Vicor 4kW MegaPAC AC-DC Switchers - Manual (Page 26)

UG:107 vicorpower.com Applications Engineering: 800 927.9474 Page 26
Current Share Boards - Optional Feature
"Current sharing" also know as Load Sharing, is the ability to divide the output current
evenly across all active power supplies. This greatly reduces stresses on each power
supply and allows them to run cooler, resulting in higher reliability. Standard "current
sharing" techniques typically utilize shunt resistors or Hall Effect devices to measure
the current from each power supply. Power shunt resistors continually dissipate power
and require cooling especially when dealing with high output currents of >100Amps.
Hall Effect devices measure magnetic fields generated by current flowing through a
conductor and, although they dissipate no power, they tend to be large and expensive.
First developed by Westcor Engineering for paralleling MegaPAC supplies, the Box-to-
Box Current Share Board or CSB allows two or more Vicor power supplies to current
share by utilizing the inherent voltage drop produced in the negative output return
cable. This eliminates the need for additional shunt resistors or expensive Hall Effect
devices and provides a simple 5 wire connection method to achieve a +/-1mV accuracy
between the Negative Output power rails. This accuracy translates to a 1% current
sharing if there is a total of 100mV conductional voltage drop in the negative return
path.
Constructed as a current source to drive the Trim pin of a Vicor module, the design
uses an accurate comparator circuit to monitor the power returns. In addition, the
circuit is unidirectional and can only trim an output voltage up. The benefit is that
only the supply that is supporting less current is adjusted up. This action balances the
currents to the load by matching the output voltages of the supplies. In the case of one
supply failing, the circuit will attempt to trim the failed supply only. This will leave the
remaining functional supply alone to provide power to the load at its nominal voltage.
Thus the circuit also offers simple redundancy. In addition, because CSB functions as
a current source, the Trim outputs (T1 and T2) of the CSB can be placed in parallel to
create a summing node. This allows current sharing between more than two supplies by
paralleling the T2 output of one CSB circuit with the T1 output of the next CSB.
Please note: The CSB is not intended for use in Hotswap Applications.
Figure 1. CSB Interconnect Example
Power Supply 1
24V@1kW
+OUT
+S
-OUT
-S
TRIM
Power Supply 2
24V@1kW
+OUT
+S
-OUT
-S
TRIM
T1
-V1
T2
-V2
Power
+VOUT
-VOUT
D*
D*
CSB02
Black
White
Brown
Yellow
Red
Figure 12.
CSB Interconnect Example