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Circuit Descriptions, Abbreviation List, and IC Data Sheets
EN 141
Q523.1U LA
9.
Reduced / Maximum Power Mode
When there is no overload and when the supply has reached
SOPS mode, the start-up power consumption is limited to
approximately 1 A in order to ensure a slow start current control
across the MOSFET. This will last as long as diode 6B14 is not
(yet) conducting while transistor 7B01 is conducting, thus
keeping the supply in Reduced Power Mode.
In general, the “on” time of the transistor 7B05 is a function of
the output current. Resistor 3B51 can be seen as sense
resistor with a voltage of V
drop
. If there is a demand for more
power, the negative voltage created from the auxiliary winding
and diode 6B11 will cause diode 6B14 and transistor 7B01 to
conduct. Transistor 7B01 will put resistor 3B49 in parallel to
resistors 3B50 and 3B51. This will result in V
drop
across resistor
3B51 to be lower, which causes the “on” time of transistor 7B05
to be longer, which enables the supply to deliver more power.
This brings the supply in Maximum Power Mode.
9.3.3
Peak current control
The peak start-up current flowing across the MOSFET also
influences the voltage across the sense resistor 3B51 and will
cause transistors 7B02 and 7B00 to conduct (via resistor
3B50). The voltage across this resistor is sensed in order to
control the maximum power.
9.3.4
Output voltage control
The voltage at the +12 V output supply line will increase until
the zener voltage of diodes 6B02 and 6B13 is reached. The
output voltage is controlled via a feedback-loop formed by
components 3B54, 6B02, 6B13, 7B04, 3B47, 7B02 and 7B00.
When the voltage exceeds 12.6 V the zener diodes 6B02 and
6B13 will conduct and will trigger opto-coupler 7B04. After a
while transistors 7B02 and 7B00 will start to conduct and this
will switch “off” MOSFET 7B05. The feedback-loop will become
stable after a while, thus controlling the output voltage.
9.3.5
Over-voltage protection
In case of malfunctioning of the output voltage control
feedback-loop as described above, the supply goes into over-
voltage protection. When the negative primary auxiliary voltage
(present at the anode of diode 6B11) reaches the zener voltage
of diode 6B03, MOSFET 7B05 will switch off. This causes
transistors 7B02 and 7B00 to conduct which will result in an
output voltage drop.
9.3.6
Audio protection / DC protection
When a fault occurs in the audio amplifiers (e.g.a short-circuit),
a voltage is sent via the AUDIO-PROT line which will trigger
thyristor 7B50. This will cause the +12 V output line to drop to
approxi3 V. The only way to reset the thyristor is to
disconnect the set from mains. After re-connect to mains, the
supply will restart normally, if the defective audio amplifier has
been repaired.
9.3.7
Service Tips
After replacing some components in the primary circuit of the
Power Supply, a variable transformer has to be used to ramp
up the mains voltage from 0 to 30 V. This will result in the
Power Supply to start-up. Monitor the +12 V output voltage and
increase the mains voltage until the regulation and feedback
loop are working.
Connector 1B40 can be used by Service to power the SSB
directly by an external power supply. This enables the SSB to
start-up without the use of the LCD Power Supply. The power
consumption of the SSB is:
•
1.5 to 2.0 A for the +12 V line.
•
2.0 A for the +5 V line.
It should be noted that, by using this method, the audio
amplifier and the audio protection are not tested!
9.4
On-board DC/DC Converters
In this platform, on-board DC/DC converters have been
foreseen. See also diagrams B02A, B02B and B02C.
9.4.1
PSU Start-up Sequence
1.
If the input voltage of the DC/DC converters is around 12 V
(measured on the de coupling capacitors 2U01/2U02/
2U93) and the ENABLE signals are “low” (active), then the
output voltages should have their normal values.
2.
First, the Stand-by Processor activates the +1V2 supply
(via ENABLE-1V2).
3.
Then, after this voltage becomes present and is detected
OK (about 100 ms), the other voltage of +3V3 will be
activated (via ENABLE-3V3).
4.
The current consumption of controller IC 7U00 is around 20
mA (that means around 200 mV drop voltage across
resistor 3U01).
9.4.2
+12V Switch
•
The +12V switch is activated when the POD-MODE signal
is “low”.
•
The rise time of the output voltage is set by components
2U12, 3U42, and 3U64 and is about 30 ms.
•
The switch “off” is fast, because there can be fault currents
that must be interrupted.
•
When the input voltage (+12VS) is higher than 12.6 V, the
switch is disabled via circuit 6U01, 3U09, 3U18, 2U14, and
7U14-2.
9.4.3
Internal Protection
•
Provides a SUPPLY-FAULT signal (active “low”), when the
output voltage of any DC/DC converter is out of its limits (
±
10% of the normal value). In such cases, the Stand-by
Processor will immediately stop the supplies by sending a
“high” control signal towards the external and internal
supplies: ENABLE-xVx, POD-MODE, ON-MODE, and
STAND-BY.
Note:
The SUPPLY-FAULT control signal is “low” when
any DC/DC converter is disabled by its control signal
(ENABLE-xVx) and +12VSW is present, therefore it is
ignored during start-up!
•
The internal protection works together with the output over-
voltage detector transistors 7U15-1, 7U15-2, 7U29-1, and
7U29-2.
9.4.4
1.2V and 3.3V DC/DC Converters
Introduction
The circuit used is a so-called “synchronous buck converter”.
Some characteristics:
•
Switching frequency: approximately 250 kHz.
•
Efficiency: approximately 90%.
•
Built-in output over-voltage and over-current protections
•
Soft start.
•
Software controlled “on/off” (via ENABLE line).
