KEPCO MBT Series - Manual (Page 109)

MBTSVC111609
4-3
FIGURE 4-3. CURRENT COMPARISON EQUIVALENT CIRCUIT
The current feedback is proportional to the flow of output current and is developed through a
current sensing resistor R
s
in series with the Output Terminal lead. The sensed voltage across
R
s
is amplified differentially by the current sensing amplifier and then calibrated by a current full
scale adjustment amplifier to a voltage signal of 0 to -5V. The later signal being proportional to
the output current.
A condition of balance exists if: G I
o
R
s
/R
f
= E
i
/R
i
and ε approaches zero, where G is the gain of
the current sensing amplifier chain. A change in E
i
or I
o
in the balance equation produces an
error signal that, when amplified by the current comparison amplifier, becomes a control signal
for the driver stage. The control signal is applied to the pass element (power transistors) to
increase or decrease its conductance so as to maintain a desired value of output current.
4.1.5 VOLTAGE/CURRENT REFERENCE
The internal feedback components in the voltage and current control channel of each MBT
Power Supply are selected to control the voltage and current full scale output range. This is
made possible by the 0 to +5V d-c signal applied to either the voltage comparison amplifier or
current comparison amplifier. These two 0 to +5V d-c Reference Potential signals are gener-
ated by the A3 Digital Board in response to a command received either from local controls, or
remotely through the IEEE 1118 bus via the A3 Digital Control Board.
4.1.6 A/D CONVERSION
The output voltage and current are continuously monitored by the voltage and current sensing
amplifiers. Two channels of an Analog Digital Converter (ADC) convert these analog voltages (0
to+5 V d-c) into equivalent digital signals. These digital signals are stored in buffers and updated
every two milliseconds. On demand from the remote Controller, the contents of the output volt-