PFC

PF

Impact on

PFC

Type

Appearance

Weight

Value

Environment

Cost

With input

voltage,

None

switch or

None

50~60%

Bad

None

fixed input

voltage

With input

voltage,

Passive switch or

Heaviest

70~80%

Better

Normal

fixed input

voltage

Without

Active

input voltage

Normal

90~99.9%

Best

Expensive

switch

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 50-60Hz 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 in a converter without active PFC.

Boost Inductor

Inductor Selection

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 100

μH 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.

PFC

B oost

L ine

M odule

DC/DC

Converter

3.3V

5V

F1

L2

L3

A C

C1

L1

C2

C3

F2

F3

+

+

DC/D C

Converter

ou t

ou t

FFiigguurree 3

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 =

VpKin (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:

d(max) =

1- VpKin (min)

Vo

where Vo is the output voltage

The rms boost inductor current is expressed as:

IL (rms) =

Iin (pk) A

2

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.

TTaabbllee 1

1:: Comparison of passive and active PFC versus no PFC.