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  Power Factor Correction PFC  

Power Factor Correction is an active rectifier, that draws a sinusoidal current from the line. New laws and restrictions are in effect since 1992, requiring active power factor control on all power supplies above 200VA. Conventional Rectifiers draw pulsed currents from line resulting in many disadvantages and problems: creation of harmonics and RF. high losses, require overdimensioning of parts. reduced max. power, that can be drawn from line.


General Description:
All rectified AC sinewave signals with capacitive filtering draw high amplitude current pulses from their source. Usually, the current peak value is in order of six (6) times the current necessary for the same power on an ohmic load! Conventional AC rectification is thus a very inefficient process, resulting in high electricity costs, and distortion of line voltage. It produces a large spectrum of harmonic signals. These could interfere with other equipment and the power factor is degraded to about 0.45. This kind of circuit is used with all conventional and switched mode power supplies. At higher power levels (200 to 500 Watts and higher) severe interference and other problems result. The power line cabling - the installation - the transformers - all must be designed to withstand these peak current values. Large voltage drops result in distortions that have to be compensated;

Harmonics must be filtered; Rectifiers have to be over-designed and need cooling. This just shows a few problems power generating plants are confronted with. They charge for all the investment and costs incurred by operating such systems on their lines; not to speak about environmental problems. Current on an ohmic load flows during the full time of a half cycle, starting from zero to peak and back to zero. This current is sinusoidal and in phase with the Source Voltage. A much smaller average current flows compared with the pulsed peak currents of conventional rectifiers. Power Factor Correction simulates this ideal "Ohmic Load" condition through an electronically controlled active rectification process. In order to comply with these requirements Schock offers active Power Factor control on most of its products.



Consider the following:

Agencies like FCC, VDE and others are guarding against pollution of power lines and RF emissions. Filtering, shielding and other precautions are necessary to comply with their requirements. When line voltages reduces after culminating the peak value, the rectifier is turned off, emitting a high frequency spectrum. Harmonics appear around 10kHz and higher, exceeding the limits enforced by the agencies in most of the cases. Expensive LOW FREQUENCY FILTERS, which are relatively large and hard to design, and also HIGH FREQUENCY FILTERS are necessary. The risk of smooth agency approval is jeopardized and can result in relatively high costs and other related problems.


Schock Power Corporation offers Power Factor Correction in many of its power supplies and battery chargers. The simulation of an ohmic load, results in a sinusoidal current, which is drawn from line. This method achieves a Power Factor of close to ONE. Universal Input Voltage Range from 90 to 270VAC (50/60Hz) can be accommodated - without range switching Much Higher Power (1.65kW) can be drawn from a 15A wall plug. With regular rectification only about 0.8kW are available, causing all mentioned disadvantages Conduction Angle is almost 180O, respectively 360O Power Factor > 0.9 (usually 0.97 to 0.99) Overall Lower Power Installation Costs No Phase Shifts are caused No or Low RF, due to: a.) lower peak current levels b.) the basic arrangement of circuit elements c.) shift of RF spectrum to much higher frequencies, and therefore easier to filter Common RF Filter for paralleled supplies are possible. Lower Risk and Costs for agency approval Competitive Advantages Advanced state of the art Products Smaller physical size of parts High Efficiency.