Control of Active Clamp Forward Converters by Independent Primary and Secondary ASICs

This paper proposes an ASIC solution to control a multi-output control-driven active-clamp forward converter devoted to PC power supplies. The primary- and secondary-side switches of the forward converter are driven by dedicated ASIC units. In detail, regulation of the main-switch duty-cycle is achieved by a primary IC implementing feed-forward control, while the outputs of the converter are locally regulated by two ASICs independently from the primary-switch duty-cycle. Since no communication is required during normal mode, the duty-cycle is defined by the the primary IC which controls the main switch is such a way that the primary duty-cycle is larger than the secondary one. Only during stand-by mode, communication between the primary controller and the secondary ones is required to increase the efficiency up to 50%. An opto-coupler device informs the primary side of the converter about the situation occurring at secondary side. Accordingly, the optocoupler communicates to the primary control ASIC the required duty-cycle, or that the primary switch has to be turned off at the occurrence of a failure at secondary side. The low-power mode efficiency is at least 50% at 500 mW load to meet the 1W stand-by requirement, and increases substantially at higher load. Experimental results carried out on a 300W demoboard are reported in this paper.