Buck Regulator Control
See function block H of A6 Power Supply Schematic Diagram in the Component-Level
Information binder.
The Buck Regulator Control pulse-width modulates the Buck Regulator and provides a
synchronized signal to the DC-DC Converter Control circuitry. The Buck Regulator Control
has two feedback paths. The first is the output of the Buck Regulator, which provides coarse
regulation. The second is the Feedback Circuit which samples and compares the
+5
Vdc
output of the Output Rectifier.
U202B
and associated circuitry sense the output of the Input Rectifier and will turn off
U203
if the voltage at TP108 goes below approximately i-170 Vdc. Also, it will not allow
U203
to
start up until this voltage exceeds +215 Vdc. A low on the output of
U202B
will also clear
the overcurrent latch in the DC-DC Converter Control circuitry.
Thermal shutdown occurs when RT201, mounted on the main heatsink, reaches a temperature
of 100 C. When this occurs, the voltage at
U203
pin 13 exceeds 0.6 V and inhibits pulses to
the Buck Regulator.
R203, R204, U211, and associated circuitry provide feedforward for
U203.
This makes the
loop gain independent of input line voltage and cancels 120 Hz ripple by more than 10 dB.
U202C and its associated circuitry permit the power supply to start up at low line voltages
at low temperatures. At low line voltages U202C will draw charge away from
C206
through
R205. This allows the Buck Regulator to turn on and draw current through the thermistors in
the Input Rectifier. This warms up the thermistors, thereby decreasing their resistance and
increasing the voltage at TP108. When the voltage is sufficiently high at TP108, the output of
U202C
will open and
C206
will be allowed to charge normally.
U202A
converts the sawtooth at TP204 to a squarewave to drive the DC-DC Converter
Control circuitry. The frequency of the sawtooth is determined by the resistance at pin 7 of
U203
and the capacitance at pin 8 of
U203.
DC-DC Converter Control
See function block I of A6 Power Supply Schematic Diagram in the Component-Level
Information binder.
The DC-DC Converter Control circuitry divides the 80 kHz squarewave from U202A
and generates two complementary 40 kHz squarewaves to drive the
FETs
in the DC-DC
Converter. Also, U202D and its associated circuitry monitor the voltage across sense resistor
R116 in the DC-DC Converter. When the current through the
FETs
in the DC-DC Converter
exceeds 1.8 A, the voltage across R116 will cause the output of U202D to go high. This sets a
latch in
U204
which turns off
U203.
12-16 Display/Power Supply Section
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