Application Note 42047 Power Factor Correction (PFC) Basics
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What is Power Factor?
Power factor (pf) is defined as the ratio of the real power (P)
to apparent power (S), or the cosine (for pure sine wave for
both current and voltage) that represents the phase angle
between the current and voltage waveforms (see Figure 1).
The power factor can vary between 0 and 1, and can be either
inductive (lagging, pointing up) or capacitive (leading, pointing
down). In order to reduce an inductive lag, capacitors are
added until pf equals 1. When the current and voltage waveforms
are in phase, the power factor is 1 (cos (0°) = 1). The
whole purpose of making the power factor equal to one is to
make the circuit look purely resistive (apparent power equal
to real power).


Understanding Recent Regulations
In 2001, the European Union put EN61000-3-2, into effect to
establish limits on the harmonics of the ac input current up to
the 40
th
harmonic. Before EN61000-3-2 came into effect,
there was an amendment to it passed in October 2000 that
stated the only devices required to pass the rigorous Class D
(Figure 2) emission limits are personal computers, personal
computer monitors, and television receivers. Other devices
were only required to pass the relaxed Class A (Figure 3)
emission limits.


Modes of Operation
There are two modes of PFC operation; discontinuous and
continuous mode. Discontinuous mode is when the boost
converter’s MOSFET is turned on when the inductor current
reaches zero, and turned off when the inductor current meets
the desired input reference voltage as shown in Figure 9. In
this way, the input current waveform follows that of the input
voltage, therefore attaining a power factor of close to 1.


Conclusion
Power companies do not get excited over low power factor
driven devices, plus the extra cost of unused or wasted power
can be quite large. This is why PFC on the device side has
become an important part of the final power system design
for so many products. There are many standards in place
(example, EN 61000-3-2) to drive power consumption to a
power factor of 1 and keep total harmonic distortion to a
minimum. Depending on the output power and the designer’s
needs, a SMPS can be designed with either a discontinuous
or continuous mode stand alone PFC controller, or a continuous
PFC/PWM mode device can be used. PFC controllers
are forecasted to grow to $175 million in 2006, and standards
are reducing the minimum power limits on systems
that require PFC, more and more PFC controllers will be
used.