Specifications

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PFC MegaPAC Technical Description
The PFC MegaPAC chassis consists of an off-line single phase, power factor corrected 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 “B” specifications of FCC
Part 15 and VDE 0871 and EN55022 level “B”. At start-up, inrush current is limited by a PTC
thermistor prior to being passed to the power rectifiers. The PTC is shunted out shortly after
initial power-up by a DC bus voltage sense circuit driving a relay. After rectification, the input
voltage is put through a boost converter that keeps the AC input current sinusoidal and
synchronized with the input AC voltage (in compliance with EN61000 at nominal line
voltages). The boost converter delivers an unregulated 370Vdc to the hold-up capacitors and 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 PFC MegaPAC outputs are disabled to limit the inrush current 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. The
internal housekeeping Vcc comes up within 1 sec after the application of input power. Once the
high voltage bus is within its limits, 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. An
auxiliary Vcc output of 5Vdc sourcing up to 0.3A is provided for peripheral use on interface
connector J10-9.
An output Enable/Disable function is provided by using an optocoupler to control the Gate In
pins of the ConverterPAC assemblies. If the Enable/Disable control pin is pulled low, the
optocoupler turns on, pulling the Gate In pin low and disabling the ConverterPAC output. The
nominal delay associated for an output to come up when measured from release of the
Enable/Disable pin is 5-10 ms. The General Shutdown function controls all outputs
simultaneously, and works in a similar manner.
The ride-through (holdup) time is the amount of time the load can be supported before loss of
output regulation after the loss of input power. Detecting the loss of input power takes a finite
time period after which the AC Power OK signal goes from a TTL “1” to “0”. This signal is
available for use within 1.2 seconds after initial power-up and may be used to indicate an
impending loss of power. Approximately 3 ms of warning time is obtained. Following the loss
of input power, the outputs are disabled after AC Power OK goes low.
A fault-clearing device such as a fuse is required per safety agency conditions of acceptability.
It should be sized to handle the specific load conditions but not to exceed 20A. 6 to 20A
fast-acting ceramic body type fuses should be used, 3AB-6 to 3AB-20 respectively. For current
ratings less than 6A, use a 3AB slow-blow type fuse. Fuses should be sized with sufficient
voltage rating as well as current rating.
Applications Manual