Active Power Factor Correction Technique for Single
Phase Full Bridge Rectifier
Suma Umesh,
L.Venkatesha,
Usha A
Dept. of EEE, BMSIT
Bangalore INDIA
sumaumesh@yahoo.com
Dept. of EEE, BMSCE
Bangalore INDIA
l.venkatesha@bmsce.ac.in
Dept. of EEE, BMSCE
Bangalore INDIA
ushaajoshi@yahoo.com
Abstract—This work addresses the power factor correction
technique adopted for a high output power, single phase full
bridge rectifier with a large capacitor at its output stage. Full
bridge rectifier with a large capacitor at its output stage is the
most commonly used circuit topology in AC-DC conversion to
get a constant DC output voltage required for various
applications. This topology is popular because of its simple
construction, low cost and high reliability. However this
topology will have a distorted input current waveform shape
with lot of lower order harmonics and low power factor which
needs to be corrected. The power factor can be improved either
by using passive or active components. In this work, a
comparative study and analysis of passive and active power
factor correction methods has been demonstrated. The
simulation of passive power factor correction for a single-phase
bridge rectifier with large output capacitor for 1 kW output has
been carried out using PSpice simulation software. The active
power factor correction for a single-phase, 1 kW output full
bridge rectifier has been designed and tested and results are
presented. Further the results of both the schemes are compared
and advantages of active power factor correction is recorded
and demonstrated in this work.
II. FULL BRIDGE DIODE RECTIFIER
The Fig.1 shows a simple diode bridge rectifier circuit with
an output capacitor. The supply current flows to charge the
capacitor only during a small interval of the input half cycle
when the supply voltage (Vs.) exceeds the voltage on the
capacitor (Vo). The corresponding waveforms are shown in
Fig.2. Such a current will have periodic non-ideal sinusoidal
waveforms. This current will have lot of lower order
harmonics and the power factor will be very poor because of
the distorted input current.
Fig.1: Full Wave Bridge Rectifier.
Keywords— Power factor Correction, full bridge rectifier with
large output capacitor, Active and Passive power factor correction
methods, Boost converter topology.
I. INTRODUCTION
For the control of electric power or power conditioning,
the conversion of electric power from one form to another is
necessary. The static power converters perform these
functions of power conversions. A diode bridge rectifier
circuit converts AC voltage into a fixed DC voltage and is the
most commonly used topology because of its simple
construction, low cost and high reliability.
The diode bridge rectifier forms a part of the power circuit
in many applications. At lower power levels, the application
is in the area of computers, telecommunications, airconditioning, battery charging etc. At higher power levels, the
application is in industries like for AC and DC drives. In case
of AC drives, a diode bridge rectifier provides the necessary
DC bus voltage which acts as an input to the inverter.
In all these applications, a large capacitor is normally used
at the output stage of the bridge rectifier to reduce the DC
output ripple. This diode bridge rectifier with a large output
capacitor will have a highly distorted, non-sinusoidal input
current with a lot of lower order harmonics and very poor
power factor [1], [2].
In this work, the input current to the diode bridge rectifier
is made sinusoidal and in phase with the input voltage to get
unity power factor.
c
978-1-4799-2206-2/14/$31.00 2014
IEEE
Fig.2: Waveforms of Bridge Rectifier
III. SIGNIFICANCE OF POWER FACTOR
The power factor indicates how effectively the equipment
draws power from the utility. At lower power factor operation
for a given voltage and power level, the current drawn by the
equipment will be large, thus requiring increased V-A ratings
of the utility equipment such as transformers transmission
lines and generators. The efficiency of the distribution
network is reduced by both reactive and distortion powers
which produce extra RMS currents. The resulting extra losses
lead to oversize the copper area of the distribution power
wires. The importance of high power factor has also been
recognised by the residential and office equipment
manufacturers for their own benefits to maximize the power
available from a wall outlet [2].
130
This shows that both the user and the electricity
distribution company take advantage from unity power factor.
The phase displacement angle and the harmonic distortion
are the main causes of poor power factor. The displacement
power factor should be high to yield a high power factor. The
harmonics should be low to yield power factor. The IEC5552, a specification [2] from the World standards organization
responsible for the quality of power restricts the amount of
current permissible at each harmonics up through and beyond
the 15th harmonic. Therefore, the input current of the bridge
rectifier should be made sinusoidal and in phase with the
input voltage to get unity power factor.
There are essentially two methods of power factor
correction-Passive correction method and Active correction
method [3].
IV. PASSIVE POWER FACTOR CORRECTION
Inductors and capacitors can be used in conjunction with
the diode bridge rectifier to improve the waveform of current
drawn from the utility grid.
The simplest approach is to add an inductor on the AC side
of the rectifier bridge. This added inductor improves the
power factor and reduces the harmonics.
The other approach is to add an inductor (Ld) on the DC
side of the bridge. This inductor increases the conduction
angles of the diodes, thereby reducing the amount of peak
current drawn from the line.
An effective approach using passive components is to add
tuned harmonic filters on the AC side to remove some
characteristic harmonics of the current drawn from the utility.
Fig.3 shows a bridge rectifier unit with passive filters,
where, Ld is the DC-side inductor, Ls is the AC-side side
inductor, Zfh is the filter tuned for lower order harmonic and
Zhpf is the high pass filter.
In active power factor correction, a power electronic
converter is used in conjunction with the diode bridge
rectifier and output capacitor for current waveform shaping.
Some of the basic power electronic topologies used for this
purpose are boost, buck, buck-boost and fly-back converters
[4].
In this work we have considered Boost topology because
of the natural advantage of having output slightly higher than
the input and also the advantage of cost, size and power
losses.
The basic Boost converter circuit is shown in Fig.4. It is a
DC-DC converter. As the name implies the output voltage is
always greater than the input voltage.
Fig. 4: Basic Boost Converter Topology.
When the switch is ON, the diode is reverse biased, thus
isolating the output stage. The input supplies energy to the
inductor. When the switch is OFF, the output stage receives
energy from the inductor as well as from the input. An output
capacitor is used to ensure a constant output voltage. The
relation between input and output voltage is given by
௏
௏௜௡
்௦
ଵ
(1)
ൌ ௧௢௙௙ ൌ ଵି஽
Where D is the duty factor of the switch expressed as
௧௢௡
ଵ
And ܶ‫ ݏ‬ൌ
‫ܦ‬ൌ
்௦
௙௦
(2)
Where, fs is the switching frequency of the converter.
The basic scheme used to implement the power factor
correction [5] using boost converter topology is shown in Fig.
5. First it eliminates the output filter capacitor of the rectifier
bridge.
Fig.3: Bridge Rectifier with Passive Filters.
V. DESIGN AND SIMULATION OF PASSIVE POWER FACTOR
CORRECTION
The bridge rectifier shown in Fig.3 with the fallowing
specifications is simulated using PSpice simulation software.
Input Voltage=230Vac, output capacitor (Co) =3000μF,
load =90Ω (1kW), Ld=2mH, L3=56.3mH, C3 =20μF,
L5=40.5mH, C5=10μF.
The passive method of power factor correction is very
simple and straight forward. But the disadvantage is the size
of the filters. For a compact system, these bulky filters are not
suitable. Moreover, these passive filters are not very reliable
because of saturation problem as they have to carry full line
current.
VI. ACTIVE POWER FACTOR CORRECTION
Fig.5: Basic scheme for active power factor correction using Boost
topology.
By removing the filter capacitor Co, the line current flows
continuously and sinusoidally, avoiding the narrow current
pulses, which otherwise would be there due to output
capacitor. The resulting half sinusoidal voltage drive a
continuous- mode boost converter.
The first task of the power factor correction circuit is to
use the boost converter to convert the varying input voltage
up and down the half sinusoids to a constant, fairly well
regulated DC voltage slightly higher than the input sine-wave
peak. The input-output voltage relation is given by
2014 International Conference on Advances in Energy Conversion Technologies (ICAECT)
131
ܸ‫ ݋‬ൌ
௏௜௡
ଵି
೅೚೙
೅
(3)
Throughout the half sinusoid of Vin, the Q1 ON time(Ton)
is width modulated to yield a constant DC voltage. The ON
time throughout the half sinusoid is controlled by a PFC
control chip which senses Vo, compares it to an internal
reference in a Dc voltage error amplifier, and in a negative
feedback loop sets Ton to keep Vo constant at the selected
value.
From the above equation, it is evident that at the lower
voltage portions of the half sinusoids, the Q1 ON time will be
large to boost the low input voltage to a value higher than the
peak of the sinusoid. And as Vin rises towards its peak, the
PFC control chip will automatically decrease the Q1 ON time
so that each voltage level along the rising half sinusoid is
boosted to that same DC level. The progression of the ON
time throughout the half sinusoid is shown in Fig. 6.
Fig.7: Continuous mode boost converter.
Continuous mode boost converter operates in an odd way [6]
to correct for load current changes. From Eq. 3, it is evident
that Vo and Ton are independent of load current. Yet if the DC
load current changes, it is obvious that the output diode
currents must change despite a constant ON time.
The circuit responds to a change in load current in the
fallowing way. Prior to say, an increase in the load current,
assume the Q1 current is like ABCD in Fig.8.
Fig. 6: Progression of ON time throughout half sinusoid.
The second task of the power factor correction circuit is to
sense input line current and force it to have a sinusoidal wave
shape in phase with the input line voltage. This is also done
by width modulation of the same boost regulator’s ON time.
This ON time is determined in a negative feedback loop
which compares a sample of the actual input line current to
the amplitude of a clean reference current sine wave. The
difference between these two sine waves is an error voltage
that is used to modulate the ON time to force the two sine
waves to be equal in amplitude.
The final voltage that controls the boost regulators ON
time must be a mix of the DC output voltage error and the
input line current error voltage. This mixing is done in a
multiplier block whose output is proportional to the product
of the output voltage error voltage and input current error
voltage.
The boost converter can be operated in dis-continuous
mode or continuous mode. In this work, a continuous mode
boost converter is selected to yield relatively smooth, ripple
free half sinusoids of the input current.
Fig.7 shows the schematic of the continuous mode boost
converter. The output voltage regulation is achieved by
changing Ton in accordance with the Eq. 3 as Vin changes.
This is done with pulse width modulator. If Vin changes
momentarily, so does Vo. A fraction of Vo is sensed by error
amplifier EA and compared to a reference voltage Vref to
yield an error voltage Veao. This DC error voltage is compared
to a built in triangle voltage Vt in voltage comparator Vc. The
Vc output is a square wave, which is high for the time from
the start of the triangle to the instant the triangle crosses the
error voltage output Veao. .And Q1 is turned on via totem pole
driver for high time of the Vc output.
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Fig. 8: Regulation against load current changes in continuous mode boosttopology.
Now for a small increase in the load current, assume the Q1
current, in steady state, Q1 current will move up to say
AB1C1D. And for a larger load current change, the Q1
current will move up to, say, AB2C2D. To cause these
changes, Ton changes over a few switching cycles but returns
to its original value in the steady state. The output load
current is the sum of IQ1 and ID1.
The increased value of the ramp on a step waveform for
increased DC load occur over a number of switching cycles
as fallows. If DC load current increases, Vo goes down
momentarily because of the source impedance looking back
into it. Then Veain goes down, Veao goes up, the VT triangle
crosses Veao later in time, and Ton increases. Now the IQ1
current ramps up for a longer time to a higher value. Then ID
starts later in time from a higher value and with a shorter OFF
time, has a higher value at the end of the OFF time.
This progresses over a number of cycles with the average
currents at the centre of IQ1, Id ramps increasing until they
equal the increased DC load, at which time Ton and Toff
slowly fall back to their initial values, as called for by Eq. 3.
Thus in a qualitative way, it can be seen that the bandwidth of
the output voltage error amplifier must not be large. If it is
large, it will respond too quickly and does not permit the
output voltage to shift for too long a time from its normal
value at a fixed input voltage. The output voltage must be
permitted to shift from its value dictated by the input voltage
for a time sufficient for the above described current build up
to occur over a number of switching cycles.
2014 International Conference on Advances in Energy Conversion Technologies (ICAECT)
All the required functions described above are currently
doable with a power factor correcting integrated-circuit chip
available from a number of manufacturers. The functions that
These chips implement are mostly the voltage and current
sensing error amplifiers, mixing of these error signals and
generating of width modulated boost transistor turn-on
control pulses.
In this work, UC3854 from Unitrode is used for active power
factor correction. This IC essentially uses boost topology to
correct power factor.
This device implements all the control functions necessary to
build a power supply capable of optimally using available
power-line current while minimizing line-current distortion.
UC3854 contains a power MOSFET compatible gate driver, a
7.5 reference, load-enable comparator, low supply detector
and over-current comparator.
Fig. 9 and 10 show the basic block diagram of UC3854 and
basic control circuit with UC3854 for active power factor
correction respectively.
and‫ ܮ‬ൌ
௏௜௡஽
(5)
௙௦οூ
Output Capacitor is selected based on the fallowing
equation.
ଶ௉௢௨௧ο௧
‫ ݋ܥ‬ൌ ௏௢మ
(6)
ି௏௢ሺ௠௜௡ሻమ
Switch and Diode must have ratings that are sufficient to
ensure reliable operation. The switch must have current rating
at least equal to the maximum peak current in the inductor
and a voltage rating at least equal to the output voltage. The
same is true for output diode.
Current sensing is done through a current sensing resistor
in the ground return. The power dissipation in the resistor
may become very high at higher current levels and I that case
the current transformers are used for current sensing, one for
the switch current and one for the diode current.
Peak current limit on UC3854 turns the switch OFF when
the instantaneous current through it exceeds the maximum
value. The current limit value is set by a simple voltage
divider. The equation for voltage divider is given below:
ܴ‫ ʹ݇݌‬ൌ
௏௥௦ோ௣௞ଵ
(7)
௏௥௘௙
Multiplier Set-up- the multiplier / divider is the heart of the
power factor corrector. The output of the multiplier programs
the current loop to control the input current to give a high
power factor. The multiplier output current is related to three
inputs by the fallowing equation:
‫ ݋݉ܫ‬ൌ
Fig.9: Basic block diagram of UC3854.
Fig. 10: Basic circuit with UC3854 for Active Power Factor correction.
DESIGN OF ACTIVE POWER FACTOR CORRECTOR.
The design process starts with specifications [5] for the
converter like, input voltage range, line frequency range,
maximum power output, output voltage. And generally the
output is selected 5 to 10 per cent higher than the maximum
input voltage.
Switching frequency fs is selected arbitrarily. The switching
frequency must be high enough to make the power circuit
small and minimize the distortion.
Inductor L is selected based on the allowable amount of
high frequency ripple current in the input.
Inductor is selected using the fallowing equations.
‫ܦ‬ൌ
௏௢ି௏௜௡
௏௢
(8)
௏௙௙ మ
Where, Km is a constant in the multiplier circuit.
The voltage divider for the Vff input has three resistors
(Rff1, Rff2, Rff3) and two capacitors (Cff1 and Cff2). The
equation is given by
௏௜௡ሺ௔௩ሻோ௙௙ଷ
ܸ݂݂ ൌ ோ௙௙ଵାோ௙௙ଶାோ௙௙ଷ ൌ ͳǤͶͳͶܸ
(9)
And ܸ݊‫ ݁݀݋‬ൌ
VII.
௄௠ூ௔௖ሺ௏௩௘௔ିଵሻ
௏௜௡ሺ௔௩ሻሺோ௙௙ଶାோ௙௙ଷሻ
ோ௙௙ଵାோ௙௙ଶାோ௙௙ଷ
(10)
Solving these two equations simultaneously, we can get
the values of Rff1, Rff2, and Rff3.
The Gain of the voltage error amplifier is given by:
‫ ܽݒܩ‬ൌ Ψܴ݅‫ ݈݁݌݌‬ൈ
௏௩௘௔
௏௢ሺ௣௞ሻ
(11)
Where, Vopk is the peak value of second harmonic voltage.
The feedback capacitor Cuff is designed using the equation:
ଵ
‫ ݂ݒܥ‬ൌ ଶగ௙௥ோ௩௜ீ௩௔
(12)
The feed forward voltage divider filter capacitors are
determined using the equations:
ଵ
(13)
‫ ͳ݂݂ܥ‬ൌ ଶగ௙௣ோ௙௙ଶ
‫ ʹ݂݂ܥ‬ൌ
(4)
ൌ ͹Ǥͷܸ
ଵ
ଶగ௙௣ோ௙௙ଷ
2014 International Conference on Advances in Energy Conversion Technologies (ICAECT)
(14)
133
With these major components designed the active power
factor corrector circuit is built for the fallowing specifications:
Input voltage-100 VRMS to 130 VRMS, supply frequency 50Hz, output voltage – 220V, power output- 1kW, switching
frequency 50KHz and a desired power factor of unity.
The experimental set up is shown in the Fig. 11
Fig.13: Input Voltage and Current Waveforms with Active power factor
correction.
TABLE 2: PERFORMANCE COMPARISON OF ACTIVE POWER
FACTOR CORRECTION.
Parameter
Fig. 11: Experimental Set-Up for Active Power Factor Correction.
VIII.
RESULTS AND DISCUSSIONS
The simulation result for passive power factor correction
circuit of Fig. 3 is shown in Fig. 12 and Table.1 gives the
performance comparison of bridge rectifier without and with
tuned filters.
With tuned filters, the power factor has improved to 0.974
as compared to a power factor of 0.42 without the tuned
filters.
Current THD
Voltage THD
Reactive power
Power factor
With
only
capacitor.
52.01 %
14.8%
107 VAR
0.742
output
With active power
factor correction
9.2 %
3%
60.6 VAR
0.986
From the above table, it is clear that the power factor has
improved tremendously and also the current harmonic
distortion has reduced from 52.01 % to 9.2%.
From the results recorded for passive and active power
factor correction methods, it is evident that both the methods
are effective in improving the power factor of a diode bridge
rectifier.
However, with active power factor correction a power
factor of 0.99 can be realized, which may not be possible with
passive filters as tuning the passive filters to the exact values
is practically difficult.
Though the design of active power factor circuit elements
are not as simple and as direct as the passive power factor
filters and number of components are more in active power
factor correction circuit, the size of the circuit will be very
compact which makes it more suitable for compact power
supply systems.
IX. CONCLUSION AND FUTURE SCOPE
Fig.12: Waveforms of bridge rectifier with passive tuned filters.
TABLE 1:PERFORMANCE COMPARISON OF PASSIVE POWER
FACTOR CORRECTION WITH AND WITHOUT TUNED FILTERS.
Parameter
Line current peak
DC voltage ripple
Power factor
With
only
output
capacitor. (No filters)
48 A
10.3 V
0.42
With Tuned filters and
AC, DC side inductors
5.7 A
6V
0.974
The experimental set up for 1 kW, single phase active
power factor correction circuit as in Fig.11 is tested and the
result is shown in the Fig. 13.
The power factor is found to be 0.986 and the input current
waveform shape has improved to a neat sinusoidal waveform.
The Table.2 shows the performance comparison of
experimental setup with and without active power factor
correction.
134
The active power factor correction technique using UC 3854
for a 1 kW output circuit, which has been implemented in this
work, has advantages over passive power factor correction
method in terms of size, volume and weight of the circuit
elements. The active power factor correction circuit is more
compact and weighs less compared to passive power factor
correction circuit.
The active power factor correction technique using UC 3854,
Which has been implemented in this work for a single phase
system can also be extended to the three phase system.
Further, the power density can be increased by increasing the
switching frequency of the boost topology. This increase in
switching frequency also allows a further reduction in the
filter component size. However raising the switching
frequency, increases the system switching losses and reduces
efficiency.
In order to increase the switching frequency while
maintaining acceptable efficiency, some soft switching
techniques have to be adopted.
2014 International Conference on Advances in Energy Conversion Technologies (ICAECT)
In future, a zero voltage turn-on (ZVT) of the main switch
and zero current turn-off (ZCT) of the boost diode may be
implemented to get good system efficiency.
[4]
[5]
ACKNOWLEDGMENT
Authors sincerely acknowledge Mr. G.S.N Raju, Scientist,
LRDE, for his guidance during execution of this work. We
also acknowledge Mr. B.V Ramesh, Scientist, LRDE, Mr.
N.C. Saha, Scientist, LRDE, Mrs. Meera Das, Scientist,
LRDE and Mr. VenkateshaPrabhu, Scientist, LRDE for their
support and technical guidelines during the execution of this
work. We also thank Dr. T.C. Balachandra, BMSIT, for his
constant support in implementing this work.
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