78

CHAPTER 4

PFC BUCK-BOOST CONVERTER FED PMBLDC MOTOR

4.1 GENERAL

The digital simulation of a Power Factor Correction (PFC) buck

boost converter based adjustable speed voltage controlled VSI fed PMBLDC

motor, is presented in this chapter. A single-phase AC-DC converter topology

followed by the buck-boost bridge converter, has been employed for the PFC

to ensure a near unity power factor over a wide speed range. The proposed

speed control scheme works on the concept of the DC link voltage control

proportional to the desired speed of the PMBLDC motor. The speed is

regulated by a PI controller. This drive ensures high accuracy and robust

operation from near zero to high speed.

This chapter presents an account of the average current control

technique used to design the cascaded buck-boost converter. Two loops are

attached to the buck boost converter in this design. The inner current loop has

a current error amplifier, which improves the power factor of the circuit

comparing the input current with the sinusoidal current reference. The outer

loop regulates the output voltage and also minimizes distortion. This drive

ensures high accuracy, improved power factor, robust operation from near

zero to high speed, higher efficiency, reduced component stresses, and the

ability to arbitrarily choose the DC output voltage. This converter also has

lower voltage stresses compared with the boost converter.

79

Single stage PFC circuits presented serious challenges recently,

when they sought to increase the output power capability with optimized

component ratings. A non-inverting buck-boost based PFC converter

operating in the conduction-mode for wide input voltage range applications

has been proposed. Unlike other PFC converters, the proposed non-inverting

buck-boost based PFC converter has both step-up and step-down conversion

functionalities to provide positive DC output-voltage. It is also well known,

that distorted voltages and current waveforms produce additional power

losses, and high frequency noise that can affect not only the power load, but

also the associated controllers.

Power factor correction can be achieved by any one of the

numerous techniques available. The most popular method used in the industry

is boost topology. This topology can operate in the continuous conduction

mode for high power application, or in the discontinuous conduction mode for

a lower power output. The buck regulator can also be used but the resulting

efficiency is lower than that of the boost converter. The drawback of this

approach is the sharp turn off power conversion, as the instantaneous line

voltage falls below the output voltage and these results in the rise of

harmonics. The applications of the boost converter are limited, since the

basic requirement is that the output voltage must be higher than the input

voltage. When a buck boost converter is used a higher power factor can be

obtained over a wide range of output voltage.

The proposed method combines the buck and boost modes in one

power stage and provides a simple solution for the unity power factor AC/DC

converter. A Buck Boost converter based PFC converter for a PMBLDC

motor will be discussed in this chapter. There are two loops attached to the

buck boost converter. The inner loop is the current loop with a current error

amplifier, to improve the power factor of the system, comparing the input

80

current with the sinusoidal current reference. The voltage error amplifier in

the outer loop regulates the output voltage and minimizes distortion. The

basic requirement of a Boost converter is that the output voltage must be

higher than the input voltage, and this factor limits its application to a narrow

range of output voltage. When a buck boost converter is used, a higher power

factor is obtained over a wide range of output voltage.

The conventional active PFC converter has to employ an

uncontrolled rectifier, and a costly boost inductor, but these power

components result in power loss, low efficiency and high cost. Additionally,

in the conventional active PFC converter, the power switches are in an ON

and OFF state in the whole mains period, enduring high voltage and current

stresses, which produce considerable switching loss and conduction loss

limiting the efficiency. Based on the operation of the switches, the buck-boost

converter has two operational modes to provide both step-up and step-down

voltage conversion functionality, including the buck plus boost and buck-

boost modes.

4.2 MATHEMATICAL MODEL OF THE PFC BUCK- BOOST

CONVERTER

A typical topology of the non-isolated (transformerless) buck-boost

converter is shown in Figure 4.1, and it is constructed with an uncontrolled

diode bridge. This is followed by a buck-boost converter (BBC), which

consists of an AC input supply voltage, diode bridge rectifier, 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, depending on the duty ratio ‘d’ .

The storage elements in the circuit are the inductor and capacitor. With the

switch ON, the inductor current increases, and the diode D maintains OFF.

When the switch S is OFF, the diode provides the path for the inductor

current.

81

Figure 4.1 PFC buck boost converter topology

The state-space models provide a general and strong basis for the

dynamic modeling of various systems including power converters. The state

space models are useful for designing the linear control loops; they can also

be used to simulate the steady state, as well as the dynamic behavior of the

power converter, fitted with the designed feedback control loops and

subjected to external perturbations. The state-space averaging and

linearization provides an elegant solution for the application of widely known

linear control techniques to most power converters.

The voltage transfer gain of the buck-boost converter is

(1 )oV d

E d (4.1)

The corresponding current transfer gain is

m

(1 )I

oI dd (4.2)

82

In the on-duration circuit configuration, the switch conducts and the

diode does not conduct. State equations denoting the on-interval circuit

configuration are shown as Equations 4.3 – 4.9.

Ldi Edt L (4.3)

1cc

dV Vdt RC (4.4)

0 0 110 0

L

L

c Cf

diidt EL

dV VRCdt (4.5)

In the off-duration circuit configuration, the switch opens and the

diode conducts. The corresponding state equations for the off-circuit

topology are given as

L Cdi Vdt L (4.6)

1 1CL c

dV i Vdt C RC (4.7)

10

1 1

L

L

C C

diidt L

dV VC RCdt

(4.8)

Using the state space averaging model the system model is written

as

83

10

1 1 0

L

L

C C

di d didt L ELddV VC RCdt

(4.9)

The PFC buck-boost converter is designed for a supply voltage of

230V, L = 150 mH, C = 220 µF, R= 5 ..

4.3 SIMULATION RESULTS

The technical specifications of the drive system are as follows:

C= 2200 microfarad.TON= 5.88 µsecs. TOFF= 5.88µsecs.T= 11.76

µsecs. Stator Resistance is 2.875 ohms, Stator Inductance is 8.5e-3mH, and

Motor inertia is 0.8e-3J.

With the aid of the designed circuit parameters, MATLAB

simulation was done and the results are presented here. The speed was set at

1800 rpm and the load torque disturbances were applied at time t=1 sec.

Speed regulations were obtained at this speed, and the simulation results are

shown.

4.3.1 PMBLDC Motor Fed from the PFC Buck-Boost Converter

The Simulink model of the buck-boost converter is shown in

Figure 4.2. The Simulink model of the closed loop controlled PMBLDC

motor with PFC buck boost converter and a PI controller is shown in

Figure 4.3. A buck boost converter is used at the input to improve the power

factor. The AC input voltage and current waveforms are shown in Figure 4.4.

The step change in the load torque is shown in Figure 4.5.

84

Figure 4.2 Buck-boost Converter

Figure 4.3 Closed Loop Speed Control of the PMBLDC Motor with PFC buck-boost Converter

85

Figure 4.4 Input voltage and current waveforms

Figure 4.5 Step Change in the load torque applied at t=1 sec

The MATLAB simulation of the PFC buck-boost converter fed

PMBLDC motor is carried out, and the simulation results are presented. From

Figure 4.6 it can be seen that the closed loop system brings the speed to the

normal value, and is maintained constant with the disturbance in the load

torque. The input voltage and current of the PFC buck-boost converter fed

PMBLDC motor is shown in Figure 4.4 and it can be seen that the power

factor is improved by using the buck- boost PFC converter. The FFT analysis

in Figure 4.7 shows that the THD is only 4.71 %. Hence, the power factor is

found to be higher with the buck-boost converter fed PMBLDC drive, and it

is higher than that of the PFC bridgeless boost converter fed PMBLDC drive.

86

Figure 4.6 Speed Response Curve

Figure 4.7 FFT Analysis of the source current

4.4 EXPERIMENTAL RESULTS

After the simulation studies, the buck-boost converter fed

PMBLDC motor was fabricated and tested. The top view of the hardware is

depicted in Figure 4.8. The hardware consists of a power circuit, control

circuit and the PMBLDC motor. The experimental setup is shown in

Figure 4.9. The input voltage and current waveforms are shown in

87

Figure 4.10. The harmonic spectrum of source voltage is shown in

Figure 4.11.

The technical specifications of the drive system are as follows : L =

150 mH, C = 220 µF, R= 5 . Input voltage is 48V and the bridgeless boost

converter output is 58V .Other components are: Diode IN4007,

Microcontroller AT89C2051, MOSFET IRF840, Driver IR2110, Voltage

(0-500V) and Current is 8A.

Figure 4.8 Top View of the hardware

88

Figure 4.9 Experimental setup

Figure 4.10 Voltage and current waveforms

89

Figure 4.11 Harmonic Spectrum of the Source Voltage

4.5 CONCLUSION

Since the basic requirement of the popular boost converter is that

the output voltage must be higher than the input voltage, its applications are

limited. When a buck- boost converter is used, a higher power factor can be

obtained over a wide range of output voltage.

A PFC buck-boost converter based PMBLDC motor drive is

simulated, and the results compared with the experimental results. Feedback

signals from the PMBLDC motor representing the speed and position were

utilized to get the driving signals for the inverter switches through a PI

controller. The hardware was fabricated and tested. The experimental results

are in line with the simulation results. It has been found that the power factor

has increased with the use of the buck- boost converter. Efficiency increases

because of the increase in the power factor. The PFC feature of the buck-

boost converter has ensured that the power factor is close to unity.