Agilent Technologies E3620A Manual De Funcionamiento Y Servicio página 77

The input ac line voltage is first applied to the preregulator
which operates in conjunction with the preregulator control
circuit to rectify the tap switched AC voltage. This preregu-
lator minimizes the power dissipated in the series regulating
elements by controlling the dc level across the input filter
capacitors depending on the output voltage.
To achieve this, tap switching of the V1 output is accom-
plished by a TRIAC(Q5), a bridge diode (CR4), two
diodes(CR5 and CR6) and the preregulator control circuit.
This circuit allows the input capacitors to charge to one of two
discrete voltage levels depending on the output required.
When the output voltage exceeds the reference level, the pre-
regulator control circuit fires the TRIAC Q5 that causes the
input capacitors to be charged to the voltage which is neces-
sary for full output of the supply. When the TRIAC is not fired,
the bridge diode CR4 conducts and half the voltage is applied
to series pass transistor Q2.
The regulator of the V2 output is, in turn, similar to that of the
V1 output regulator except that it has two TRIACs(Q3 and
Q4) and two bridge diodes(CR2 and CR3) to allow the input
capacitors to be charged to one of four discrete voltage levels
depending on the output required in order to minimize power
dissipation in the series regulator transistor Q1.
The main secondary winding of the power transformer has
three sections (N1, N2, and N3), each of which has a different
turns ratio with respect to the primary winding. At the begin-
ning of each half-cycle of the input ac, the control circuit
determines whether one, both or none of the TRIAC will be
fired. If neither TRIAC is fired, the bridge diodes (CR2 and
CR3) receive an ac input voltage that is determined by N1
turns (tap 18 and 19 of the power transformer) and the input
capacitors charge to a corresponding level. If TRIAC Q3 is
fired, input capacitors charge to the voltage determined by
N1+N2 turns. Similarly, if TRIAC Q4 is fired the capacitors are
charged by N1 + N3. Finally, if all TRIACs are fired simulta-
neously, input capacitors charge to its highest voltage level
determined by N1 + N2 + N3 turns (tap 17 and 20 of the
power transformer).
The TRIAC control circuit determines which TRIACs are to be
fired by monitoring the output voltage and comparing these
values against a set of three internally derived reference lev-
els. These three reference levels are translated into boundary
lines to allow the output characteristic to be mapped into four
operating regions (Figure A-2). The boundary lines, which are
invisible to the user, are divided into four operating regions
(V1, V2, V3, and V4) to minimize the power dissipation in the
series pass transistors. Whenever the output voltage is below
the sloping V1 line, the control circuit inhibits two TRIACs and
the input capacitors charge to a voltage determined by N1.
Figure A-2 indicates the windings that are connected as a
result of the other voltage decisions.
The reference and bias supplies of each supply power the
operational amplifiers and provide reference and bias volt-
ages for the output regulators. The display power circuit pro-
vides voltage which is used by the A/D converter and display.
Figure A-2. Output Power Plot
Diode CR13 and CR19 are connected across the output
terminals in reverse polarity. They protect
electrolytic capacitor and the series regulator transistors from
a reverse voltage applied across the output terminals.
Two meter pushbutton switches select which supply (V1 or
V2) has its output voltage and current indicated on the front
panel meters.
MAINTENANCE
INTRODUCTION
This section provides performance test, troubleshooting infor-
mation, and adjustment and calibration procedures. The fol-
lowing operation verification tests comprise a short procedure
to verify that the power supply is performing properly, without
testing all specified parameters.
If a fault is detected in the power supply while making the
performance check or during normal operation, proceed to
the troubleshooting procedures. After troubleshooting, per-
form any necessary adjustments and calibrations. Before
returning the power supply to normal operation, repeat the
performance check to ensure that the fault has been properly
corrected and that no other faults exist.
Test Equipment Required
Table A-1 lists the equipment required to perform the various
procedures described in this section.
Operation Verification Tests
The following tests assure that the power supply is per-
forming properly. They do not, however, check all the speci-
fied parameters tested in the complete performance test
described below. Proceed as follows:
a. Perform turn-on checkout procedure given in page 1-7.
b. Perform the load regulation performance tests given in
the following paragraphs.
A-2
the output