AN-42047 Datasheet, PDF (1/11 Pages) Fairchild Semiconductor – Power Factor Correction

Application Note 42047
Power Factor Correction (PFC) Basics
www.fairchildsemi.com
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, point-
ing down). In order to reduce an inductive lag, capacitors are
added until pf equals 1. When the current and voltage wave-
forms 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).
Real power (watts) produces real work; this is the energy
transfer component (example electricity-to-motor rpm).
Reactive power is the power required to produce the mag-
netic fields (lost power) to enable the real work to be done,
where apparent power is considered the total power that the
power company supplies, as shown in Figure 1. This total
power is the power supplied through the power mains to pro-
duce the required amount of real power.
“Total Power”
Apparent Power
(S) = Volt Amperes = I²Z
Reactive Power
(Q) = vars = (X_L – X_C) | ²
θ
Real Power
(P) = Watts = (I²R)
Figure 1. Power Factor Triangle (Lagging)
The previously-stated definition of power factor related to
phase angle is valid when considering ideal sinusoidal wave-
forms for both current and voltage; however, most power
supplies draw a non-sinusoidal current. When the current is
not sinusoidal and the voltage is sinusoidal, the power factor
consists of two factors: 1) the displacement factor related to
phase angle and 2) the distortion factor related to wave
shape. Equation 1 represents the relationship of the displace-
ment and distortion factor as it pertains to power factor.
PF = ^Irms(1) cosθ = Kd ⋅ Kθ
(1)
Irms
Irms(1) is the current’s fundamental component and Irms is
the current’s RMS value. Therefore, the purpose of the
power factor correction circuit is to minimize the input
current distortion and make the current in phase with the
voltage.
When the power factor is not equal to 1, the current wave-
form does not follow the voltage waveform. This results not
only in power losses, but may also cause harmonics that
travel down the neutral line and disrupt other devices con-
nected to the line. The closer the power factor is to 1, the
closer the current harmonics will be to zero since all the
power is contained in the fundamental frequency.
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.
Figure 2. Both Current and Voltage Waveforms are in
Phase with a pF =1 (Class D)
Figure 3: This is What is Called Quasi-PFC Input,
Achieving a pF Around 0.9 (Class A)
Causes of Inefficiencies
One problem with switch mode power supplies (SMPS) is
that they do not use any form of power factor correction and
that the input capacitor C_IN (shown in Figure 4) will only
charge when V_IN is close to V_PEAK or when V_IN is greater
REV. 0.9.0 8/19/04