Fluke 93 Service Manual page 67

CIRCUIT
DESCRIPTIONS 3-33
F/ybeck
ccn
verier
See
ligure
3.19
and
circuit
diagram A2c
(figure
107).
The
main components
of this flyDack converter are
V2532
(convener-switch),
L2504 and L2505
(windings),
R2582
(sense
resistor),
and
G2536
and V2533
(secondary
circuit).
The main
regulating
element
is
N2503
(see
figure 3.19),
-VcH
Figure
3.
19
Schematic diagram
flyback converter
N2503
incorporates
an
oscillator,
the
frequency
of
which
is
determined
by
R2548 and C2527
(fixed
frequency
of
100
kHz), This
oscillator
drives
a
sawtooth
generator.
The
produced sawtooth
voltage
is
compared
to
a
DC
voltage.
This
DC
voltage
is
made
by
ar;
internal
error amplifier (voltage
regulator),
which compares
the
produced
converter voltage
>V_CH
to
a
stable
5V
reference
voltage.
This
Is
done
with
a
bridge
circuit
(R2564,
R2555, R2S57, R2568).
Figure
3.20 IntemaJ
N2503
voltage
waveforms
When
the
sawtooth
voltage
is
larger than
the
DC
voltage,
the output
signal
(CA.
CB
on
pins 12,13)
is
"high ".
When
the sasvtooth voltage
is
less
than the
DC
voltage, the ou4>u1 signal
is "low"'.
In this
way
the
duty cycle
of
N2503's
output signal
can be changed, thus changing
the
energy
transferred
to
the
secondary
converter
circuit.
The
output signal
is
level shifted
by
transistor
V2526 and
related
circuitry.
Now
this
square
wave
signal
is
used
to
drive converter switch
V2S32, which
is
bootstrapped
via
V2526, V2529, R2546,
R2562, and C2537.
Charging
current
limiter
N2503
limits
the
voltage
difference
between CL+
(pin
4)
and CL-
(pin
5) to
200 mV.
If
the voltage
between these two
inputs
starts to
rise,
the
irxtamaf
DC
voHage
will rise,
and
the duty cycle
of the
output
square
wave
voltage
will
decrease
(see voltage regulation described
earlier).