The 5th Power Electronics, Drive Systems and Technologies Conference (PEDSTC 2014), Feb 5-6, 2014, Tehran, Iran fljlPEDSYc-J'

Implementation and Comparison of Two Common Power Factor Correction Techniques in AC/DC

Switching Converters

Seyed Mohsen Hoseini Electrical Engineering Department

of Semnan University Sm _ hoseini@sunlight.semnan.ac.ir

Y ousef Alinejad Beromi Electrical Engineering Department

of Semnan University Yalinejad@semnan.ac.ir

Seyed Mohammad Sadeghzadeh Faculty of Engineering Shahed

University Sadeghzadeh@shahed.ac.ir

Abstract- In this paper, two prevailing techniques for active

power factor correction (PFC) including critical conduction

mode control (CRM) and average current mode control (ACM)

are designed and applied to a single-stage AC/DC converter with

flyback structure. The operation principles, control design

procedure, and control system simulation are presented. Then, a

comparison between two methods is carried out in terms of three

categories involving compensation quality, efficiency and

economic saving as well as control design requirement. The

experimental results based on a laboratory lOOW prototype are

provided to test the practical performance of control strategies.

The results evaluate the ability of each method to improve the

power factor and reduce the total harmonic distortion (THD).

They show that the ACM method presents a better compensation

performance than the CRM method in the same condition.

However, the design requirements and the converter power level

are important in the selection of appropriate control method.

Keywords - Power Factor Correction, ACIDC Switching

Converter, Harmonic Compensation, CRM control Method, ACM

control method.

I. INTRODUCTION

The fast growing of the power switching converters consist of semiconductor devices has been a critical power quality issue in recent years. These power electronic-based equipments cause a lot of power quality problems in the electrical power system. The semiconductor devices like diodes, MOSFETs, and IGBTs show nonlinear behavior, leading to harmonic distortion and power factor reduction. In order to overcome the input current harmonic distortion and fulfill the harmonic limitation standards such as IEEE 519, some various passive and active PFC methods are suggested. Although the passive PFC methods are simple and cost effective, they possess some disadvantages such as large size and weight, low flexibility, inadequate compensation performance, and depending on the load characteristics. These have limited their widespread applications for PFC purposes. The mentioned drawbacks can be overcome by using active PFC techniques, which are a good alternative for passive methods. Some of the active PFC methods are peak current mode control [1], sliding mode control [2], time delay feedback [3], inductor voltage detection control [4], etc. The mentioned methods cannot be prevalent

978-1-4799-3479-9/14/$31.00 ©2014 IEEE

285

due to some disadvantages, such as high switching losses and stresses, complexity of design, low accuracy, and subharmonic oscillations. The two most common control methods are critical conduction mode control (CRM) [5-7] and average current mode control (ACM) [8-10] which widely used in the PFC switching power supplies. The popularity of these methods is due to their implementation simplicity, high compensation performance, easy access to required ICs, lower switching losses, etc. Despite the widespread use of these PFC methods, it seems that there is not an exhaustive comparison between them in the literatures.

This paper provides design guidelines for the control structure by using CRM and ACM methods, and presents a comparison between two cited methods in terms of different characteristics. Both the simulation and experimental results demonstrate the correctness of the control design, and the comprehensive comparison contributes to select suitable method for the respective converter regarding to the different criteria and situations.

II. POWER FACTOR CORRECTION IN SWITCHING CONVERTERS

The ideal AC/DC switching converter should present a resistive load to the power source, so that the sum of the bridge rectifier and switching converter can be seen as an equivalent resistance. According to Fig. 1, the line current is sinusoidal and in phase with the applied input voltage. Therefore, the unity power factor is achieved. The relationship among the input current and voltage of the converter should be as:

(I)

Where VacCt) and iacCt) are the input voltage and current of the converter, respectively. Req is known as the constant of proportionality or effective resistance [ll]. Both of the PFC control methods applied to a single-phase AC/DC converter with flyback structure as Fig. 2.

Figure I. The input voltage and current waveforms of the ideal PFC converter

The flyback structure has some advantages such as transfonner isolation, simple structure, and buck-boost operation in wide conversion range. Generally, the pulse width modulation (PWM) is used to vary the duty cycle of the switch, in order to control the input current and regulate the output DC voltage. It is desirable to compensate both of the distortion and displacement factor in the power factor expression. It can greatly improve the power factor.

III. CRITICAL CUNDUCTION MODE CONTROL (CRM)

A. Operation Principle and control algorithm The primary magnetizing current can work in three

operation modes. If the magnetizing current is always continuous and greater than zero, the converter will operate in the continuous conduction mode (CCM). When the inductor value or the load current decreases, the converter goes into discontinuous conduction mode (DCM) in which the inductor current during some intervals of the switching period is equal to zero. The critical conduction mode (CRM) is based on keeping input current in boundary of CCM and DCM.

In the CRM control method, the magnetizing current peak value follows a reference current (ire!). The reference current is a sinusoidal signal in phase with the input source voltage. During the on-time of the MOSFET, the magnetizing inductor (Lm ) stores energy from the source as well as the inductor current increases as follow:

(2)

Where iLm is the magnetizing current in the primary side of the transformer at the on-time of the switch, Vg (t) is the input voltage of the converter. Lm and Np/ Ns are the magnetizing inductance of the transfonner primary winding and the transfonner ratio, respectively. Ts is the switching period, and d is the duty cycle (d = tan/Ts).

i. i, --->- --->-

+

Vg

Do

�I il.oad -

=F �ot

R

Figure 2. The single-phase flyback converter with PFC control

286

When the inductor current reaches to the reference current, the transistor becomes off-state. The maximum inductor current Cir;::X) can be expressed as:

(3)

Where tON is on-time of the switch over a switching cycle. Then, the transistor is turned on again as soon as the inductor current reaches to zero as follow:

Where va is the output voltage. In the CRM, the switching frequency is variable but on-time of the switch is constant on each cycle. The switching period and frequency are equal to:

2 T. _

4Np LmP 1 s - N/VJi (1_NSV9(t))2 Np Va

F _ 1

_ Ns Vm 1 Ns Vm I . ( ) I

2 2 ( )2 i s --- 2 -- . - sm wt Ts 4Np LmP Np Va

(5)

(6)

Therefore, an acceptable range of the switching frequency can be expressed as:

(7)

Equation (7) can be used to select the appropriate value of the inductance Lm and output voltage Va for the CRM operation. The control scheme of the CRM method is presented in Fig. 3. Thus, according to it, the average current wavefonn can be similar to the source voltage. This leads to the reduction in the harmonic contents and improvement in the power factor.

B. Controller Design The controller design process is composed of two steps.

First, a PI controller is used to regulate the output voltage. Second, the input voltage and current are sensed for PFC control to achieve near unity power factor. The PI controller maintains the output voltage at the desired value and improves the steady-state response. This controller adds a pole in the origin of the complex plane and increases the system type by one [12]. Moreover, it can improve the disturbance rejection performance, limit high-frequency noise and reduces the steady-state error:

ire!

Figure 3. The reference signal versus switch current in flyback converter with CRM control method

u(t) = Kpe(t) + K[ f; e(t)dt

G ( ) - K + K[ _ Kps+K[ c S - P -----s s

(8)

(9)

Where u(t) is the output signal, e(t) is the input error signal, and Gc(s) is the transfer function. Kp and K[ are proportional and integral gains, respectively. Increasing K[ leads to faster but more oscillatory response. In the switching converters, the output voltage variations (v) is a function of three independent inputs: the control input variations (d), the AC line variations (vg), and the load current variations (ZZoad) [11]. Thus, v can be expressed as:

v(s) = Gvd(s)d(s) + Gvg (s)vg (s) -Zout(S)ZZoad(S) (10)

Where GVd (s) is the control to output transfer function, Gvg is the line to output transfer function, and Zout is the converter output impedance. For simplicity, it can be assumed that the small signal variations in the input voltage and DC output current is low, thus v is particularly dependent on the control input variations and GVd (s) can be determined as follow:

G s = 17(S) I � vd() des) vg=o lZoad=O

(11 )

For the tlyback converter operated at the CRM, Gvd(s) can be explained as [13]:

G (s)=�(s)=G .Ns.(1+ctlJ(l-�) (12) vd d do Np 1 +_s_+� woQ w�

Where the expressions of the parameters of the Equation (12) are illustrated in the Table. I. The control algorithm of the CRM method is shown in Fig. 4. The block diagram of the CRM control method can be seen in Fig. 5. The controller transfer function is equal to:

Gc(s) = k. (1+sWL) (13)

A inverted zero at frequency tL is added to the loop gain. If tL is sufficiently lower than the loop crossover frequency (tc), the phase margin is unchanged. It is employed to increase the low frequency loop gain and improvement regulation in the output voltage at frequencies less than of the loop cross over frequencies [11]. Equations (14) and (15) indicate the feedback gain H(s) and control signal vc(s) , respectively.

TABLE I. THE PARAMETERS OF TRANSFER FUNCTION

Parameter Title Expression Angular corner 1-D

UJa frequency -JLsecXC (1-D)XR

Q Quality factor � TF Zero(l)

1 UJz1 -R xC

TF Zero (2) (1-D)'xR

UJz2 ---DXL"Rr Cda DC Gain

Vg (1 D)'

287

generation

Output voltage control

II �. V:"

if V�ec > 0 --+ Sw: no change if V;:'ec < 0 ----) Sw: on

Do �I

iload -->

��O�R

Gate drive d(t)

Figure 4. Control algorithm for the CRM method

H (s) = Vref (14) Vo Vc(s) = Gc(s)(vret(s) - H(s) . vo(s)) (15)

Thus, the reference signal which should be compared with the switch current can be expressed as:

iret(s) = Hi (s) . Vg(s) . vc(s) (16)

In order to achieve the CRM operation, the inductor current should be sensed continuously and follows the sinusoidal reference signal. The tum-on command applies to the switch gate as soon as the inductor current reaches to zero. Due to the measured current in the tlyback converter is discontinuous, the secondary transformer voltage sensing is needed to determine the moment of zero-crossing of the current.

C. Simulation Results To analyze and evaluate the PFC performance of the CRM

method, the simulation on a single-phase tlyback converter is carried out in PSIM environment. The network and converter parameters are given in Table 2. The converter waveforms, including the output voltage, the input current, the AC line current, and the switch voltage versus secondary transformer voltage are shown in Fig. 6(a-d), respectively.

Vsee

Figure 5. Block diagram of the CRM control method

6O c--�----�-�----,--, 50 40 Output Voltage=50V

(a) O.05-�O.1-0 ;-5 -O:2--:O0.:O25-:':0.3�

Time(s)

Time(s)

0.06

0.074

0.07 Time(s)

0.078 0.08Z Time(s)

0.086

Figure 6. The simulation waveforms of the CRM method; (a) output voltage, (b) reference current vs input current, (c) line current, (d) switch and secondary

transformer voltages

TABLE 2. THE NETWORK AND CONVERTER PARAMETERS

Parameter Value Supply Parameters 220Vrms ISOHz

Converter Output Power lOOW

Converter Output Voltage SOV

Switching Frequency 30-S0KHz

Transformer Ratio 0.3

Transformer Leakage Inductance SOIlH

Magnetizing inductance 0.47mH

IV. AVERAGE CURRENT MODE CONTROL (ACM)

A. Operation Principle and control algorithm Unlike the CRM, the ACM method works in both of

continuous and discontinuous conduction mode. It adopts twoloop control structure for control the output voltage and input current. This technique forces the input current average to follow reference signal by current control loop. Generally, in this operation mode, the initial inductor current is non-zero on each period. The inductor current can be expressed as:

izn (t) =

vg(t) t + iL (0); (iL (0) =/=. 0) ; (0 < t ::::; dTs) (17) m Lm m m

The switching frequency is constant and is dependent to PWM carrier signal frequency. Equations (19-21) can be used to calculate the minimum range of Lm for continuous conduction mode operation in the flyback converter.

(19)

(20)

D _ 1

- (NSV9 ) Np'Vo +1 (21)

Where lLm is the average of the inductor current, Ifm is the boundary current, and D is the constant duty cycle value. Using above equations, the inductor value range can be derived from:

288

(22)

Fig. 7 shows the control scheme of the ACM method, where the average of the inductor current follows the sinusoidal reference current.

B. Controller Design The closed-loop controller requires designing two PI

compensator to minimize the output voltage ripple and the error between reference signal and inductor current. The control algorithm of the ACM method is shown in Fig. 8. Here, it is not required to measure the transformer secondary voltage. The three input signals to controller are the output voltage (va )'

the input current (ig) and the rectified input voltage (vg). The block diagram of the ACM method is presented in Fig. 9.

The main steps to calculate the transfer function of the system is similar to the section 3(B). The reference current and small-signal variations of the duty cycle can be expressed as below:

, Gcz(S).(iret(S)-isw(S)) des) =

VM (24)

if·,!

Figure 7. The reference signal versus magnetizing inductor current with ACM control method

00 iLoad --+ .1

+ II �. v:" ��O�R

v,

Pulse width modulator

Current control loop

Reference current generation

1 i,w

ierr = ire! - isw G () - k (1+",L') c2 S - 2' -,-

d(t)

;-------'----, Voltage control

loop VerT = HVa - Vret

Gel (s) = kd1+:",)

Vg = IVs sin wtl

�rY4

Figure 8. Control algorithm for ACM method

Where GC1(s) and Gc2(s) are the voltage compensator transfer function and current compensator transfer function, respectively. VM is the peak amplitude of the carrier signal.

C. Simulation Results The simulation waveforms of the ACM control method on

the tlyback converter include the inductor current versus reference current, the compensated input current, and the AC line current are shown in Fig. 10(a-c), respectively. [t is obvious from Fig 10 that the peak amplitude of the input current is less than of the CRM method. Since the input current is discontinuous, the magnetizing current waveform IS

displayed to show the current follows the reference signal.

Y. EXPERIMENTAL RESULTS

In order to evaluate the performance of two methods, the PFC controller is implemented on a laboratory prototype tlyback converter, and the experimental results are provided. The test operating parameters of the converter are as: rated out power: Po =lOOW, RMS input voltage: Vin,rms =220Y, line frequency: fline =50Hz, output DC voltage: Vo =50Y, magnetizing inductance: Lm =470IlH, and switching frequency: fsw =30-50KHz. The experimental waveforms of the output voltage in 50Y, input current resulted from lack of harmonic compensation, with CRM, and ACM compensation techniques are shown in Fig. ll(a-d), respectively. Fig ll(e) shows the zoomed switch current and confirms the softswitching in the CRM method.

voltage compensator

Figure 9. Block diagram of the ACM control method

1:�' l 8-6 -

4 � Iref J o ��A�� i � 1iJ: _W __ IIli_ ....... aw a -2 -

-4 - -4 -

(a) -6- (b) -8 - .--�---'---' -8 0.06 0.07 0.08 0.09 0.1 0.05 0.06 0.07 0.08 0.09 0.1

Time(s) Time(s)

8

i a -2

-4 -6 -8

0.05 0.06 0.07 0.08 0.09 0.1 Time(s)

Figure 10. The simulation waveforms of the ACM method; (a) inductor current, (b) compensated input current, (c) line current

289

II +--- +--

t----i [Sms/div]

(a) H

T I .....,. ", .� ,JPI' �. lr fi.. I�,

---t i [ ms/divl [ ms/div]

I (c) \U}

H

I

I

n I I

/' I / I .. / . I Y I. / / , I

[O.015msjdiv]

1+

I I I :-(E)

H

Figure I I. The experimental waveforms of the converter; (a) output voltage, (b) unfiltered input current, ( c) compensated current with CRM method, (d)

Compensated current with ACM, (e) soft-switching in switch current

YI. EVALUATION AND COMPARISON

The simulation and experimental results can validate the performance of these two methods. The comparison can be performed based on compensation quality, efficiency and economic saving as well as control design requirement. The input current THD and power factor value, presented in Table. 3, are suitable indexes for PFC performance analysis. Despite both of these techniques presents a good compensation quality, the THD value in the ACM is less than of the CRM in the nominal power, and the power factor is higher. One of the advantages of ACM is constant operating frequency, while the switching frequency in the CRM is variable which causes subharmonic injection, switching losses and noise generation. In the CRM technique, the input current peak is high, and ripple current is maximum, thus it is appropriate only for low power converter. [n addition, both high input current peak and ripple lead to increase electromagnetic interference (EMI) and the size and weight of the EM[ filter. With the ACM method, the peak and rms currents can be reduced to 50% and 25%, respectively. However, unlike the ACM, the natural soft switching is realized in the CRM in turn-off and turn-on states of the switch. [t leads to reduce switching stresses during MOSFET turn-on and diode turn-off. Also, the transient response of the ACM is typically slower than of the CRM.

ACM method requires an additional PI compensator loop to reduce current error. The maximum transistor and diode voltages are equal in two methods, while maximum transistor and diode currents are higher in the CRM method. Table. 3 and

Fig. 12 are presented a comparison between two methods in terms of different parameters. Although the ACM can be presented a better compensation performance, but some issues such as converter power level, design equipments, and type of application are important in the selection of appropriate control method.

TABLE 3. THE COMPARISON BETWEEN CRM AND ACM METHODS

Parameter CRM ACM THD 18.3% 15.7%

Power Factor 0.987 0.993

Economic Saving 33.81% 34.4%

Loss Reduction 43.7% 44.5%

Switching Frequency Variable (25-50KHz) Fixed (30KHz)

Operation Mode CRM(BCM) CCM/DCM

Natural Soft Switching Yes No

Input Current Ripple High (maximum) Low

Vg dmax Ts (�.-�-} + Maximum Switch Current Np 1-0 0

Lm 3LDT 2Lm s Maximum Switch Voltage

Np Vg,peak + "N;' Vo

Np Vg,peak + "N;' Vo

PI Compensator I 2

Pulse Width Modulator No Yes

Secondary Transformer Yes No

Voltage Sensing

Conventional Power Level Low (::::; 300w) High (more than 1000w)

-0.995 \,It;,,,,.=22Q,: � [ 0.99 J\l �.,....

j /"" 0.985 � v,r/ ? CR"" !1M • 0.98

� I?' :'1 0.975

0.97 ' t 20 30 40 50 60 70 80 90 100

% Output Power (PLo•d)

11 vtfL2201i-c: � 27 '"-p--g 25 -.�

- H'" M 23 �1 � 21 "

"g 19

� 17

� 15

20 30 40 50 60 70 80 90 100

% Output Power (P\'Q.d)

88

86 ''1-,1,,, 84 'ITTF � ;!:;' ,cll'l,*

� ';, � F -:;;-� 82

J 80

� 78

76

74

72 � 70

20 30 40 50 60 70 80 90 100

% Output Power (P'G.dl

0.999 F¥ -� �."� �w J 0.997

[ 0.995 �,

j 0.993 �. � IU f.'W� R F-�

0.991 • �� � 0.989 'CrR

0.987 1111 11 :'\ 0.985 '\. E'"

180 190 200 210 220 230 240

Input Voltage (VII

c 20 � � 19

g 18

!� W"�"'00W� J b. ....,. A=e£}� A=1 .� 17

� 16 CM�

" 15

"g 14 / � 13 Z. � 12 >-P"

180 190 200 210 220 230 240

Input VoltCige IVl)

89 .J c(: H Ip�,.;;rw'$ 87 r:-;.. ",,- �J - �-A � 85 f� F � J 83

'" 81 "-. II

1f.

'F,;� 79

77

75

180 190 200 210 220 230 240

Input Voltage (V,)

Figure 12. variation of power quality indexes in different output loads and input voltages; (a) power factor, (b) total harmonic distortion, (c) efficiency

290

VII. CONCLUSION

This paper evaluates and compares the performance of two more common power factor correction (PFC) techniques involving the ACM and CRM methods. The operation principles, the controller design procedure, and the simulation of the control system in both of methods have been presented. The practical performance of the control strategies has been tested by using a prototype lOOW single-stage flyback converter. Then, a comparison between two methods is carried out in terms of compensation quality, efficiency and economic saving as well as control design requirement. The results show that the ACM method represents a lower THD (15.7% versus 18.3%) and higher power factor (0.992 versus 0.985) rather than the CRM method in the same condition. However, the design requirements and the converter power level are important in the selection of appropriate control method.

REFERENCES

[I] B. Bryant and M. K. Kazimierczuk, "Modeling the closed-current loop of PWM boost DC-DC converters operating in CCM with peak currentmode control," Circuits and Systems, IEEE Trans. on, Vol. 52, Issue. II, pp. 2404-2412,2005.

[2] P.P. Lung, c.T. Lih and K.J. Kuang, "Sliding mode control for PWM single phase boost power factor correction," Industrial Electronics and Applications (lCIEA), IEEE Conf. on, Taiwan, pp. 1247-1252,2010.

[3] A. El Aroudi and M. Orabi, "Stabilizing technique for AC-DC boost PFC converter based on time delay feedback," Circuits and Systems, IEEE Trans. on, Vol. 57, Issue. I, pp. 56-60,2010.

[4] T. Tanitteerapan and S. Mori, "An input current shaping technique for PFC flyback rectifier by using inductor voltage detection control method," Electrical and Electronic Technology, Int. Conf. on, Singapore, pp. 799-803,2001.

[5] M. Marvi and A. Fotowat-Ahmady, "A fully ZVS critical conduction mode boost PFC," Power Electronics, IEEE Trans. on, Vol. 27, Issue. 4, pp. 1958-1965,2012.

[6] T. L. Chern, L. H. Liu, C. N. Huang, Y. L. Chern, and J. H. Kuang, "High power factor flyback converter for LED driver with boundary conduction mode control," Industrial Electronics and Applications

(lCIEA), IEEE Conf. on, Taiwan, pp. 2088-2093, 2010.

[7] J.W. Shin, G.S. Seo, B.H. Cho and K.C. Lee, "Modeling and implementation of digital control for critical conduction mode power factor correction rectifier," Control and Modeling for Power Electronics

(COMPEL), IEEE Work. on, Japan, pp. 1093-5142,2012.

[8] N. Yadaiah, A. Suresh Kumar and YM. Reddy, "DSP based control of constant frequency and average current mode of Boost converter for power factor correction (PFC)," Advances in Power Conversion and Energy Technologies (APCET), Int. Conf. on, India, pp. 1-6,2012.

[9] Y. Van, F. C. Lee, and P. Mattavelli, "Analysis and design of average current mode control using describing function-based equivalent circuit model," Power Electronics, IEEE Trans. on, Vol. 28, Issue. 10, pp. 4732-4741,2013.

[10] D. Jayahar and R. Ranihemamalini, "Inductor average current mode control for single phase power factor correction buck-boost converter," Emerging Trends in Electrical and Computer Technology (ICETECT), Int. Conf. on, India, pp. 274-279, 2011.

[II] R.W. Erickson, "Fundamentals of power electronics," Kluwer Academic Publishers, 2st Edition, 2000.

[12] Lab sheet, "Power system operation and control," Faculity of Engineering, Multimedia University, 2013.

[13] E. Rogers, "Understanding buck-boost power stages in switch mode power supplies, Application Report, Texas Instrument, 2002.

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Implementation and comparison of twocommon power factor correctiontechniques in AC/DC switchingconverters

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Conference Location :

Tehran, Iran

Digital Object Identifier :

10.1109/PEDSTC.2014.6799387

Publisher:

IEEE

In this paper, two prevailing techniques for active power factor correction (PFC) including critical

conduction mode control (CRM) and average current mode control (ACM) are designed and applied to a

single-stage AC/DC converter with flyback structure. The operation principles, control design

procedure, and control system simulation are presented. Then, a comparison between two methods is

carried out in terms of three categories involving compensation quality, efficiency and economic saving

as well as control design requirement. The experimental results based on a laboratory 100W prototype

are provided to test the practical performance of control strategies. The results evaluate the ability of

each method to improve the power factor and reduce the total harmonic distortion (THD). They show

that the ACM method presents a better compensation performance than the CRM method in the same

condition. However, the design requirements and the converter power level are important in the

selection of appropriate control method.

Published in:

Power Electronics, Drive Systems and Technologies Conference (PEDSTC), 2014 5th

Date of Conference:

5-6 Feb. 2014

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Hoseini, Seyed Mohsen ; Electrical Engineering Department of Semnan University, Iran ; Alinejad Beromi, Yousef ; Sadeghzadeh, Seyed Mohammad

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Abstract