Specifications

12
1-800-927-9474
20-13
DC MegaPAC Technical Description
The DC MegaPAC chassis consists of an EMC filter, cooling fan, customer interface and
associated housekeeping circuits.
Input DC voltage (+Vin, –Vin and GND) is applied to the input connectors. The input current is
passed through an EMC filter designed to meet British Telecom specifications. At start-up,
inrush current is limited by a thermistor. The thermistor is shunted out shortly after initial
power-up by a relay driven by a DC bus voltage sense circuit. The DC voltage is then fed to the
backplane. The backplane supplies power to a variety of ConverterPAC assemblies that provide
the desired voltage, regulated outputs.
At initial power-up, the DC MegaPAC outputs are disabled to limit the inrush current and to
allow the DC bus potential to charge to the operating level. A low-power flyback converter
operating with PWM current-mode control converts the voltage DC bus into regulated low
voltage to power the internal housekeeping circuits and DC cooling fan. The internal
housekeeping Vcc comes up within three seconds after the application of input power. Once the
input range is within specification, the Vin OK signal asserts to a TTL “1” indicating that the
input voltage is OK, and allows the power outputs to be enabled. The power outputs will be in
regulation 500 ms after the Vin OK signal asserts to a TTL “1”. An auxiliary Vcc output of 5Vdc
sourcing up to 0.3A is provided for peripheral use on interface connector J10-16 and J10-17.
DC MegaPAC Interface Connections
Chassis Input Power Terminals (J9)
Input DC power is applied to solderless lugs J9, using a wire size of 2 AWG. J9-1 is the +DC
Voltage IN connection and J9-3 is the -DC Voltage IN connection. The Earth Ground is
accessed via J9-2, a size 10-32 self-locking PEM nut. Max. torque recommended is 25 in-lbs.
A fault clearing device such as a fuse or circuit breaker at the power supply input is strongly
recommended. For an output of 1600W with operation on 48Vdc (and low line operation
EMI Filter
Under, Over & Reverse
Voltage Protection
Inrush Current Limiting
Logic Power
Supply
Under, Over &
Reverse Voltage
Control
Housekeeping
Circuits
DC
Fan
DC Bus Sense
Customer Interface
(Otpo-Isolators)
Power
Input
DC-DC Output
Assembly #1
DC-DC Output
Assembly #2
DC-DC Output
Assembly #3
Power
Output
Power
Output
Power
Output
DC-DC Output
Assembly #5
Power
Output
Power
Output
DC-DC Output
Assembly #4
Control
DC-DC Output
Assembly #6
Power
Output
DC-DC Output
Assembly #8
Power
Output
DC-DC Output
Assembly #7
Power
Output
Figure 18.
DC MegaPAC
Architecture
MegaPAC™ Family AC-DC, DC-DC Switchers