Panasonic Z8 / Chassis Service Manual / Other ▷ View online
3DQDVRQLF
3.2.
Power Supply Circuit
The STR-F6523, IC801 is used in the Z8 power
supply to control and regulate the power supply
operation.
supply to control and regulate the power supply
operation.
This
device
features
over-voltage
protection and thermal shutdown. The output stage of
the IC incorporates a built-in MOSFET switching
transistor.
the IC incorporates a built-in MOSFET switching
transistor.
3.3.
Operation
The supply voltage for the main power supply circuit
is fed via the standby relay RL801 to the bridge
rectifier D802 where the AC voltage is fully rectified
and smoothed by capacitor C809.
is fed via the standby relay RL801 to the bridge
rectifier D802 where the AC voltage is fully rectified
and smoothed by capacitor C809.
This smoothed DC voltage of approximately 300V
then feeds the supply to pin 3 of the switched mode
power supply IC IC801, where the DC voltage is held
at the drain of the internal MOSFET, by its parallel
zener diode.
then feeds the supply to pin 3 of the switched mode
power supply IC IC801, where the DC voltage is held
at the drain of the internal MOSFET, by its parallel
zener diode.
3DQDVRQLF
3.4.
Start Up
A start-up circuit is used to start and stop the
operations of the control IC IC801 (STR-F6523), by
detecting the voltage appearing at the V
operations of the control IC IC801 (STR-F6523), by
detecting the voltage appearing at the V
IN
terminal,
pin 4.
At start-up, capacitor C810 is charged via R804,
which causes the voltage at pin 4 of IC801 to
increase. Once V
which causes the voltage at pin 4 of IC801 to
increase. Once V
IN
terminal pin 4 voltage reaches
approximately 16V, IC801 begins to operate and drive
the internal power MOSFET, causing current to flow
through the drain/source terminals at pins 3 and 2,
and to the winding B1-B2 of switching transformer
the internal power MOSFET, causing current to flow
through the drain/source terminals at pins 3 and 2,
and to the winding B1-B2 of switching transformer
T802 via L803 and R809. The current at terminal B1
is split into two paths.
is split into two paths.
The first path follows the current being fed to the
winding B1-B2 and back to pin 4 of IC801 via R811
and D805. Once the control circuit starts operation,
the voltage at the V
winding B1-B2 and back to pin 4 of IC801 via R811
and D805. Once the control circuit starts operation,
the voltage at the V
IN
terminal pin 4 of IC801 starts to
decrease. However, the drive winding voltage
reaches the set value before pin 4 voltage drops to the
shutdown voltage of 10V. Hence the voltage supply to
pin 4 is maintained.
reaches the set value before pin 4 voltage drops to the
shutdown voltage of 10V. Hence the voltage supply to
pin 4 is maintained.
The second path is connected from terminal B1 to P2
of the FBT. This causes current to flow via the winding
P2-P1, which provides the +B supply to the FBT T552
pin 9.
of the FBT. This causes current to flow via the winding
P2-P1, which provides the +B supply to the FBT T552
pin 9.
$&
LQ
$&
LQ
3DQDVRQLF
3.5. Oscillator and Constant Voltage
Control Circuit
The oscillator within IC801 makes use of the charging
and discharging of internal capacitor C1 (4700pF)
and generates pulse signals which turn the internal
power MOSFET On and Off. The constant voltage
control of a switch mode power supply is performed
by fixing the OFF time of the MOSFET (around 50uS)
and changing the ON time in the pulse width control
operation.
and discharging of internal capacitor C1 (4700pF)
and generates pulse signals which turn the internal
power MOSFET On and Off. The constant voltage
control of a switch mode power supply is performed
by fixing the OFF time of the MOSFET (around 50uS)
and changing the ON time in the pulse width control
operation.
3.5.1. ’ON’ Condition and Time
When the switching power MOSFET is ON, C1
begins to charge.
begins to charge.
3.5.2. From ’ON’ to ’OFF’
When the voltage on C1 reaches (approx.) 6.5V, the
output from the oscillator is reversed, and the internal
switching power MOSFET switches OFF.
output from the oscillator is reversed, and the internal
switching power MOSFET switches OFF.
3.5.3. ’OFF’ Condition and Time
With the power MOSFET now OFF, Capacitor C1
starts discharging through R1, at the fixed time
determined by the time constant C1, R1.
starts discharging through R1, at the fixed time
determined by the time constant C1, R1.
3.5.4. From ’OFF’ to ’ON’
When C1 voltage has dropped to around 3.7V, the
output from the oscillator is reversed again and the
power MOSFET again turns ON, thus repeating the
cycle.
output from the oscillator is reversed again and the
power MOSFET again turns ON, thus repeating the
cycle.
3DQDVRQLF
3.6. Regulation
The power supply ON time is changed by controlling
the the charge current of the internal capacitor C1.
D804 is a photocoupler, which provides the drive
current to the ’FB’ (Feedback) terminal of IC801 pin
1 via D812 and R806. The photocoupler current
varies in response to the output from pin 2 of
comparator IC IC802.
IC802 pin 1 monitors the +B supply voltage via the
zener diode D814 by comparing it with a reference
voltage established internally within IC802.
the the charge current of the internal capacitor C1.
D804 is a photocoupler, which provides the drive
current to the ’FB’ (Feedback) terminal of IC801 pin
1 via D812 and R806. The photocoupler current
varies in response to the output from pin 2 of
comparator IC IC802.
IC802 pin 1 monitors the +B supply voltage via the
zener diode D814 by comparing it with a reference
voltage established internally within IC802.
If the AC mains input voltage to the switched mode
power supply increases, the +B voltage level tends to
rise. This results in an increased current flow to the FB
terminal, pin 1 of IC801 via the photocoupler D804,
diode D812 and resistor R806. Increasing the rate at
which C1 charges, causes the power MOSFET ON
time to reduce. This in turn causes the +B level to
return to its nominal value.
power supply increases, the +B voltage level tends to
rise. This results in an increased current flow to the FB
terminal, pin 1 of IC801 via the photocoupler D804,
diode D812 and resistor R806. Increasing the rate at
which C1 charges, causes the power MOSFET ON
time to reduce. This in turn causes the +B level to
return to its nominal value.
3.7. Drive Circuit
The drive circuit charges and discharges the
capacitance between the gate and the source
terminals of the internal power MOSFET, by receiving
pulses
capacitance between the gate and the source
terminals of the internal power MOSFET, by receiving
pulses
from
the
oscillator.
The
basic
circuit
configuration is a totem-pole type connection of
transistors. Since the maximum sink current (0.3A)
can become active even when the V
transistors. Since the maximum sink current (0.3A)
can become active even when the V
IN
voltage is lower
than the shutdown voltage, the drive circuit turns off
the MOSFET without fail.
the MOSFET without fail.
Click on the first or last page to see other Z8 / Chassis service manuals if exist.