AN1059 APPLICATION NOTE DESIGN EQUATIONS OF HIGH-POWER-FACTOR FLYBACK CONVERTERS BASED ON THE L6561

Three different configurations that an L6561-based flyback converter can assume have been identified. They are illustrated in fig. 1. Configurations a) and b) are basically conventional flyback converters. The former works in TM (Transition Mode, i.e. on the boundary between continuous and discontinuous inductor current mode), therefore at a frequency depending on both input voltage and output current. The latter works at a fixed frequency, imposed by the synchronisation signal, and is therefore completely equivalent to a flyback converter based on a standard PWM controller. Configuration c), which most exploits the aptitude of the L6561 for performing power factor correction, works in TM too but quite differently: the input capacitance is so small that the input voltage is very close to a rectified sinusoid. Besides, the control loop has a narrow bandwidth so as to be little sensitive to the twice mains frequency ripple appearing at the output.