily demonstrated by stacking
multiple evaluation boards and interconnecting the inputs and outputs with standoffs to create a
parallel array. The DCM uses a negative load-line to implement wireless droop-sharing in an array. Each
DCM in an array operates in the same way as it does as a stand-alone unit. With equal trim conditions,
the load is effectively shared across multiple DCMs. Mismatches in this case are modest, and are further
canceled by an effective negative voltage vs. temperature coefficient. See the DCM datasheet for more
detail on load line and tempco. DCMs in an array require no derating of maximum output power or
current.
DCMs in an array with mismatched trim conditions will not share the load equally at light- to
moderate-load conditions. As the load increases, one or more DCMs (starting with those with the
highest programmed output trim voltage) will go into current limit and their contribution to the overall
output current will plateau. For DCMs, current limit is not a fault condition, rather it is a valid constant-
current mode of operation and a DCM in current limit will provide constant current to the load. As long
as the load does not exceed the maximum load rating of the array of DCMs, the output voltage will
continue to be regulated by any remaining DCMs still in constant voltage mode. Even with mismatched
trim conditions, the array can be safely loaded up to the full rated array capacity.
The following connections and settings should be used for an array of DCM evaluation boards:
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All DCMs in a parallel array must be the same model.
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The boards should be physically stacked using metal standoffs at the +IN & –IN lugs, the +OUT &
–OUT lugs, and the CHASSIS_GND lug. This also connects these nodes electrically so that a single
source, single load, and earth ground connection can be made to the system.
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The +IN lugs are not required to be connected together for an array of DCMs. The wireless
sharing does not require the same differential input voltage be present on all DCMs in the array.
In some applications dissimilar input voltages may be needed, which is fully supported.
