Philips 286NS-05, Service Manual

Page: 10 / 39

9 CIRCUIT DESCRIPTION
24 Comet
9 CIRCUIT DESCRIPTION
9.1 Description of the power supply:
9.1.1 General
Note: The vo96S and +96s are not mains isolated.
The power-supply used in this chassis is a self-oscillating down
converter with an auxiliary winding to help the FET to switch.
9.1.2 Principle of the down-converter (Fig 8.1):
When switch TS7504 is closed, the voltage on L2-3 is Vin-Vout.
During this time, energy is stored in the coil and energy is
delivered to the load. When switch TS7504 opens, the energy
stored in the coil will be stored in the output capacitor (C2515).
This is due to the fact that the current through the coil has to
decrease linear. When the switch is open the current is floating
through D6504, L2-3 and C2515. By controlling the duty-cycle
of the switch, the output voltage can be regulated.
9.1.3 Start-up (see diagram A1):
When the switch TS7504 is closed, the input voltage is placed
over winding 2-3 of transformer 5500, which acts as coil L2-3
in Fig 8.1. Via resistors R3513,R3518 and R3512 the switch is
turned on for the first time. Zener diode D6502 prevents that
the Ugs of the FET becomes higher than 15V. When the input
voltage is on winding 2-3, there is also a voltage on winding 1-
2. Via winding 1-2 the correct switching voltage is obtained.
The DC-part of this voltage is blocked by capacitor C2503.
Diode D6510 acts as a protection in start-up and in short-circuit
situations. During start-up the output capacitor C2515 is empty.
It takes a relative long time to charge the gate to a voltage high
enough to switch on the FET. This is due to the fact the diode
D6510 is conducting. When this diode is conducting, the
current that would normally flow into the gate of the fet to switch
on the FET, is now flowing into C2515. In this way a smooth
start-up is guaranteed.
9.1.4 General way of working (Fig 8.2):
The state of the power-supply can be divided into three areas:
– T-on;In this state the FET is conducting and energy is
stored in the coil and in the output capacitor.
– T-off;In this state the fet is non conducting and the energy
stored in the coil is fed to the output capacitor.
– T-dead;Fet is out of conduction and there is no energy in
the coil.
T-on; In the T-on state, switch TS7540 is switched on. When
the switch is on the voltage over resistors R3514-R3515 is a
direct measure for the current through winding 2-3. This is a
negative voltage. When this voltage becomes below a certain
level, TS7501 starts conducting and will switch off the fet. In
this way it is prevented that the coil can go into saturation. This
could be the case when the output voltage is very low. (long on
time of the FET). When the output-voltage becomes too high
during T-on the FET will be switched off. (see Output-voltage
regulation)
T-off; Due to the stored energy a current will start to flow
through D6504, C2515 and winding 2-3. Due to the fact that the
current is flowing through this circuit, a voltage with reverse
polarity is on winding 1-2. In this way the fet remains off until
the current through winding 2-3 reaches zero. Now a new cycle
will start. The fet will be switched on and all starts over again.
T-dead; If the output voltage is too high (for example in a low
load situation) the FET remains off till the output-voltage is not
to high anymore.
9.1.5 Output voltage regulation:
This is done by the circuit D6501, R3509, TS7502, R3505,
R3507, R3510. Transistor TS7502 can only conduct when the
voltage on the base is 0V7 lower than the voltage the voltage
on the emitter. This means that the voltage drop over resistors
R3505 and R3507 should be 5V6(zenerdiode) + 0V7(base-
emitter). This is reached when the output voltage exceeds the
100V. Now transistor TS7502 starts conducting, which brings
transistor TS7501 in conduction. As a consequence the gate
voltage of the fet becomes very low and the fet stops
conducting. As long as the output voltage is too high the fet
stays out of conduction.
9.2 Protections:
9.2.1 Overvoltage protection:
A disadvantage of a down converter is that if the switch
becomes a short-circuit, the output voltage will increase to the
input voltage. This could damage circuits. In this power-supply
there is a protection to prevent this. If the output voltage
becomes higher than 130V, zenerdiode D6514 starts to
conduct. The Vin will be short circuited. This will blown the main
fuse 1501 and protect in this way all the other circuits.
9.2.2 Short-circuit and start-up protection:
The short-circuit protection works the same as the start-up
protection. If the output-voltage is very low in case of a start-up
or a short-circuit condition, The gate will be charged very slowly
due to the fact that zenerdiode D6510 is conducting. So the
current is not only charging the gate but is also flowing into the
output capacitor. In this way it takes a few milliseconds to
switch on the fet. Diode D6510 takes also care that the fet
never remains in his power consuming (linear) area.
9.2.3 Undervoltage protection
If the output voltage is very low, it also takes a large time before
the current through winding 2-3 reaches zero. The power
supplied to the circuit is in this way very low and protects in this
way the circuit.
9.2.4 Other output voltages:
The output vo8S, +9S and +5S are made by winding 5-
6. During the time that the fet TS7504 is not conducting, energy
is transformed to this winding (flyback principle) and the
voltages mentioned above are created. From the +9S, the +5S
voltage is derived. This voltage is stabilized by transistors
TS7505, TS7500 and zenerdiode D6500. D6500 is the
reference voltage and TS7505 is delivering the current. When
zenerdiode D6500 starts conducting, the voltage over resistor
R3502 becomes high and a POR signal is created.
9.3 Degaussing:
R3516 is a dual PTC (2 PTC's in one housing). After switching
"on" the set, the PTC is cold, so low ohmic. This makes the
degaussing current high. After degaussing the PTC is heated,
so high ohmic. This makes the degaussing current low. After
degaussing the PTC remains heated by the mains.
9.4 Line-circuit (Diagram A1):
The primary side of the line-circuit and the deflection coil are
connected to the hot earth. The driver-circuit contains an opto-
coupler to create isolation between the low-signal parts and the