A FAMILY OF PWM SOFT-SWITCHING CONVERTERS WITH LOW STRESSES OF VOLTAGE AND CURRENT

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

Universidade Federal de Uberlilndia

Departamento de Engenharia Eletrica

Campus Santa M6nica - blow E

38400-902 - Uberldndia - MG - Brasil

ABSTRACT This paper presents a new family of PWM

Sojl-Switching Converters which can operate without switch voltage and current stresses in high switching frequency for a wide range of load

To illustrate the operating principle of this new

converters a detailed study including simulations and

experimental tests 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, this

yields high switching losses and consequently, low efficiency

in hard switching converters. To solve this problem the

quasi-resonant converters (QRC) were proposed in [ 11.

However, some of their characteristics such as load limitations control difficulties due to variable frequency operation, and high switch voltage and current stresses

restrict the practical use of these converters.

Since the Pulse -Width- Modulated Quasi-Resonant Converters (PWM-QRC) proposed in [3] operate with fixed switching frequency they do not present the control problem

as the QRC’s. On the other hand they present all the

disadvantages of the QRC’s which limit their applications.

Nowadays there are several converters, such as those present 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 half.

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 - OPERATING PRINCIPLES OF THE 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.

0-7803-3044-7196 $5.00 0 1996 IEEE 299

Vi T vo

Fig. I - Circuit diagram of the Boost converter.

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

2nd stage [t&] 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.

9

2 I I I (h)

Rg. 2 - Equivalent circuirs for dyferent operation stages of

?he Boost converter.

3rd stage [t&J The switch SI turning on process has a negligible &ect 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 S1. This can prevent voltage V, rise to

300 1

Vo, which can provoke hard switching for the main switch.

Thus, this time interval must be as short as possible. 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 (f).

Fig. 3 - Theoretical waveforms

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 iLR decreases linearly until it

reaches zero, delivering all its stored energy to the load.

7th stage [t69t7j 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 [t7,ts] 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 occur 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.

Auxiliary switch S2 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 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 foiloMing

assumptions:

30 1

Supply current Ii and load voltage Vo are ripple-free.

All components and switches are ideal.

Furthermore the following definitions were used:

(3)

The final result was the voltage gain given by the following equation:

V" 1 _-

1- D+- -a+- ( v , - r 1 T a ,

Where:

(4)

3 - FAMILY OF CONVERTER

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 (S2), a resonant inductor (LR), two diodes (D1 and D2)

, . -

f ig . 4 - Fmnicy of so#-&ching converter with low voltage and a resonant capacitor which are connected as follows: and current stresses,

302

4 - SIMULATION RESULTS

Rg. 5 - Normalized phase plane

Switch Sz and inductor LR are in series and both are in

parallel with main switch SI. Diodes DI and D2 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 DI and D2. In a similar way all the

converters of the family shown in fig.4 can be obtained.

The phase of 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.

1 ! vi+ Io vo I I

Tab.1 - Values of I, and V, all converters of the family shown infig.4

From fig. 4, it can be seen that the phase plane, for this

family of converters, shapes almost like a square . This

means the resonance stage is short, contributing for high

300+ ....'..........'..........+..........+..........t................ +

' , I ' I

, I

8 ) , I

I . . I

I . . I

1oo+----+----------i----------*----------+.----------'----------~---- .?"U, . .

. a I .

. I

Fig. 5 - Simulation waveforms

For additional illustration the Boost converter was studied

by simulation where the following parameter set was used.

Fig. 5 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 efficiency of these converters stresses.

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5 - EXPERJMENTAL RESULTS 6 - CONCLUSION

A new family of PWM Soft-Switching Converters has

been presented As a conclusion, the following

characteristics ca.n be referred to these converters:

0 No device stays under a voltage greater than load or

supply x70ltage.

0 The maximum current through any component is not

greater than input or load current

0 They maintain their Soft-Switching characteristics in

6 - REJRERENCES

Fig. 6 - experimental waveforms Time: 1.25pS/di~ volrage:20 Vdiv current: W d i v [I] Fred c. Lee, “E& Frequency Quasi-Remnant Converter

Technologies”, proceedings on the IEEE, vol. 76 ne 4

A@ 1988.

[2] G. HIM, C. S . L ~ U and F. C . ~ e e , “Novel Zero-Voltage-

Transition PWM converters”, IEEE - PESC’92, record,

Fig. 6 shows some important waveforms obtained with the

prototype implemented in laboratory for this converter with the following parameter.

Vi=ZOV Lfz25OpH L ~ = 0 3 f l C ~ = 0 9 @ &= pp55-61. -~

2o cf = look@ Fs = loo = [3] I. Barbi, J. C. Bolacell, D. C. Martins and F. B. Libano,

S1.2 E IRF640 “Buck Quasi-Resonant Converter Operating at Constant Frequency: Analysis, Design and Experimentation”, IEEE - PESC’89 record, pp 873-880.

It can be seen through these waveforms that this

converter operates in a soft-switching way with low voltage

and current stresses. [41 L. C. de Freitas, N. P. Filho and V. J. Farias, “A Novel As it was expected the experimental results obtained are

family of DC-DC PWM Converter Using the Self- Resonant Principle”, EEE-PESC’94, record, pp 1385- 1391.

close to the theoretical results seen before, validating the converter proposed.

304