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Mar 7, 2015

Power-factor correction (PFC) Converter



    Introduction :
In recent years, there have been increasing demands for high power factor. Among power factor correction techniques, the analogy control method is now attractive in industry, but the digital control method is the trend in the future. With the stringent requirements of power quality, power-factor correction (PFC) has been an active research topic in power electronics, and significant efforts have been made on the developments of the PFC converters. In general, the continuous-conduction mode (CCM) boost topology has been widely used as a PFC converter because of its simplicity and high power capability. It can be used with the universal input voltage range.  Recently, in an effort to improve the efficiency of the PFC rectifiers, bridgeless PFC circuit topologies are used. Generally, the bridgeless PFC topologies may reduce the conduction loss by reducing the number of semiconductor components in the line current path. So far, a number of bridgeless PFC boost rectifier implementations and their variations have been proposed.

    

     Conventional Boost Converter :
Diode rectifiers are the most commonly used circuits for applications where the input is the ac supply. The power factor diode rectifiers with a resistive load can be as high as 0.9 and it is lower with a reactive load. With aid of a modern control technique, the input current of the rectifiers can be made sinusoidal and in phase with the input voltage, thereby having an input power factor of approximate unity. A unity power factor circuit combines a full bridge rectifier and a boost converter.

    


      Bridgeless Boost Converter :                         
The boost inductor is split and located at the AC side to construct the boost structure. In this first half line cycle, MOSFET M1 and boost diode D1, together with the boost inductor construct a boost DC/DC converter. Meanwhile, MOSFET M2 is operating as a simple diode. The input current is controlled by the boost converter and following the input voltage. During the other half line cycle, circuit operation as the same way. Thus, in each half line cycle, one of the MOSFET operates as active switch and the other one operates as a diode: both the MOSFET’s can be driven by the same signal.
The difference between the bridgeless PFC and conventional PFC is summarized. Comparing the conduction path of these two circuits at every moment, bridgeless PFC inductor current only goes through two semiconductor devices, but inductor current goes through three semiconductor devices for the conventional PFC circuit.



      

        Comparison:

PFC converter
Slow diode
Fast diode
MOSFET
Conduction path On/(Off)
Conventional
4
1
1
2 slow diode, 1 MOSFET/(2 slow diode, 1 fast diode)
Bridgless
0
2
2
1 body diode, 1 MOSFET/ (1 MOSFET
body diode, 1diode)

               Table : Differences between conventional PFC and bridgeless PFC



As shown in Table 4.1, the bridgeless PFC uses one MOSFET body diode to replace the two slow diodes of the conventional PFC. Since both the circuits operating as a boost DC/DC converter, the switching loss should be the same. Thus the efficiency improvement relies on the conduction loss difference between the two slow diodes and the body diode of the MOSFET. Besides, comparing with the conventional PFC, the bridgeless PFC not only reduces conduction loss, but also reduces the total components count.





         Automatic Power Factor Controller  :
This can be achieved by using microcontroller based power factor controller .The main core of this work is to design power factor controller. This system will be able to control the power factor of both linear and nonlinear load system. The design aims to monitor phase angle continuously and in the event of phase angle deviation, a correction action is initialized to compensate for this difference by continuous changing variable capacitors value via switching process. The overall system requires only one chip, a few power electronic components and a bank of capacitors.



 Figure : Block Diagram microcontroller based PFC


 





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