For laptop adaptors, power density is important for portability whereas for ‘static' adaptors,such as those for LCD TVs or mini-PCs, power density can also mean lower system cost bysaving on PCB area and plastic housings.Traditional low power systems use Discontinuous Current Mode (DCM) PFC which is asimple, low system cost implementation. However, in DCM the peak current increases withpower, increasing the size of diode required and especially making the EMI filter very largeand so around 250W, Continuous Conduction Mode (CCM) PFC is used due to the lowerpeak currents. However, CCM required a large ‘multiplier'-based IC with AC-line sensingand many peripheral components and so took more design effort and PCB area.In ‘One-Cycle Control' (OCC) the information required to correct the current waveform isextracted only from the DC bus voltage and the return current and processed by OCC's‘integrator-with-reset'.For lower power systems like laptop adaptors, the benefits of OCC vs. DCM are shown interms of a small reduction in control component count, system solution cost and a majorincrease in power density. Comparative circuit schematics with Power Factor and THDresults are presented.
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