Table of Contents
Advertisement
the Control
board
are t he
error
amplifier,
the pulse-
width
modulator,
and
the
Switcher
drive-circuitry
porti on
of t
he
converter's
regulation control
loop.
There
is
also a
pulse-width modulator which
drives
the
switching
transistor for
the
High-Voltage P ower
Sup-
ply.
The Control
board
protects
the
converter with
lockouts
under four
conditions: output overvoltage,
input
ac
and
de under/overvoltage, out
put overload or
short-circuit,
and high internal
temperatures.
A block diagram
of
the Control
board
is
shown at
the
end of
this
section in Figure
8-5,
a
schematic
in
Figure
R-6,
and
the
printed
wiring
board
assembly
and
parts
list
in
Figure
8-7.
8.2.2.2
AC/DC
Switch-Over
The ac/dc
switch-over section
provides 1)
the
volt-
age
for
t he
Control
board,
and
2)
the switch-over
cir-
cuitry
to
change between
ac
and
de
operation.
For
de
operation,
the
de input is
connected to
DC BUS input
via
the
input
filter
(C
l
,
C2
and Ll) on
the
Switcher
board. During
switch-over,
trans
istor
Q2 is off,
which
means
there is no AC
VOLTAGE
SENSE,
which
ena-
bles
transistors
Ql and
Q3. Q3
then
energizes relay
Kl
on the Switcher
board. This switches
the
de primary
to
the
chopping
transformer
and
disconnects the ac
primary.
Ql then
switches the DC
BUS
through
CR16
to the FREQUENCY
STD
SUPPLY
line
and the
voltage
regulator (U9 and UlO). U9 and UlO
supply
power
to
Vs
and
V
1
•
on
the
Control
board.
For ac
oper-
ation,
the FREQUENCY STD SUPPLY
line
supplies
power
to the two regulators,
U9 and
UlO.
AC
opera-
tion overrides de
operation through
the
AC
VOLT-
AG
E SENSE
line,
which
turns Q2
on and
Q3 off. This
pulls
the
RELAY
ON line
high,
up to
the DC
BUS,
causing relay
Kl to switch the
ac primary
to
the
chop-
ping t ransformer.
8.2.2.3
Loop
Filter
The
loop
fil ter
consists
of a
combination error
amplifier/
filter which
controls
t he
bandwidth and
sta-
bili ty
of
the
control
loop.
T he
error ampl
ifier
com-
pares
the
+
f>V
output with
the reference
voltage
set
by potent
iometer
R6.
R6
is
adjusted
to provide a
+:i .:ZV output. T
he
loop
fil ter
provides
a
1-kHz, 3-dB
band
width.
8.2.2.4
Drivers
and Control
T he low-voltage driver
and
control
section
contains
t
he
pulse-width
mod
ul
ator, dead-time
select
,
and
ac/
de drivers.
T
he
error
voltage
from
the loop
filter
is
connected
to the
pulse-width modulator
U2
for
duty-
cycle
control
of switchers. Pulse-width modulator
U2
provides two 40-kHz
complementa ry
outputs.
R
IO
and
C7
control
the
freq
uency.
Dead
ti me between the two
complementary drive signals ensures
that
the two
transistor
switches
(Ql and
Q2,
or
Q3
and Q4 on
the
Afi
board
)
never
turn
on at
the same ti
me.
T he dead
8-2
time
is
selected
by
Q6 via t he DC BUS line
for
0.5 1J.sec
in
the de
mode
(R63
and
C7)
and
21-1sec
in
t he ac mode
(R9
and C7).
The complementary outputs are
switched
by U3 to
either the ac
drivers
(Ull) or
the de drivers
(U 12). The
driver
-select circuitry
(Ql5
and
U5)
is
con-
trolled
by
the AC
VOLTAGE SENSE
line. U2 uses the
timing
capacitor
(C8) to soft-start the
power
supply.
8.2.2.5
Lockout for
Output Overvoltage
Out put-overvoltage
lockout
protects modules
and
boards
in
t he
System Analyzer from high
voltages
on
outputs
of
the Low-Voltage
Power
Supply. The +
12V
and
-
12V
outputs are
monitored
by
comparator
U7.
When
it detects
an
overvoltage, this
comparator
will
shu
t
d own
the
power
supply
via t he
inpu
t to the
pulse-
width
modulator. The +5V output
is
protected
against
overvoltage by
zener diode
VR3
on
the
A3
board.
The
+33V
and -
5V
out
puts
a re
protected
by
VR1 and
VR2
on
the
A5
board.
8.2.2.6
Current-Limit
Lockout
T
he
current-limit
lockout
is
used to
monitor
t he
power
the
System Analyzer
draws
from
the power
f; Upply.
This monitoring measures the
instantaneous
t
urrent
in
the switching transistors. In de
mod
e,
th
e
switching current
is
measured
by R20
(8m
VI
A)
on
the
Switcher hoard and t
hen
compared
by
U6B
to
a
de
refere nce,
thus
providing a cu
rrent
limit
of
25A.
In
ac
mode,
the
switching
current
is
monitored
by
T1
(1
00/1
turns
ratio) on
the
Output
board. T he
signals
AC
CURRENT
SE NSE and
AC CURRENT
SE
NSE
are
full-wave rectified
by
CR5-CR8,
which
are
loaded
with
100
ohms
hy R29. T he resulting voltage
is
1 V/A
of
the de switching
current.
U6A
compares
this
volt
-
age to a de
reference,
providing a current
limit
of
1.5A.
When the
current
limit
is exceeded,
U6A
sets
the
cur-
re nt-limit
latch
(U84),
shutt ing
d own
the
power sup
-
ply via
t he
shut -down pin on
U2.
The
current
latch
is
reset
by toggling
the
P OWER switch on
the
front
panel.
8.2.2.7
Lockout for Input Over/Undervoltage
For
voltages outside the
specified
input
range
of
100
to
130
Vac,
200
to
260
Vac,
or
11 to 19
Vdc,
the circuit-
ry
for
over/undervoltage lockout
shu
ts
down
the power
supply via the soft-start
pin on
U2.
Lockout
of ac
and
de
in
put undervoltage
uses
the same
circuitry. The
ac
in put
voltage
is
monitored via the FREQUENCY
ST
D
SU
PPLY
line,
and
the de input voltage
is
monitored
via
the
DC
BUS line.
When t he out put of U9
d
rops
below 8.2V,
the
zener
diode VR4
will drop
out,
turning
Q4 otr. T
his
wi
ll
allow
CR19 to
turn
Q5
on,
causing the
power
supply to
shu
t down
via
the
soft-start
pin
on U2.
W
he
n
the DC
I
line
goes
above
19V,
VR3
wi
ll
zener,
t urn
ing
Q5
on. For
ac
overvoltage,
the AC VOLTAGE
SENSE
signal
is
divided
by R30
and
Rl and t
hen
Advertisement
Table of Contents
Troubleshooting
