Use of current controlled mutual inductor to limit recycling current in the AHB-Flyback converter

Owing to its low component count and easy scalability towards high frequency operation, the Asymmetrical Half Bridge (AHB) Flyback converter is considered one of the most attractive solutions in compact offline, low power and low cost LED driving applications. Such converter can be properly designed to operate with a significant input voltage ripple so as to minimize the input filter capacitance and to guarantee the compliance with the EN 61000-3-2 standard without any kind of power factor correction (PFC). However, despite its inherent simplicity, this converter suffers the high primary side recycling current due to the need of guaranteeing the zero voltage commutation of the half bridge switches in the whole line half period. This paper presents a solution to this problem based on the idea of modulating the mutual inductor magnetizing inductance to compensate for the DC-link voltage variation at twice the line frequency. The aim is to keep, through the use of a current controlled variable inductance, the magnetizing current peak to peak ripple constantly equal to the minimum necessary to guarantee Zero Voltage Switching (ZVS) operation. A prototype, to drive, from the European line voltage (230V_RMS at 50Hz) a 10W LED lamp, has been built and tested in order to verify the effectiveness of the proposed technique and converter performance. Experimental results are provided.