Specifications

38 Maxim Industrial Solutions
Programmable logic controllers (PLCs)
Isolated power-supply functions
Overview
Typically PLCs have a backplane
power rail of about 24V DC, although
the actual voltage can differ, usually
from 12V to 48V. The power comes
from an isolated DC-DC converter
connected to a factory AC-DC supply.
A PLC can be equipped with an
auxiliary battery with a special
OR-ing controller. Together, this
configuration forms an uninterrupt-
able power supply (UPS) to ensure
continued operation in the event of
an AC power-line brownout or failure.
During AC faults the battery supplies
the power rail to the backplane.
Power functions
The whole PLC power network is
quite complicated with a variety
of protection, isolation, and post-
regulation functions. This power
function can also be duplicated in a
system to allow hot backup and hot-
swapping in case of a power fault.
PLC functions take power from the
power rail and are organized into
separate functional modules that
have hot-swap controllers to prevent
inrush current surges during hot
installation/removal. The modules’
power inputs can usually withstand
a higher voltage than the power rail,
because high-voltage spikes can
occur at those inputs. Each functional
module has its own local converters
to generate regulated 5V, 3.3V,
and other local power rails. The
CPU typically requires even lower
voltages for a high-performance
controller’s core and/or FPGA. Analog
I/Os can require ±15V or higher
voltages for op amps and/or analog-
output conditioners. A PLC can also
provide a regulated power output of
24V for smart sensors, other remote
equipment, and analog current-loop
interfaces.
Isolated power-supply functions
Block diagram of isolated power-supply functions. For a list of Maxim's recommended PLC solutions, please go to: www.maxim-ic.com/plc.
CALIBRATION, MARGINING, AND
ADJUSTMENT
VOLTAGES VOLTAGES
VOLTAGE TO
FIELD
INDUSTRIAL DC BUS
HOT-SWAP
CONTROLLER
MOSFET/
RECTIFIER DRIVER
OVERVOLTAGE
PROTECTION
DIGITAL
POTENTIOMETER
CURRENT
LIMITER
ISOLATED
FIELD POWER
ISOLATED
INTERFACE
POWER
INTERFACE
MODULE
GALVANIC
ISOLATION
BARRIER
GALVANIC
ISOLATION
BARRIER
CPU
DC-DC
POWER
PRECISION
RESISTORS
ISOLATED
I/O
POWER
ISOLATED
I/O
POWER
VOLTAGE
MONITORS
BACKUP BATTERY
MANAGEMENT
THERMAL
MANAGEMENT
PUSHBUTTON
CONTROLLER
I/O CARDS
= MAXIM SOLUTION