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Investigation on Light Load Efficiency Improvement for PFC

Fig. 1. On time and frequence profiles of adaptive on-time PFC.
With the fast growing information technologies, high efficiency AC-DC front-end power supplies are becoming more and more due to the energy conservation consideration. For the power factor correction (PFC) stage, the conventional constant frequency average current mode control has very low efficiency at light load due to high switching frequency related loss. Inspired by the constant on-time control widely used in DC/DC application, this on-time control concept can also apply to PFC. At heavy load condition, the PFC circuit is controlled to operate exactly the same as the conventional constant frequency PFC. So the on time profile of the boost switch in half line cycle is shown in Fig.1(a), and the frequency is a constant value in heavy load, CCM condition. The key of this control method is to maintain the same on-time profile when load is changing. If the load is reduced to medium load range, the two sides of half line cycle will enter DCM operation first, and the switching frequency of the DCM region is reduced. While in very light load condition, the whole half line cycle will enter DCM and the switching frequency will decrease significantly. By keeping the on time profile of the boost switch unchanged at all load conditions, the switching frequency will automatically decrease at light load, leading to greatly improved light load efficiency.

Fig. 2. Efficiency comparison.
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