Vicor Autoranging MegaPAC AC-DC Switcher - Manual (Page 21)

UG:106 vicorpower.com Applications Engineering: 800 927.9474 Page 21
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 +/-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.
Requirements:
1. For proper operation, the power supplies being paralleled should be enabled
at the same time.
2. -Out conductors must be of equal length and wire gauge. Separate -Out conductors
must be used from each supply to the load, or the use of a "Y" connection to a
common point must be used as shown in figure 1. Each leg of the "Y" must have a
minimum of a few millivolts of drop in order for proper operation. 50mV to 100mV
of drop will provide from 5% to 1% accuracy.
3. -V1 and -V2 for all Box-to-Box circuits must be connected directly at the negative
output power studs or terminals to achieve accurate current sharing.
4. D* can be added if redundancy is needed. If redundancy is not required, D* can be
replaced with direct wire connections.
5. When using D*, the Power input should be connected on the cathode side of the
paralleling diodes as shown above.
6. Terminate Sense Leads either locally or remotely as shown in Figure 1.
7. For paralleling more than 2 supplies consult factory for assistance.
End of Life - Not Recommended for New Designs