On-Board Dc/Dc Converters; Reduced / Maximum Power Mode; Peak Current Control; Output Voltage Control - Philips 32PFL9432/98 Service Manual

Circuit Descriptions, Abbreviation List, and IC Data Sheets
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.
9.4.1 Start-up sequence
When the Platform 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).
9.4.2

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 appr.
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
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
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.
9.4.3

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.
9.4.4

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.
9.4.5 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.
. If there is a demand for more
drop
across resistor
drop
Q528.1A LA
9.4.6 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
appr. +3 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.
9.4.7 Service Tips
After replacing some components in the primary circuit of the
Platform Supply, a variable transformer has to be used to ramp
up the mains voltage from 0 to 30 V. This will result in the
Platform 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 Display 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!
9.5

On-board DC/DC Converters

In this platform, on-board DC/DC converters have been
foreseen. See also diagrams B02A, B02B and B02C.
9.5.1 PSU Start-up Sequence
1. If the input voltage of the DC/DC converters is around 12 V
(measured on the decoupling 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).
9.5.2 +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.
9.5.3 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
9. EN 171