Fecg/Iup Board Theory Of Operation - GE Corometrics 170 Series Service Manual

FECG/IUP Board Theory of Operation

Isolated Power Supply
5-36
Theory of Operation: FECG/IUP Board Theory of Operation
Patient isolation for the FECG and the IUP catheter is accomplished by providing
floating power supplies to power the FECG and IUP front end electronics.
Integrated circuit U12 provides non-overlapping drive signals to dual FET U9 which
in turn drives the primary of transformer T1. Resistor R94 and capacitor C12
provide dampening for the primary of T1 so as to limit the voltage at the drains of
U9 to a level below their breakdown voltage. The frequency at which the primary of
T1 is switched is 35968.75 Hz which is derived by dividing the signal 144 kHz by
four. The actual frequency of this signal is 143875 Hz and is derived from the PAL
U50 in the ultrasound circuit. Thus the transformer switching frequency is
synchronized to the ultrasound circuit. The secondary of transformer T1 drives
rectifiers D9 and D10. These rectifiers are filtered by C8 and C10 for the positive
power supply, and by C9 and C11 for the negative power supply. Integrated circuit
U3 and its associated components provide a regulated positive voltage (+10 V for
Model 173; +9.3 V for Model 174), while integrated circuit U4 provide a regulated
negative voltage (–10 V for Model 173; –9.3 V for Model 174). Integrated circuit
U1 is a positive precision 2.5 volt reference. The output of U1 is input to the non
inverting input of amplifier U2 (pin 3). The output of this amplifier U2 (pin 1) drives
the emitter follower Q1. The gain of the amplifier-emitter follower is 1.6 and is set
by resistors R3 and R4. The output of Q1 emitter is 4 V (1.6 x 2.5). This voltage is
used to power either an IUP transducer or tocotransducer. Resistor R1 in the
collector of Q1 is used to sense the current drain from other manufacturers of IUP
transducers who don't have an IUP enable associated with them. When the voltage at
the junction of resistors R36 and R37 falls below 1.164 V
(4 mA) the output of the voltage comparator U16 (pin 8) switches from a high state
to a low state, which in turn causes transistor Q3 to turn on and transfer the IUP
enable across the isolated barrier. This signal is named IUPEN*. However If a
tocotransducer is present, this signal is ignored. Transistor Q3 can also be turned on
by the signal PEN*.
For Model 173 and 174 Monitors, integrated circuit U33 and associated components
provide regulation of the power supplied to the FECG/IUP Board circuitry (+12 V
for Model 173; +11.6 V for Model 174).
For a Model 174 only, the CLEAR signal synchronizes the counter in the PAL; this
in effect stops the power supply during the receive cycle of the ultrasound process
for noise reduction purposes.
170 Series Monitor
2000947-004
Revision C