Specifications
12
1-800-927-9474
20-6
Autoranging MegaPAC/Mini MegaPAC Technical Description
The MegaPAC and Mini MegaPAC chassis consist of an off-line single phase AC front end,
EMC filter, cooling fan, customer interface and associated housekeeping circuits.
Input AC mains voltage (L1, L2/N and GND) is applied to a terminal block. The input current is
passed through an EMC filter designed to meet conducted noise limit “A” specifications of FCC
Part 15 and VDE 0871. At start-up, inrush current is limited by an NTC thermistor prior to
being passed to the power rectifiers. The NTC is shunted out shortly after initial power-up by a
relay driven by a DC bus voltage sense circuit. The sense circuit also controls the input
autoranging selection relay on the autoranging MegaPAC. The power rectifiers and filter
capacitors are arranged in a conventional full wave bridge rectifier/voltage doubler configuration.
This operates as a full wave bridge rectifier on 230Vac, and voltage doubler on 115Vac,
delivering unregulated 300Vdc to a high voltage backplane. The backplane supplies power to a
variety of ConverterPAC assemblies that provide the desired low voltage, regulated outputs.
At initial power-up, the MegaPAC outputs are disabled to limit the inrush current, reduce peak
currents in the autoranging relay contacts, and to allow the DC bus potential to settle out to the
correct operating level. A low-power flyback converter operating with PWM current-mode
control converts the high voltage DC bus into regulated low voltage to power the internal
housekeeping circuits and DC cooling fan. When operating on 115Vac, the internal housekeeping
Vcc comes up within 1,000 ms after the application of input power. On 230Vac, it comes up
within 500 ms. The input range selection circuit in the Autoranging MegaPAC may take up to
200 ms to select the range if 115Vac is applied. When 230Vac is applied, the circuit
immediately selects for operation on 230Vac. The Mini MegaPAC must be manually strapped
for 115Vac or 230Vac operation. Once the input range selection has taken place, the AC Power
OK signal asserts to a TTL “1” indicating that the input power is OK, and allows the power
outputs to come up within 15-30 ms later. An auxiliary Vcc output of 5Vdc sourcing up to 0.3A
is provided for peripheral use on interface connector J10-9.
Applications Manual
FCC/VDE "A"
EMI Filter
AC-DC Power Rectification,
Input Autoranging,
Inrush Current Limiting
Logic Power
Supply
Inrush Current &
Autoranging
Control
Housekeeping
Circuits
DC
Fan
DC Bus Sense
Customer Interface
(Optoisolators)
High Voltage Unregulated
300Vdc Bus
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 #4
DC-DC Output
Assembly #5
Power Output
Power Output
Control
Figure 8.
MegaPAC and
Mini MegaPAC
Architecture