Buck Regulator Control - Agilent Technologies 8564EC Service Manual

Display/Power Supply Section
A6 Power Supply Assembly

Buck Regulator Control

See function block H of the 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 senses the output of the input rectifier
and will turn off U203 if the voltage at TP108 is less than
approximately +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.
664 Chapter 13