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Technology |
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Power
Factor Correction PFC |
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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.
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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.
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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.
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Features:
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.
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