Load Sharing - KEPCO HSM 3.3-230 Operator's Manual

2.7.6

LOAD SHARING

When operating two or more power supplies in parallel, either for capacity or redundancy, it is
desirable to distribute the load equally among all of the power supplies in order to improve perfor-
mance, reduce stress and increase reliability. HSM power supplies incorporate active circuitry
which forces multiple power supplies wired in parallel to share load current, both in voltage- and
current-mode regulation. The HSM employs a single wire connection between paralleled power
supplies, forming a master-slave relationship as follows: the highest voltage unit becomes the
master, and all of the remaining units are slaved to it via the load share signal (ISHARE), which
boosts the slave outputs in order to increase load share. A maximum boost limit of 5% prevents
the slave units from following a defective master into an overvoltage condition, or from creating a
load hazard if either the slave itself or the load sharing system is defective.
When implementing load sharing, the user must ensure that all power supplies are attempting to
regulate to the same voltage at the same location, and must minimize the possibility of load
share signal corruption; the power supplies should, as nearly as possible, emulate a single large
power supply. To this end, the following rules apply:
a) If possible, remote error sensing should be employed, with all error sensing connections ter-
minated at the same physical point, and as close to the power supplies as possible; if local
error sensing is required, power lead voltage drops must be minimized. Provide local noise
decoupling capacitors across all sense wire termination points.
b) The power supplies should be located as near to each other as possible, with power termina-
tions bussed together using adequately sized interconnections; the power supply/load inter-
connections should be distributed evenly along the power supply output interconnection
busses. This is especially important in high-current systems employing several power sup-
ply modules in parallel, where voltage drops in the interface connections can be significant in
comparison to the load share signal voltage and introduce both d-c and a-c errors.
c) All power supply output voltages should be adjusted as closely as possible, and in any case
within a 2% error band. Additionally, the current limit setpoints should be identical and high
enough to support the load requirements; for (N+M) systems, this means setting the current
limits high enough to tolerate loss of M power supplies and still support the load.
d) Minimize the load share signal wire interconnection lengths to reduce risk of noise influence.
HSMSERIES OPR 052912
2-11