Short Circuit Protection; Input Filter/Isolation Barrier - GE Corometrics 126 Service Manual

Short Circuit Protection

Input Filter/Isolation Barrier

Revision B
Theory of Operation: Isolated Power Supply Board
Short circuit protection for the entire supply is provided by R14, R30 and Q2. When
the voltage at the Vc pin of U4 reaches approximately 1.33 V, Q2 will start
conducting current from its emitter to collector which limits the amount of power
the supply can deliver. During a short on any output, the Vc pin wants to increase so
that the duty cycle of the supply can increase and supply more power to the load.
This circuit limits the maximum power the supply can deliver to any load to
approximately 10 W.
This circuit also helps reduce the peak in-rush current this type of supply sees during
its power-up sequence. As with the short circuit protection mentioned above, this
circuit limits the amount of current the supply can "dump" into the output filter
capacitors during power-up thus reducing the overall in-rush current.
An LC filter (C1,L3) is placed between the +20VDC input supply and the +20I
voltage that powers the switching supply. This filter prevents noise from the
switching supply from getting out on the +20VDC supply bus. This reduces the
radiated RF produced from the switching supply.
A spark gap (SG1) bridges the isolation barrier and is connected with two 10 k
resistors (R22, R23) that connect to each ground. This circuit provides a high-
voltage path from the isolation ground to the earth ground. This guarantees that if
the isolated ground sees a high voltage (i.e. defibrillator pulse) the current will go
through the spark gap instead of breaking down the linear optocoupler or the fly-
back transformer. The optocoupler and the transformer have breakdown voltage
ratings higher than that of the spark gap.
120 Series Maternal/Fetal Monitor
2015590-001
Ω
4-79