Philips 7603 series, Service Manual

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Figure 2
Introduction
The supply is a Switching Mode Power Supply (SMPS). The frequency of operation varies with the circuit load. This 'Quasi-
Resonant Flyback' behavior has some important benefits compared to a 'hard switching' fixed frequency Flyback converter.
The efficiency can be improved up to 90%, which results in lower power consumption. Moreover the supply runs cooler and
safety is enhanced.
The power supply starts operating when a DC voltage goes from the rectifier bridge via T5520, R3532 to pin 8. The
operating voltage for the driver circuit is also taken from the 'hot' side of this transformer.
The switching regulator IC 7520 starts switching the FET 'on' and 'off', to control the current flow through the primary winding
of transformer 5520. The energy stored in the primary winding during the 'on' time is delivered to the secondary windings
during the 'off' time.
The 'MainSupply' line is the reference voltage for the power supply. It is sampled by resistors 3543 and 3544 and fed to the
input of the regulator 7540 / 6540. This regulator drives the feedback optocoupler 7515 to set the feedback control voltage
on pin 3 of 7520.
The power supply in the set is 'on' any time AC power goes to the set.