Chapter 3 — Theory of Operation
CV30 Fixed Mount Computer Service Manual 49
The input voltage to the heater system came from the Output 2 of the Dual
output DC/DC power supply. HEATER_MAIN is the external DC source
rated to accept 12 VDC. This source is channeled through a ferrite bead; a
5 A fuse and a common mode choke to V12P0_HEATER. The same
technique is applied to the HEATER_NEUTRAL to HEATER_GND.
The heater functions within the input voltage range of 12 VDC +/- 25%
and a maximum wattage of 10.145 W. The heater system consists of boost
converter LT1619, hardware heater ON/OFF control circuit, software ON/
OFF, and heater status query control.
A thermister is mounted on this heater. The resistance change in the
thermister with temperature is reflected in the hardware control circuit. A
comparator LM2903 in this control circuit compares the voltage across the
thermister with a fixed voltage of 6 V. When temperature at the thermister
measures approximately 15°C, the voltage across the thermister will be
greater than 6 V. The output of the comparator will then turn on the
MOSFET controlling the input power (12 V) to the boost converter
LT1619. This converter steps up the voltage to 24 V and supplies power to
the heater. Conversely, the heater voltage will be automatically cut off when
the temperature at the thermister measures below 15°C. In normal
operation, the heater is automatically turned ON/OFF through hardware
control.
The software is also available to query the status of the heater (on/off)
through the nHEATER_STATUS signal. Logic “0” indicates the heater is
ON, vice versa for OFF status. Signal HEATER_ON will allow the
software to ON the heater, overwrites the hardware control. Logic “1” on
this signal line will enable the heater ON.
Logic Power System
The overall logic power system is illustrated in the power system
illustrations on pages 46, 47, and 48. The MAIN_PWR Power
Distribution System illustration indicates the power flow in the system and
the control signal to each power plane.
V12P0
V12P0_SYS To V12P0
All the logic power originating from the input voltage of V12P0_SYS (from
external power source), is channeled through a ferrite bead, a 5 A fuse, and
a common mode choke to V12P0. The same manner is applied to the
SYS_NEUTRAL to GND. Subsequently the V12P0 is used in the battery
charger circuit and stepped down to other power rails. The fuse used in the
over current protection is a polyswitch resettable type. This type of fuse
latches into a high resistance state when a fault occurs and it automatically
resets to its original state once the fault is removed. This V12P0 will remain
ON if an external power source is attached.
V12P0_CCFL
V12P0 To V12P0_CCFL
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