Service manual

Ultraview 1030/1050 Monitors — Service Manual
3-13
Theory
The power supply has these features:
• 7 output DC voltages (+5 V, +3.3 V, +12 V, -12 V, -5 V, +20 V, +15 V)
• Automatic battery switch over operation
• Self shut-OFF when on battery operation and battery voltage is low
• Over current protection
• Wide input voltage range, <12 V to 24 V
• >46 W output capability
Slow Turn-On — The slow turn-ON circuit is needed to reduce the possibility of arcing when the
cable from the AC/DC power supply is plugged into the PCBA.
Battery Input Comparator — The minimum start up and operating voltage provided by the
AC/DC power supply is 11.5 VDC. If the battery voltage is above the input voltage and the input
voltage from the AC/DC power supply is below 12 V, the system will operate from the battery
power via a comparator which switches between using an AC/DC power supply or the batteries for
power.
Linear Regulator — The +12 V linear regulator is needed to operate the charger pre-regulator
circuit and also to provide “house keeping power.” This +12 V is the only source of power to
operate the +20 V pre-regulator IC chip. Without this +12 V linear regulator power, the +20 V
pre-regulator circuit and, therefore, the battery charger does not operate. This is why the battery
charger works from the AC/DC power supply power but not from the batteries.
±12 V, -5 V Power Converter — This is a single-ended flyback converter topology based on a
circuit designed around the MICREL 38HC43 pulse width modulation (PWM) controller. It operates
at 100 KHz. The circuit is regulator based on feedback from the +12 V output. The - 12 V and the
-5 V outputs are unregulated and consist of only the transformer output, a diode rectifier, and a
capacitor.
The output power rating for the +12 V (±0.3 V) section is 24 W. The power rating for the -12 V
(±1.21 V) is 1.2 W. The power rating for the -5 V (±0.9 V) is 0.5 W.
Varying power input requirements, such as DC from an ambulance, Spacelabs Medical power
supplies (+24 VDC and +18 VDC), and battery voltage (10.6 VDC to 13.5 VDC), require the
converter to be functional from 10.6 VDC to 24 VDC input.
+5 V, +3.3 V Power Converter — This is a step down DC-to-DC converter (also called a “buck”
converter) dual output power system. The design is based on MAX796 PWM (pulse width
modulation) controller. The frequency if fixed at 150 KHz. The circuit is regulated via a combination
of 3.3 V voltage feedback and +5 V current feedback.
The +5 VDC is capable of 15 W continuous. The +3.3 V is capable of 6 W continuous. The circuit
has a wide input voltage requirement and capability as does the ±12 V/-5 V converter.
This converter provides power at better than 80% efficiency.