Triple-Mode Control Scheme for an Active-Clamp Flyback Converter to Improve Efficiency at Light-to-Ultralight Loads

Active-clamp flyback (ACF) converters are widely adopted for low-power applications due to their simplicity and low cost. The advantages of conventional ACF converters include a low component count, leakage energy recycling, and soft switching. However, widely used complementary control strategies achieve high efficiency only at heavy loads, but not at light-to-ultralight loads, due to the conduction loss of the auxiliary switch. This paper proposes an ACF converter, specifically designed to operate in Discontinuous Conduction Mode (DCM), with a triple-mode control method to enhance efficiency across a wide load range, especially at light-to-ultralight loads. The proposed control strategy employs both complementary and non-complementary control methods tailored to specific load conditions. For medium-to-heavy load conditions, when switching loss in the auxiliary switch is more significant than conduction loss, the complementary control method is implemented. Conversely, under light-load conditions, when conduction loss is more significant than switching loss, the non-complementary control method is adopted to minimize the conduction time of the auxiliary switch. This approach ensures soft switching for the main switch across light to full loads and for the auxiliary switch during medium-to-heavy load conditions. At ultralight loads, a burst mode is employed to further enhance the converter’s efficiency. Operating principles and design considerations are explained in detail. A 65-W ACF prototype with a 155 Vdc input and a 19 Vdc output, operating at a 65 kHz switching frequency, has been developed to verify the feasibility of the proposed control strategy. Experimental results demonstrate a peak efficiency of 90.16% at light load (defined as below 30% of rated load) and 88.82% at ultralight load (defined as below 12% of rated load), both surpassing conventional ACF converters.