Specifications

Pg 24 Vicor 800-735-6200 Westcor Division 408-522-5280 Applications Engineering 800-927-9474 Rev. 4/2010
PFC MegaPAC Design Guide
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" tech-
niques 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 mag-
netic 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 par-
alleling MegaPAC supplies, the Box-to-Box
Current Share Board or CSB allows two or more
Vicor power supplies to current share by utiliz-
ing the inherent voltage drop produced in the negative
output return cable. This eliminates the need for addi-
tional 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 cir-
cuit 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 cur-
rents 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 sum-
ming 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 paral-
leled 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 pro-
vide from 5% to 1% accuracy.
3. -V1 and -V2 for all Box-to-Box circuits must be con-
nected directly at the negative output power studs or ter-
minals to achieve accurate current sharing.
4. D* can be added if redundancy is needed. If redun-
dancy 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.
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
Ye l lo w
Red