HP 8562E, Service Manual

Page: 467 / 482

7. If the sawtooth is correct, check the base of Q401 for 30 pulses.
8. If the duty cycle is high, but there is no Vdc, suspect the bridge rectifier, CR401
through CR404.
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 j-6.8 Vdc
output of the output rectifier.
and associated circuitry sense the output of the input rectifier and will turn off
if the voltage at TP108 goes below approximately Vdc. Also, it will not allow to
start up until this voltage exceeds Vdc. A low on the output of will also clear
the overcurrent latch in the DC-DC converter control circuitry.
Thermal shutdown occurs when mounted on a large heatsink, reaches a temperature of
100 C. When this occurs, the voltage at pin 13 exceeds 0.6 V and inhibits pulses to the
buck regulator.
R203, R204, R226, and associated circuitry provide feedforward for This makes the
loop gain independent of input line voltage and cancels 120 Hz ripple by more than 10
and its associated circuitry permit the power supply to start up at low line voltages
at low temperatures. At low line voltages will draw charge away from 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
will open and will be allowed to charge normally.
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 and
the capacitance at pin 8 of
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 squarewave from and
generates two complementary 40 squarewaves to drive the in the DC-DC converter.
Also, and its associated circuitry monitor the voltage across resistors R112 through
R117 in the DC-DC converter. When the current through the in the DC-DC converter
exceeds 1.8 A, the voltage across the resistors will cause the output of to go high. This
sets a latch in which turns off
Display/Power Supply Section 13-17