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2-6
Theory of Operation
The main power transformer has another secondary winding that provides ac power to the second-
ary board. This power is used to generate the unregulated
+
19V and - 19V which in turn generate
all the other secondary supplies.
The low line detector circuitry monitors the voltage on the secondary winding of the main power
transformer. That voltage is transferred to the supervisor board where it is compared to a reference
voltage. If the voltage on the secondary winding drops too low, a power-fail-warning (NPFW) is
sent to the processor. This provides a 4 to 8 ms warning before the power is actually shut down by
the fault circuitry.
The peak power monitor is a current sense transformer in series with the main power transformer
primary winding. Its purpose is to protect the switching transistors from damage due to an excessive
power condition. When the current exceeds 9A, the monitor generates a fault signal to shut down
the power supply assembly.
A
+
16V bias supply provides power to various monitor and control circuits throughout the power
supply assembly. This supply stays active when the power supply is shut down due to a fault
condition. It controls the power supply LEDs and governs power supply start-up.
Secondary Board
Power from the main power transformer on the primary board is transferred via bus bars to the
secondary board. There a tapped rectifier converts the raw ac power into unregulated
+
19V and
- 19V dc. These voltages in turn operate several switching regulators and one linear regulator. The
voltages produced, which are used throughout the computer, are
+
12V,
+
12VMM, - 12V,
+
6V,
+
3V, and - 2V. The
+
6V,
+
3V, and - 2V outputs are used exclusively by the processor stack.
A temperature-sensing thermistor is mounted on the secondary board heat sink. This thermistor is
monitored by the supervisor board and controls the power supply fan and the processor stack fan.
These two dc box fans operate at three speeds: low, medium, and high.
When the power supply assembly thermistor detects that the temperature of the secondary board
heat sink has risen above 39°C, the circuitry steps the fans to medium speed. A further rise of
temperature above 51°C causes stepping of the fans to high speed, and the user is notified that the
fans have reached their maximum speed. At 97°C, a power fail warning (NPFW) is transmitted, and
4 to 8 ms later the power supply is shut down. The power supply assembly is also shut down if a
temperature sensor on the CPU finstrate in the processor stack detects a temperature over 100°C.
When cooling off, the fans step from high to medium speed when the temperature drops below
46°C, and from medium to low speed when the temperature drops below 34°C.
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