4-78 120 Series Maternal/Fetal Monitor Revision B
2015590-001
Theory of Operation: Isolated Power Supply Board
potential, the parallel combination of C24 and R27 was connected across the two
grounds.
Isolated Feedback
The feedback for the supply crosses the isolation barrier via a linear optocoupler
(U2). The complete feedback circuit is comprised of U1–3, VR4, R7–13,
C10,19,20,21,25.
The IL300 linear optocoupler consists of an AIGAs IRLED irradiating an isolated
feedback and an output PIN photodiode in a bifurcated arrangement. The feedback
photodiode captures a percentage of the LED’s flux and generates a control signal
that can be used to servo the LED drive current. This technique compensates for the
LED’s non-linear, time, and temperature characteristics. The output PIN
photodiode produces an output signal that is linearly related to the servo optical flux
created by the LED.
As stated in “Controller”, the VFB pin of U4 wants to be at 1.244 V to close the
loop. R10 and R11 divides down the main +15 V unregulated output to 1.245 V. U1
and R9 source current into the IRLED. R12 and the internal PIN photodiode of U2
close the loop around U1. C21 provides some high frequency roll-off for amplifier
U1.
VR4 and C19 provide a steady +5 Vdc supply that is used by the isolated PIN
photodiode of the linear optocoupler and powers operational amplifier U1.
On the non-isolated side, R7, R13, and the current being sourced from the second
PIN photodiode, re-create the feedback voltage seen on the isolated side at U1–3.
The IL300s are bin sorted by the manufacturer to group the parts with common
transfer gains. The transfer gain of the IL300 can vary from 0.56 to 1.61. Instead of
paying a premium for buying only one bin, a potentiometer (R7) was used so that the
variations in the transfer gains of U2 could be corrected for. If the IL300’s gain was
1.0, the combined resistance of R7 and R13 would equal that of R12. C25 provides
some filtering for the feedback signal. U3 and R8 provide buffering between the
feedback signal and the VFB pin of U4.
To Next Page
Loading...
Need help?
General
