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
ampIlf ierN2501. 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.
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