Power Factor Correction (PFC) – Types
(COLLECTION BY PRAVIN MEVADA)
There are two types of Power Factor Correction techniques; Passive PFC and
Active PFC.
Let’s understand one by one.
• Passive Power Factor Correction
Passive components like inductors and capacitors are used in Passive PFC to
correct reduced power factors. These capacitors and inductors are tuned to the line
frequency in a low pass or band pass configuration. The Passive PFC normally
uses a simple line-frequency LC filter to extend the current conduction angle and
decrease the THD of the input current of the diode-capacitor rectifier. This type of
power factor correction method corrects the power factor in between 0.7 to 0.85.
Physical size and weight of these filters at mains frequency makes them nasty,
particularly when one considers that the circuitry size and form factor should be
small.
Types of Passive Power Factor Correction;
1. Capacitor input filter
This type is also called as the π filter; it removes unwanted frequencies from a
signal. This filter reduces the harmonic content of a current waveform by making
sure that the cut-off frequency of the filter is just above the fundamental frequency.
As a result, the best possible reduction of harmonics can be achieved. Below figure
shows a π filter;
2. Valley Fill PFC
One more way to achieve power factor improvement up to 0.85 using simple, low
cost circuitry shown below;
This power-factor corrector can be used in low-power applications, where a high
effective ripple voltage on DC output can be tolerated. It is frequently used in
electronic ballast applications. The circuit contains two capacitors and three
diodes. The two capacitors are charged in series around the line peak to half of the
peak line voltage. When the line voltage falls below the single capacitor voltage,
the bridge rectifier diodes are reversely biased, which doesn’t allow current to
flow. Valley-fill’s diodes then conduct and the capacitors are connected in parallel
to feed the load.
Let’s discuss its working in detail (Refer Figure 2); the capacitors C1
and C2 are charged to ½ of the AC peak voltage in series via the diode D2 and
resistor R1 on each half cycle of the rectified AC input. R1 is for reducing the
peaks in the current waveform as the capacitors charge. They supply output current
after the BUS voltage follows the sinusoidal waveform down to Vpeak/2. At this
time the caps are essentially in parallel an supply load current until the rectified
AC input again exceeds Vpeak/2 on the next half cycle. This valley fill passive PFC
circuit presents good power factor (>0.85) and low THD (30%), the major
drawback is the 50% DC BUS ripple witch result in a very high lamp current crest
factor. The capacitor CX is for filtering the half-bridge inverter switching spikes
which appears at DC BUS. Particularly at the light load condition or at the peak of
AC input, a big spike occurs at every switching cycle when switching frequency
decreases towards resonance causing load voltage and current to increase.
Please check my previous blog (Valley Fill PFC Circuit; January 20, 2016) for the
calculation and selection of the components value for designing Valley Fill Circuit.
Advantage of the Passive PFC is its easiness; the passive LC filter is a high-
efficiency and low-cost PFC solution that could potentially meet the IEC 61000-3-
2 Class D requirement in the low-power range. Other advantages counts simplicity,
reliability and ruggedness, insensitivity to noise and surges, no generation of high-
frequency electromagnetic interface (EMI) because of no high-frequency switching
losses.
For higher power designs, the presence of heavier and bulkier filter
inductors increases the size and weight of the passive components, which is a
disadvantage of this Passive PFC technique. Also, Passive PFC does not use the
full energy potential of the AC line. This technique is not able to effectively
eliminate line current harmonics. PFC is not possible for universal input range.
Refer below figure for Passive PFC waveforms where AC input current is lagging
with respect to AC input voltage hence both current and voltage are not purely in-
phase.
• Active Power Factor Correction
A typical switch mode power supply with no power factor correction may have a
power factor of around 0.60, with passive power factor correction, it may be
around 0.80, and with active power factor correction it would be 0.95 or better than
this.
Active PFCs use active electronics circuits, which contain devices like MOSFETs,
BJTs, and IGBTs. Active power factor correction can involve more circuitry than
other methods, but can be very effective in its result. Active PFC offers improved
THD and is considerably smaller and lighter than a passive PFC circuit. Active
PFC operates at frequencies higher than the line frequency so that compensation of
both distortion and displacement can occur within the timeframe of each line
frequency cycle, resulting in corrected power factors of up to 0.99.
The aim of active power factor correction is to make the input to a power
supply appear like a simple resistor. An active power factor corrector does this by
programming the input current in response to the input voltage. As long as the ratio
between the voltage and current is a constant the input will be resistive and the
power factor will be near to 1.0.
Distortion in an active power factor corrector comes from several sources:
the feedforward signals, the feedback loops, the output capacitor, the inductor and
the input rectifiers.
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.
Let we discuss two basic types of active PFCs;
1. Boost
The boost-circuit based PFC topology is the most popular. It is a good solution for
complying with regulations. Here the output dc voltage is greater than its input dc
voltage. Also, the input current is continuous and it generates the lowest level of
conducted noise and the best input current waveform. The boost regulator input
current must be programmed to be proportional to the input voltage waveform for
power factor correction. Feedback is necessary to control the input current. It
contains at least two semiconductor switches and at least one energy-storage
element. All boost PFC circuit uses a controller IC for the purpose of ease of
design, reduced circuit complexity, and to control the cost. Below figure shows a
Boost Type PFC Circuit;
Let’s discuss its working in detail; When the MOSFET (S) is closed, the inductor
(L) output is connected to ground and the voltage (Vi) is placed across it. The
inductor current increases at a rate equal to Vi/L. When the switch is opened,
however, the voltage across the inductor changes and is equal to VL-Vin. Current
that was flowing in the inductor decays at a rate equal to (VL-Vi)/L.
The boost converter has the filter inductor on the input side, which provides a
smooth continuous input current waveform. The continuous input current is much
easier to filter, which is a major advantage of this design because any additional
filtering needed on the converter input will increase the cost and reduces the power
factor due to capacitive loading of the line.
Disadvantage to this technique is that the output voltage is
always greater than the peak input voltage.
The boost regulator input current must be forced or programmed to be
proportional to the input voltage waveform for power factor correction. Feedback
is necessary to control the input current and can be done by below mentioned
methods;
· Peak current mode control: This has a low gain, wide bandwidth current loop
which generally makes it unsuitable for a high performance power factor corrector
since there is a significant error between the programming signal and the current.
This will produce distortion and a poor power factor.
· Average current mode control: It is based on a simple concept. An amplifier is
used in the feedback loop around the boost power stage so that input current tracks
the programming signal with very little error. This is the advantage of average
current mode control and it is what makes active power factor correction possible.
Average current mode control is relatively easy to implement.
2. Buck
It is a converter in which the output voltage is less than the input voltage. It is like
a voltage step-down converter. It is also a current step-up converter. In this
technique energy is stored in the inductor. Below figure shows a Buck Type PFC
Circuit;
Let’s discuss its working in detail; When the MOSFET is in the ON state the
current flows to the load and energy is stored in both the inductor (L) and the
capacitor (C) and no current flows through the diode as it is reverse-biased. When
the MOSFET is in the OFF state, the energy stored in (L) is released reverse back
into the circuit and the current flows via the load and diode. At some point when
the load voltage begins to fall, the charge stored in C becomes the main source of
current until the switch is ON again.
Advantage with Buck type PFC is that the inductor limits the rate of change of load
current. But, the input current is discontinuous and a smoothing input filter is
required. Buck converter provides one polarity of output voltage and unidirectional
output current. It has high efficiency, more than 90%. Also as it generates the low
voltage at output, we can apply this lower input voltage to the DC-DC output stage
where we can use and achieve lower voltage rated semiconductors, optimized loss
and size of isolation transformer and better performance.
Advantage of the Active PFC is its extensive suppression of line current
harmonics. Also, it is able to operate in Universal Input range. Disadvantage is
Complex and costly circuitry.
Refer below figure for Active PFC waveforms where AC input current is almost
perfectly in-phase with AC input voltage.
