Specifications

31PLCs
Overview
Typically PLCs have a backplane power
rail of +24V DC, although the actual
voltage can dier, usually from 12V
to 48V. The power comes from an
isolated DC-DC converter connected
to a factory AC to DC supply. The PLC
can be equipped with an auxiliary
battery with a special ORing controller.
Together, this conguration forms an
uninterruptable power supply (UPS)
to ensure continued operation in the
event of an AC powerline brownout
or failure. During AC faults the battery
supplies power 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. In
some cases isolated power supplies
are implemented on a modular card.
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. In
cases where a regulated DC-DC supply
is not required, Maxim’s transformer
drivers can be used to enable a designer
to quickly and simply design a highly
ecient isolated DC-DC converter. The
CPU or FPGA typically requires even
lower voltages for the high-performance
core. 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
4–20mA current-loop interfaces.
Nonisolated and Isolated Power-Supply Functions
CALIBRATION, MARGINING, AND ADJUSTMENT
VOLTAGES VOLTAGES
VOLTAGE TO
FIELD
INDUSTRIAL DC BUS
HOT-SWAP
CONTROLLER
OVERVOLTAGE
PROTECTION
OVERVOLTAGE
PROTECTION
DIGITAL
POTENTIOMETER
CURRENT
LIMITER
ISOLATED
POWER
ISOLATED
POWER
INTERFACE
MODULE
GALVANIC
ISOLATION
BARRIER
GALVANIC
ISOLATION
BARRIER
CPU
DC-DC
POWER
AC-DC
SUPPLY
PRECISION
RESISTORS
ISOLATED
POWER
ISOLATED
POWER
VOLTAGE
MONITORS
BACKUP BATTERY
MANAGEMENT
THERMAL
MANAGEMENT
PUSHBUTTON
CONTROLLER
I/O CARDS
= MAXIM SOLUTION
Block diagram of a typical PLC power chain.