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PFC-Fundamentals 1. Sinusoidal Current Sourcing from the Public Power Grid 1
PFC-Fundamentals
1. Sinusoidal Current Sourcing from the Public Power Grid
Michael Frisch, Tyco Electronics / Power Systems, May 04

Associated with power electronics connected to the public power grid obtrudes more and more the
required power factor correction. The power factor reflects to what extent effective power is sourced
from the power grid. With a power factor less then 1 additional non-reactive power cause additional
power dissipation in the mains supply and the power cable. A maximal power factor is aimed for. The
term power factor associated with power electronics isnt wrong but misleading. The traditional electro-
mechanic power factor is displacement of the sinusoidal current to the also sinusoidal voltage (cos(
)).
For most power electronics applications is the non-sinusoidal shape of the current the problem to
solve. It is more precise to call this challenge as the target of a sinusoidal current sourcing.
The Problem:
In power electronics applications usually the
AC voltage is converted into DC voltage via
Rectifier Bridge and capacitor. The capacitor is
charged only in the time interval were the input
voltage exceeds the DC-voltage at the
capacitor.
This leads to short heavy current pulses
causing interferences and increased power
dissipation in the supply line.

The trigger level of the mains fuse is linear
dependent with the average losses in the
supply line. Applications exceeding 4kW are
not running at the usual for 16A designed and
fused installations.
These current pulses cause not only
interferences and losses in the individual home
installation also in the power grid interferences
with other consumer loads and power losses in
the electrical supply line. Therefore the
electricity suppliers push for a sinusoidal
current sourcing as regulated e.g. in Europe by
standards as EN61000-3-2.
The PFC-Boost Circuit as a Solution:
The sinusoidal current sourcing might be
achieved with a PFC boost. This circuit
contains additionally to the input rectifier and
the capacitor a choke, a transistor and a diode.


Function:
Is the transistor switched on energy is stored in
the choke. After switching OFF the energy will
flow via diode into the capacitor. With an
appropriate sequence of pulses at the
transistor control current is pumped during the
complete half wave into the capacitor. With this
boost topology an almost sinusoidal current
sourcing is achievable.
2


Example:
A power application requires 3.7kW effective power from the power grid. The half wave
duration in a 50Hz grid is 10ms. With sinusoidal current sourcing the current in a 230V
power line is 16A
RMS
. The power losses in the 50m
home installation results to:
P
V(sin)
= I
2
* R = 12,8W

With a pulsed current sourcing without correction the capacitor is charged during 2ms per
half wave only. In 1/5 of the time 5 times higher current have to charge the capacitor to
keep the 3,7kW power level. But the current influences the losses by square, the time
shortage is only linear. The losses are increased by factor 5!

In this example:
P
V(noPFC)
= (1/5 * (5*16A)
2
* 0,05
) = 64W.

Result:
The application without PFC cannot be used at a usual 16A installation. The not reactive
power causes in the home installation additional losses of 61,8W and additional losses in
the electrical supply line. Supply line losses were one factor for the US electricity blackout
in 2002.