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PFC Stage with Improved Light-load Behaviour Disclosure Number: IPCOM000031459D
Original Publication Date: 2004-Oct-25
Included in the Prior Art Database: 2004-Oct-25

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Switched-mode power supplies (SMPS) require a power-factor correction (PFC) at the input to reduce the amount of line-bound harmonics sent back into the net. There are two basic functional principles for PFC: a passive one using a large filter inductor or an active one using a boost converter. The active PFC stage is being widely used due to its advantages, such as reduced volume, lower cost and weight, efficiency and capability of wide input voltage ranges. The basic principle of active PFC stage is shown in figure 1. In the Critical Conduction Mode, the control IC switches the MOSFET Q1 at a high frequency (much higher than the input frequency) causing a triangular current in the inductor L1. During turn-on of Q1, energy is stored in the inductor, and during turn-off, energy is transferred to the output. Depending on the duty cycle, the output power can be controlled. The generated output voltage is always higher than the input voltage. Diode D1 prevents the current from flowing backwards from the output. In current implementations, the output voltage (also called bus voltage) is kept constant for offline power supplies at around 400V. It is usually followed by a DC/DC-converter operating e.g. with a switch and a transformer. Various topologies, e.g. fly-back, forward etc., are well-known. The disadvantage of the state of the art is that the duty cycle becomes rather small, and efficiency becomes low at lower loads. There are however many applications requiring also a good light-load efficiency, which is difficult to achieve using existing approaches.