KEPCO MBT Series - Manual (Page 111)

MBTSVC111609
4-5
4.1.8.4 PATH FAULT PROTECTION The path fault protection circuit is activated if the connection
between a power lead and corresponding sensing lead is interrupted or if the power and sens-
ing leads are too long. The path fault detector monitors the voltage drop between the Power
Supply output terminals (OUTPUT and COMMON) and the corresponding sense leads (OUT S
and COM S). When this voltage drop reaches some point in excess of 0.5V per lead for longer
than approximately 6 milliseconds for M and MG Models, or longer than 20 milliseconds for MR
and MGR Models, the path fault detector operates as follows:
(MR, MGR MODELS) The output voltage and current are programmed to zero, and the relays
are commanded to Open, disengaging the load from the power supply.
(M, MG MODELS) The output voltage and current are programmed to zero and/or the Crowbar
and Turn-off circuit is activated, shutting the power supply down.
In any case, activation of the path fault detector generates a corresponding error message
which is supplied to the controller before MBT power is shut off.
This feature also ensures that current through the sense leads cannot exceed a small value
(1mA) so that the load can not be supplied through the sensing leads, and the load is protected
against an uncontrolled voltage at the output when the sense leads are interrupted.
4.1.8.5 CROWBAR AND TURN-OFF CIRCUIT Activation of the Crowbar and Turn-off circuit causes a
Crowbar error message to be issued, stored in the error buffer, and sent to the remote controller.
This circuit short-circuits the output terminals of the Power Supply to protect the load (rapidly
discharging the output capacitor), and sends a signal to turn off the POWER front panel circuit
breaker.
4.1.9 RESPONSE TIME
An important feature of MBT programmable Power Supplies is a fast response to input com-
mands. The programming speed is determined primarily by the size of the output capacitor. For
fast programming speed it appears that the output capacitor should be removed or reduced in
value. However, a large output capacitor at the output is desirable to maintain low output ripple
and noise, to increase stability and to provide a large reservoir of energy for fast recovery to
step load changes.
When the output voltage is increased as a result of a SET command, the output capacitor will
charge up. The charging time is determined by the Power Supply current limit setting. The larger
the current limit setting the shorter the charging time.
For a SET command calling for a reduction in output voltage, the output capacitor will discharge.
The discharge time is determined by the load resistance and not by the Power Supply current-
limit setting. Even if the output capacitor has a small value, for light loads the discharge time will
be very long.
The MBT Power Supply is designed with a special output capacitor discharge circuit that will
discharge the output capacitor to zero within approximately 25 milliseconds. The circuit is auto-
matically activated anytime the power transistors are being cut-off. To limit the power dissipa-
tion in the discharge circuit, a maximum rate of two discharges per second is permitted. This
rate is established at no load (worst case); at maximum load a faster rate may be obtained.