Novametrix Medical Systems MARS PO2 TECH 2001, Service Manual

Page: 17 / 85

2775 Main Board
Section 4
Theory of Operation
Rev. 01 Model 2001 Service Manual
11
Power On/Off Control Circuitry
See page 4 of 4 on schematic.
The Model 2001 power on/off control circuitry consists of the VBACK supply (regulated by
IC12), IC10, IC11 and the POWER key.
When the battery or AC Mains is first applied to the power supply board (via VIN J102 pin 1),
VBACK goes to +5 volts, provides power to IC10 and IC11, and through the C26 and RP4 (pins
3,4) network at IC10 pin 8, sets IC10 pin 2 to a logic Low.
The ON/OFF line is brought Low each time the front panel POWER key is pressed. This sends
the output at IC11 pin10 High. This Low-to-High transition clocks the (#1) D flip-flop portion of
IC10. The Q1 output at IC10 pin 2 goes High and with each successive press of the POWER
key, this output toggles to the opposite level (Low or High). A High turns the Model 2001 on and
a Low shuts it off.
While the Q1 output at IC10 pin 2 is High, the MOSFET Q7 is turned on and pulls the gate of
MOSFET Q8 to ground, thus causing Q8 to conduct as well. With Q8 conducting, the currently
active monitor power source-either the AC Mains derived supply or the battery supply will flow
through Q8 to the voltage input (pin 7) of the Pulse Width Modulator IC9. The output IC9 pin 6
will oscillate (at the frequency set by R13 and C15). This causes Q5 to switch on and off and
provide a path to ground through the primary coils of T1 for the supply (Mains or battery) at T1
pin 12. Current flowing in the primary is measured at IC9 pin 3 and the duty cycle of the pin 6
output will vary with the load on the transformer.
Current flow in the transformer primary induces current in the three secondary coils and creates
the 12 volt analog supplies (+V12 and -V12), +VA, the VRAWI that powers the isolated RS232
circuitry, and the +5 volt VCC supply that power the remaining circuits in the monitor. The Model
2001 turns on. The +V12 and -V12 supplies are rectified and filtered by D2, D4, C10, C11, C12
and C13. The +V12 is regulated by IC7 and the -V12 by IC8. The Vcc supply is rectified by D3,
filtered by L1, C9 and C20 and fused at F1, and in addition, a feedback loop to IC9 contains
VR1 which is factory adjusted to produce a +5.00 volt ± 0.05 volt VCC supply (measured under
load).
Once the monitor powers up, a SYNC signal toggles Q9 on and off causing a timing pulse to
be transmitted across C19 and C15 to the input at IC9 pin 4. This has the effect of synchronizing
the output of the pulse width modulator with the data sampling operations of the analog board
and keeps power supply switching spikes from interfering with those operations.
Power Supplies
The secondary pins 7, 8, 9 of T1 form a center tap transformer, the voltage is rectified by D2
and D4 then filtered by C10, C11, C12 and C13. The resistor and capacitor combinations R66,
C113 and R68, C115 across the rectifying diodes D2 and D4 respectively are for EMI reduction.
The dual 12 volt supplies, +V12 and -V12 which are generated from this voltage are regulated
by IC7 and IC8 respectively. The secondary winding of pins 5 and 6 of T1 are rectified by D3
(R67 and C114 for EMI reduction) and filtered by C9, L1 and C20, this voltage designated as
VCC (+5 volts) acts as reference for IC9, supplies power for the opto isolator non-isolated side
and powers other circuitry on the board.
The secondary winding consisting of pins 2 and 3 are rectified and filtered by D1 and C1. The
rectified voltage at this point is approximately 7 volts DC and is regulated to 5 volts by IC2. This
isolated supply powers the isolated portion of the opto-isolators and the RS232 driver chip IC1.
The unregulated voltage VRAWI is sent to the rear panel connector J101.