NOVEL ZERO CURRENT TURN-ON AND TURN-OFF COMMUTATION CELL

E.C. Dias, L.C.G. Freitas, V.J. Farias, E.A.A. Coelho, J.B. Vieira Jr. and L.C. de Freitas Universidade Federal de Uberlândia - UFU

Núcleo de Pesquisa em Eletrônica de Potência – NUPEP Uberlândia-MG, Brasil e-mail: freitas@ufu.br

Abstract— This paper presents a novel soft-commutation cell capable of providing ZCS operation keeping the main switch current equal to the load current. The operation and main properties of this cell are also included. A theoretical approach for the design of the proposed cell applied to a DC-DC Buck converter is presented and corroborated by simulation and experimental results of an 800W laboratory prototype.

Keywords - DC-DC Converters, ZCS Converters, Resonant Converters, Soft-commutation Cells.

I. INTRODUCTION

Huge technological evolution in the Power Electronics field lead to the development of new topologies of power converters dedicated to a large variety of applications such as telecommunications, motor drives, automotive, etc.

The main goals have been the reduction of weight and volume, increase of efficiency, and hence, increase of power density. Thus, it becomes essential for power supplies to become smaller.

The power supplies mainly get smaller by increasing their operating frequency to decrease the size of the power transformer and output LC or capacitive filter [1]. Besides, increasing the power supplies efficiency provides the reduction of heat sinks, which also contributes to decrease the power supplies weight and size. However, increasing the switching frequency also increases the switching losses at both turn-on and turn-off. Other problems such as emission of electromagnetically interference (EMI) are dependent on the switching frequency f and increases at proportion of f4 [2, 3]. Thus, to operate at high frequency in order to allow smaller power supplies designs, the switching losses at both turn-on and turn-off must be mitigated.

In this context, in the beginning of 1985 the first topologies of quasi-resonant converters (QRC’s) were presented. The quasi-resonant converters were obtained basically associating specific resonating LC circuits with the switch to render its current sinusoidal rather than square wave in shape. Then, the switch can be turned-on and off during the zero crossing of current eliminating the overlap between voltage and current, which causes the switching losses. This technique was named zero current switching (ZCS) [4-23].

The association of pulse width modulation (PWM) to quasi-resonant converters and resonant converters provided great contributions in the power electronics field allowing the increase of the switching frequency without compromising the power converters efficiency. Another advantage achieved deploying quasi-resonant PWM converters in power supply topologies was the reduction

in irradiated and/or conducted EMI which allowed increasing the switching frequency without compromising the operation of the control circuit or the operation of other electronic equipments nearby [2, 3], [7, 8], [19].

In ordinary quasi-resonant ZCS converters the main switch current is a result of the combination of the load current with the resonating current, i.e. load current plus resonating current; which forces the designers to choose a switch with higher current capability [4-6], [15-18].

Therefore, the purpose of this paper is to present and evaluate a novel soft-commutation cell capable of providing ZCS operation keeping the switch current equal to the load current. Substituting the typical PWM cell found in classics power converter structures by the presented ZCS cell and taking to account the invariance principle [9-12], a new ZCS converter family could be achieved [23].

II. NOVEL SOFT-COMMUTATION CELL

The proposed soft-commutation cell, portrayed in Fig. 1, was developed to operate at fixed frequency deploying MOSFETs as active switches and to provide both zero current turn-on and turn-off (ZCS) of the active semiconductors. As one can observe, the presented cell is composed by two resonating inductors (Lr1 and Lr2), three diodes (D1, D2, and D3), three switches (S1, S2, and S3), and one resonating capacitor (Cr). These components are arranged in a way that all switches are commutated in ZCS conditions.

Fig. 1. The new On-Off ZCS cell.

Comparing to quasi-resonant converters [14], this new soft-commutation cell presents the following advantages: fixed and high switching frequency without switching losses; however, deploying this soft-commutation cell to other converters, additional advantages can be achieved, i.e. soft-commutation for wide load ratio and better current distribution among the auxiliary switches S2 and S3 and the main switch S1. It is important to emphasize that the main switch S1 is designed to support just the rated

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current which provides cost reduction. Besides, in the proposed cell, the current stress is reduced since the resonating current does not flow through the main switch S1, as illustrated in Fig. 2.

(a)

(b)

Fig. 2. (a) Theoretical waveforms for Buck PWM-ZCS-QRC presented in [14] (b) Theoretical waveforms of the proposed On-Off ZCS Buck.

The proposed cell can be applied to any ordinary converter if terminals “a” (active), “c” (common) e “p” (passive) are connected correctly. Thus, in [22], the authors presented the application of the proposed soft-commutation cell to ordinary DC-DC converters, i.e. Boost, Buck-Boost, Sepic, Cúk, Zeta, Flyback, and Forward with one and two switches.

In this paper, the authors focused their attention on the analyses of the proposed soft-commutation cell applied to an ordinary Buck converter operating in continuous conduction mode (CCM), as portrayed in Fig. 3. Experimental results of a 800W laboratory prototype corroborate the complete theoretical analyses also included and presented in the next sections.

Fig. 3. The On-Off ZCS Buck Converter.

III. PRINCIPLES OF OPERATION

To simplify the analysis, the main filter is considered as I0 current source. Based on the above assumption and considering a single switching period, the operation of the proposed circuit can be illustrated by six topological stages, as shown in Figs. 4 to 9. First Stage [t0, t1]: This stage begins when switches S1 and S2 are turned on and ends when current iLr1 reaches I0. During this stage, the current flowing through inductor Lr1 linearly increases and the resonance among voltage source Vin, inductor Lr2, and capacitor Cr begins (Fig. 4); Second Stage [t1, t2]: This stage begins when iLr1 = I0 and ends when iLr2 = 0. During this stage, the oscillation in series resonant circuit composed by the input voltage Vin, inductor Lr2, and capacitor Cr remains, which makes current iLr2 increase, passing through its maximum value and decreasing until zero. Diode D2 does not allow current circulation during the negative semi-cycle of current iLr2, hence, switch S2 is turned off with zero current (Fig. 5). Third Stage [t2, t3]: This stage begins when iLr2 = 0 and ends when switch S3 is turned on. During this stage there is energy transference from the DC source Vin to the load R0 (Fig. 6). Fourth Stage [t3, t4]: This stage begins when the switch S3 is turned on and it ends when the switch S1 is turned off. During this stage, current iLr1 decreases until reach zero. Therefore, switch S1 is zero current turned off (Fig. 7). Fifth Stage [t4, t5]: This stage begins when the switch S1 is zero current turned off, so capacitor Cr linearly discharges (Fig. 8). Sixth Stage [t5, t6]: This stage begins when switch S3 is turned off and it ends when switches S1 and S2 are turned on, beginning a new switching cycle. During this stage, the load current flows through the free-wheeling diode (Fig. 9).

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Fig. 4. Equivalent circuit of the first stage of operation.

Fig. 5. Equivalent circuit of the second stage of operation.

Fig. 6. Equivalent circuit of the third stage of operation.

Fig. 7. Equivalent circuit of the fourth stage of operation.

Fig. 8. Equivalent circuit of the fifth stage of operation.

Fig. 9. Equivalent circuit of the sixth stage of operation.

IV. ANALYSIS RESULTS

To establish the principle of operation, the following assumptions must be taken into account: 1. All components and switches are ideal; 2. The source Vin is considered a single DC source and

ripple free. By analytical study of the operation stages illustrated in

Figs. 4 to 9, the following relevant expressions are obtained.

Definitions:

0111.

wLr Cr

= (1)

0212.

wLr Cr

= (2)

0 1.I Lr

Vin Crα = (3)

011LrZ

Cr= (4)

022LrZ

Cr= (5)

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First stage (t0, t1):

1( ) .1

ViniLr t tLr

= (6)

0202

2( ) . ( .( ))ViniLr t sen w tZ

= (7)

02( ) .cos( .( ))vCr t Vin Vin w t= − (8)

101

twαΔ = (9)

Second stage (t1, t2):

01( )iLr t I= (10)

0202

2( ) . (2 .( ))ViniLr t sen w tZ

= (11)

( ) 2.vCr t Vin= (12)

2022.

twπΔ = (13)

Third stage (t2, t3):

01( )iLr t I= (14)

2( ) 0iLr t = (15)

( ) 2.vCr t Vin= (16)

3 3 2 energy transfert t tΔ = − (17)

Fourth stage (t3, t4):

0 0101

1( ) . ( .( ))ViniLr t I sen w tZ

= − (18)

2( ) 0iLr t = (19)

( )2( ) 1 1vCr t Vin α= + − (20)

( )401

1 . arccos2

tw

π α⎛ ⎞Δ = −⎜ ⎟⎝ ⎠

(21)

Fifth stage (t4, t5):

1( ) 0iLr t = (22)

2( ) 0iLr t = (23)

( )2 0( ) . 1 1 .I

vCr t Vin tCr

α= + − − (24)

5 201

1 1 1. 1tw α α

⎛ ⎞Δ = + −⎜ ⎟⎜ ⎟

⎝ ⎠ (25)

Sixth stage (t5, t6):

1( ) 0iLr t = (26)

2( ) 0iLr t = (27)

( ) 0vCr t = (28)

The equations (1) to (25) lead to (26):

012

02 3

01 1 02

01

.1 1 12 .2 21 . .

2. 1cos ( ) .cos .

ff tfG

f Tff

πα πα α

πα α

α−

⎡ ⎤+ + − + + −⎢ ⎥

Δ⎢ ⎥= +⎢ ⎥⎛ ⎞⎢ ⎥− − ⎜ ⎟⎢ ⎥⎝ ⎠⎣ ⎦

(29)

Where: f = Switching frequency. f01 = resonant frequency = w01 /2.π. f02 = resonant frequency = w02 /2.π. T = period = 1/f.

G= VoutVin

.

Figure 10 shows the relevant theoretical waveforms and figures 11 and 12 show two state space phase of the proposed converter. The first state space phase relates the parametrical current in switch S1 with voltage across capacitor Cr. The second space relates the parametrical current in switch S2 with voltage across capacitor Cr.

Fig. 10. The On-Off ZCS Buck Simulation waveforms.

11. LrILrCr

VCr

θ

01 4.w tΔ

Vin 2.Vin

01. LrI

Cr

1ª Stage

2ª Stage 3ª Stage

4ª Stage

5ª Stage

6ª Stage Fig. 11. The first state space phase.

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VCrVin 2.Vin

1ª Stage2ª Stage

3ª Stage4ª Stage5ª Stage

6ª Stage

max22 . LrILr

Cr

22. LrILrCr

Fig. 12. The second state space phase.

The static gain, obtained through equation (29), was resolved for several values of α and the variations of time Δt3 adopted as zero in order to provide a better visualization, as portrayed in Fig.13.

One can observe that in this new family of converters the static gain is sensitive to load variations.

Fig. 13. Static gain in relation to f/f0.

V. SIMULATION AND EXPERIMENTAL RESULTS

In this section the main waveforms obtained through simulation analyses using PSpice® and the experimental results obtained with an 800 W Buck converter prototype are presented. The simulation analysis was developed with the following parameters set:

TABLE I

Simulation parameters of the proposed ZCS Buck converter.

Design Specifications

Average output voltage, V0 (avg) = 400 V Average output power, P0 = 800W Input voltage, Vin (DC) = 180 V Switching frequency, f = 100 kHz

ZCS Buck converter

Ressonating Capacitor Cr = 35nF Capacitor Cf = 33µF

Ressonating Inductors Lr1 = 5µH, Lr2 = 20µH Inductor Lf = 20µH

Swithces, S1, S2, S3 Ideal Diodes, D0, D1, D2, D3 Ideal

One can observe that the proposed new soft-commutation cell provides zero current turn on (ZCS) in all switches and eliminates a common current stress observe in the quasi-resonant converters [3],[5] once that the peak of the resonance current is deviated to the auxiliary switch S2.

It is important to emphasize that, despite the fact that an extra auxiliary switch is needed, it is possible to achieve better power distribution among semiconductors, as a consequence, it can be precisely designed. In conclusion, it is possible to reduce cost and/or achieve higher power densities. The main current and voltages waveforms are portrayed in figures 14 to 16 illustrating what has just been stated.

In Fig.14 it can be observed that switch S1 is turned on and off with zero current, characterizing the operation in ZCS.

Fig. 14. Voltage and current on the switch S1.

The current and voltage waveforms of switch S2 are shown in Fig.15 illustrating that a soft-commutation can also be achieved. Figure 16 shows the waveforms related to switch S3 illustrating the operation in ZCS conditions was achieved.

Fig. 15. Voltage and current on the switch S2.

Fig. 16. Voltage and current on the switch S3.

Besides the ZCS operation achieved in all switches, it can also be observed that the voltage across the switches does not exceed, in any moment, the input voltage Vin (disregard parasites oscillations).Thus, this new cell also

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has the great advantage of not causing additional voltage stress across the main switch.

To demonstrate the performance of the proposed soft-commutation cell an 800 W Buck converter prototype was implemented in laboratory with the following parameters set.

TABLE II Prototype parameters of the proposed ZCS Buck converter.

Design Specifications

Average output voltage, V0 (avg) = 400 V Average output power, P0 = 800W Input voltage, Vin (DC) = 180 V Switching frequency, f = 100 kHz

ZCS Buck converter

Ressonating Capacitor Cr = 35nF Capacitor Cf = 33µF

Ressonating Inductors Lr1 = 5µH, Lr2 = 20µH Inductor Lf = 20µH

Swithces, S1, S2, S3 IRFP460, IRF840, IRF840

Diodes, D0, D1, D2, D3 APT15D100K, APT15D100K, HFA08TB60, HFA08TB60

The oscilograms are shown through Figs. 17 to 19 and

the waveforms were acquired using a THS 720 Tektronix oscilloscope and a Tektronix Tm 502A current gauge.

Figure 17 shows the current and voltage waveforms of switch S1 illustrating the soft-commutation achieved. Figure 18 illustrates the current and voltage waveforms of switch S2 and Fig.19 illustrates the current and voltage waveforms of switch S3. It can be seen that all switches are zero current turned on characterizing, therefore, ZCS operation condition.

(a)

Fig. 17. Voltage and current of switch S1 (a) Turn-on (b) Turn-off.

(a)

(b)

Fig. 18. Voltage and current of switch S2 (a) Turn-on (b) Turn-off.

(a)

(b)

Fig. 19. Voltage and current of switch S3 (a) Turn-on (b) Turn-off.

Therefore, one can observe that the experimental results corroborate with theoretical and simulation analyses,

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proving that ZCS operation with low voltage stress can be achieved with this new soft-commutation cell.

Finally, the efficiency characteristic of the proposed On-Off ZCS Buck converter analyzed in laboratory is portrayed in Fig. 20. These values were obtained using a Yokogawa® wt230 Multimeter. In order to provide a comparative analysis about the efficiency levels achieved with the laboratory prototype, a Buck converter without the proposed soft-commutation cell was also built in laboratory using the same layout and the same components. Thus, in this situation, one can conclude that significant efficiency improvements can be achieved with the application of the proposed soft-commutation cell, as depicted in Fig. 20.

Despite the great number of components, one can observe that the efficiency achieved at rated load was around 95%, which could be higher if better MOSFET, i.e. with lower series resistance; and SCHOTTKY diodes were deployed.

Fig. 20. Efficiency of the proposed Buck converter.

Figures 21 illustrates the prototype setup implemented in laboratory.

Fig. 21. Experimental setup.

VI. CONCLUSION This paper presents a new soft-commutation cell

capable to provide non-dissipative operation conditions of power electronics converters for a wide load ratio. The

authors presented important simulations and experimental results for an 800W Buck converter corroborating with theoretical analyses also included. The main characteristics of the proposed cell are: • Zero current turn on and off of all active

semiconductors; • Elimination of current stress in the active

semiconductors, which is commonly found in Quasi-resonant converters;

• The maximum voltage value across the switches is equal to the input voltage source.

• Lossless commutation at full load range; • High switching frequency with high efficiency; • Low noise level; • This cell can be applied to any ordinary converter.

As disadvantages it presents:

• Higher number of components when compared to Quasi-resonant converters;

• Additional voltage stress across diode D0.

ACKNOWLEDGMENT The authors would like to thank CNPq, FAPEMIG, and

CAPES for their financial support.

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