Theory
of
Operation
2-5
LEDs
on
the assembly indicate overvoltage or undervoltage conditions for dc supplies, as well as
overtemperature, peak power exceeded, and
110
door or processor stack door open.
The ac module consists of an on/off power switch/circuit breaker assembly, line
filter,
and
an
attached ac power cord. Interconnection
is
made with two barrier strips
and
qUick-disconnect
terminals. A primary voltage change requires changing the switch assembly, power cord,
and
cable
that interconnects the line
filter
and
power supply.
The power supply assembly
is
cooled by a dc box fan mounted on the chassis. The fan
is
not
an
integral part of the assembly.
The power supply assembly has three printed circuit boards: the primary board, the secondary
board,
and
the supervisor board. The following description
is
organized according to the functions
performed by the ac module and these three boards.
The power supply assembly plugs into the motherboard
in
the computer base via six connectors,
two per board. These connectors,
and
their associated motherboard traces, interconnect the power
supply assembly boards,
and
carry voltage and signal lines between the power supply assembly
and
other computer modules.
Fifteen status-indicating, light-emitting diodes (LEDs) are contained
on
the supervisor board. They
are described
in
a
folloWing
subsection
in
this chapter.
Ac Module
The ac module has
an
attached 6-foot line cord that plugs into an ac outlet. The line filter reduces
conducted emissions from the power supply assembly.
It also presents the ac voltage to the on/off
power switch/circuit breaker which enables/disables ac power to the computer.
The trip point of the circuit breaker depends on which power configuration
is
being used. The 110V
breaker trips at
IS
amps line or line overvoltage. The 220V breaker trips at 10 amps. Reset the
circuit breaker by turning the switch off
and
then turning
it
back on.
Primary Board
After being rectified, the ac to the primary board charges two
2800~.Lfd
capacitors. Each capacitor
is
charged to lS0-200V, depending on the line voltage
and
load.
The rectified dc voltage from the capacitors feeds a switching regulator. The regulator drives current
through the main power transformer.
Rectifiers at the output of the main power transformer develop the
+
SV
switching supply. This
supply provides
+ S volts for use throughout the computer. The +
SV
output
is
monitored by the
supervisor board for overvoltage
and
undervoltage.
Also,
the +
SV
output feeds the pulse width
modulator. The modulator output controls the base drive which
in
turn controls the switching
transistors. The pulse-width-modulator-enable signal
(PWM
ENABLE)
from the supervisor board
enables the modulator as long as no fault signal
is
present. A fault signal disables the modulator,
shutting down the power supply assembly.
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