KEPCO MST Series - Manual (Page 39)

MST SERIES 061813
3-7
3.7.3 OVERTEMPERATURE DETECTORS
Any one of a number of internal or external conditions (i.e., elevated ambient temperature,
clogged vent holes, cooling fan failure, etc.) can cause elevation of internal heat sink tempera-
tures to unsafe levels. If unattended, the resultant temperature rise of the power devices will
lead to eventual destructive failure. MST Power Modules incorporate two internal thermostatic
switches to detect unsafe internal operating temperatures. One is located on the heat sink
related to the input preregulator and dc-dc converter, while the second monitors the heat sink
related to the output stabilizer. If the temperature of either heat sink rises above a predeter-
mined safe temperature, the related thermostat will change state, initiating a fault shutdown and
generating a Device Overtemperature status flag. The thermostat will reset upon return of the
heat sink to acceptable operating temperature, however the module recovery procedure (see
PAR. 3.7.1) must still be performed.
3.7.4 A-C LOSS DETECTOR
Although MST Power Modules are designed to provide a minimum of 21.5 milliseconds of out-
put hold-up time (one full cycle of 47Hz power), loss of source power for extended periods will
eventually result in complete Power Module shutdown. Additionally, certain power loss intervals
can create a condition where the internal bias supply for the digital control circuitry decays par-
tially, but not enough to force a system interrupt and reset; when this occurs, it is possible that
digital communication and processing can be corrupted without warning or notice. To prevent
this, MST Power Modules incorporate a power loss detector which will initiate a fault shutdown
with an accompanying Device Power Loss status flag if the internal bulk d-c storage capacitor
voltage falls to a level where quality of the internal bias voltages is questionable.
3.7.5 OPEN SENSE/POWER WIRE PROTECTION
In the event that the error sense leads (+S, -S) become disconnected from the power leads, the
output voltage will attempt to rise uncontrollably; the MST Power Module fault detection circuitry
continuously monitors the error sensing leads to protect against this occurrence. If an open
sense lead or power lead condition is detected, the fault detector immediately issues a string of
commands which reset the output status to Voltage Mode, Output Disabled (all output relays
open) and programs the output voltage and current limit to zero. At the same time, a fault mes-
sage is sent to the controller (Load Path Fault in CIIL, Relay Error in SCPI). Once the open
sense or power lead condition is corrected, the power module is ready to accept additional com-
mands without any further recovery measures; manual reset is not required.
The open sense/power wire protection circuitry can also be triggered by a power lead voltage
drop in excess of the 0.5V maximum specified headroom. If the error sense lead connections
appear to be correct, the user should double-check the size of the power leads to verify ade-
quate margin. Contact Kepco Applications Engineering for assistance if necessary.