Sanyo AVM-2550S Training Manual page 69

( lll ) On Operation (T2 Period in Figure 2)
The current produced from the output coils decreases in inverse proportion to
time and when it reaches 0, T1 period finishes and T2 period begins. At this
time, the distributed capacitance of converter transformer T601 is equivalently
input in parallel with the inductance of input coil 5-8 in the converter
transformer T601, and a resonance will occur. The resonance current will
travel from the terminals 8 to 5 of the input coil. Therefore, because a current
is trying to be produced from the feedback coil terminals 2 to 3, Q601 will
remain Off. The resonant current will be largest at the end of T2 period and
the current coefficient will become 0. (See T2 Period in Figure 2-(B).) After
this, the current coefficient will reverse the polarity. Therefore, a current will
be produced in the feedback coil terminals 3 to 2 and the current (terminal 2
C612 R614 R613/R618
causing Q601 to go On. Even if this current is very small, due to the operation
of positive feedback in the feedback coil, Q601 will momentarily go On.
( lV ) On Period (T3 Period in Figure 2)
When Q601 is On, its collector current will linearly increase. Accordingly a
constant voltage will be produced by the feedback coil, pins 2-3 and due to
the positive feedback to the base of Q601, it will remain On. At this time, the
energy (the amount determined by the formula 1/2 Ll
coil pins 5-8. With Q601 On, the greatest amount of energy will be stored at
the end of T3 period.
CONTROL CIRCUIT
The output voltage from the output coil pins 16-14 is rectified and smoothed
by the rectified output circuit (D625, C625) and a +B voltage of 130V will be
produced. To detect and produce a stable +B voltage, a control circuit has
been added.
( l ) Circuit Operation
As shown in Figure 1, the +B voltage is applied to pin 3 of IC601, and resistive
divided by R1 and R2, and coupled to the base of Q1. The zener diode Z1
connected to the emitter of Q1 is set to provide a stable emitter voltage. At
this point, when the +B voltage is higher than 130V, the base voltage of Q1
will be higher than that at the +B voltage of 130V, causing more base current,
and more collector current will flow. When more current flows through the
Q601 base/emitter terminal 3) will flow
2
) will be stored in input
collector, the output from the photo diode inside D612 will also increase. The
output of this photo diode will be received by its photo transistor and the
impedance between the collector and emitter of the transistor will decrease.
When the impedance decreases the current from the collector of the photo
transistor will increase. This will cause an increase in the collector current of
Q605 and the base voltage of Q604. However, because the DC voltage from
the photo transistor D612 would only serve to keep Q604 On, having no way
to make the switching transistor go Off, the voltage will be fed into the
feedback coil pins 2-3. This voltage will be a saw tooth wave integrated by
R622 and C613 and merged with the DC output voltage from the photo
transistor.
Therefore, the DC saw tooth wave shaped output voltage applied to the base
of Q604 comes from the photo coupler and the integrator (R622, C613).
When the output voltage reaches the triggering value (0.6~0.7V) for the
base/emitter of Q604, it will turn Q604 On and the base current of the
switching transistor Q601 will be bypassed by the collector/emitter of Q604
and Q601 will go Off quickly. Since the +B voltage varies up from 130V, it is
necessary that the period Q601 is On is as short as possible. In this way, by
alternating Q604 On, and Q601 Off control is achieved. In other words, when
the +B voltage is higher than 130V, the saw tooth wave DC level will increase,
T3 period (Q601 is On) will become shorter and Q601 will go Off quickly.
When the +B voltage is lower than 130V, the DC level will decrease and T3
period will become longer and Q601 will go Off slowly.
( ll ) Control Operation
When the AC input voltage becomes higher than 120V, unstable DC voltage
produced from C609 will also increase and the output voltage will be trying to
increase. However, because the amount of feedback will increase, the period
Q601 is On will be shorter, and a stable output voltage will be produced.
Contrarily, when the AC input voltage becomes lower, the amount of feedback
will decrease and the On period of Q601 will be longer, producing a stable
output voltage. When the load of the secondary side increases, the output
voltage will be trying to decrease. However, the amount of feedback will
decrease and the On period of Q601 will be longer, producing a stable output
voltage. When the load of the secondary side decreases, due to the opposite
operation of the increase in the load, a stable output voltage will be produced.
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