Fluke 93 Service Manual page 69

CIRCUIT
DESCRIPTIONS 3-35
This
self-oscillating
flyback converter consists
of:
•
V2509
(converter-switch)
-
R2509...R2517
(sense-
resistors)
•
V2502
(thyristor svsotch)
•
R2544
(Start-up resistor)
-
T2501
(windings)
-
3 separate secondary
circuits for -30V,
-5V,
and
+5V
The
main
regulating
component
Is
operational
amp
Ilf
ierN 2501. This op-
amp compares
the
produced
secondary
*SV
voltage
with
a
rofarence voltage,
produced by zener
diode N2502.
If
the
secondary
•(•SV
increases, the
fault
signal
generated by
the
N25Q1
will
produce a
current that
causes an
extra
voltage
drop over R2508.
Because
of
this,
thyristor
V2502
will fire earlier.
The
switching
frequency
Of
the flyback converter Inaeases and
the
secondary
-t-5
V
voltage decreases.
When
the
ScopeMeler
Is
switched
on (RSSLSTN
is
"active low'),
V2544
(see
circuit
diagram A2c,
figure 10.6) connects
the Inverting Input of
N2501
to
ground.
When
the
ScopeMeler
starts up,
capacitor
C2509
causes
the reference voltage
and
therefore the output voltage, to
rise ^owly,
limiting
the Inrush
("starting')
current
drawn from
the
batteries or
line
voltage.
Undervottage
detection
and
protection
circuit
When
the flyback converter
is
oscillating,
capacitor
C2532
is
charged every
period via
R2543
and
V2516. During normal operation C2532
Is
discharged
by V2517, which
Is
driven via
R2641, V2511,
R2529, and V2509.
If,
for
example,
the
secondary +5V
voltage
becomes
too
low,
C2532
ts
not
discharged by V2517.
This
will
activate
the
RESPOWHT
signal,
and
the povsrer
wllf
be
switched
off
compietely, preventing
further
damage
of circuits.
(The
+5V
voltage
can
become
too low
because
the
Input
voltage -VbaVs
Is
too
low.
or
the
power
output
to
the
ScopeMeler
circuitry Is
too
high.)
R2542, C2531
,
and
diode
V2508
will
reset
C2532
during the
start
up
of
the
power
supply
(the
voltage
across
C2532
will
become
zero).
This
is
necessary because V251 7
cannot be
driven
via
V2541,
just
after
the
ScopeMeler
Is
switched
on.
Reference source
The
reference
source provides a
stable positive
(-t-Vref)
and
negative reference voltage
(-Vref)
used
in
other parts
of
the
ScopeMeter.
It
also
uses
the voltage across zener diode
N2502
as
an
input
voltage.
NOTE:
The
flyback converter,
used
in
the battery
charger seot/on
(section 3.4.8)
has
a fixed
osdllating
frequency
of 100 kHz.
The amount
of
energy supplied
is
regulated
by
varying the
duty
cycle.
The
flyback
converter
used
in this
power
supply,
however,
Is
seff^scUlatlng
and
operates on
a
variable oscillating frequency
and a
fixed duty
cycle.
For
alkaline batteries, for
example,
the oscillating
frequency
s
about
62
kHz.