NCP1650
www.onsemi.com
14
THEORY OF OPERATION
Introduction
Optimizing the power factor of units operating off of AC
lines is becoming more and more important. There are a
number of reasons for this.
There are a growing number of government regulations
requiring Power Factor Correction (PFC). Many of these are
originating in Europe. Regulations such as IEC1000−3−2
are forcing equipment to utilize input stages with topologies
other than a simple off−line front end which contains a
bridge rectifier and capacitor.
There are also system requirements that dictate the use of
PFC. In order to obtain the maximum power from an
existing circuit in a building, the power factor is very critical.
The real power available from such a circuit is:
P
real
+ V
rms
I
rms
PF
A typical off−line converter will have a power factor of
0.5 to 0.6, which means that for a given circuit breaker rating
only 50% to 60% of the maximum power is available. If the
power factor is increased to unity, the maximum available
power can be obtained.
There is a similar situation in aircraft systems, where a
limited supply of power is available from the on−board
generators. Increasing the power factor will increase the
load on the aircraft without the need for a larger generator.
Figure 31. Voltage and Current Waveforms
v, i
v, i
OFF−LINE CONVERTER
PFC CONVERTER
t
t
V
V
I
I
Unity power factor is defined as the current waveform
being in phase with the voltage, and undistorted. Therefore,
there are two causes of power factor degradation – phase
shift and distortion. Phase shift is normally caused by
reactive loads such as motors which are inductive, or
electroluminescent lighting which is highly capacitive. In
such a case the power factor is relatively simple to analyze,
and is determined by the phase shift.
PF + cos q
Where q is the phase angle between the voltage and the
current.
Reduced power factor due to distortion is more
complicated to analyze and is normally measured with AC
analyzers, although most circuit simulation programs can
also calculate power factor. One of the major causes of
distortion is rectification of the line into a capacitive filter.
This causes current spikes that do not follow the input
voltage waveform. An example of this type of waveform is
shown in the upper diagram in Figure 2.
A power converter with PFC forces the current to follow
the input waveform. This reduces the peak current, the rms
current and eliminates any phase shift.
The NCP1650 accomplishes this for both continuous and
discontinuous mode power converters.
PFC Operation
The basic PWM function of the NCP1650 is controlled by
a small block of circuitry, which comprises the DC
regulation loop and the PFC circuit. These components are
shown in Figure 26.
There are three inputs to this loop. They are the fullwave
rectified input sinewave, the instantaneous input current and
the DC output voltage.
The input current is forced to maintain a near unity power
factor due to the control of the AC error amplifier. This
amplifier uses information from the AC input voltage and
the AC input current to control the power switch in a manner
that provides good DC regulation as well as an excellent
power factor.
The reference multiplier sets a reference level for the input
fullwave rectified sinewave waveform. One of its inputs is
connected to the scaled down fullwave rectified sinewave,
and the other is connected to the output of the DC error
amplifier. The signal from the DC error amplifier adjusts the
level of the fullwave rectified sinewave on its output without
distorting it. To accomplish this, it is necessary for the
bandwidth of the DC error amp to be less than twice the
lowest line frequency. Typically it is set at a factor of ten less
than the rectified frequency (e.g. for a 60 Hz input, the
bandwidth would be 12 Hz).
