Current Limit Characteristic - KEPCO HSM 3.3-230 Operator's Manual

The signal generated by the OVP detector is gated with a signal from the fault detector circuit to
produce a selective overvoltage shutdown function which prevents shutdown of operational
power supplies in a parallel-redundant power system configuration. The OVP latches of any
working power supplies are disabled, allowing only the faulty modules to be latched off; system
operation can then return to normal, assuming sufficient load capacity in the remaining modules
to support the load. This function is critical in fault-tolerant power systems, otherwise a single
overvoltage failure could ripple through all of the operating supplies and result in a complete
power system loss.
NOTE: The overvoltage protection circuit senses the voltage directly at the output terminals
of the power supply, not at the error sensing point. When selecting an overvoltage set
point, the user must take into account the expected power lead voltage drop and, if
applicable, the transient response overshoot in order to avoid false shutdowns. The
HSM design is such that the power supply cannot generate an output voltage high
enough to cause internal damage, regardless of OVP setting.
3.9

CURRENT LIMIT CHARACTERISTIC

HSM power supplies provide two different current limiting modes for different applications;
selection of the desired mode is accomplished via switch S1-3, accessed through the top cover
of the HSM (see Figure 2-1). The following describes the operational differences and selection
method of each:
a) Continuous Limiting: This is the factory-set (default) mode of operation. When the output
current of the power supply reaches the programmed current limit, the output regulator
switches to current mode operation and maintains the output current by modulating the out-
put voltage; this operating mode is maintained indefinitely, and recovery to voltage regulation
mode is automatic upon reduction of the output current below the current limit point. This
mode is ideal for high-power battery chargers and applications where operation in current
regulation mode is normal, or where immediate recovery from an overload condition of any
duration is critical.
NOTE: HSM power supplies are designed to maintain continuous delivery of 110% of rated
current indefinitely. When operating parallel/redundant power supply configurations in
continuous limiting mode, the user must size the power supply/load interconnection
conductors to withstand the total maximum load current available from all of the paral-
leled power supply modules.
b) Undervoltage Lockout: This mode is enabled when S1-3 is set to ON (Figure 2-1). The
crossover from voltage- to current-mode operation is the same as for Continuous Limiting;
however, after approximately 15 seconds, the output load regulation circuit is locked off via
the overvoltage protection latch, requiring the user to recycle source power to restart the
power supply (see PAR. 3.8). This mode permits automatic recovery from short-term over-
loads, but eliminates the danger of overheating and damage to the load and load wiring due
to continuous exposure to high current; this is especially useful in redundant power systems,
where the continuous overload current of all of the paralleled power supply modules can be
in excess of twice the normal load current. As the circuit is triggered by the occurrence of an
output undervoltage condition, this circuit can also protect circuits which may be unduly
stressed in the presence of an extended undervoltage condition; an example of this are bat-
teries, which can be damaged by discharge voltages below a specified minimum.
NOTE: When undervoltage lockout mode is enabled, it is necessary to also enable the
Remote Reset function (see PAR. 3.12) in order for the Remote Inhibit function to oper-
ate properly.
HSMSERIES OPR 052912
3-5