Specifications

Rev. 6/2004 Vicor 800-735-6200 Westcor Division 408-522-5280 Applications Engineering 800-927-9474 Pg. 21
PFC MegaPAC-EL/PFC MegaPAC-High Power Low Noise
Current Share Boards - Optional Feature
"
Current sharing" also known as Load
Sharing, is the ability to divide the out-
put 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 continu-
ally dissipate power and require cooling
especially when dealing with high out-
put currents of >100Amps. Hall Effect
devices measure magnetic fields gener-
ated by current flowing through a con-
ductor and, although they dissipate no
power, they tend to be large and expen-
sive.
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 cur-
rent 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 conduc-
tional 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 l o w
Red