Power Factor Correction (PFC) Application Notes
Background
More than 90 million notebook computers, 150 million LCD monitors
and 50 million LCD televisions are expected to ship in 2008. With
such a fast growing number of power electronics, actions must to be
taken to ensure the functionality of the nationwide power grid.
In 2001, the European Union put EN61000-3-2 into effect to set the
harmonic regulation standard on any power grid supplied application
with power consumption over 75 watts. This essentially requires
power factor correction (PFC). Additionally, a standby power
dissipation limit is set to conserve power when a load is OFF.
“80 PLUS” is an initiative funded by electric utilities to integrate more
energy efficient Power Supply Units (PSUs) - especially for desktop
computers and servers. 80 PLUS certifies to more than 80% energy
efficiency at 20%, 50% and 100% of rated load. To meet the 80
PLUS certification, PSUs require a PFC of 0.9 or greater at 100%
load. This means PSUs that waste 20% or less electric energy (as
heat at the specified load levels) will lead to reduced electricity
consumption and lower bills. Rebates are sometimes given to
manufacturers who use 80 PLUS certified PSUs.
Implementing power factor correction (PFC) into switch mode power
supplies will maximize:
• The power handling capability of the power supply
• Current handling capacities of power distribution networks
Figure 1: A passive PFC circuit requires only a few components to increase efficiency,
but they are large due to operating at the line power frequency.
PF C Inductor
+
AC
-
D C Bus
Input power factor (PF) is defined as:
PF =
Real Power (watts)
Apparent Power (VA)
Although pleasantly simple and robust, a passive PFC rarely achieves
low Total Harmonic Distortion (THD). Also, because the circuit operates
at the low line power frequency of 50Hz or 60Hz, the passive
elements are normally bulky and heavy.
Active PFC
Active PFC offers better THD and is significantly smaller and lighter
than a passive PFC circuit. To reduce the size and cost of passive
filter elements, an active PFC operates at a higher switching
frequency than the 50Hz/60Hz line frequency.
P FC
Inductor
+
AC
-
P FC
C o ntrol
PF is expressed as decimal number between zero and one (0 and 1).
A non-corrected power supply with a typical PF equal to 0.65 will
draw approximately 1.5 times greater input current than a PFC
supply (PF = 0.99) for the same output loading. The non-corrected
supply requires additional AC current to be generated which is not
consumed by the load, creating I
2
R losses in the power distribution
network.
There are two types of PFCs:
1. Active
2. Passive
DC
Bus
Passive PFC
The simplest form of PFC is passive (Passive PFC). A passive PFC
uses a filter at the AC input to correct poor power factor. The passive
PFC circuitry uses only passive components — an inductor and
some capacitors (Figure. 1).
Figure 2: An active PFC circuit produces low THD and uses relatively small passive
components.
Active PFC functions include:
• Active wave shaping of the input current
• Filtering of the high frequency switching
• Feedback sensing of the source current for waveform control
• Feedback control to regulate output voltage
Inductor Selection
Cooper Bussmann Coiltronics
®
PFC inductors are available for use with a
wide variety of PFCs from 100W to 250W. They operate with controllers
from several IC manufacturers to provide PFC supply solutions that utilize
either passive or active PFC boost topology.
Coiltronics PFC inductors range from 100μH to 6.2μH, 100kHz. The
standard input voltage range is 85V to 385V with different toroid materials
such as ferrite, powder iron and Kool-Mu™ to provide significant low core
loss. The toroidal geometry allows using thicker wire to decrease DC
resistance and yield higher current capacity. Many vertical or horizontal
through-hole mounting options are available with an operating temperature
range of –20°C to +105°C.
Buck, boost, flyback and other converter topologies are used in active
PFC circuits.
The DC-DC converter input capacitor also benefits from active PFC.
The capacitor can be sized to filter the high frequency ripple of the
active PFC circuit instead of a much larger capacitor that would be
required to smooth the 50-60Hz input. The regulated input of the DC-
DC converter also demands a lower range of duty cycle from the DC-
DC converter. Other benefits of active PFC include increased “hold-
over-time.” Hold over (brownout protection) benefits from always
starting at the maximum voltage; and because energy in the
capacitor is related to
1
/
2
CV
2
, the capacitor can be much smaller than a
capacitor in a converter without active PFC.
Boost Inductor
F1
L2
L1
AC
C1
C2
C3
PFC
B oost
L ine
M odule
F2
C
out
D C /D C
Converter
3 .3V
ou t
+
PFC
Type
Appearance
With input
voltage,
switch or
fixed input
voltage
Weight
PF
Value
Impact on
Environment
PFC
Cost
None
None
50~60%
Bad
None
5 V
ou t
L3
F3
D C /D C
Converter
+
With input
voltage,
Passive switch or
fixed input
voltage
Active
Without
input voltage
switch
Heaviest
70~80%
Better
Normal
Normal 90~99.9%
Best
Expensive
Figure 3: PFC Boost - Typical application circuit, 3.3 & 5V, 60W combined output power.
The boost-circuit based PFC topology is the most popular. It is an
economical solution for complying with regulations. The inductance
value is selected based on the desired current ripple in the boost
inductor. The inductance value is expressed as follows.
L=
where:
• VpKin (min) is the peak minimum input voltage
• fs is the switching frequency
•
Δi
is the ripple current
• d(max) is the maximum duty cycle expressed as:
Table 1: Comparison of passive and active PFC versus no PFC.
Fuses
AC Input Line Fuse
Product safety standards written by Underwriters Laboratories (UL) and the
International Electrotechnical Commission (IEC) require fuses for primary AC
power protection and secondary protection against any catastrophic failure
within the input filter capacitors, PFC boost module, output electrolytic
capacitors (Cout) or the DC-DC converters. The PFC boost module usually
does not contain overcurrent protection; if a short-circuit is applied across
its output terminals, there is no internal circuit opening device to safely
interrupt the power. Without fuse protection in the AC input line (see
fuse F1 in Figrure 3), the boost converter is not protected.
Fusing the DC-DC converter input lines is essential for protection against a
catastrophic DC-DC converter failure (see fuses F2 and F3 in Figrure 3).
Protecting the DC-DC Converter
Although the primary input line fuse will eventually activate, DC fuses
positioned right at the input to the DC-DC converters will limit the energy
delivered by the hold-up capacitors (Cout) and will prevent failure to the
PFC boost module.
VpKin (min) * d(max)
fs *
Δi
pK
d(max) = 1- V in (min) where Vo is the output voltage
Vo
The rms boost inductor current is expressed as:
I (pk)
IL (rms) = in
A
2
Fuse time current curves (I [amps] versus t [time]) should be consulted
for verification of the primary line fuse selection. The DC fuse should not
open as a result of normal inrush currents flowing at supply startup.
Inrush current is limited within most PFC modules to 5A peak
(3.54Arms) by an active inrush current-limiting circuit. Inrush current
duration (t) increases with increasing output capacitance (Cout) and can
be approximated by t = (50)x(Cout).
Common Cooper Bussmann
®
fuses applied to the overcurrent protection
points in the circuit of Figure 3 are:
• F1: RoHS compliant S501-2-R fast-acting 5 x 20mm ceramic tube fuse
rated for 2A @ 250Vac
• F2 & F3: RoHS compliant PC-Tron fast-acting PCB through-hole fuse
rated up to 250Vac/450Vdc. (Product codes PCB, PCC, PCD, PCE, PCF,
PCH and PCI.)
Figure 4: S501-2-R and PC-Tron Fuses
Coiltronics Inductors for Power Factor Correction
PFC Type
Passive
Part Number
CTX01-15789
CTX02-12236
CTX02-12378
CTX22-16885
CTX08-13679
CTX16-15954
CTX16-17309
CTX16-17769-R
CTX16-18405-R
CTX22-15557
Inductance
1mH @ 3.4A
500μH @ 2.6A
769μH @ 2.6A
200μH @ 7.0A
1.2mH @ 4.7A
1.0mH @ 3.1A
200μH @ 2.0A
340μH @ 5.3A
140μH @ 3.2A
1.0mH @ 1.3A
Aux Wdg
Yes
No
Yes
Yes
Yes
Yes
No
Yes
No
Yes
Geometry
Core Material
ER28L
Ferrite
Toroid
Powder Iron
Toroid Kool-Mu™/Sendust
Toroid
Powder Iron
EE42x15
Ferrite
ER28L
Ferrite
Toroid
Powder Iron
Toroid
Powder Iron
Toroid Kool-Mu™/Sendust
EFD25
Ferrite
Mounting
Verticle
Horizontal
Horizontal
Verticle
Horizontal
Verticle
Verticle
Horizontal
Horizontal
Horizontal
Size (mm)
30 x 24 x 36
31.8 x 31.8 x 19.5
34.3 x 34.3 x 18.3
43 x 22 x 47
51 x 48 x 39
30 x 24 x 36
35.2x33.5x22
40 x 40 x 19.8
21x21x11
31 x 27 x 13
Output Power
-
-
-
-
250W
100W
200W
150W
150W
100W
Active
Other inductors available for prower factor correction.
For other inductor values and technical application assistance contact
Cooper Bussmann at
InductorTech@cooperindustries.com.
The Cooper Bussmann Coiltronics
®
brand of magnetics specializes in standard and custom solutions, offering the latest in state-of-the-art
low-profile high power density magnetic components. We remain at the forefront of innovation and new technology to deliver the optimal mix of
packaging, high efficiency and unbeatable reliability. Our designs utilize high frequency, low core loss materials, and new and custom core shapes in
combination with innovative construction and packaging to provide designers with the highest performance parts available on the market. The
Coiltronics Brand product line of power magnetics continually expands to satisfy shifts in technology and related market needs. Standard Product
Categories include:
• Shielded Drum Inductors
• Toroidal Inductors
• Unshielded Drum Inductors
• Specialty Magnetics
• High Current Inductors
• Custom Magnetics
Please visit
http://www.cooperbussmann.com/datasheets/elx
to see data sheets on the wide variety of inductor solutions we have to offer.
For techncial inquiries e-mail
InductorTech@cooperindustries.com.
Order samples online -
www.cooperbussmann.com
© 2009 Cooper Bussmann
St. Louis, MO 63178
636-394-2877
www.cooperbussmann.com
Reorder # 4040
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