PHILIPS 25PT4457/58
Manual de Servicio

 9  9

�

Figure 9-9 Degaussing When the set is switched on, the degaussing relay 1515 is immediately activated as transistor 7580 is conducting. Due to the RC-time of R3580 and C2580, it will last about 3 to 4 seconds before transistor 7580 is switched off. 9.6.2 Basic IC Functionality For a clear understanding of the Quasi-Resonant behaviour, it is possible to explain it by a simplified circuit diagram (see Figure below). In this circuit diagram, the secondary side is transferred to the primary side and the transformer is replaced by an inductance LP. CD is the total drain capacitance including the resonance capacitor CR, parasitic output capacitor COSS of the MOSFET and the winding capacitance C of the
W

�

transformer. The turns ratio of the transformer is represented by n (NP/NS).

Frequency Behaviour The frequency in the QR-mode is determined by the power stage and is not influenced by the controller (important parameters are LP and CD). The frequency varies with the input voltage VIN and the output power POUT. If the required output power increases, more energy has to be stored in the transformer. This leads to longer magnetising tPRIM and demagnetising t times, which will decrease the frequency.
SEC

See the frequency versus output power characteristics below. The frequency characteristic is not only output power-, but also input voltage dependent. The higher the input voltage, the smaller tPRIM, so the higher the frequency will be.