HP 6296A - Page 23

•
•
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SECTION
IV
PRINCIPLES OF
OPERATION
H
R£1'£RENCE
CIRCUIT
r---
REFERtNCE
VOLTAGES
T
BIAS
8JAS
-
SUPPLY
-VOLTAGES
AC
FOWD
TRANS
TORM
DI
I
RECTIFIER/
-
PllEREGUIJI
lOR,
....
MOTE:
DENOTES
VOLTAGE
FEECIACl
PATH
---
DENOTES
CURRENT
FtEDIACl
MTH
nLTER
I.
SCA
-
COtmlOL
CJllCUJT
'
•
-
-
~
SERIES
REGULATOR
"
ERROii
AMPL.
L<J
CONSTANT
-
-
CURRENT
INPUT
CIRCUIT
....
±
...
-
CURRENT
SAMPlJNG
RESJS10R
...
GATING
CONStAHT
CIRCUIT
VOL?AGE
-
.,_
DC
INPUT
OUTPUT
T
ClllCUIT -
~
FAULT
...
INPUT
MtTER
CJRCIJIT
....
CIRCUIT
•
l
-
,..
-
Figure
4-1.
Overall
Block Diagram
4-1
OVERALL
BLOCK
DIAGRAM
DISCUSSION
4-2
The power
supply,
as
shown on
the
overall
block diagram
on
Figure
4-1,
consists
of
a power
transformer, a
rectifiei-preregulator-filter,
pre-
regulator
(SCR)
control
circuit,
series
regulator,
error
amplifiers.
gating
circuit,
a
constant
volt-
age input
circuit.
a
constant
current
input
circuit,
a
fault
input
circuit,
a
reference
circuit,
bias
supply,
and
a
meter
circuit.
4-3
The
input
line
voltage
is
reduced
to
the
proper
level
by
the
power transformer
and
coupled
to a
rectifier
bridge
consisting
of
two
rectifier
diodes and two
SCR's.
The bridge
simultaneously
performs
the
necessary
rectifying
and
preregu\at-
ing
functions.
The SCR's,
oper~ting
in
conjunc-
tion with a
control
circuit,
minimize the power
dissipated
by
the
series
regulator
by
keeping
the
yoltage
drop
across
the
regulator
at
a low
and
constant
level.
The
SCR
control
circuit
accom-
4-1
plishes
this
by
issuing
a
firing
pulse
to
one of
the
SCR's
once
during
each
half
cycle
of
the input
ac.
The
control
circuit
continuously
samples
the
out-
put
voltage,
the
input
line
voltage,
and
the
volt-
age
across
the
series
regulator
and,
on
the
basis
of
these
inputs,
determines
at
what
time during
each
half
cycle
that
the
firing
pulse
will
be
gen-
erated.
4-4
The
series
regulator.
part
of
another
feed-
back
loop,
is
made
to
alter
its
conduction
to
main-
tain
a
constant
output
voltage
or current.
Its
con-
duction
varies
in
accordance
with
feedback
control
signals
obtained
from
the
error
amplifier.
It
should
be
noted
that
the
series
regulator
provides
fine and
"fast"
regulation
of
the
output, while
the
preregulator
handles
large
relatively
"slow"
regu-
lation
demands. The
de
current
from the
series
regulator
passes
through a
current
sampling
re-
sistor
before
reaching
the
positive
output terminal.