Supervisor Board - HP 9020 Service Manual

Theory of Operation
2-7
- 2V. The
+
6V,
+
3V, and - 2V outputs are used exclusively by the processor stack. Also, a
separate
+
12V output is produced to drive mass storage devices.
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.
Supervisor Board
The supervisor board contains most of the monitoring, control, and fault-indicating circuitry for the
power supply assembly.
The overvoltage and undervoltage monitor checks the voltage outputs of all secondary board
power supplies except the
+
12MMV supply for mass storage devices. The monitor also checks the
+
5 V output and the peak power monitor from the primary board. If a fault is detected, the pulse
width modulator is disabled, the shutdown sequence is enabled, and the appropriate LEDs are lit.
Temperature monitors check the power supply assembly thermistor, described in preceding para-
graphs, and the processor stack thermistor. When the processor stack temperature exceeds 100°C
or the power supply temperature exceeds 97°C, this board generates the power fail warning
(NPFW) signal and initiates the shutdown sequence.
A
+
10V reference supply provides the voltage condition references for over and under voltage
monitoring and temperature sensing. This supply is enabled by the
+
16V bias supply from the
primary board.
The fan control circuitry provides a constant -10V to the 110 card cage fan (FANIon Figure 2-3)
whenever power is on. This fan never changes speed. The fan control circuitry also provides - 10V
to the power supply assembly fan (FANP) and processor stack fan (FANF) as long as the power
supply assembly temperature does not exceed 39°C. When the power supply assembly thermistor
detects between 39°C and 51°C, the fan control circuitry provides - 12V to the two fans, stepping
them to medium speed. Between 51°C and 97°C, the fans are stepped to high speed at -14V. The
fan high signal (NFANHI) is also sent to the processor which sends a message to the user, indicating
that the computer is performing maximum cooling and that a further temperature increase could
result in shutdown.
If either the 110 door or processor stack door is opened while the power supply assembly is
operating, the shutdown sequence is initiated.