Fluke 93 - Page 67

CIRCUIT DESCRIPTIONS 3-33
F/ybeck ccnverier
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).