Philips 32PF9968/10 Service Manual page 140

更多难得资料请到江南家电维修论坛免费下载！
http://bbs.520101.com
EN 140 9. Q523.1U LA
The +24 V (or 295 V in case of a scanning backlight TV) and
+18V voltages are generated through a so-called resonant DC-
DC power converter (LLC-converter) (item no. 7005 to 7008 in
diagram A2). The output of +18 V requires an additional DC to
DC converter which is built around a so-called "Buck
converter".
A Buck converter is in fact a step-down DC to DC converter.
The operation of a Buck converter is fairly simple: It uses two
switches (a transistor and a diode) and an inductor and
capacitor. It alternates between connecting the inductor to
source voltage to store energy in the inductor and discharging
the inductor into the load. Refer to figure "Principle of Buck
Converter".
ON STATE
OFF STATE
Figure 9-6 Principle of Buck Converter
Figure "Buck converter - application" describes the application
which is used in this platform.
18V -> 12V DC-DC converter
7402
+18V
3407
2020
6401
+30V
Gate driver
Figure 9-7 Buck converter - application
In diagram A3 the DC-DC converter which is built around the
Buck converter is depicted. In this application, the controller
(item 7401) controls the gate driver (built around item no. 7403,
7404, 7405 and 7407). Transistor 7403 switches the gate of the
MOSFET (item 7402) "on", and 7405 switches it "off".
Transistors 7404 and 7407 avoid cross-conductance of the
driver. The output of the gate driver circuitry can be measured
at testpoint I604. An "low-voltage lock-out" circuitry is built
around item no. 7406 and activates when the input voltage of
+18 V drops below 15 volt.
An over-voltage protection circuitry is built around item no.
6020 and 6018 and will activate if the feedback circuitry is
defect (see diagram A2).
A primary over-voltage protection is performed by item no.
7307 (opto-coupler) (see diagram A1) and will activate in case
Circuit Descriptions, Abbreviation List, and IC Data Sheets
H_16770_108.eps
260307
+12V
5401
2402
Controller
7401
H_16770_107.eps
260307
the primary input voltage (mains) is too high. The software will
detect an incorrect signal on the Power-Good line and the TV
will go into protection.
An secondary over-current protection circuitry is built around
item no. 2014 and 3021 (diagram A2). If a short-circuit occurs
in the output circuitry (+12 V, +24 V), the switching frequency
of the DC/DC converters will increase. This means that the
soft-start voltage on pin 11 of the resonant mode controller
(item 7001) will be "low" and the output current will reduce.
The following signals interface with the SSB via connector
1P11 (see diagram A2):
• DIM-control: adjusts backlight intensity (pulse-width
modulated signal of 3.3 V).
• Power-Good (described above).
• Light on_off: switches the LCD backlight (0 - 3.3 V).
• Boost (fixed: or 3.3 V or left "open" depending on
commercial request or display specification).
• Stand-by: stand-by signal and 5.2 V from SSB to switch
relay on Power Supply Unit on/off.
9.2.2 47 and 52" Sets
The 47 and 52" sets in this chassis come with a buy-in Sanken
Display Supply unit and is a black-box for Service. When
defective, a new panel must be ordered and the defective panel
must be sent for repair, unless the main fuse of the panel is
broken. Always replace a defective fuse with one with the
correct specifications! This part is available in the regular
market.
The supply unit works together with the on-board platform
supply (see section "On-Board Platform Supply").
The supply unit in this platform has an output voltage of +24 V.
Refer to the Spare Parts list for the correct order number.
The supply unit delivers the following voltages to the chassis:
• Mains voltage (filtered) for SSB (connector 1P02)
• +395 V DC (connector 1P22).
• +24 V DC (connectors 1319/1316).
• +12 V DC on connectors 1M09, 1M12 and 1P15.
When all these voltages are present, the Power-Good signal
becomes "high" (+5 V) (connectors 1P10.2 and 1P11.2).
9.3 On-Board Platform Supply
In this platform, an on-board platform supply has been
foreseen. This means that the mains voltage, after filtering, is
fed to the SSB.
The supply is a Self Oscillating Power Supply (SOPS) and
working according to the Quasi Resonant Conversion (QRC)
principle. Refer to diagrams B01A and B01B for details. For the
on-board DC/DC converters refer to diagrams B02A, B02B and
B02C.
9.3.1 Start-up sequence
When the main supply is switched "on", the voltage across
capacitor 2B50 increases. This will trigger the gate of MOSFET
7B05 (via resistors 3B60 and 3B34). When the voltage on pin
1 of the MOSFET reaches the threshold level, the MOSFET
starts conducting. As result, current will flow in the primary
winding of transformer 5B01. The output voltage increases but
the supply will not start oscillating because the auxiliary voltage
is still too "low" to drive the MOSFET autonomously. Oscillating
will start only when then auxiliary voltage across capacitor
2B32 is high enough to drive the gate of the MOSFET. This
brings the supply in SOPS mode (self-oscillating).