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CHAPTER 2

POWER FACTOR CORRECTION CONVERTERS

2.1 GENERAL

A detailed analysis of the various power factor correction

converters used with the PMBLDC drive is presented in this chapter.

The PMBLDC drive consists of a VSI and the PMBLDC motor,

which is usually powered through a diode bridge. The basic requirement of

almost all applications involves power converters with improved power

quality. Further, power quality standards for low power equipment such as

IEC 61000-3-2, emphasize low harmonic contents, and a near unity power

factor current to be drawn from the AC mains with these equipments. Hence,

the use of a PFC converter has become inevitable for a PMBLDC motor

drive. High quality converters are used to interface the AC line and the DC

load. Their aim is to make the load appear resistive, so that a unity power

factor is achieved even in the presence of the distorted line voltage. Power

factor correction can be done using one of the various topologies available.

Two stage power factor converters are known as power factor pre regulators.

These types of PFC converters make use of a boost converter at the front end,

and a forward converter at the second stage for voltage control. This results

not only in a huge cost, but in complexity as well.

Several AC-DC converters have been introduced to achieve the

demanded power conversion, reduction in harmonics and improvement of the

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power factor. Various types of converters can be used for power factor

correction. They are the converter, canonical switching cells (CSC),

bridgeless boost converters and zeta converters. The CSC converter forms the

main block of all high frequency switching converters. It has been found that

the CSC converter has minimum components, and is suitable for the single

phase rectifier circuit with power factor correction. The basic types of DC-DC

converters, when operating in the discontinuous conduction mode, have the

property of self-power factor correction. Power Factor pre-regulators which

are two-stage PFC converters, and the voltage control of DC-DC converters

have been discussed in the literature.

Traditional linear-type power regulators have largely given way to

switching power supplies, which are more efficient but bulky. However,

switching power supplies, cause substantial harmonic distortion to the line

current, and produce electromagnetic noise via conduction and radiation,

interfering with the working of the equipments nearby.

The attention given to the quality of the current drawn from the

utility line by the electronic equipment is increasing due to several reasons.

Low power factor reduces the power available from the utility grid, while a

high harmonic distortion of the line current causes EMI problems and cross

interferences, through line impedance between different systems connected to

the same grid. Many efforts are being made to develop interface systems, to

improve the power factor of standard electronic loads.

Basic DC-DC converters, when operating in the discontinuous

conduction mode, have the property of self-PFC. If these converters are

connected to the rectified AC line, they have the capability of giving higher a

power factor by the very nature of their topologies. The input current

feedback is unnecessary when these converters are employed to improve the

power factor. To improve the power factor, a PFC circuit is designed and

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placed in the front end of the converter, which in turn, is interfaced with the

load. This power factor correction circuit may be an independent unit.

There exists between the input power of the PFC circuit and its DC

output power an imbalance of instantaneous power. Therefore, the operating

principle of a PFC circuit is to process the input power in ways to store

excessive input energy, when input power is higher than the DC output

power, and release the stored energy when the input power is less than the DC

output power. To accomplish this process, at least one energy storage

element must be included in the PFC circuit.

In most of the PFC circuits, an input inductor is connected to the

bridge rectifier. The input inductor can operate either in the Continuous

Conduction Mode (CCM) or in the Discontinuous Conduction Mode (DCM).

The peak inductor current samples the line voltage automatically. In the

DCM, the input inductor is no longer a state variable, since its state in a given

switching cycle is independent of its value in the previous switching cycle.

This property of the DCM input circuit can be called, a self power factor

correction, because no control loop is required on its input side. This is an

advantage over the CCM PFC circuit, in which the multi-loop control strategy

is essential. But the input inductor operating in the DCM cannot hold

excessive input energy before the end of each switching cycle. As a result, a

bulky capacitor is used to balance the instantaneous power between the input

and the output. When the CCM is used, the input current is normally a train

of triangle pulses with a nearly constant duty ratio. Hence, an input filter is

necessary for smoothing this pulsating input current. To ensure a high power

factor, the average current of the pulsating current should follow the input

voltage in both shape and phase.

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One of the basic converter topologies is the DCM input circuit,

when they are applied to the rectified line voltage. However, they may draw

different shapes of the averaged line current.

A power factor corrector is the interface between the AC line and

power converter, that takes power from the supply but does not give any

power back to the supply. An ideal power factor corrector should emulate a

resistor on the supply side, while maintaining a fairly regulated output

voltage. The PFC converter is the resistive load to the AC source, which

provides a regulated DC output as input to an ordinary converter. In the case

of sinusoidal line voltage, this means the converter must draw a sinusoidal

current from the utility. To this end, a suitable sinusoidal reference is

generally needed, and the control objective is to force the input current to

follow this current as closely as possible.

Typical power supplies have a full-wave bridge rectifier on the

input side, followed by a storage capacitor. While the bridge diodes conduct,

the line drives as an electrolytic capacitor, i.e., a nearly reactive load for the

line. This causes the line voltage and current to be out of phase, which is sub-

optimal for power distribution. The maximum power is delivered only when

they are in phase with each other. Since the power factor is the cosine of the

phase angle, a resistive load with a phase angle of zero will have the power

factor of one. Further, instead of dissipating power, reactive loads store

power and return it to the supply sometime later. This results in waveform

distortion and harmonics on the AC line. Line noises and surges reduce the

power quality. A PFC converter appears resistive to its source. This implies

that the input current must differ from the sinusoidal source voltage by only a

scaling factor. Their waveforms must be identical, though scaled by the

effective input resistance of the PFC, according to Ohm’s Law.

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2.2 CONTROL SCHEME FOR THE PFC CONVERTER BASED

PMBLDC MOTOR

A detailed block diagram of a PFC converter based PMBLDC drive

is shown in Figure 2.1. A PMBLDC motor develops torque proportional to

its phase current, and its back-EMF is proportional to the speed. Therefore, a

constant current in its stator windings with the variable voltage across its

terminals, maintains a constant torque in a PMBLDC motor under variable

speed operations.

Figure 2.1 Schematic of the PFC converter fed PMBLDC drive

The speed control scheme is based on the control of the DC link

voltage reference as an equivalent to the reference speed. For the speed

control of the PMBLDC motor, rotor positions are acquired by the Hall Effect

Sensors and they are utilized by an electronic commutator to generate the

switching sequence for the VSI, which feeds the PMBLDC motor. The speed

of the motor is compared with the reference speed. The resulting speed error

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is passed through the PI controller and is used to correct the pulse width of the

converter. A PFC converter is used at the input to improve the power factor.

Modeling and simulation play an important role in the design of a

power electronics system. The classic design approach begins with an overall

performance investigation of the system, under various circumstances through

mathematical modeling. The electronic commutator uses signals from the Hall

Effect position sensors to generate switching sequences for the VSI, as given

in Table 2.1.

Table 2.1 Output of the electronic commutator based on the signals from Hall Effect Sensor

Hal Signals Switching Signals

Ha Hb Hc Sa1 Sa2 Sb1 Sb2 Sc1 Sc2

EMF_a EMF_b EMF_c Q1 Q2 Q3 Q4 Q5 Q6

0 0 0 0 0 0 0 0 0

0 -1 +1 0 0 0 1 1 0

-1 +1 0 0 1 1 0 0 0

-1 0 +1 0 1 0 0 1 0

+1 0 -1 1 0 0 0 0 1

+1 -1 0 1 0 0 1 0 0

0 +1 -1 0 0 1 0 0 1

0 0 0 0 0 0 0 0 0

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2.3 VOLTAGE SOURCE INVERTER FED BLDC MOTOR

The poor quality of the voltage and current of a conventional

inverter fed BLDC motor is due to the presence of harmonics, and hence,

there are significant levels of energy loss. The inverters with a large number

of steps can generate high quality voltage waveforms. A voltage source

inverter can run the BLDC motor, by applying three phase square wave

voltages to the stator winding of the motor. A variable frequency square

wave voltage can be applied to the motor by controlling the switching

frequency of the power semiconductor switches. The square wave voltage

will induce low frequency harmonic torque pulsation in the machine.

Variable voltage control with variable frequency operation is also not possible

with square wave inverters, for the total harmonic distortion of the classical

inverter is very high. In this study, the total harmonic distortion has been

analyzed for the VSI (with and without filters) fed BLDC. Power electronic

devices such as power rectifiers, thyristor converters and static VAR

compensators contribute considerable level of harmonics. Even the updated

Pulse-Width Modulation (PWM) techniques are used to control modern static

converters as machine drives. The power factor compensators do not produce

perfect waveforms and they strongly depend on the switching frequency of

semiconductors. Voltage or current converters, force the use of machines

with special isolation, as they generate discrete output waveforms, and in

some applications large inductances, connected in series with the respective

load.

Simulation results of VSI fed BLDC motor with and without a

filter, have been compared with those of the bridgeless boost converter fed

BLDC motor.

The VSI fed BLDC motor with the capacitor filter is shown in

Figure 2.2. The input voltage and current waveforms of the VSI fed BLDC

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motor with a capacitor filter are shown in Figure. 2.3. It can be seen that

current is displaced by 88°, and hence, the power factor is much less with the

capacitor filter system.

Figure 2.2 VSI fed BLDC motor

Figure 2.3 Input voltage and current waveforms of the VSI fed BLDC motor

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The VSI with input L-filter fed BLDC motor is shown in

Figure 2.4. The input voltage and current waveforms of the VSI with input L-

filter fed BLDC motor are shown in Figure 2.5. It can be seen that the current

is displaced by 80°, and hence, the power factor has been found to have

improved a little with the L filter.

Figure 2.4 VSI with input L-filter fed BLDC motor

Figure 2.5 Voltage and Current of the VSI fed PMBLDC Motor with L-Filter

The VSI with input T-filter fed BLDC motor system is shown in

Figure 2.6. The input voltage and current waveforms of the VSI with input T-

filter fed BLDC motor system are shown in Figure 2.7. It can be seen that the

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current is displaced by 66°. Hence the power factor has been further

improved with the T-filter.

Figure 2.6 VSI fed BLDC motor with input T-filter

Figure 2.7 Voltage and Current of the VSI fed PMBLDC Motor with T-Filter

2.4 PFC CONVERTER TOPOLOGIES

Some of the common Power Factor Correction Converter

topologies are :

Boost Converter

Fly Back PFC

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Forward Converter

Half Bridge Converter

Buck- Boost Converter

Zeta Converter

SEPIC Converter

Cuk Converter

In this chapter the basic Buck, Boost, Buck-Boost, Fly back,

forward, Cuk, SEPIC and ZETA converter topologies will be discussed. The

results of the analysis show that only some of them are suitable for PFC

applications.

2.4.1 Boost Converter

Certainly the most popular topology in PFC applications is the

boost topology as shown in Figure 2.8. A boost converter or step-up

converter shown in Figure 2.9, is a power converter with an output DC

voltage higher than its input DC voltage. It is a class of switching-mode

power supply (SMPS) containing at least two semiconductor switches, that

are a diode and a transistor; it has also one energy storage element. Filters

made of capacitors are normally added to the output of the converter, to

reduce the voltage ripple.

AC/DC conversion is effected by a diode rectifier while the

controller operates the switch to properly shape the input current ig according

to its reference. The output capacitor absorbs the input power pulsation,

allowing only a small ripple of the output voltage VL. The boost topology is

simple, and allows low-distorted input currents and an almost unity power

factor with different control techniques. The output capacitor is an efficient

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energy storage element, and the ground-connected switch simplifies the drive

circuit.

Figure 2.8 Principle of PFC Boost Converter

A large inductor in series with the source voltage is essential for a

conventional boost converter. The input current flows through the inductor

and the switch, when the switch is turned ON. The inductor stores energy at

this stage. When the switch is OFF, the inductor current does not die down

instantaneously. During the OFF period, the current is forced to flow through

the diode and the load. As the current tends to decrease, the polarity of the

EMF induced in the inductor is reversed. Hence, the voltage across the load

is the sum of the supply voltage and the inductor voltage, and is higher than

the supply voltage. The voltage impressed across the inductor during the on-

period is Vd. At this stage, the current rises linearly from the minimum level

I1 to the maximum level I2. Therefore, the voltage across the inductor is VL

=Vd.

Figure 2.9 shows the Boost Power Factor Correction Converter. It

is comprised of a diode rectifier, boost inductor, switching device, boost diode

and boost output capacitor. Boost converters have the potential for the

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highest efficiency and the lowest component stresses, if their conversion

characteristics meet the input/output specifications. It is well-known that the

boost topology is very effective in PFC applications, provided the DC output

voltage is close to but slightly higher than the peak AC input voltage.

Figure 2.9 Boost PFC Converter

The main drawbacks of this topology are

• Start-up overcurrent, due to the charge of the large output

capacitor

• Lack of current limitation during overload and short circuit

conditions, due to the direct connection between the line and

load

• Difficult insertion of a high-frequency transformer for

insulating the input and output stages

• The output voltage is always higher than the peak input

voltage

• Not a naturally isolated structure

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In universal-input applications, with the RMS input line voltage in

the 90-305V range, the output voltage has to be set to about 450V. At a low

line input, the switch conduction losses are high, because the input RMS

current has the highest value and the highest step-up conversion is required.

The inductor has to be oversized for large RMS currents at a low line input,

and for the highest volt-seconds applied throughout the input-line range. As a

result, a boost converter designed for universal-input PFC applications is

heavily oversized, compared to a converter designed for a narrow range of

input line voltages. Due to the large energy storage filter capacitor at the

output, the boost converter has inrush current problems that can only be

mitigated using additional components. Conventional single-switch buck-

boost topologies, including the plain buck-boost, Fly back, SEPIC and Cuk

converters have greatly increased the component stresses, component sizes

and reduced efficiency compared to the boost converter.

2.4.2 Fly Back Converter

The fly back converter is an isolation converter. Its topology is

shown in Figure.2.10, and Figure 2.11 shows its current waveform. The input

voltage-input current is shown in Figure 2.12.

Figure 2.10 Fly back Converter

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Figure 2.11 Driving Pulses and Input Current

where Lm is the magnetizing inductance of the output transformer. The fly

back converter has all the advantages of the buck-boost converter without any

limitation, and it also provides Input-Output isolation. This property makes

the fly back converter most suitable for power factor correction with the DCM

input technique. Figure 2.12 shows the Input V-I characteristics of the basic

fly back converter operating in DCM. The limitations of the flyback

converters are the gapped transformer inductance, which results in zero in the

right-half-plane, and it makes closed loop compensation in the CCM very

difficult. It has a very slow transient response. It also requires a high output

capacitor, because of the lack of a second-order low-pass inductor/capacitor

filter at the output.

Figure 2.12 Input V-I Characteristics

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Input Current 2

1, 1( ) ( )2

savg

m

D Ti t v tL (2.1)

2.4.3 Forward Converter

The forward converter with a non-dissipative snubber is a topology,

that eliminates the turn-off losses, by charging a capacitor with magnetic

energy, which is stored in the magnetizing and leakage inductances of the

transformer in the OFF period. In the ON period this charge resonates and

reverses the polarity across the capacitor, and provides zero turn-off losses at

the onset of the primary switch ON period.

Compared with flyback converters, forward converters require one

additional output inductor even though they have reduced requirements for

the output capacitor. The circuit shown in Figure 2.13 is a forward converter.

To avoid transformer saturation, the forward converter needs the third

winding to demagnetize the transformer. When a forward converter is

connected to the rectified line voltage, the demagnetizing current through the

third winding is blocked by the rectifier diodes. The resetting circuit

increases the count and cost, and therefore, the forward converter is not

available for PFC purposes.

Figure 2.13 Forward Converter

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2.4.4 Half Bridge Converter

The basic topology of the bridgeless PFC boost rectifier is shown in

Figure 2.14. Compared to the conventional PFC boost rectifier, shown in

Figure 2.9, one diode is eliminated from the line-current path, so that the line

current flows simultaneously through the two semiconductors, and these

results in reduced conduction losses.

SB2SB1

RL

LB

D2D1

CAC

Figure 2.14 Bridgeless Boost Converters

However, the bridgeless PFC boost rectifier in Figure 2.14 produce

significantly more common-mode noise than the conventional PFC boost

rectifier. The above analysis shows that the bridgeless PFC circuit not only

simplifies the circuit topology but also improves the efficiency. The

bridgeless PFC topology removes the input rectifier conduction losses, and is

able to achieve higher efficiency. Figure 2.15 shows the input current and

voltage waveforms of the half bridge converter.

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Figure 2.15 Input Current and Voltage Waveforms

The bridgeless PFC uses one IGBT body diode to replace the two

slow diodes of the conventional PFC. Since both circuits operate as boost

DC/DC converters, the switching loss should be the same. Efficiency

improvement thus relies on the conduction loss difference between the two

slow diodes and the body diode of the IGBT. Compared with the

conventional PFC, the bridgeless PFC not only reduces conduction loss, but

reduces the total component count; it thereby increases the efficiency of the

drive.

2.4.5 Buck Boost Converter

A typical topology of the PFC Buck boost converter is shown in

Figure 2.16, and it is constructed by the uncontrolled diode bridge. This is

followed by a Buck Boost Converter (BBC). It consists of an AC input

supply voltage, capacitor C, inductor L, power switch S, and load resistance

R. It allows the output voltage to be higher or lower than the input voltage.

This depends on the duty ratio d. The storage elements in the circuit are the

inductor and the capacitor.

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Figure 2.16 PFC Buck-Boost Converter Topology

The evaluation of the performance of the PFC buck-boost fed PMBLDC drive in Figure 2.17, shows that the power factor has been found to be higher with the use of the buck-boost converter. The results obtained show an improvement in the power quality at the AC mains. The PFC feature of the buck-boost converter has ensured the power factor close to unity in a wider range of input AC voltage, and hence a wider range of speed. The buck-boost converter can be a promising PFC converter.

Figure 2.17 Input voltage and current waveforms

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2.4.6 Zeta Converter

The Zeta Converter as shown in Figure 2.18, originally of the

buck-boost type, can be regarded as a flyback type when an isolated

transformer is incorporated. This converter is safe at the output side, and is

flexible for adjustment. A Zeta converter performs a non-inverting buck-

boost function similar to that of a SEPIC converter. The topology is also

similar to that of the SEPIC converter, in that it uses two inductors, two

switches and a capacitor to isolate the output from the input. The only

difference is that Zeta conversion requires a P-Channel MOSFET as the

primary switch, while SEPIC convertion uses an N-Channel MOSFET. A

Zeta converter operating in the discontinuous mode can be used for the PFC.

The source voltage and current waveform of a Zeta Converter shown in

Figure 2.19, results in a greater improvement in the power angle. It is said to

be attractive, since it operates both in the step up and step down functions. It

is also a naturally isolated structure prossessing power at one single stage.

But, in high power applications, high RMS values of the current cause a high

level of stress in the switches.

Figure 2.18 Zeta Converter

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Figure 2.19 Source Voltage and Current waveforms of the Zeta Converter

2.4.7 Sepic Converter

The Single-Ended Primary-Inductor Converter (SEPIC) as shown in Figure 2.20, is a type of DC-DC converter, that allows a voltage output greater than or less than or equal to that of its input. The output of the SEPIC is controlled by the duty cycle of the switching devices. A SEPIC is similar to the traditional buck-boost converter, but has the advantages of having a non inverted output. The isolation between its input and output is provided by a capacitor in series, and it has a true shutdown mode. SEPIC converters are useful in applications, where the battery voltage can be above and below that of the intended output of the regulator. Its self-PFC capability is poor.

Figure 2.20 SEPIC Converter

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2.4.8 Cuk Converter

It can be seen that the Cuk, SEPIC and Zeta converters have the same input V-I characteristics. Each of these converter topologies has two inductors, one located on its input and the other on its output. The performance of the Cuk converter is also suitable for both the SEPIC and the Zeta converters.

The circuit diagram of the Cuk Converter is shown in Figure 2.21, and the waveforms of the Cuk converter for the input inductor current, output inductor current and the current through capacitor C, are shown in Figure 2.22.

Figure 2.21 Cuk Converter

Figure 2.22 Current Waveforms

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It could be seen in Figure 2.23 that the input waveform is distorted.

The distortion is due to the offset present in the characteristics.

Figure 2.23 Input V-I characteristics of the basic Cuk converter operating in DCM

2.4.9 Bridgeless Cuk Converter

The bridgeless topologies of the Cuk converter shown in Figure 2.24, require an isolated gate-drive.

Figure 2.24 Topology of the Bridgeless Cuk Converter

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The input voltage and current waveforms of the bridgeless Cuk

converter are shown in Figure 2.25.

Figure 2.25 Input voltage and current waveforms

It could be seen from Figure 2.25, that the input waveforms of the

voltage and current are almost in phase. Therefore, it can be concluded that

the bridgeless Cuk converter has a good self-PFC property.

2.5 CONCLUSION

VSI fed PMBDC motor drives with different types of filters have

been presented. Various PFC converters like the Flyback, Forward, Buck

Boost, Zeta, SEPIC and Cuk converters have also been presented. A few

converters like the boost , bridgeless boost, buck-boost , bridgeless Cuk and

Zeta converters were simulated and their performance was studied. The

simulation results were compared in terms of the power angle, THD and

closed loop performance.