PFC aramkor
SMPSRM
Power Factor Correction
requiring all electrical equipment connected to
a low voltage distribution system to minimize
current harmonics and maximize power factor.
2. The reflected power not wasted in the
resistance of the power cord may generate
unnecessary heat in the source (the local
step–down transformer), contributing to
premature failure and constituting a fire hazard.
3. Since the ac mains are limited to a finite current
by their circuit breakers, it is desirable to get
the most power possible from the given current
available. This can only happen when the
power factor is close to or equal to unity.
The typical AC input rectification circuit is a diode
bridge followed by a large input filter capacitor. During
the time that the bridge diodes conduct, the AC line is
driving an electrolytic capacitor, a nearly reactive load.
This circuit will only draw current from the input lines
when the input’s voltage exceeds the voltage of the filter
capacitor. This leads to very high currents near the peaks
of the input AC voltage waveform as seen in Figure 33.
Since the conduction periods of the rectifiers are small,
the peak value of the current can be 3–5 times the average
input current needed by the equipment. A circuit breaker
only senses average current, so it will not trip when the
peak current becomes unsafe, as found in many office
areas. This can present a fire hazard. In three–phase
distribution systems, these current peaks sum onto the
neutral line, not meant to carry this kind of current, which
again presents a fire hazard.
Power Factor (PF) is defined as the ratio of real power
to apparent power. In a typical AC power supply
application where both the voltage and current are
sinusoidal, the PF is given by the cosine of the phase
angle between the input current and the input voltage and
is a measure of how much of the current contributes to
real power in the load. A power factor of unity indicates
that 100% of the current is contributing to power in the
load while a power factor of zero indicates that none of
the current contributes to power in the load. Purely
resistive loads have a power factor of unity; the current
through them is directly proportional to the applied
voltage.
The current in an ac line can be thought of as consisting
of two components: real and imaginary. The real part
results in power absorbed by the load while the imaginary
part is power being reflected back into the source, such
as is the case when current and voltage are of opposite
polarity and their product, power, is negative.
It is important to have a power factor as close as
possible to unity so that none of the delivered power is
reflected back to the source. Reflected power is
undesirable for three reasons:
1. The transmission lines or power cord will
generate heat according to the total current
being carried, the real part plus the reflected
part. This causes problems for the electric
utilities and has prompted various regulations
VOLTAGE
Power
not used
Power used
I
110/220
AC VOLTS IN
+
CURRENT
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IAV
Figure 33. The Waveforms of a Capacitive Input Filter
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DC To Power
Supply
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SMPSRM
pulses generate more heat than a purely resistive load of
the same power. The active power factor correction
circuit is placed just following the AC rectifier bridge. An
example can be seen in Figure 34.
Depending upon how much power is drawn by the unit,
there is a choice of three different common control
modes. All of the schematics for the power sections are
the same, but the value of the PFC inductor and the
control method are different. For input currents of less
than 150 watts, a discontinuous–mode control scheme is
typically used, in which the PFC core is completely
emptied prior to the next power switch conduction cycle.
For powers between 150 and 250 watts, the critical
conduction mode is recommended. This is a method of
control where the control IC senses just when the PFC
core is emptied of its energy and the next power switch
conduction cycle is immediately begun; this eliminates
any dead time exhibited in the discontinuous–mode of
control. For an input power greater than 250 watts, the
continuous–mode of control is recommended. Here the
peak currents can be lowered by the use of a larger
inductor, but a troublesome reverse recovery
characteristic of the output rectifier is encountered,
which can add an additional 20–40 percent in losses to the
PFC circuit.
Many countries cooperate in the coordination of their
power factor requirements. The most appropriate
document is IEC61000–3–2, which encompasses the
performance of generalized electronic products. There
are more detailed specifications for particular products
made for special markets.
A Power Factor Correction (PFC) circuit is a switching
power converter, essentially a boost converter with a very
wide input range, that precisely controls its input current
on an instantaneous basis to match the waveshape and
phase of the input voltage. This represents a zero degrees
or 100 percent power factor and mimics a purely resistive
load. The amplitude of the input current waveform is
varied over longer time frames to maintain a constant
voltage at the converter’s output filter capacitor. This
mimics a resistor which slowly changes value to absorb
the correct amount of power to meet the demand of the
load. Short term energy excesses and deficits caused by
sudden changes in the load are supplemented by a ”bulk
energy storage capacitor”, the boost converter’s output
filter device. The PFC input filter capacitor is reduced to
a few microfarads, thus placing a half–wave haversine
waveshape into the PFC converter.
The PFC boost converter can operate down to about
30 V before there is insufficient voltage to draw any more
significant power from its input. The converter then can
begin again when the input haversine reaches 30 V on the
next half–wave haversine. This greatly increases the
conduction angle of the input rectifiers. The drop–out
region of the PFC converter is then filtered (smoothed)
by the input EMI filter.
A PFC circuit not only ensures that no power is
reflected back to the source, it also eliminates the
high current pulses associated with conventional
rectifier–filter input circuits. Because heat lost in the
transmission line and adjacent circuits is proportional to
the square of the current in the line, short strong current
Switch Current
Input Voltage
I
Vout
Vsense
Csmall
+
Control
Conduction Angle
Voltage
Figure 34. Power Factor Correction Circuit
Current
IAVG
Figure 35. Waveform of Corrected Input
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Supply
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