Philips Colour Television Specifications download pdf (Page 45)

Circuit Descriptions and Abbreviation List

GB 97EM3E 9.

/R3118 (= 5 W). This voltage is a measure of the current

and if it exceeds 1.4 V, TS7101 will be driven into

conductivity and consequently connects the gate of

TS7102 to earth. The FET will block. The current is: 1.4

V/5

W = 0.28 A. The voltage across the secondary

winding (8,9) will be negative, diodes D6111 and D6107

will block.

– Time interval t1 - t2: The sudden current interruption in

the primary coil will induce a counter-e.m.f. that wants to

maintain the current. The voltage on the drain of the FET

will increase. The secondary voltage (8, 9) will become

positive and will charge C2104 via D6111. All energy that

was stored in L5101 during t0 - t1 will be transferred into

the load. Due to the transformer principle, a voltage will

now be induced in the primary winding (3, 5) and the co-

coupled winding (1, 2). This voltage will be N* U

SEC

(N=

winding ratio). The voltage across the co-coupled coil will

be negative, keeping the FET blocked.

– Time t2: At t2, the current through the secondary coil will

be reduced to zero, as C2104 is no longer charged. As a

consequence, the voltages will decay and will change

polarity. The gate of the FET will be again made positive,

is driven into conductivity and the cycle starts again.

Feedback and stabilisation

The Standby Power Supply always oscillates at maximum

power. The only limiting factor is the maximum primary

current, which has been pre-set with R3108//3118.

R3114, R3124, R3113 and zener diode D6122 determine

OUT

. If the voltage across R3114 exceeds the threshold

voltage of the diode of the optocoupler 7104 (

± 1V)or,in

other words, U

OUT

exceeds 5.2 V, the transistor of the

optocoupler will conduct.

Transistor TS7100 is now driven, and a negative voltage will

be transposed to the emitter of TS7101. When TS7101

conducts, the gate of the FET is at earth potential, forcing the

oscillator stop. Due to the load, the secondary voltage

OUT

will

decrease. At a certain voltage, optocoupler TS7104 will block

and the oscillator will start again.

Since there are no capacitors, and there is a high

amplification factor in the feedback circuit, the feedback is

ultra-fast. This is why the ripple on U

OUT

is minimal. The

negative supply voltage (-13 V) used in the feedback circuit,

originates from the co-coupling coil and is rectified through

D6103.

Stabilisation is not affected through duty-cycle control but

through burst-mode of TS7100.

Burst-mode is load dependent. If the power supply is less

loaded, the secondary voltage will have the tendency to

increase more rapidly. If the load on the power supply

increases, then the oscillator stops less often, right up to the

moment that the oscillator is operating continuously:

maximum load. If the power supply is now loaded even more,

the output voltage will decay. The maximum primary current

set by R3108//3118 determines the maximum load.

Protection

If the optocoupler would fail, the secondary voltage will

increase. This would have disastrous consequences since

many ICs (e.g. OTC, Flash-RAM and DRAM) are fed with this

5.2 V. In other words; very expensive repairs would be

required.

We already know that the negative supply is directly

dependent upon the secondary 5.2 V, as a consequence of

which the negative supply will increase proportionally as the

secondary voltage increases.

If the negative supply in the mean time reaches -15 V, D6106

will start to zener and as a consequence TS7101 will start

conducting. Basically, D6106 will take over the stabilisation

task of the optocoupler, however, with a considerable

spread: from -13 V to -15 V is a 15 % increase, thus

OUT

will

increase from 5.2 V to max. 6 V.

Tuner Supply

The Standby supply produces 2 voltages for the Tuner: +33V

TUN

) and +5VT.

• The +33V is the tuning voltage for the Tuner.

• The +5VT is derived from the +8V with stabiliser 7911,

and is used to supply the tuner only.

SSB Supply

There are several voltages going to the SSB: +8V, +5V and

+3V3.

• The +5V and +(always present) come directly from the

Standby power supply.

• The +3V3 is derived from the +5V with stabiliser 7910 (on

the LSP).

9.3.4 Main Supply (diagram A1)

Some important notes on beforehand:

• V

BAT

is not isolated from the main supply ('hot').

• V

BAT

is alignment free.

Principle

The Main Power Supply, generates the 141 V (V

BAT

)andthe

+/- 16 V for the audio part. It is based on the so-called 'down

converter' principle.

Figure 9-5

• After closing switch 'S', the linear in time increasing

current I

, will charge capacitor C.

• Opening switch 'S' will generate a counter-e.m.f. in coil L,

trying to maintain current I

. This is possible via diode D

(this diode is also called 'freewheel diode'). So after

opening 'S', the magnetic energy stored in coil L will be

transferred to electrostatic energy in capacitor C. The V

will only supply current during the time that 'S' is closed

while a constant current is flowing through RL.

• V

BAT

is directly proportional with V

andthetimethat'S'

is closed and reverse proportional with period time 'T'. So

by changing the duty cycle, it will be possible to control

BAT

96532156_022.eps

060100

BAT

Vin

BAT

Vin

closed

open

δT

BAT =

1 2 ... 40 41 42 43 44 45 46 47 48 49 50 ... 62 63

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Philips Colour Television Specifications Page 45

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