Active PFC functions include:
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.
• 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
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
PFC
PF
Impact on
PFC
Type Appearance Weight
Value Environment
Cost
capacitor is related to 1/ CV2, the capacitor can be much smaller than a
2
With input
voltage,
capacitor in a converter without active PFC.
Boost Inductor
None switch or
fixed input
None
50~60%
Bad
None
L2
F1
3.3Vout
voltage
F2
F3
PFC
+
L1
C2
C3
DC/DC
B oost
Line
With input
voltage,
C1
Cout
AC
Converter
M odule
Passive switch or
fixed input
Heaviest 70~80%
Better
Best
Normal
5Vout
L3
voltage
+
DC/DC
Converter
Without
Active input voltage Normal 90~99.9%
switch
Expensive
Figure 3: PFC Boost - Typical application circuit, 3.3 & 5V, 60W combined output power.
Table 1: Comparison of passive and active PFC versus no PFC.
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.
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
pK
V
(min) * d(max)
in
fs * Δi
L =
where:
pK
capacitors (C ) or the DC-DC converters. The PFC boost module usually
out
• V
(min) is the peak minimum input voltage
in
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.
• fs is the switching frequency
• Δi is the ripple current
• d(max) is the maximum duty cycle expressed as:
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).
pK
1- V
(min)
in
d(max) =
where V is the output voltage
Protecting the DC-DC Converter
o
V
o
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
The rms boost inductor current is expressed as:
(pk)
I
delivered by the hold-up capacitors (C ) and will prevent failure to the
in
out
IL (rms) =
A
PFC boost module.
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