Rockwell Automation PDC24 / PAC Chassis Power Supplies - Manual (Page 8)

8 PDC24 / PAC October 2005 – Issue 3
Triguard SC300E
THEORY OF OPERATION
Main power flow
The 5.4V output allows for a drop of about 0.4V across the auctioneering diodes in each
module supplied.
The output voltage is fed back to the SMPS control circuits via the +ve and -ve sense lines.
The SMPS then compensates for any voltage drop along the power supply lines by increasing
its output voltage. The rated output voltage is thus present at the load rather than at the PSU
output terminals. The -ve sense input to the SMPS is modified by the power share system.
The 5V supply current sensor is a 0.005-ohm resistance formed by two resistors in parallel.
The voltage developed across the resistors is used as a measure of output current by the
power share and fault alarm circuits.
Various system voltages are connected by current limiting resistors to individual pins of the
test socket on the front panel. The ‘current limiters’ protect the system against accidental
short circuits on any of the test socket pins.
WARNING
Plant shutdown - Adjusting the power supplies in a running system could result in a
plant shutdown.
Power share system
The power share control circuit accepts inputs from the 5V supply current sensor, the +ve and
- ve sense lines, the output adjustment trimmer (O/P ADJ) on the front panel, and the power
share line. It drives the power share line, the power share LED on the front panel, and the
voltage on the -ve sense line input to the SMPS.
Normally the voltage at the -ve sense input to the SMPS is close to that on the -ve sense line
from the backplane. When the power share control circuit detects that an increase in output
current is required to maintain power balance, the -ve sense SMPS voltage drops, so that the
voltage between the +ve and -ve sense lines is now greater. The SMPS then increases its
output voltage.
Fault alarm system
The power fail output from the SMPS switches from logic high to low just before the SMPS
output fails (i.e. to give advance warning following a failure of the ac/dc input supply). The fault
alarm circuits de-energise the fault relay in response to:
• The power fail signal
• An input from a 70ºC thermal cut-out on the SMPS
• An undervoltage output condition detected on the +ve and -ve sense lines
A set of changeover contacts controls the Fault and Normal LEDs on the front panel. Another
pair of relay contacts is available to operate an external alarm. These contacts open under fault
conditions.