Section Iv Principles Of Operation; Overall Description; Detailed Circuit Description - HP 6236B Operating And Service Manual

4-1

OVERALL DESCRIPTION

4-2 This section presents the principles of operation
of the Models 6236B and 6237B Triple Output Power
Supply. Throughout this section refer to the combined
schematic diagram of Figure 7-1.
All information in this section applies to both
models unless otherwise indicated.
4-3 The two primary windings of the power transformer
are connected in one of four different ways by setting the
two slide switches mounted on the circuit board. These
switches select one of the nominal ac input voltages for
which the supply is designed: 100V, 120V, 220V, or 240V.
4-4 The transformer secondaries, together with
rectifiers and capacitor filters, provide raw dc for the three
output regulator circuits and for another regulator that pro-
vides reference and bias voltages to the output regulators.
4-5 By comparing its output to a high-stability refer-
ence, the 0 to +6-volt regulator (6236B) or 0 to +18-
volt regulator (6237B) holds its output voltage at the
value determined by a front panel control. Any error in
the actual output as compared to the desired output is
amplified by an operational amplifier and applied as
feedback to control the conduction of a series regulator
transistor. As a result, the voltage across the series
transistor varies so as to hold the output voltage constant
at the desired level. The high gain of the voltage
comparison amplifier and the stability of the reference
voltage ensure that input voltage or load current
variations have little effect on the output voltage.
4-6 The 0 to +6-volt output in the Model 6236B is
protected by a current foldback limiter to minimize
dissipation in the series regulator transistor during
overloads. In a current foldback circuit, the current limit
depends on the output terminal voltage and in this regulator
ranges from 2.75A±5% at 6 volts to 1A±15% with the
output shorted. (An output of 2.75A is 110% of the 2.5A
rated maximum at 6 volts.) The operating region of the
+6-volt regulator output is enclosed by a heavy line in
Figure 3-3. If the operating point reaches the diagonal
current limit line, a decrease in load resistance moves the
operating point down the line, reducing the output voltage
PRINCIPLES OF OPERATION
NOTE
SECTION IV
and current. Current foldback is controlled by a second
operational amplifier in the regulator that monitors the dc
output current. This current comparison amplifier takes
control of the output away from the voltage comparison
amplifier when the current reaches the design limit.
Removing the overload restores constant voltage
operation automatically.
4-7 The +20-volt regulator has a fixed current limit at
110% of its 0.5 amp maximum rated output but is
otherwise similar to the +6-volt regulator.
4-8 The 0 to -20-volt regulator is, in turn, similar to the
+20-volt regulator except that it resembles a
complementary mirror image of the latter. The output
voltages of the +20volt and -20-volt supplies are both set
by the same front panel control and track each other within
1% in the fixed tracking ratio mode. Precise tracking of the
two outputs is achieved by controlling the positive output
conventionally and using that output as the reference
voltage for the negative output.
4-9 The 0 to +18-volt regulator in the Model 6237B
is similar to the +20-volt regulator. It has a fixed current
limit at 110% of its 1.0 amp output.
4-10 The reference and bias supply provides reference
and bias voltages for the output regulators.
4-11 The turn-on/turn-off control circuit prevents output
transients when the supply is turned on or off. It does this by
delaying the application of certain bias and reference voltages
at turn-on and removing them shortly after turn-off.
4-12 A three-position meter switch selects which of
the supplies has its output voltage and current indicated
on the front panel meters. The proper range of the dual-
range meters is selected automatically
4-13

DETAILED CIRCUIT DESCRIPTION

4-14 0 To +20-Volt Regulator
4-15 Voltage Comparison Amplifier. The voltage com-
parison amplifier in the +20-volt supply controls the conduc-
tion of series regulator transistor Q1 so that the voltages at
the two inputs of the amplifier remain equal. A fixed voltage
divider holds its inverting input (U1-2) at -16mV. Its non-
inverting input (U1-3) monitors the output voltage in series
4-1