Philips DVD 580M, Руководство по обслуживанию, Страница: 48 / 60

Circuit description
EN 48 DVD580M 9.
9.8 Circuit Operation And Trouble Shooting Guide
9.8.1 Power Supply Circuit
Operation Analysis
The function of the power supply unit is DC to DC converter, the
input of the circuit is 12V DC, the output of the supply unit
provides the voltages of VCC for CPU, +5V & 3.3 V for Digital
decoder, -24V, AC1 & AC2 for Heater of VFD, +12V & 5V for
Slot loader, and +9.3V, -9.3V for Audio amplify.
The primary winding of the power transformer is connecting to
the 12V from external power Unit, and the Drain terminal of the
MOSFET transistor. The ON / OFF behavior of the MOSFET
generating a switching voltage with variable frequency, the
individual voltages supplying to the DVD system is converted
by the transformer. Two of the output (+5V, +3.3V) from
secondary winding of the transformer is to be used as feedback
control (circuit constructed with U203, Q205, Q202). The
switching frequency is controlled within 45kHz to 50kHz
according to the loading condition.
Note that the driving and feedback circuit is forming a close
loop circuit that maintains the stability of the power supply; any
circuit link in the closed loop to be opened will cause the
damage of high voltage components in the system. Hence,
precaution at the control loop has to be taken when proceed the
service work.
Key Components
High precision Variable voltage stabiliser TL 431
TL431 is constructed as an error amplifier for detecting the
voltage fluctuation from power transformer.
Figure 9-8 TL431 Block diagram
The TL431 includes 4 part of circuit:
•
Error amplify A
• Internal reference voltage V ref of 2v5.
• A NPN transistor VT, the function of this transistor is to
adjust the loading current
• Protection diode VD, the diode is used to prevent the
damage of K - A terminal when reverse connection.
TL431 operation principle:
When the voltage at K (pin 3) starts to grow, the VREF is also
growing. When VREF grows higher than the internal Vref
(2V5), the error-amplify output a positive voltage that driving
the transistor VT to make the terminal voltage at K (cathode)
falls. On the other hand, when the VREF drop lower than the
internal Vref, the error amplify outputs a lower voltage to VT
(error amplify transistor), hence making the terminal voltage at
K to be higher.
The operation of TL431 in DVD580M system:
In the power supply circuit of DVD580M, pin 1 of U203 (TL431)
is used to detect the voltage fluctuation of +3.3V; the varying
voltage at pin 1 will be transfer to current output from pin 3 of
TL431 that driving to Q205 then Q202 as a feedback path of
closed control loop. The feedback signal is finally controlling
the ON -OFF timing of Q203 (power MOSFET) hence changing
the switching frequency to make a stable voltage output to DVD
system.
Fault condition judgment on TL431:
Since the TL431 is a single conduct device between K -A (pin3
to pin 2), the forward resistance between K-A terminal should
be 5 kohm to 5.1 kohm. Under this condition, the device is
judged to be good.
Schottky diodes:
The characteristics of Schottky diode is having large forward
rectify current, low forward voltage drop, short reverse recovery
time, and low reverse surge voltage. The typical forward
voltage drop on the Schottky diode is within 0.4V.
In DVD580M, the Schottky diode is used with 31DQ06 in
which, the forward rectify current is 3A, and reverse voltage is
60V.
MOSFET transistor
Q203 is a N-channel MOSFET power transistor. The Drain to
Source breakdown voltage is 55V; conducting resistance is
less than 0.1 ohm. If any replacement is need, the same model
has to be applied in order to maintain the product reliability.
Output Voltage Table On Power Supply Board:
Figure 9-9 Output Voltage
Major Test Waveforms Pattern:
Figure 9-10 Test Waveforms
(Pin 1) VREF
K Cathode (pin 3)
A ANODE (pin 2)
A
Vref
CL 26532085_033.eps
180702
Location 1 2 3 4 5 6 7 8 9 10 11 Unit
CN204 +12 0 +5 0 V
CN203 NC 0 0 0 - 9.3 + 9.3 0 + 3.3 0 + 5 0 V
CN205 0 0 0 0 0 + 9.3 - 9.3 V
CN202 +5 0 - 22 - 18.5 - 15.6 0 0 + 5.6 0 V
U203 Gate terminal: 2.48 v, Source terminal: 4.10 v V
Q208 Base to Emitter: 0v7, Collect to Emitter: 0v3 V
Q207 Base to Emitter: 0v, Collect to Emitter: 0v7 V
CL 26532085_055.eps
180702
Standby Power start Normal operation
Normal operation Stand By
21Vpp
Frequency 45K ~ 50KHz 12us 27us
22Vpp
Frequency 45K ~ 50KHz
CL 26532085_056.eps
180702
+5V
+5V
CN202 P7
CN202 P9
Q203 1-3
2-3