A FAMILY OF PWM SOFT-SWITCHING CONVERTERS WITHOUT SWITCH VOLTAGE AND CURRENT STRESSES

M. S. Vilela E. AA Coelho J. B. Vieira Jr. L. C. de Freitas V. J. Farias

Universidade Federal de Uberlhdia Departamento de Engenharia Eldtrica

Campus Santa Mdnica - bloco E 38400-902 - Uberlhdia - MG - Brasil

ABSTRACT This paper presents a new family of PWM Sofi-Wching Converters which can operate wititout switch voltage and current stresses in high switching Jiequency for a wide range of load

To illustrate the operatng principle of this new converters a det&d study including simulations and aperimental tea3 is carried out for the boost converter.

The validity of these new converters is guaranteed by the obtained results.

1 - INTRODUCTION

High switching frequencies are necessary to reduce the size and the weight of the DC-DC converters. However, thts yields high switching losses and consequently, low efficiency in hard switching converters.

To solve this problem the quasi-resonant converters (QRC) [ 1) and the Pulse-Width-Modulated Quasi-Resonant Converters (PWM-QRC) [3] were proposed inicialy, but some of their characteristics such as load limitations and high switch voltage and current stresses restrict the practical use of these converters

Nowadays there are several converters, such as those presented in [2] and [4], that do not have the limitations described above. On the other hand such converters still contain some disadvantages as exemplified as follows.

The converters presented in [4] have several advantages but they can not operate in a soft switching way with duty cycle less than ha@.

To overcome these draw-back a new family of PWM Soft- Switching Converters with low voltage and current stresses is being proposed in this paper. The main characteristics of these converters will be illustrated through a detailed study for the Boost converter as follows.

2 - TEE BOOST CONVERTER

Fig. 1 shows the circuit diagram for the Boost Converter of the family which is object of study in this work. To emphasize the operating principle of this converter, its working cycle is divided in eighth stages which are shown in figure 2 and described as follows

1st stage [t&] This stage begins when auxiliary switch S2 is turned on at time b. During This stage the resonant inductor current ( I d increases linearly, as it is shown in fig.3, until it reaches the input current ( I, ) at time tl.

0-7803-3073-0/96/$5 .OO @1996 IEEE 5 3 3

2nd stage [tl,t2] When current iLR reaches the current I, diode Do turns off and the main switch voltage becomes zero propitiating good conditions to turn on the switch SI. This stage ends when main switch SI. is turned on. It must be as short as possible to improve the converter efficiency.

Fig. 1- Circuit diagram of the Boost converter.

3rd stage [tz,t3] The switch SI turning on process has a negligible effect to the current flow when inductor LR is considered an ideal element. But this do not really happen and during this stage some energy stored in LR is dissipated on its own resistance, making possible current transference from LR to switch SI. This can prevent voltage V, rise to V, which can provoke hard switching for the main switch. Thus, this time interval must be as short as possible.

4th stage [t&] At time t3, the auxiliary switch is turned off in a ZVS way. During this stage resonance between inductor LR and capacitor CR occurs until the resonant capacitor voltage reaches load voltage Vo.

5th stage [t&] The diode D2 turning on clamps resonant capacitor voltage V, in load voltage Vo. Within this stage resonant inductor current im decreases linearly until it reaches zero, delivering all its stored energy to the load.

6th stage [t&] At this stage the main switch is the unique semiconductor device that is conducing current and the supply current free-wheels at network shown in fig.2 (0.

7th stage [t&] This stage initiates when main switch SI is turned off in a ZCS way. During this stage, resonant capacitor CR discharges linearly, delivering all its stored energy to the load.

8th stage [t&] The tuning on process of diode Do determines the beginning of the last stage at time t7. At this stage power transference from source to load occurs until the

auxiliary switch is turned on again, initiating another operation cycle.

According to the operation principles described above, some important characteristics of this converter can be related as follows.

It can be seen that the turning on and the turning off processes occuf with soft switching for all switches without the inconveniences which were seen in the previous section.

Fig. 3 shows that no device of this converter stays under a voltage greater than load voltage Vo. In addition the maximum current through any component of this converter is not greater than supply current I,. This is an important feature of this converter.

1 L L I- L

(h) Fig. 2 - Equivalent Circuits for different operation stages of the Boost converter.

ILR

f ig . 3 - Theoratical waveforms

Auxiliary switch S 2 is used to guarantee ZVS turning on for main switch. However the turning on of SI can be used to propitiate soft turning off for switch S2. Therefore main switch SI can be considered as an auxiliary switch and vice- versa. In any case, the switch considered as auxiliary switch

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must stay on a short time, thus it will be cheaper than the main switch.

To determine the gain voltage of the this converter, several equations were written based on the following assumptions: 0 Supply current Ii and load voltage Vo are ripple-free. 0 All components and switches are ideal.

vi

Furthermore the following definitions were used:

(1)

1, v,

The final result following equation:

Buck Boost Buck-Boost Cuk SEPIC ZETA Io I, I Ii+b I + I, I + Io v, vo v,+vo v, V,+VO vi+vo

(2)

(3)

was the voltage gain given by the

D= - t6 - t o T

3 - FAMILY OF CONVERTERS

(5)

The Boost converter shown in fig.l was obtained by addition of a resonant network to the conventional Boost PWM converter. this resonant network consist of a auxiliary switch (SZ), a resonant inductor (LR), two diodes and D2) and a resonant capacitor which are connected as follows:

Switch S2 and inductor LR are in series and both are in parallel with main switch SI. Diodes D1 and DZ are in series, forming a branch connected between to a terminal of diode DO and to the common node of the LR and SZ. Capacitor CR is connected to the other terminal of diode Do and common node of the diode D1 and D2. In a similar way all the converters of the family shown in fig.4 can be obtained.

The phase plane for all converters of this new family can be represented by a single diagram as shown in fig.5, where V, and I, are generic parameters which assume different values for the diverse converters as shown in tab. 1.

Tab.1- Values of I, and V, for a11 converters of the family shown inBg.4

Fk. 4 - Famizy of sofi-dching converter with Imu voltage and current stresses.

Rg. 5 - Nodzedphaseplane

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From fig. 4, it can be seen that the phase plane, for this famly of converters, shapes almost llke a square This means the resonance stage is short, contributing for high efficiency of these converters II 1 - SIMULATION RESULTS

t + c t c 300

‘I. . . / \ ’ . . ’ ’ ’ ’ 1 . . ‘

Fig. 7 - experimental waveforms I . Time: 1.25p!Ydiv voltage:20 V/div current: 2A/div

. ,

. , 6 - CONCLUSION 0 :

I .

I . . I

Fig. 6 - Simulation waveform

For additional illustration the Boost converter was studied by simulation where the following parameter set was used.

Vi=lOOV & = 2 0 . 0 0 C ~ = 2 0 n F L p O 3 pH Cf= l o o p Lf=2OOpH Fs=200KHz

Fig. 6 shows the waveforms obtained by this simulation. It can be seen from this figure that the converter operates in soft switching way without high switch voltage and current stresses.

5 - EXPERIMENT& RESULTS

Fig. 7 shows so” important waveforms obtained with the prototype implemented in laboratory for this converter with the following parameter. V,=20V L f = 2 5 0 m L ~ = 0 3 C ~ = 0 9 @ & = 20 C2 Cf = 1OOfl FS = 100 KHz DO,J,~ = U1620 S1,z = IRF640

It can be seen through these waveforms that this converter operates in a soft-switching way with low voltage and w e n t stresses.

As it was expected the experimental results obtained are close to the theoretical results seen before, validating the converter proposed.

A new family of PWM Soft-Switching Converters has been presented. As a conclusion, the following characteristics can be referred to these converters: 0 No device stays under a voltage greater than load or

supply voltage. 0 The maximum current through any component is not

greater than input or load current 0 They maintain their Soft-Switching characteristics in

high frequencies and power. They operate with constant fkequency.

obtained results. The validity of these new converters is confrmed by the

6 - REFERENCES

[ 11 Fred C. Lee, “High Frequency Quasi-Resonant Converter Technologies”, proceedings on the IEEE, vol. 76 n2 4 April 1988.

[2] G. Hua, C. S. Leu and F. C. Lee, “Novel Zero-Voltage- Transition PWM converters”, IEEE - PESC’92, record, pp 55-61.

[3] I. Barbi, J. C. Bolacell, D. C. Martins and F. B. Libano, “Buck Quasi-Resonant Converter Operating at Constant Frequency: Analysis, Design and Experimentation”, IEEE - PESC’89 record, pp 873-880.

[4] L. C. de Freitas, N. P. Filho and V. J. Farias, “A Novel family of DC-DC PWM Converter Using the Self- Resonant Principle”, IEEE-PESC’94, record, pp 1385- 1391.

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