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 to90%, 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 transformer5520. The energy stored in the
primary winding during the ‘on’ time is delivered to the secondary windings during the
‘off’ time.
The ‘Main Supply’ line is the reference voltage for the power supply. It is sampled by
resistors 3543 and3544 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’ anytime AC power goes to the set.
Derived Voltages
The voltages supplied by the secondary windings of T5520 are:
• ‘MainAux’ for the audio circuit (voltage depends on set execution, see table
below),
• 3.3 V and 3.9 V for the microprocessor and
• ‘Main Supply’ for the horizontal output (voltage depends on set execution, see
table below).
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