PSM control technique for primary‐side regulating fly‐back converters

Abstract

A pulse skipping modulation (PSM) strategy is introduced to the primary-side regulating (PSR) fly-back converters operating under light load. The discrete-time state equations of the PSR fly-back converter works in discontinuous conduction mode are discussed. Based on the iterative map, the stability of PSR fly-back converter modulated by PSM scheme is analysed. A self-adaptive modulation factor control technique is proposed in response to the primary side sampling characteristics of the PSR fly-back converters. Theoretic analysis and simulation results show that by utilising the proposed PSM strategy, the power dissipation under light loads can be reduced, and that the modulation factor tolerance is 1.1% by maximum in comparison with its ideal value within 10-100% load range. By optimising the proposed technique, the output voltage ripple under light loads can also be well controlled to <1% (50 mV). The technique is realised by a 1 μm 5 V/40 V/700 V BCD process, and a 5 V/1 A PSR fly-back AC-DC converter prototype is built. The experimental results match the theoretical analysis. The technique proposed is believed to help improving the efficiency for refined regulating method in PSR fly-back converters under light loads. 2 Principles of PSR fly-back PSM modulation PSR fly-back converters use sense voltage divided from the auxiliary winding, which is coupled with the primary side winding, as feedback voltage to form a closed-loop control. Typical PSR flyback PSM converter and the proposed technique control loop with sampling characteristic are shown in Fig. 1.

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