KEPCO MST Series - Manual (Page 37)

MST SERIES 061813
3-5
3.5 PARALLEL OPERATION
MST Power Modules are specifically designed for operation in parallel, either for increased
power or for fault redundancy. The output stabilizer incorporates an active load-sharing scheme
to ensure equal distribution of load current among all paralleled modules, resulting in reduced
operating stress and higher reliability. The load-share circuitry employed is a single-wire type
with the positive error sense connection providing the signal return path.
To determine the number of Power Modules needed for a specific application, the user divides
the total required load current (including any transient peaks) by the Power Module’s rated cur-
rent, rounding the result up to the next whole number if necessary. This method determines “N”,
the minimum number of power modules needed to support the load. For redundant applications
this number is increased based on the desired redundancy factor (N+1, N+2, etc.). For non-
redundant applications, the user can reduce overall power system operating stress by increas-
ing the number of modules above N.
For parallel operation, all Power Modules should be same model. The module d-c outputs and
error sense leads are wired in parallel to the load; the load share terminals from each DC Output
connector are daisy-chained together. The user must ensure that the load and sense wiring min-
imizes loop inductance to prevent stray pickup from injecting noise into the load share signals.
Twisted pairs (shielded, if necessary) should be used for both power and error sense leads
when remote error sensing is used, with all error sensing lines terminated to the same physical
location; if possible, the user should employ local error sensing.
Program each module for Voltage Mode operation at identical output voltage and current limit.
The load sharing circuitry will automatically select the “load master” as the module with the high-
est “effective” voltage supplying power to the output bus; all other modules operate as slaves
and are indicated as such by the CURRENT SHARE indicator LED on the front panel of the
power module. Since MST Power Modules are enabled sequentially, the enabled modules will
generate output voltage errors until the minimum number of modules (N) are turned on; the user
should ignore these error flags until all modules are enabled (SCPI provides a method of mask-
ing these flags using software). The same is true if modules are disabled sequentially.
3.6 SERIES OPERATION
MST Power Modules are designed to allow series operation to achieve higher output voltage if
desired. Series operation requires detailed investigation into all possible output conditions which
may result from either normal or abnormal operation to ensure that the power supplies are not
exposed to undue stress, especially voltage. This need is increased for MST Power Modules
which, because of the sequential turn-on characteristic, may be exposed to greater risk during
output enabling and disabling sequences. Users wishing to operate MST Power Modules in
series are directed to contact Kepco Applications Engineering for specific guidance.