Table of Contents
Advertisement
compared by
U6D
to a
de
reference.
When an
overvol-
tage
occurs,
the
output
ofU6D will
shut
down the
sup-
ply
via the
soft-start.
8.2.2.8
Lockout for
High Internal
Temperature
When
the internal temperature of the
power
supply
rises above
85°C,
switch
S1 will close,
shutting
down
the power
supply
via the
soft-start
on
U2.
8.2.2.9
High-Voltage
Driver
The oscillator
output
of
the pulse-width modulator
{U2)
is
fed to
a
divide-by-two
frequency
divider
(U8B).
This output is
used
to
synchronize the
constant
duty
cycle
pulse-width
modulator
(U4) that
drives
the
switching transistors
for the High -Voltage Power
Supply.
This
modulator
operates
at
a constant
duty
cycle of
20
kHz,
wit
h
a
dead
time of 2
JlSec. R47
and
Cl
3
control
the
operating frequency,
while
R27
and
CI3
control
the
dead
time.
Modulator
U4 provides
±5V complementary
output, allowing
more
efficient
drive
of the
switching transistors; this
is
because
the
bases
are
switched
negative to sweep
out
the charge for
turn-oft'.
8.2.2.1 0
Control
Logic
Functions
Table
8-1 shows
the control
logic
functions
for
the
A4
board.
8.2.3 OUTPUT
BOARD
(AS)
8.2.3.1
General
The Output
board
contains
the
chopping trans-
fo rmer. rectifiers
and
filters for
the
Low-Voltage Power
Supply.
The
chopping
transformer
(Tl)
has
two
pri-
mary
windings (one
for ac
and
one
for
de
operation)
and
a multiple-output
secondary
which
provides ±
5V,
±
12V, ±
llOV,
and
+33V. Also
included on
this
board
is a current-sensing transformer
(T2) for
sensing pri-
mary current
on the ac off-line
side of
the
chopping
transformer.
A
schematic
of
the Output
board
is
shown at
the end
of
this
section
in
Figure
8-8,
and the printed wiring
hoard
assembly and
parts
list
in Figure 8-9.
8.2.3.2
AC Operation
During
ac operation, the
half-bridge
switching
tran-
sistors
(Q1
and
Q2)
on
the Switcher
board (A6)
drive
the
chopping transformer
via the XFMR
AC DR
and
XFMR
AC
DR
primary
winding. The transistors
drive
current
into
and
then
out
of the
primary winding.
To
ensure that
both Q1
and
Q2 are
off, dead time
between
these
two transistors is
important.
Without dead
time,
there
will be
a
short-circuit
across the
primary
wind-
ing.
The
rectified-average ac
voltage
at
the primary
winding is
a
nominal 87V. During
ac operation, the
primary
current
is
monitored
by
the current-sensing
transformer
(T2).
The ratio for
the current-sensing
transformation
is
100
to 1,
and
the
secondary is
loaded
with
100 ohms
on the
Control board
(A4). This
pro-
duces
a
secondary
output of
1
V
/A
of
primary
current
in
the chopping
transformer.
If
the
primary
current in
the
chopping
transformer rises above 1.5A
(due
to an
excessive
load
or
a
short
on the secondaries),
the
power
supply
will shut down.
8.2.3.3
DC
Operation
During de
operation,
the
push-pull switching
tran-
sistor
(Q3 and
Q4)
on Switcher board
A6 drives
the
chopping
transformer via DC CENTER
TAP,
XFMR
DC
DR,
and
XFMR DC DR.
This
primary winding
is
center-tapped.
A de
voltage is applied to
the
center tap,
and XFMR
DC
DR
and XFMR
DC DR
are alter-
nately
pulled
low
by
switching
t ransistors.
The
nomi-
nal
de voltage
at
the
DC
CENTER
TAP
is 11.7
Vdc
for an input
of
12.0
Vdc.
The
primary
current in
de
operation is
monitored
on
Switcher
board
A6.
Table
8-1.
Control
Logic
Functions
INPUTS
OUTPUTS
PWR
ON
PWR OFF
AC
DEAD
PULSE
AC
DC
AC
DC
RELAY
VOLTAGE
TIME
WIDTH
DRIVER
DRIVER
LED
LED
ON
SENSE
MODULATOR
DRIVER
DRIVER
HIGH
HIGH
HIGH
2
~-tSEC
OFF
OFF
OFF
ON
OFF
HIGH
HIGH
HIGH
LOW
0.5
~-tSEC
OFF
OFF
OFF
OFF
ON
LOW
HIGH
LOW
HIGH
2
~-tSEC
OFF
OFF
OFF
ON
OFF
HIGH
HIGH
LOW
LOW
0.5
~-tSEC
OFF
OFF
OFF
OFF
O
FF
HIGH
LOW
HIGH
HIGH
2
~-tSEC
ON
ON
OFF
ON
O
FF
HIGH
LOW
HIGH
LOW
0.5
~-tSEC
ON
OFF
ON
OFF
ON
LOW
LOW
LOW
HIGH
INVALID
STATE
LOW
LOW
LOW
INVALID STATE
36930·
1
24
8-3
Advertisement
Table of Contents
Troubleshooting
