PHILIPS L72EAA CHASSIS
Service Manual

9
9

Circuit diagram description
Introduction For a quick overall view of all diagrams see the block diagram on sheet 4.
Circuit diagram description

L7.2E
9.1.5 Primary current (I-prim) sensing (pin 7 of IC7520)

29

The current sense voltage Vpin 7 is a measure for the I-prim through FET 7518. The I-prim is converted into a voltage by R3518. The current sense voltage Vpin 7 is used to control both the secondary output voltages and the maximum I-prim.

This chassis is executed with:

9.1.6
� � � � A mains isolated SMPS (switched mode power supply) A single chip TV processor with software controlled picture geometry adjustments Micro computer with teletext function (execution depended)Separate audio module (multi mono or NICAM) with output amplifier

Demagnetization control (pin 8 0f IC7520)
The voltage across winding 1 - 2 has the same polarity as the voltage across the secondary windings. As a result the voltage across this winding is negative during the FET is conducting, and positive during the FET is not conducting. The so called demagnetization "DEMAG" function in IC7520 (input pin 8) is used for blocking the output Vpin3 during the time that there is still energy in the transformer (Isec not zero). This is realized by delaying the switch "on" point of the FET until the demagnetization is completely finished.

9.1
9.1.1

Power supply (Diagram A1)
Mains input and degaussing 9.1.7
The mains voltage is filtered by L5500, L5501 and L5502, full wave rectified by a diode bridge (6502-6505) and smoothed by C2508. The DC voltage for the SMPS is applied at pin 7 of T5545 (e.g. 300V DC for 220V AC mains). The degaussing current is applied via dual PTC resistor R3504 After switching "on" the set, the PTC is cold so low-ohmic and therefore the degaussing current is very high. During degaussing, the PTC is heated up and is getting high-ohmic, as a result the current through the PTC becomes very low.

Standby mode
In the standby mode the load decreases under a certain threshold level. The SMPS is than switching to the so called "reduced frequency mode". The switching frequency is than reduced to 20 kHz. The minimal load threshold level is determined by R3532 connected to pin 12. In normal operation mode the internal oscillator is adjusted at 70 kHz. This frequency is determined by C2531 and R3537 connected to pin 10 and pin 16 respectively of the IC7520. In standby mode the internal oscillator is adjusted at 20 KHz. This frequency is determined by R3536 connected to pin 15 IC7520.

9.1.2

Switched mode power supply
The switched mode power supply (SMPS) is mains isolated. The control device IC7520 (MC44603AP) delivers duty cycle controlled pulses for driving switching FET 7518. The pulses have a fixed frequency of 70 kHz in normal operation. For a detailed block diagram of IC7520 (MC44603) see Fig. 9.1.

9.1.8

FET 7518 gate regulation
D6524 prevents pin 3 of IC7520 from becoming negative (this will destroy the IC) due to stray inductance in the gate part of the FET. The safety resistor R3525 limits the drive current to the gate of the FET 7518

9.1.3

Start up and take over circuitry. 9.1.9
Via the start-up circuitry R3530 and R3529 one side of the 220V AC mains is used to start-up IC7520 via the supply pin (Vpin 1). As long as Vpin 1 has not reached 14V5, IC7520 does not start up and only sinks 0.3mA. As soon as Vpin 1 reaches the 14V5, IC7520 starts driving FET 7518 into conduction and pin 1 sinks a typical supply current of 17mA. This supply current can not be delivered by the start-up circuit, so a takeover circuit has to be available. If no take-over take's place, the voltage on pin 1 will decrease and IC7520 switches off . In that case the restart will start again. During start-up a voltage across winding 1 - 2 is built up. At the moment the voltage across winding 1 - 2 reaches approx. (12V, D6540 start conducting and takes over the supply voltage Vpin 1 of IC7520 (take over current is approx. 17mA).

Over voltage protection of the secondary voltages
After start-up is the supply voltage Vpin 1 taken over by positive winding 1 - 2, and so after start up Vpin 1 is a measuring point for the secondary output voltages. After start-up (via an internal switch) this Vpin 1 is internally tapped (voltage divided) to a voltage which can be measured at pin 6 (so Vpin 6 is also a measuring point for the secondary output voltages). As soon as the voltage Vpin 6 > 2V5 the logic in IC 7520 will shut down the output at pin 3. This 2V5 threshold at Vpin 6 is equivalent to a Vpin1 of 16V DC which is equivalent to a voltage at the supply voltage (VBATT of approx. 95V DC (normal operation) and 102V DC (standby). After switching "off" because of over voltage protection, the IC starts up again In case an over voltage situation is sensed at the secondary output voltages, the SMPS will go in over voltage protection. In case the over voltage situation remains present, the SMPS will give over voltage protection slow-start, over voltage protection slowstart, etc. ( a very good audible hick-up mode).

9.1.4

Secondary output voltages sensing (pin 14 of IC7520)
Winding 1 - 2 has the same polarity as the secondary windings witch are supplying the load. During the FET is not conducting the secondary windings and winding 1-2 are positive. D6537 conducts and charges C2537; the DC level across C2537 is a reference for the secondary output voltages e.g. the +95V((VBATT). This control voltage (feedback voltage) is applied via voltage divider R3538, R3539 and potentiometer R3540 (for adjusting the +VBATT) to the error amplifier input IC7520 pin 14.

9.1.10 Undervoltage protection of the secondary voltages
If the supply voltage Vpin 1 < 9V DC the output pulse at pin 3 will be shut down. As soon as Vpin 1 < 7V5, the IC7520 will be totally shut "off". Vpin 1 of 9V DC is equivalent to a voltage at (VBATT of approx. 70V DC (normal operation) and 95V DC(standby). Vpin 1 of 7V5 is equivalent to a voltage at