Active Filters for Power Quality Improvement in Power Supply Design

Martin Butler, Prof. W. G. Hurley

The information age in which we live has led to an exponential increase in use of consumer electronic goods such as personal computers. At the same time the growth in the amount of harmonic pollution in the ac supply has been steadily rising. A common factor between both of these trends is the switch mode power supply. Rectification of the ac mains was traditionally achieved using a standard full bridge rectifier with capacitor filter. This method although simple, allowed for the production of low cost, efficient power supplies. Unfortunately this technique is characterised by distortion of the input ac current waveform and flat topping of the input ac voltage waveform. This together with the increased use of switch mode power supplies in many of today?s electronic goods, has led to an undesirable level of harmonic pollution in the supply. For this reason, manufacturers of switch mode power supplies are now looking at ways of improving the power factor rating. A common approach used is to insert a high power factor (HPF) preregulator between the full bridge rectifier and the filter capacitor. The HPF preregulator actively shapes the input current to be sinusoidal and in phase with the input voltage waveform, achieving near unity power factor. A downstream dc-dc converter then transforms the raw dc voltage at the output of the preregulator to a tightly regulated and controllable dc level. This project concerns itself with the operation of the HPF preregulator. Issues such as mathematical modelling and control of the HPF preregulator are discussed in great detail along with the drawbacks of this two stage approach to power factor correction. Since there are two converters used, all the power is processed twice. This leads to reduced efficiency, increased size and cost. A single stage power supply topology is presented which overcomes the disadvantages of the above approach, by using a PFC compensator. Using a suitable control scheme the PFC compensator absorbs the reactive component of the voltage at the output of the preregulator, yielding a well regulated dc level. The PFC compensator is a new concept which has the benefit of only processing a small amount of power, hence overall size and cost are reduced with no ill effect on the input power factor.

Start date: 1st of October 1995. This project has been completed.

Project Publications

Conference
M. Butler, W. G. Hurley, W. Wolfle, "Active Power Factor Correction of a Single Stage AC-DC Converter with Reactive Power Control", 7th European Conference on Power Electronics and Applications, EPE'97, Trondheim, Norway, vol. 1, pp. 176-181, September 1997.
M. Butler, W. G. Hurley, W. Wolfle, "A Novel Scheme for Active Power Factor Correction of a Single-stage AC-DC Converter", 31st Universities Power Engineering Conference, UPEC'96, Crete, vol. 1, pp. 421-424, September 1996.

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