Vicor Westcor PFC Mini Power Factor Corrected AC-DC Switcher - Manual (Page 24)

UG:115 vicorpower.com Applications Engineering: 800 927.9474 Page 24
Current Share Boards - Optional Feature
"Current Sharing" also known 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 >100 Amps. 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 +/-1 mV accuracy
between the negative output power rails. This accuracy translates to a 1% current
sharing if there is a total of 100 mV 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 Hot-Swap applications.
Figure 12.
CSB Interconnect Expample
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