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Published in IET Power Electronics

Received on 17th November 2011

Revised on 18th December 2012

Accepted on 24th December 2012

doi: 10.1049/iet-pel.2012.0092

ISSN 1755-4535

Soft single switch resonant buck converter withinherent PFC featureAmin Emrani, Mohammad Reza Amini, Hosein Farzaneh-Fard

Electrical and Computer Engineering, Isfahan University of Technology, Isfahan, Iran

E-mail: [email protected]

Abstract: In this study a new fully soft switched converter with a single switch is presented. The relatively small value of required

passive circuit elements in this converter makes its implementation simple and cost-effi

cient. Since this converter operates at fullysoft-switching conditions without any extra switches and low number of circuit components, the proposed converter is highlyefficient. The other advantage of this converter is its inherent power factor correction (PFC) feature. In this study, the

proposed converter is introduced and its theoretical analysis is presented. Also a prototype converter is implemented and theexperimental results are presented to verify the theoretical analysis.

1 Introduction

Switch-mode DCDC converters are employed to varyvoltage levels and consequently they are indispensable inalmost every electrical device. Size and efficiency of DC

DC converters are two signifi

cant issues in topologyselection. Increasing switching frequency would reduceconverter volume and weight but would result in higherswitching stress, more switching loss and electromagneticinterference (EMI). To overcome these limitations and toincrease operating switching frequency, soft-switchingtechniques have been highly developed and widely applied[111].

Soft-switching PWM converter topologies [13]commonly employ extra switches to provide soft-switchingconditions resulting in higher complexity. In [4], a familyof PWM converter topologies is presented in whichsoft-switching condition is obtained without any auxiliaryswitch. This reduction is achieved at the cost of extra

voltage and current stresses of the main switch. In addition,employing coupled inductors has created leakageinductance that has deteriorated the soft-switching condition.

Another category of soft-switching converters are thequasi-resonant converters which are frequency controlled.Although these converters do not suffer from extraswitches, their output filter cannot be optimally designed[510]. Also the operation of the converter is load dependent.

Both of the topologies explained so far require relativelylarge inductors as input or output filter. Switched capacitorconverters (SCC) are introduced to eliminate the magneticcomponents of DCDC converters and hence reduce thefabrication complexity, volume and cost [11, 12]. In these

converters, the circuit capacitors are charged and dischargedvia the switches. This is the inherent drawback of thesetypes of converters, which increases the switch currentstress and drastically limits the converter power handling.

In addition, when a capacitor is connected to anindependent voltage source with a different voltage level, afixed amount of energy is dissipated. This phenomenon iscommon in SCCs and would reduce the efficiencydramatically. In [12] a method is presented to alleviate this

problem. However, this adds complexity to the converter.To improve SCCs, a small inductor is added in series withthe switched capacitor to provide zero current switching atturn on and reduce switch current peak and EMI [1316].

The main disadvantage of all the mentioned SCCconverters is its fixed gain to a specific coefficient (1/nwhere n is 1, 2, 3, ) of the input voltage. In addition,these converters employ high number of semiconductorelements and capacitors.

Another similar method derived from SCC is switchedresonator converters (SwRC). These converters are

presented as a new family of soft-switching DCDCconverters with small passive elements [17, 18]. Theseconverters have many advantages compared with their

aforementioned counterparts, such as fully soft-switchingconditions for all semiconductor elements and notransferred power restriction. Also, their range of voltagegain is reasonably wide. Nevertheless, the main drawbackof these converters is that they must employ at least twounidirectional switches. Unidirectional switches are not asaccessible and if a diode is placed in series with a common

bidirectional switch to make it unidirectional, theconduction losses would increase.

In this paper, a fully soft switched SwRC converter ispresented which not only enjoys the merits of thepreviously introduced converters [17, 18], but also employsonly a single switch that reduces the converter cost and

complexity. In addition, the employed switch can be acommon bidirectional switch, further improving efficiency.The maximum voltage on the resonant capacitor is lowerthan that of SwRC and the presented converter has inherent

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PFC feature because of its discontinuous conduction modeoperation.

In the next section, the converter operation is describedcomprehensively. In Section 3, design consideration isdiscussed. In Section 4, simulation and experimental resultsfor a 230 W prototype based on the theoretical design are

presented to verify the analysis. Finally, the converterinherent PFC feature is discussed.

2 Circuit description and operation

In this section, the proposed converter shown in Fig. 1 isintroduced and analysed. The converter steady-statewaveforms are shown in Fig. 2. The equivalent circuit ofeach operating mode is shown in Fig. 3. At each intervalthe parts of circuits which carry current are highlighted.Assuming all circuit elements to be ideal, the converteroperating modes are as follows:

Mode I (t0 t1): Assume that before Mode I, currents ofthe inductors are zero and VCr is equal to 2Vs Vo. At t0,

S is turned on under zero-current switching (ZCS) and Crdischarges through a resonance with L2.

iL1(t) = 0 (1)

iL2(t) =2Vs Vo

Z1sin v1 t t0

(2)

Vcr(t) = 2Vs Vo

cos v1 t t0

(3)

v1 =1

L2Cr (4)

Z1 =

L2Cr

(5)

Mode II (t1 t2): Att1, the sum of the VCrand output voltage

becomes less than the input voltage, thus D turns on underZCS and IL1 initiates to increase

I1 = iL2 t1

= VsZ1

3 2 Vo

Vs

(6)

iL1(t) = L2

L1 +L2I1 cos v2 t t1

Vs Vo

L1 +L2

v2

sin v2 t t1

+ Vs VoL1 +L2

t t1

+ L2L1 +L2

I1

(7)

iL2(t) =L1

L1 +L2I1 cos v2 t t1

+ L

21 Vs Vo

L1 +L2 2

Z2

sin v2 t t1

+ Vs VoL1 +L2

t t1

+ L2L1 +L2

I1

(8)

Vcr(t) =L1

L1 +L2Vs Vo

cos v2 t t1

Z2I1 sin v2 t t1 +L2

L1 +

L2

Vs Vo (9)

Z2 =

L1L2

L1 +L2

Cr

(10)

v2 =1

L1L2

/ L1 +L2

Cr

(11)

Mode III (t2 t3): At t2, VCr reaches the negative value ofoutput voltage and then the auxiliary diode Da turns onunder ZVS. Although the auxiliary diode is conducting, the

resonant capacitor voltage stays constant, so only iL1 goesthrough the switch. In this interval, the switch can beturned off at ZVS condition. The best time to turn off theswitch is at t2 because at this moment the switch current is

Fig. 1 Soft single switch ZVS Buck converter

Fig. 2 Waveforms of the proposed converters

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the lowest.

I1 = iL1 t2

(12)

I2 = iL2 t2

(13)

iL1(t) =Vs

L1t t2

+I1 (14)

iL2(t) = VoL2

t t2

+I2 (15)

Vcr(t) = Vo (16)

Mode IV (t3 t4): Att3, the switch is turned off and thereforethe resonant capacitor begins to resonate with L1. At the sametime, L2 keeps on discharging linearly to the output capacitorthrough Da. At t4, the auxiliary diode current reaches zero and

Da turns off under ZCS condition.

I2 = iL1 t3

(17)

I3 = iL2 t3

(18)

iL1(t) = I2 cos v3 t t3

+ VsZ3

sin v3 t t3

(19)

iL2(t) = V

oL2

t t3 +I3 (20)Vcr(t) = Vsos v3 t t3

+ Z3I2 sin v3 t t3

+ Vs Vo (21)

Mode V (t4 t5): During this mode, the resonance continuesbetween L1 and Cr until iL1 reaches zero and D turns off atZCS

iL1(t) = 0 (22)iL2(t) = 0 (23)

Vcr(t) = 2Vs Vo (24)Mode VI (t5 t6): In this mode, all semi-conductor devicesare off and the load is being supplied by the output

capacitor. The duration of this mode is decided by thecontrol circuit in order to regulate the output voltage.

3 Design consideration

Based on the theoretical analysis of the proposed converter inthe previous section, design considerations are explained asfollowing

3.1 1-Resonant elements (L1, L2 and Cr)

To design the converter, Pout, Vin andVo should be identified,based on the switch limitations. fmax is the maximum allowed

switching frequency which is selected practically about a fewhundred kilo Hertz. fsw is the switching frequency whichvaries in such a way to regulate the output voltage againstload and line variations.

Ein =t5

t1

Vs iL1 dt= 2CrV2S (25)

Eout=V2oR T (26)

Eout = Ein (27)

A =VoVS= 2CrRfsw (28)

Cr= Pout2V2Sfsw

(29)

The parameter Ein is the input source energy delivered to theconverter and Eout is the consumed energy by the load. The

parameter A is the converter voltage gain.When the resonant capacitor voltage becomes equal to the

output voltage (at the end of Mode II), the voltage across theswitch becomes zero and thus it can be turned off at ZVS butat this point its current is equal to L1 current. Also, the currentof L1 has to be minimised to achieve ZCS condition.

Although L1 current increases in Mode II and the durationof this mode is related to L2, thus, L1 should be larger than

L2. Larger L1 would further restrict the converter frequency.Thus, the value of L1 can be selected twice that of L2.

Fig. 3 Equivalent circuit of each operating interval, from

af Modes IVI

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To control the output voltage, the converter frequency mustbe changed. As the duration of Modes IV is approximatelyconstant, the length of Mode VI can be varied. To calculatethe maximum frequency, the duration of Modes IV must

be assessed.Modes I and II occur when the switch is on and in these

modes L2 resonates with Cr. The duration of these modescan be estimated as

ta = p

L2Cr

(30)

Modes IIIV happen when the switch is off and in thesemodes, L1 resonates with Cr. The modes are approximatelyequal to

tb= p

L1Cr

(31)

To obtain maximum frequency, the duration of Mode VI hasto be zero.

Tmin = tb+ ta = p L1Cr + L2Cr (32)L2 = 2L1 (33)

fmax =1

tb+ ta= 1p 1 +

2

L2Cr

= 0.132L2Cr

(34)By considering equations (32)(34), L1 andL2 are calculated.

3.2 2-Semiconductor elements rating (Sm, Daand D)

Assuming that the current ofL1 att3 is almost zero, the switchmaximum voltage would reach 2VS Vo. However, becauseof the small initial current of the L1, the switch voltage stresswould be somewhat more than 2VSVo. Practically the switchmaximum voltage is

VSW,max = 1.1 2VS Vo

(35)

The maximum switch current occurs in Mode II, which is themaximum current of Lr2 and can be estimated as

ISW,max = 1.12VS Vo

Z1(36)

The current and voltage stress of diodes D and Da are

VD, max = 1.1VS (37)

ID,max = 1.1VSVS

(38)

VDa ,max = 2.2VS (39)

IDa ,max = ISW,max (40)

4 Simulation and experimental results

To justify the validity of the theoretical analysis, the proposedconverter is simulated by PSIM. Fig. 4 shows the switchcurrent and voltage. It can be observed from the figure thatZCS and ZVS conditions are provided at turn on and turn

off, respectively. Figs. 5 and 6 illustrate the soft-switchingconditions at turn on and turn off for diode (D) andauxiliary diode (Da), respectively.

A 230 W prototype of the proposed converter has beenbuilt. Input and output voltages are 100 and 48 V,

respectively. The switch and diodes are IRF460 andBYW29, respectively. The inductors and capacitors aredesigned as L1 = 20 H, L2 = 10 H, Cr= 100 nF and C=100 F. The inductor cores are EI2519 with 15 turnswinding and an air gap to obtain the desired inductance.

The experimental switching waveforms of semi-conductorelements are shown in Figs. 79. These waveforms illustratethe achieved soft-switching conditions for all semi-conductorelements as explained for the simulation results. As shown inFig. 7, switch is turned on under ZCS and turned off underzero-current zero-voltage switching (ZCZVS) because ofauxiliary diode turning on. The presented experimental

Fig. 5 Current and voltage of auxiliary diode Da

Fig. 4 Current and voltage of switch (S)

Fig. 6 Current and voltage of diode D

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results confirm the theoretical analysis and the simulationresults.

The major ringing in Figs. 7 and 8 is basically because ofresonance between inductor L2 and the inherent capacitanceof the off-turning diode Da. There is another minor ringingoccurring when the main switch is turned off. This ringingis damped because Da is already on.

In Fig. 9, there is a decaying oscillation because of the

resonance between the junction capacitance of diode D andinductor L1.

Fig. 10 shows the converter voltage gain with variousloads. It can be observed from this figure that for a specificload, as the input voltage decreases/increases, the switchingfrequency would be increased/decreased by the feedbackloop to regulate the output voltage. Note that similar toZCS quasi-resonant converters (and many other resonantconverters with a constant switch on time), the convertercannot provide regulation at no load. If the converter load isreduced abruptly, the controller rapidly reduces theswitching frequency and the output voltage remainsconstant. The result of this test is shown in Fig. 11. As can

be observed from this figure, the dynamic response of theconverter is very fast. This is because of the small size of

Fig. 10 Vo/Vs against normalised switching frequency

Fig. 7 Switch current and voltage (time scale is 1 s/div)

Top is the current waveform (7 A/div) and bottom is voltage waveform(50 V/div)

Fig. 8 Current and voltage of Da (time scale is 1 s/div)

Top is the current waveform (14 A/div) and bottom is the voltage waveform(50 V/div)

Fig. 9 Diode current and voltage (time scale is 1 s/div)

Top is the current waveform (4 A/div) and bottom is voltage waveform(50 V/div)

Fig. 11 Top is the output voltage ripple (1 V/div) and bottom is the

output current (2 A/div and time scale is 25 s/div)

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the resonant components which results in a very smallconverter time constant. Fig. 12 illustrates the efficiency forvarious operating loads, showing that the converterefficiency is approximately 96% in a wide load range. Thisis because of soft-switching conditions for allsemi-conductor elements and low number of passiveelements.

5 Inherent PFC characteristics

The linear relationship between input voltage andLr1 current,confirms the inherent converter power factor correction (PFC)feature. To verify this, the circuit shown in Fig. 13 isimplemented at AC input voltage of 110 V and DC outputvoltage of 50 V. By employing the same elements andoperating at the same switching frequency as the previous

prototype, the obtained output power is around 230 W.Since there is no need for feedback to illustrate the properconverter operation as a PFC, in this case the prototypePFC converter is operated without feedback. To ensure thatthe 100 Hz output voltage ripple would be < 2%, an 1800F electrolytic capacitor is chosen for output filter. A smallLC filter is used at the input of the converter to filter thehigh-frequency switching ripple. The values of filtercomponents are 10 H and 1 F.

Fig. 14 illustrates the input current and voltage of theconverter and verifies the theoretical assertion of almostunity PF. The total harmonic distortion is < 0.35 and the

power factor is 94%. To regulate the converter outputvoltage based on the above mentioned specifications, theswitching frequency has to be changed from 50 to 120 kHz.The voltage of resonant capacitor and its current are shownin Fig. 15. Also, the current of the resonant inductors L1and L2 are shown in Figs. 16 and 17, respectively. As it can

be observed from these figures the energy in the resonanttank, which is related to the resonant capacitor voltage andthe current in the resonant inductors, varies in accordanceto the input voltage. The efficiency of the PFC circuit at

Fig. 12 Converter efficiency against output power

Fig. 13 Soft single switch ZVS Buck converter as a PFC Converter

Fig. 14 Top is the input line current (3 A/div) and bottom is the

input line voltage (100 V/div and time scale is 2.5 ms/div)

Fig. 15 Top is the resonant capacitor current (10 A/div) and

bottom is the resonant capacitor voltage in PFC circuit (100 V/div

and time scale is 2.5 ms/div)

Fig. 16 L1 current in PFC circuit (10 A/div and time scale is

2.5 ms/div)

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various loads is illustrated in Fig. 18. It can be observed thatPFC efficiency at full load is 93%.

6 Conclusions

A fully soft switched converter with a single switch ispresented in this paper. The required passive elements arevery small which makes it suitable for most applications.The converter efficiency is approximately 96% in wide load

range. This is primarily because of the presence of fullysoft-switching conditions for all semi-conductor elements,

the low number of circuit components and the small size ofinductors. The proposed converter is also appropriate forPFC purposes because of the inherent PFC feature of theconverter. The presented experimental results confirm thetheoretical analysis and demonstrate superior converter

performance.

7 References

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14 Cheng, K.W.E.: New generation of switched capacitor converters.IEEE PESC, 1998, pp. 15291535

15 Yeung, Y.P.B., Cheng, K.W.E., Ho, S.L., Law, K.K., Sutanto, D.:Unified analysis of switched-capacitor resonant converters, IEEETrans. Ind. Electron., 2004, 51, (4), pp. 864873

16 Lin, Y.C., Liaw, D.C.: Parametric study of a resonant switchedcapacitor DCDC converter. Proc. IEEE TENCON, 2001, vol. 2,pp. 710716

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Fig. 18 PFC ef ficiency against output power

Fig. 17 L2 current in PFC circuit (10 A/div and time scale is

2.5 ms/div)

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