Technical Note
4040
Effective December 2017
Supersedes 2010
Power Factor Correction
(PFC) application notes
Power Factor Correction (PFC)
application notes
Overview
Every year, millions and millions of notebook computers, LCD
monitors and LCD televisions are produced. With such a fast
growing number of these and other electronic devices using more
and more power, 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:
•
Power handling capability of the power supply
•
Current handling capacities of power distribution networks
Input power factor (PF) is defined as:
PF =
Real Power (watts)
Apparent Power (VA)
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:
•
Active
•
Passive
Technical Note
4040
Effective December 2017
Power Factor Correction
(PFC) application notes
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).
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 50 Hz or 60 Hz,
the passive elements are normally bulky and heavy.
PF C Inductor
+
AC
-
D C Bus
Boost inductor
The boost-circuit based PFC topology is the most popular. It is an
economical solution for complying with regulations
(Figure 3).
The
inductance value is selected based on the desired current ripple in
the boost inductor. The inductance value is expressed as follows:
pK
L = V in (min) * d(max)
fs *
Δi
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:
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
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
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
+
Active PFC
Active PFC offers better THD and is significantly smaller and
lighter than a passive PFC circuit (Figure 2). To reduce the size and
cost of passive filter elements, an active PFC operates at a higher
switching frequency than the 50 Hz/60 Hz line frequency.
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
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 50-60 Hz 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 1/2CV
2
, the
capacitor can be much smaller than a capacitor in a converter
without active PFC.
•
•
•
•
5 V
ou t
L3
F3
D C /D C
Converter
+
Figure
3.
PFC Boost - Typical application circuit, 3.3 & 5 V, 60 W combined
output power.
Inductor selection
Eaton's
PFC inductors are available for use with a wide variety of
PFCs from 100 W to 250 W. They operate with controllers from
several IC manufacturers to provide PFC supply solutions that
utilize either passive or active PFC applications
(Table 1).
Eaton's
PFC inductors range from 200 µH to 1.2 mH. The
standard input voltage range is 85 V to 385 V with different core
materials such as ferrite, iron
powder
and Kool-Mu™ to provide
significant low core loss. The E-core and toroidal geometries
allow using thicker wire to decrease DC resist-ance 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
(Table
2).
P FC
Inductor
+
AC
-
P FC
C o ntrol
DC
Bus
Figure
2.
An
active PFC circuit produces low THD and uses relatively small
passive components.
2
EATON
www.eaton.com/electronics
Power Factor Correction (PFC) application notes
Technical Note
4040
Effective December 2017
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 Figure 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 Figure 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 (C
out
) and will prevent failure to the
PFC boost module.
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 5 A peak (3.54
Arms) by an active inrush current-limiting circuit. Inrush current duration
(t) increases with increasing output capacitance (C
out
) and can be
approximated by t =(50)x(C
out
).
Common Eaton Bussmann Series fuses applied to the overcurrent
protection points in the circuit of Figure 3 are:
•
F1: RoHS compliant S501-2-R fast-acting 5 x 20 mm ceramic
tube fuse rated for 2 A @ 250 Vac
•
F2 & F3: RoHS compliant PC-Tron fast-acting PCB through-
hole fuse rated up to 250 Vac/450 Vdc. (Product codes PCB,
PCC, PCD, PCE, PCF, PCH and PCI.)
(Figure 4)
Table 1. Comparison of passive and active PFC versus no PFC.
PFC
Type Appearance Weight
With input
voltage,
None switch or
xed input
voltage
With input
voltage,
Passive switch or
xed input
voltage
PF
Value
Impact on
Environment
PFC
Cost
None
50~60%
Bad
None
Heaviest 70~80%
Better
Normal
Figure
4.
Eaton's Bussmann Series S501-2-R and PC-Tron Fuses
Without
Active input voltage Normal 90~99.9%
switch
Best
Expensive
Table
2. Eaton inductors for Power Factor Correction.
PFC Type
Passive
Active
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
Eaton
1000 Eaton Boulevard
Cleveland, OH 44122
United States
www.eaton.com/electronics
© 2017 Eaton
All Rights Reserved
Printed in USA
Publication No.
4040
December 2017
Eaton is a registered trademark.
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of their respective owners.
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