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91369 Service Manual
3-7
Theory
DC-DC Converters
The internal power supply derives six output voltages for different functions of the CPU PCBA.
• +12 V @ 24 W
• -12 V @ 1.2 W
• -5 V @ 0.5 W
• +15 V @ 3 W
• +5 V @ 15 W
• +3.3 V @ 6 W
In the event of mains power failure, all supplies are maintained through power derived from up to two internal
12 V NiMH batteries.
The power supply subsection has a slow power-ON feature, which prevents high inrush currents when power
is switched ON. The power supply will also control which input source to use, either the external power supply
or the batteries. The minimum input voltage required to operate the monitor is 11.5 VDC. If the external power
supply input is less than 11.5 VDC, and the batteries greater than 10.75 VDC, the monitor will run on battery
power.
Power Failure Operation
The power supply on the CPU PCBA provides a digital signal PFAIL to the CPU to indicate that a power failure
condition is imminent. This signal is asserted by the power supply if its input power fails, or if the monitor is
switched OFF. This signal is provided through a power supervisor IC to the MPC8270 as an interrupt, causing
the processor to take immediate power failure action.
Power failure operation of the monitor takes advantage of the fact that this monitor’s design requires at least
one charged battery to be present at all times. This battery provides power for the monitor far in excess of the
intended three-minute data retention requirement for the module data.
Battery Chargers
The battery chargers have the following features:
• Front panel LED battery charging/failure indicator.
• Automatic “battery care” recharge cycles to ensure a maximum charge.
• Individual battery chargers for each of the two batteries, so that a bad battery will not degrade the other
battery.
• If the monitor is OFF, one or two batteries can be fully charged in 90 to 120 minutes. If the monitor is ON,
batteries are charged in four to five hours.
Charger Pre-Regulator
— The +20 VDC output provides up to 60 W of battery charging power. This DC
output voltage is then used as the main power source for charging batteries. The circuit uses a single-ended
flyback converter topology. This pre-regulator circuit is active at all times when the monitor is powered from the
external power supply voltage.
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