UG:123
Page 3
MegaPAC™ Dos and Don’ts
n
n
Do not leave ConverterPAC
™
Sense lines open. Always terminate them locally or at the load. Use
twisted pair 22-24AWG wire.
n
n
Always fill all output slots of the MegaPAC. If a slot is not filled with a ConverterPAC, it should be
filled with an airblock. Failure to do so can result in overheating and damage to the power supply.
n
n
Do not unplug ConverterPACs while input power is applied. They are not designed for
hot-plug applications.
n
n
Do not restrict airflow to the unit. The cooling fan draws air into the unit and forces it out at the
output power terminals.
n
n
For power expansion use booster ConverterPACs. Viewing the unit from the output terminal side,
always insert boosters to the right side of the driver.
n
n
Run the output (+/–) power cables next to each other to minimize inductance.
n
n
Wait five minutes after shutting off power before inserting or removing ConverterPACs.
n
n
Do not attempt to repair or modify the power supply in any manner other than the exchange of
ConverterPACs as described in this manual.
n
n
Insert proper fault protection at power supply input terminals (i.e., a fuse).
n
n
Use proper size wires to avoid overheating and excessive voltage drop.
n
n
Never loosen the inner nut on a ConverterPAC.
n
n
Verify output nuts are tight before powering up.
Technical Description
The DC MegaPAC chassis consists of an EMI 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 EMI filter designed to meet conducted 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 low-voltage, regulated outputs.
Voltage conversion in the output assemblies is achieved by the Vicor family of Zero-Current Switching
(ZCS) DC-DC converters. These are forward converters in which the main switching element switches
at zero current. This patented topology has a number of unique attributes: low switching losses; high
frequency operation resulting in reduced size for magnetics and capacitors; excellent line and load
regulation; wide adjustment range for output; low EMI/RFI emission and high efficiencies.
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 DC bus into regulated low voltage to power the internal
housekeeping circuits and DC cooling fan. When operating on 48V
DC
, the internal housekeeping VCC
comes up within 3s after the application of input power. Once the input range is within specification,
the Input Power OK signal asserts to a TTL "1" indicating the input power is OK, and allows the power
outputs to be enabled. The power outputs will be in regulation 500ms after the Power OK signal asserts
to a TTL "1." An auxiliary VCC output of 5V
DC
sourcing up to 0.3A is provided for peripheral use.
