TI200-PFC
8
2
Product Overview
2.8 – Active Power Factor Correction
In electric power systems, a power supply with a low Power Factor will draw more current than the same
power supply with a high Power Factor while doing the same work.
Power Factor (PF) in ac systems is defined as the ratio of the real power W (watts) flowing to the load over
the apparent power VA (volts-amps) in the circuit. This is represented by a number between 0 and 1. For
example: this is a percentage .75PF=75%PF.
A power supply is considered to be a non-linear load in which the ac power is rectified and then filtered. It is
these non-linear loads that reshape the current waveform into something different introducing harmonics
and distortion known as THD (Total Harmonic Distortion).
THD is defined as the ratio of the sums of all the powers of the harmonics to the power of the fundamental
frequency (i.e. the fundamental frequency would be the line frequency 60 Hz and the 2nd order harmonic=120
Hz, the 3rd order=240 Hz, etc.).
When the mains instantaneous voltage exceeds the voltage of the Input Capacitors the Rectifiers conduct
which causes a current spike (see Figure 2.8.1). These spikes induce harmonics and distortion. These
additional harmonics over the fundamental frequency are what contribute to a poor Power Factor. The higher
order harmonics in the ac current cause the skin effect of the conductors carrying the ac currents to the load
to increase.
Skin effect in ac circuits is where the higher frequency currents do not penetrate the entire conductor due to
the opposing eddy currents causing them to ride along the surface of the conductor. It is these magnetic fields,
generated by the eddy currents, which cause the resistivity of the conductor to increase with frequency.
This means the conductor needs to carry additional currents plus the load current to compensate for the
higher order harmonics. These extra currents generate magnetic fields and are stored in the power lines, the
switch gear and the power supply. They then return back to the power grid during the off periods of the cycle
resulting in wasted energy in the form of heat.
Tesla™ Industries was able to develop a wide bandwidth active Power Factor Correction that runs from
40 Hz to 450 Hz which pulls unity power only at the fundamental frequency. This was achieved by forcing
the current to follow the voltage waveform (see Figure 2.8.2) so that current is pulled through the entire
sinusoidal waveform on a cycle-by-cycle basis. This eliminates the current spikes and strips out the additional
harmonics causing a massive increase in efficiency.
THD =
I n
2
PF =
1
n = 1
I
2
1
1 + THD
2
