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Analysis and Design of a Novel Single-Phase PFC

AC-DC Step-Up/Down ConverterLung-Sheng Yang Tsorng-Juu Liang

Green Energy Electronics Research Center

Department of Electrical Engineering, National Cheng Kung University, Taiwan.E-mail: tjliang@mail.ncku.edu.tw

Abstract A novel single-phase power-factor-correction AC-DCstep-up/down converter is presented in this paper. This converteris operated in discontinuous conduction mode to achieve purelysinusoidal line current, almost unity power factor, low totalharmonic distortion of line current, and step-up/down DC outputvoltage. The proposed converter employs a coupled-inductor withsame winding-turn in the primary and secondary side, which ischarged in series during the switch-on period and is discharged inparallel during the switch-off period. Therefore, the discharging

time can be shortened. The proposed converter can be operated inlarger duty-ratio range than the conventional single-phase PFCbuck-boost converter for DCM operation. Thus, the proposedconverter is suitable for universal line voltage (90 264 V) andwide output-power range. Moreover, the steady-state analyses ofvoltage gain and the boundary operating condition are discussed

in detail. The selections of coupled-inductor and output capacitorare also presented. Finally, a prototype circuit is built in thelaboratory to verify the performance.

I. INTRODUCTIONThe DC power source is widely used in many applications,

such as DC power supplies, uninterruptible power supplies,battery chargers, and inverters. Traditionally, either the diode

bridge or the thyristor rectifiers are used for AC-DC powerconversion. These rectifiers have some advantages, which

include simple circuit configuration and low cost. But, these

rectifiers will result in the power pollutions, such as pulsatingline current, low power factor, and high total harmonic

distortion of line current (THDi). In order to improve these

problems, many power-factor-correction (PFC) powerconverters are presented to achieve purely sinusoidal linecurrent, high power factor, and low THDi to comply with the

61000-3-2 requirement. For various DC-voltage applications,the PFC power converters are categorized as step-up voltageconversion [1, 2], step-down voltage conversion [3], and step-

up/down voltage conversion [4, 5]. Because these convertersare operated in continuous conduction mode (CCM), the

control scheme is complicated and the cost is high. For costreduction, some topologies with discontinuous conduction

mode (DCM) are presented by using a simple control scheme.

However, in order to avoid higher current stress on powersemiconductor devices, these converters are suitable for lowoutput-power applications. The PFC boost converters with

DCM operation are used for step-up voltage conversion.However, these converters exist some issues, such as higher

start-up inrush current and THDi [6]. In order to reduce theTHDi, the DC output voltage must be sufficiently larger than

the peak value of line voltage. To avoid that the DC outputvoltage is too high beyond practical applications, some

modulation techniques are researched [7]. The PFC buckconverters with DCM operation are presented for step-down

voltage conversion [8]. But, the power factor is not high andthe THDi is rather high. The conventional PFC buck-boostconverter with DCM operation is employed for step-up/down

voltage conversion. The power factor is almost unity and theTHDi is rather small [9]. However, it is not suitable foruniversal line voltage (90 264 V) and wide output-power

range.

In this paper, a novel single-phase PFC step-up/downconverter with DCM operation is proposed. This converter can

be operated in larger duty-ratio range than the conventional

single-phase PFC buck-boost converter. The proposed

converter can achieve almost unity power factor, low THDi,

and adjustable step-up/down output voltage for universal linevoltage and wide output-power range.

II. OPERATIONAL PRINCIPLE OF PROPOSED CONVERTERThe circuit configuration of the proposed single-phase PFC

step-up/down converter is shown in Fig. 1. This converter is

operated in DCM by using the pulse-width modulation

technique. Switches, S1 and S2, are triggered with same control

signal. A coupled-inductor with same winding-turn in theprimary and secondary side is adopted in the proposed

converter. The primary and secondary winding of the coupled-

inductor are charged in series from the line source during the

switch-on period and are discharged in parallel during the

switch-off period. Therefore, the discharged time can be

shortened. The duty-ratio of the proposed converter can be

operated in wide range than the conventional single-phase PFC

buck-boost converter. Thus, the proposed converter can be

applied for universal line voltage and wide output-power range.

Some typical waveforms in a half line source period are shown

in Fig. 2. Owing to the symmetrical characteristics of the

single-phase system, the operating principle is analyzed for 0 > 1, the above equation can be approximated as2 2 2 2

2

,0

[ sin ] .4(1 ) 8(1 )

m s m sc avg o o

o o

d V T d V T i t i dt i

k Lv k Lv

= = + +

(19)

Then, the differential equation of output voltage is given by

2 21[ ].8(1 )

m s oo

o

d V T vdv

dt C k Lv R=

+

(20)

Thus, the equation of DC model can be written as follows:

2 2

.8(1 )

m s o

o

D V T V

k LV R=

+

(21)

where Vo andD are the DC quantities ofvo and d, respectively.

Next, the normalized inductor time constant is defined as

.L

s

L

RT

(22)

Substituting (22) into (21), the voltage gain is given by

.8(1 )

o

m L

V DG

V k= =

+

(23)

C. Boundary Condition

In order to ensure that the proposed converter is operated in

DCM, iL1 and iL2 must go to zero in each switching period.

From Figure 2, the time duration ts,h is obtained as follows:

, ,

(2 ( ) )

.2

s o s h

s h on r h

o

DT V e t

t t t V

+

= + =

(24)

When the maximum ofts,h is equal to Ts and |es(th)| is equal to

Vm, the proposed converter is operated in boundary conduction

mode. From (24), the boundary voltage gain can be found as

.2(1 )

obc

m

V DG

V D= =

(25)

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Using (23) and (25), the curves of voltage gain and boundary

voltage gain are shown in Figure 4. It is seen that the voltage

gain of the proposed converter has a linear characteristics.

Moreover, the boundary normalized inductor time constant LB

can be derived when the voltage gain G is equal to its boundary

voltage gain Gbc. Then, LB can be obtained as

2(1 ).

2(1 )LB

D

k

=

+

(26)

Thus, the curve ofLB is plotted in Figure 5. It is seen that the

proposed converter is operated in DCM when L < LB.

0 0.2 0.4 0.6 0.8 D0

1

2

G

3

CCM

DCM

2(1 )bc

DG

D=

0.005L =

0.008L

=

0.015L

=

Figure 4. Voltage gain and boundary voltage gain of the proposed converter

(assuming k= 0.95).

00

0.2

0.05

0.4

0.1

0.6

0.15

0.8

0.25

D

CCM

DCM

LB

0.2

Figure 5. Boundary conditions of the proposed converter (assuming k= 0.95)

IV. SELECTIONS OF COUPLED-INDUCTOR AND OUTPUTCAPACITOR

A. Selection of Coupled-Inductor L1 and L2

In order to ensure that the proposed converter is operated in

DCM, the appropriate LB can be selected under the required

voltage gain. Therefore,L must satisfy the following inequality:

.LBs

RL

f