Circuit Descriptions and List of Abbreviations
EN 92 FM249.
9.2.3 Standby Supply (Diagram P2)
Introduction
The standby supply is a separate power supply, meant to
reduce power consumption of the Flat TV monitor in standby
mode. The standby supply operates on the AC voltage from the
input filter part, and has to deliver a stable 9 V voltage.
It has three mains isolated outputs, and one 'hot' output:
• +9VSTBY (called +9VSTBY_SW after the 'on/off' switch
1101), to power the 'on'/'off' relays in the pre-conditioner.
• +5VSTBY_SW derived from the +9V
STBY
via voltage
regulator 7540.
• +3V3STBY_SW, to supply the µ P of the PDP.
• 25V_HOT, to supply the LLC controller.
The standby supply is also connected to the pre-conditioner
output (400V_HOT), in order to deliver a voltage as long as
possible, after switch 'off' and at mains dips.
Operation
The standby supply is always operational when the AC input
voltage is present, so even when the POWER switch is in the
'off' position. After a small rectifier (D6512/6513) and buffer
capacitor (C2503), the DC voltage is applied to an SMPS
(switched mode power supply).
The SMPS itself is build around IC7500, a 'TINYSwitch
TNY256'. This IC contains a control circuitry and a power
MOSFET. It uses a simple 'on/off' control loop to regulate the
output voltage. The generated +9V
STBY
voltage, at the
secondary side, is rectified by D6504 and smoothed by C2508.
The supply for the TNY256 on pin 5 comes via resistor R3506
and L5500.
By using secondary sensing, a very accurate standby voltage
and high efficiency is achieved. The sensing circuit uses a
TL431 as reference voltage/error amplifier. Optocoupler 7501
is used for the mains isolation.
When the output voltage rises, the reference voltage on the
TL431 will exceed 2.5 V and the current through this device and
the optocoupler LED will increase. By this, the optocoupler
transistor will conduct more. When this current (at pin 4 of
IC7500) exceeds 50 uA, the MOSFET is switched 'off', and the
output voltage will drop. When this current drops below 40 uA,
the MOSFET is switched 'on' again.
During the time that the MOSFET is 'on', the IC has no supply
voltage. To overcome this period, the energy stored in the
bypass capacitor C2513 is used. This capacitor is charged
during the time the MOSFET is 'on'.
As the TNY256 is sensitive for transients (mains spikes), a
'peak clamp' circuit (300 V zenerdiodes 6501 and 6502) is used
to limit the voltage to a safe level.
9.2.4 Fan Control (Optional, Diagram P4)
For ceiling mount or portrait-mode use, there is foreseen in four
optional fans, grouped per two. The temperature within the
monitor is measured via a sensor (R3372, KTY81) on the PSU.
This sensor is, via A/D converter (item 7530 on the SCAVIO),
connected to the OTC. According the temperature within the
cabinet, the OTC-software will drive the two PWM outputs of
the OTC. These outputs (FAN_SP_1 and 2) are connected to
the PSU, where for each signal, a corresponding voltage is
generated to supply the fans. These voltages
(FAN_SUPPLY_1 and _2) are proportional to the duty cycle of
the corresponding PWM signals.
The OTC senses the temperature every 5 s. If it has reached
T-alarm, and this value has been measured three times
consequently, the monitor will go into protection, and a error-
code is generated.
9.2.5 Pre-conditioner (Diagram P5)
Warning: the pre-conditioner does not provide mains isolation.
Introduction
The European Law describes a reduction of mains harmonics
for apparatus with a power consumption above 75 W.
Therefore the pre-conditioner is designed. This module serves
as the interface between the mains input and the V
S
/V
A
supply.
The advantage of a pre-conditioner is (compared to a mains
input filter):
• Reduction of mains harmonics to legal limits.
• Lower mains current for the same output power.
• Power factor close to 1.
• Stable regulated output.
• Small and low weight.
The input voltage of the pre-conditioner is universal, between
95 and 264 V
AC
. The output is 400 V
DC
(400V_HOT) with a
maximum output power of 300 W. This output voltage is
delivered to the V
S
supply.
Operation
Start-up
The two relays (1450 and 1460, diagram P2) are controlled via
the SUPPLY_ON signal. This signal will become 'high' when
the +9V_STBY_SW, the STANDBY (from the OTC), and the
LATCH signal are 'ok'. It then switches indirect relay 1450 via
transistor 7460 and so enables the use of a small low voltage
switch.
To protect rectifier 6600 and relay 1450, the inrush current is
limited to a maximum of 20 A by charging the capacitor 2605
through two serial PTCs (3450 and 3451) and two NTC’s (3452
and 3453).
After approximately 0.5 sec, relay 1460 is activated. This relay
will short the PTCs. The advantage of using an NTC, is the fact
that the resistance varies with the current and hence the mains
voltage. At a high mains voltage, the current is lower for the
same power.
Two clamp diodes 6605 and 6606 charge output capacitors
C2616/2617 to the peak voltage of the mains input. During
normal operation, both diodes are blocked because of the
output voltage of 400 V
DC
, and will only conduct if there is a
mains spike or an output dip.
Capacitor 2616 delivers via R3668 the start-up voltage at pin
16 of IC7650. After the start-up cycle, IC7650 is supplied via
auxiliary winding 1-2. Capacitor C2663 is charged during the
cycle that MOSFET 7610 conducts. While MOSFET 7610 is
switched 'off', this capacitor transfers its energy via D6661 to
the input of stabiliser IC7660. The 15 V output voltage of this IC
is fed via D6665 to V
CC
pin 12 of IC7650.
The slow start function is realised by the circuit consisting of
transistor 7654, D6654, R3654, and C2654.
Normal Operation
An up-converter circuit is used for the pre-conditioner. The
switching frequency of the converter is chosen much higher
than the mains frequency. It is therefore possible to consider
the supply as constant, during every high frequency period,
and the envelope of all voltage steps during the low frequency
period approximates a half sine wave.
The output voltage of the pre-conditioner equals the input
voltage, when the MOSFET is continuous switched 'off', and
increases while the MOSFET is switched 'on'.
The rectified mains input voltage is connected to pin 5 of
IC7650 via voltage divider R3650 and R3651. This voltage is
proportional with the mains input, and is used to change the
duty cycle of the pulses, which are generated at pin 11.
Because the width of these pulses is not small enough, the
circuit around transistors 7640 and 7641 is added. It decreases
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