This application note focuses on the experimental results of a 65 W adaptor driven by the NCP1239 and on the general behavior of this controller. The NCP1239 is a fixed−frequency current−mode controller featuring a high−voltage start−up current source to provide a quick and lossless power−on sequence.
With a supply range up to 35 V, the controller hosts a jittered 65 or 100 kHz switching circuitry operated in peak current mode control. When the power on the secondary side starts to decrease, the controller automatically folds back its switching frequency down to minimum level of 26 kHz. As the power further goes down, the part enters skip cycle while limiting the peak current that insures excellent efficiency in light load condition. It features a timer−based fault detection circuitry that ensures a quasi−flat overload detection, independent of the input voltage.