Switching Power Supply - Sanyo AVM-2550S Training Manual

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The switching power supply circuits employed in the VB7C chassis is
comprised of four blocks: the smoothing circuit, oscillating circuit, control
circuit and rectified output circuit as shown is Figure 1 below.
FUNDAMENTAL OPERATION
The AC input voltage is rectified in the smoothing circuit, and an unstable DC
voltage will be produced by capacitor C609. This unstable DC voltage will be
input into the oscillating circuit. The blocking oscillator in the oscillating circuit
will turn the switching transistor Q601 On and Off, producing rectangular
pulses in the input coil at the frequency determined by the control circuit. The
generated pulses will be converted into rectangular waves according to the
turn ratio of input coil to output coil, and smoothed by the rectified output
circuits, to obtain the desired DC voltages.
STARTING OPERATION
When the power is turned On, a micro-current will be applied to the base of
switching transistor Q601, after going through the starting resistors R603 and
R615, and the drive resistors R613 and R618. A small current will flow from
the collector after passing through the input coil pins 5 to 8 of the converter
transformer T601. When the starting power is produced from the input coil, a
feedback voltage will be produced in the feedback coil pins 3 to 2. This
voltage serves to apply positive feedback to the base of transistor Q601, after
going through D609, C612, R614, R613 and R618, and turns Q601 On. The
operation is performed almost instantaneously when the power is turned On
and the state of transistor Q601 remains On. The collector current increases
in direct proportion to time as seen in equation i=(V/L*) x t.
* "L" is the inductance of input coil pins 5 to 8.
OSCILLATING CIRCUIT
The oscillating circuit uses a blocking circuit to produce oscillation by turning
the switching transistor Q601 On and Off. Therefore, we need to examine
each state (Off Operation, Off Period, On Operation and On Period) of the
switching transistor separately.

SWITCHING POWER SUPPLY

( l ) Off operation
• When control circuit is not operating:
Because the feedback voltage is determined by the turn ratio of input coil to
feedback coil, it is constant when the DC input voltage from C609 is stable.
Therefore, a constant base current is applied from the feedback coil, after
going through the drive resistors R613 and R618.
Since the collector current increases in direct proportion to time, when it is
then multiplied over the base current, Q601 can not remain On and it will
quickly turn Off.
• When control circuit is operating:
While Q601 is On, a voltage pulse is fed through the feedback coil pins 3 to
2 and integrated by an integrator circuit consisting of resistor R622 and
capacitor C613, and a saw tooth wave voltage will be generated.
The collector and emitter of transistor Q604 are connected to the base and
emitter of switching transistor Q601 as shown in Figure 1. The output voltage
from the aforementioned integrator together with the output from the detection
circuit consisting of +130V power regulator IC601, photo coupler D612 and
error amplifier Q605, will be applied to the base of Q604.
When this voltage reaches the triggering value (0.6~0.7V) for the base/emitter
of Q604, Q604 will turn On and the current from the base of switching
transistor Q601 will be bypassed by the collector/emitter of Q604, and Q601
will go Off.
( ll ) Off Period (T1 Period in Figure 2)
When Q601 is Off, the energy stored in the input coil 5-8 during its On
Operation will be supplied to the load side from the output coils through the
rectified output circuit. An output voltage determined by the turn ratio of
output coils will be produced from this energy. The current produced from the
output coils will decrease in inverse proportion to time. (See T1 period in
Figure 2(E).)
– 64 –

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