Novametrix Medical Systems 515 Service Manual page 12

Theory of OperationPower Supply Board
Service Manual: Mode
on. With Q2 on IC6 is powered up and the DC-DC converter turns the moi
on.
Pressing the POWER button again will turn the unit off by causing IC3 p
to pulse high w'hich toggles pin 1 of ICl high. Pin 11 of 1C3 will go low tur
the unit off, provided pin 12 is high via IC3 pin 10. If the OFFDIS line frorr
processor is high then the monitor will not shut off until this line is bro
low. The reason for this is to ensure that power remains while the process(
writing to memory.
The PSUST line informs the digital board the status of the DC-DC con'
er, IC3 pins 5 and 6 are tied to the gate of Q1 which goes high while the ur
on, C5 will charge up through Rll, therefore IC3 pin4 will go low. Once
unit is turned off C5 quickly discharges through Dl, then IC3 pin4 (PSL
goes high.
1.2.4 Switching regulator, DC-DC converter
Power for the remaining boards in the monitor, and the RS232 interfac
generated by a switching regulator that is configured as a DC-DC conve
When the unit is turned on IC6 is powered up, this is a current mode p
width modulator which drives the primary winding of transformer T2 via
The N-channel MOSFET Q5 is pulsed by IC6 pin 6, the switching frequent
approximately 18 KHz, with a 10% duty cycle (=5 psec) (the duty cycle
vary with load requirements).
There are three secondaries on T2. The -i-5 volt digital supply is gener
from pins 5 and 6, rectified by D9 and filtered by C22, Cl4, and L2. Resii
R29, R39, R33 and potentiometer VR3 act as feedback for IC6 enabling
output to be adjusted and regulated for -i- 5VDC. Dual analog supplies are
crated at pins 8 and 9 with pin 10 as the center tap (AGND). Diodes D7
D8 rectify the secondary voltage and CIO, Cll, C13, and C15 filter the ou
An isolated supply is generated at pins 2 and 3 for the RS232 interface cin
ry.
1.2.5 RS232 interface
The RS232 interface is isolated from the rest of the circuitry, an isol
-I-5VDC supply is generated by the DC-DC converter, and information to
from the microcomputer is handled by four opto-isolators. Pins 2 and 1
transformer T2 are rectified by Dll and filtered by C23, IC7 is a -i-5
regulator for the isolated portion of the RS232 interface circuitry.
The 25 pin D connector on the rear panel is used for the RS
communication, information from the D connector is run to IC8 w
translates the incoming RS232 levels to 5 volt logical levels. There are
lines which are transmitted from the microcomputer and are handled by