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Theory
of
Operation
AC
Input Circuit
This circuit is composed of an AC Power Switch, a fuse, a line
filter, and an inrush current limiting circuit and rectifying
smoothing circuit.
The inrush current limiting circuit controls
the charging current to electrolytic capacitors when power is ON.
The line filter reduces noise that leaks from the power source to
the AC line or that returns from the unit to the power souce; it
satisfies the specifications of noise regulations.
Control Circuit
&
Power Converter Circuit
This circuit is a self oscillation switching system, generally
called an R.C.C.
(Ringing Choke Converter).
The R.C.C.
circuit
does not fix the oscillating frequency.
Whenever input voltage
is high or the load becomes light, the oscillating frequency will
be high.
The current through R4 and R5 supplies transistor Ql's base, then
Q1 turns
ON.
When transistor Ql is On, the
Q1
current excites
the transformer T1 and voltage rises in the bias coil of Tl(2-3)
which leads transistor Q1 positive bias, then transistor
Q1
turns
ON.
When transistor Q1 turns ON, collector current charges the energy
to primary inductance of transformer T1
( 4 - 6 ) .
Increasing the
collector current of transistor
Q1
to the point of:
I
>
I .hfe
C =
B
Then, transistor Ql immediately turns
OFF.
In a moment,
transformer T1 will have negative voltage which will be supplied
to the secondary circuit through a rectifier.
A Short Circuit
Protector is provided to protect transistor
Q1
from excess
amounts of current when the secondary circuit becomes shorted.
When transistor 9 2 detects the voltage drop at R13, the collector
of 42 shorts the base and emitter of
Q1.
Then Q1 stops working
so that the circuit protects Q1 from over current.
The over current protector in the - 1 2 ~ line is provided by the
three terminal positive voltage regulators IC2
,
IC3 (built-in
current fold back protection
1 ,
which protects Q1 against
excessive current from the -12V line.
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