Power Supply Description (Service Group I) - HP 3585A Service Manual

Model 3585A
6·3 1 . POWER SUPPLY DESCRIPTI O N (Service Group I)
A 70 is the motherboard with some rectification circuitry mounted on it. A 75 is the Power
Supply Control board . It provides a 1 50 V supply, 5V and 24V supplies for use as raw sup­
plies and 1 8V reference for use on other boards . It provides the 60 Hz trigger and finally, it
provides the 20 MHz clock used on the switching regulators .
A76 is the Power Distribution board which routes the various supplies to where they are
needed . Also on this board are 5V, - 1 5V and 1 5V regulators .
The A7 1 -74 are Switching Power supplies and operate the same . The switching power supply
provides a very efficient means for regulating the voltage associated with high current de­
mand . The principal component involved is the switching regulator which, when provided
with the proper drive signal, switches between two states . When the switching regulator is
turned on, the resistance between the input and output is very low. This low resistance
dissipates very little power, even with high current flow. When the switching regulator is
turned off, the resistance between the input and output is very high . This results in complete
current cutoff and no power is dissipated by the device . With this in mind, it can be easily
realized that any prolonged delay in switching between the two states will result in high
power dissipation and failure of the device. Therefore, the switching drive current and
voltages must be of the proper magnitude to assure complete state change of the switching
regulator. The drive signals to the switching regulator are developed from a 20 kHz clock
signal modified by the current and voltage regulator sense circuits .
The output from the switching regulator consist of pulses of high voltage and current . These
pulses are filtered by a low pass network formed by a series induct or and a parallel
capacitor . The voltage output is monitored by the voltage sense circuit which compares the
monitored voltage to a known reference . If voltage output is low, the drive pulse remains on
for a greater period of time. The current output is monitored across a low resistance series
resistor located between the inductor and capacitor . The voltage drop across the resistor
signals the current sense detector which turns off the switching hybrid . If the current de­
mand is too great , such as in the case of a short circuit, the current detector will signal the
current sence latch causing a yellow indicator (current limit LED) to light and the output
current to fold back .
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Circuit Functional Descriptions
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