Vicor PFC Micro Power Factor Corrected AC-DC Switcher - Manual (Page 20)

PFC Micro Design Guide
" 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 cool-
ing 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 sim-
ple 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 con-
ductional 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 cir-
cuit 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 volt-
ages 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,
Figure 1. CSB Interconnect Example
Supply # 1
5V @ 120A
+OUT
+S
-OUT
-S
TRIM
Supply # 2
5V@120A
+OUT
+S
-OUT
-S
TRIM
T1
-V1
T2
-V2
Power
+VOUT
-VOUT
D*
D*
CSB02
Black
White
Brown
Yellow
Red
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 con-
ductors must be used from each supply to the load, or the use of a "Y" connec-
tion 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 neg-
ative 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.
CURRENT SHARE BOARD - Optional Feature
Pg. 20 Vicor 800-735-6200 Westcor Division 408-522-5280 Applications Engineering 800-927-9474 03-000048 rev A