A FAMILY OF PWM SOFT-SINGLE-SWITCHED CONVERTERS WITH LOW

VOLTAGE AND CURRENT STRESSES

L. R. Barbosa J. B. Vieira Jr. L. C. de Freitas V. J. Farias

Universidade Federal de Uberliindia

Departamento de Engenharia Eletrica

Campus Santa Manica -Bloc0 E

3 8400-902-Uberl2nd1a-MG-BRAZIL

Abstract - The use of switching regulator circuits at high power levels and high frequencies requires careful examination and control of the power dissipation during switching. In this paper it is shown a family of PWM soft-single-switched converters which, having only a single active switch, is able to operate with soft switching in a pulse width modulation way without high voltage and current stresses. In addition such converters can work at high switching frequencies for wide range of load. In order to illustrate the operating principle of these new converters a detailed study, including simulations and experimental test is carried out. The validity of these new converters is guaranteed by the obtained results.

I. INTRODUCTION

High switchmg 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 (QRCs) were proposed in [l]. However, some of their characteristics such as load limitations and control difficulties due to variable frequency operation restrict the practical use of these converters.

Since the pulse-width-modulated quasi resonant converters (PWM-QRC)operate with fixed switclung frequency they do not present the control problem llke the QRCs [2]. On the other hand they present all the other disadvantages of the QRCs which limits their applications.

converters whch does not present the limitations described above. An example of such converters is found in [3]. Although this converter presents several advantages, its main switch turns off hard and has hgh current stress.

The most part of the PWM soft switching converters proposed until now have two active switches and one that has

Nowadays there are many

a single active switch, as described in (41, presents h g h stresses.

To overcome ths draw-back, the converter in [5] was proposed. In ths paper it will be shown additional aspects of that converter and a converter family derived of it.

IT. BOOST PWM SSS CONVERTER

Fig. 1 shows the Boost PWM soft-single-switched converter of the proposed family. The inductor L R ~ is used to provide ZCS turning on of the switch Sl. L2 is magnetically coupled with L I . The turn ratio between L1 and L2 must be large enough to induce voltage in L2, makmg null the current of Lm before the next turning on of the switch SI , when D2 , D3 and D4 are simultaneously conducting.

I I I I I I-

Fig. 1 - Boost PWM soft-single-switched converter

The branch composed by LRI, D1 and C, is used to charge CR with voltage V, before the switch SI is turned off. Thus, ths switch will turn off in a ZVS way.

The operating stages are presented in Fig. 2, and they are described as follows.

First Stage [to, tl] -

load occurs through diode Dz. During thls stage transference of energy from source to

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First Stage [to , t,]

Second Stage [ti , t7.1

Sixth Stage [ts , f6]

$1 I r l I

Thud Stage [tz , t3]

Fourth Stage [t3, f4]

Eighth Stage [t7 , t ~ ]

Fig. 2 - Stages of operation ofthe Boost PWhf soft-single-switched Converter.

Second Stage (tl, tz/ - When switch SI is turned on, the L R ~ current increases

linearly and CR begins to charge through the circuit with V, , LRI , DI , CR and SI.

Third Stage [t2, t3] - This stage begins when diode D2 is turned off. At t h s

moment L2 current becomes equal to LI current, which is given by the expression IJ(1-K) where I , is the current that was in LI before SI is turned on and “K” is the turn ratio between LI and L2 . We can observe that the current in the switch SI increases with “K”. Thus its value must be chosen as small as possible, in order to avoid hgh SI current stress.

Fourth Stage (f3. t4] -

when the voltage in CR becomes equal to output voltage. This stage begms when &ode Ds is turned on. Ths occurs

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In this stage the LRI current decreases linearly until it becomes zero.

Fi$h Stage [ t 4 , ts7 - At time t4 , the current through LR1 becomes null and the

diodes DI and D3 are turned off. The duration of this stage depends on the duty cycle.

Sixth Stage [tj , ts] - The switch SI is turned off with null voltage and LI . LZ ,

C, and D3. conduct. In this stage the capacitor discharges in a linear way.

Seventh Stage (ts , t,] - When the capacitor C, voltage becomes smaller than

induced voltage in the L2, &ode D2 is turned on. In this stage the induced voltage is fixed at K (v,-VJ and L R ~ current goes down due to the resonance between CR and L R ~ . .

Eighth Stage ( t 7 , ta] - When the capacitor CR voltage becomes zero, &ode 0 4 is

turned on. The LR2 current goes down linearly due to induced voltage in Lz.

This stage finishes when the LR2 current becomes zero, starting another switching cycle.

The main theoretical waveforms are shown in Fig. 3.

i t . .

Fig. 3 - Main waveforms for circuit in Fig. 1

The resonant current between LRI and C, can be reduced by increasing value of LRI, without &sturbing the other circuit parameters. Since the branch composed by LRI and CR does not belong to the principal branch, capacitor CR can be charged during all interval that the switch Sl is conducing.

Fig. 4 shows the state plane for PWM soft-single-switched converter. It is observed that the maximum value of the CR voltage is equal to the output voltage V,, which is reached at the begtnning of the fourth stage / t3 , t4].

~

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Since the capacitor charges due to resonance between LR] and CR, the maximum voltage which appears in the CR terminals is twice the input voltage V,. This limits the output voltage at that same value, if soft switching is desired.

V O

Fig. 4 - State plane for PWM soft-singlsswk&ed converter

Fig. 5 shows four dfferent regions of the plane (a x G). In the regions 3 and 4, the converter operates hard and in the other two regons it works with soft switchmg. In the regions 2 and 3 the LRz current becomes zero before the resonant capacitor discharge completely, as indicated at t7 in Fig. 3. Despite, the converter operates soft in the region 2

"2 t

G

Fig. 5 - Converter operatingregions-

In. CIRCUIT ANALYSIS

Fig, 6 presents the model used for static gain determination.

s,

Fig. 6 - Boost converter model for static gain determination.

To determine the converter static gain of the PWM Boost converter, presented in this paper, the following considerations have been made:

- the current 1, and voltage Vu are ripple-free. - all the components and switches are ideal.

Furthermore the following definitions were used:

where

Using these definitions and several equations which was developed for all stages, we can determine the static gain, as given by the following equation.

I + A I - D + B

G =

Where:

K L l K A = - +- ( I -K) T w o

-a K K U - K ) 4. - + -U

(1-K)' [ (1-K) + $1 2aG a

( 5 )

G(I - K)2 2a

- tX

(fs - tl) D = T

T = t, - t o

From equation ( 5 ) it i!; observed that the converter static gain depends on: the tun1 ratio (K) between LI and L2 , the duty-cycle (D), resonant frequency (a,,), the normalized current (a) and the switching period.

Fig. 7 shows how the voltage gain varies with normalized current (a) for five different values of duty cycle. As it can be seen, t h s converter, except for light loads, behaves almost like the conventional boost converter.

G

-1-

0.15 0.2s 0.35 0.45 0.55 0.m 0.75 0.85 0.95

Fig. 7 - Curves of static gain characteristic vs normalized input CUt-rent, when duty cycle varied.

a

IV. A FAMILY OF PWM SSS CONVERTERS

As it can be seen, the Boost PWM-SSS converter shown in fig. 1 is consisted of the ccinventional Boost PWM converter added by the resonant nelwork composed by the elements LR2, L2, D, , D3 and D4 which are connected as shown in fig. 1.

The other converters of the family which is being proposed are obtained in similar way. as shown in fig. 8.

As it well known, all converters of a family have state planes with the same shape and voltage gains which is easily derived from each other.

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~.

rl

LI - n2 -. L1

i ~ D 3

... 1.558~4 1.56ms 1 .562~1; 1.56% 1.5LLns 1 . 5 6 8 s 1.57ms

3 1 f) T i n e VI Fig. 9 - Simulation results for the PWM soft-single-switched converter.

VO

t As it is observed from Fig. 9, SI operates under soft

switching condtions. It is turned on under zero-current and turned off with zero-voltage. The maximum voltage in this switch is equal to the output voltage, which must be lower than twice the input voltage value.

Rl

VI. EXPERIMENTAL, RESULTS

v o V. SIMULATION RESULTS t

In order to illustrate the feasibility and operation of the

A prototype circuit was constructed to verify the waveforms predcted above.

VI +Gf D,

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R, converters of the proposed family, the boost converter shown in Fig. 1, has been simulated with the following parameters:

==c 1

L3 LR2 = 2 pH (resonant inductor 2) C1 = 330 pF (output filter capacitor)

L, L1 3 ,, D4

C1 RI C, = 30 nF (resonant capacitor) == RI = 100 S2 (load resistance)

t

The values of Ll, L2, LRI, LR2* C1, CR, and Rl are the sane speclfied in simulation. The switchmg frequency is 100 KHi. The switch used was the MOSFET IRF640, and the &odes D,, Dz,, D3 and D4 were the MUR 1620.

Fig. 10 shows the waveforms of voltage in the resonant capacitor CR and the current in the resonant inductor LRI. As seen in this photograph, the voltage reaches 58 Volts, that is almost the double of input voltage.

. . . . . . . . . . . . . .

. . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . .

VCR 2 OVId iv

i mi 1 Ndiv

20.8 V

Fig. 10 - Wavefom of voltage in the resonant capacitor CR (upper trace) and current in the resonant inductor LRI (lower trace), respectively VCR (2OV/div)

and im (INdiv), 2 peddiv.

Fig. 11 shows again the waveforms of voltage in the CR, together with the current in the resonant capacitor

resonant inductor L R ~ . .

VCR

2 W)d iv

. . . . . . . . $ “ ” ] . . . . : . . . . : . . . . : . . . . : ............................ . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . Ch1 20 V 5 v

Fig 11- Waveform of voltage in the resonant capacitor CR (upper trace) and current in the resonant inductor LRZ (lower trace), respectively vat (20V/div)

and i- (1 Ndiv); 2 peddiv.

Fig. 12 shows the switch waveforms for voltage and current. As it is observed, the switch is turned on under zero-current and turned off with zero-voltage.

Fig. 13 shows the efficiency versus output power. The measured converter efficiency is between 92% and 97%.

mF., ~, ... I , [ * F . I .... f “ ,~

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

VS PDVIdiv

E.. . _ f . . . . . . . . . f . . . . . . . . . I... . I _ . . . f . . . . t . . . . f . . .

is . . . . Pwdiv

Fig. 12- Voltage (uppertrace) and current (lowertrace) waveforms in the switch, respectively VS (2:0V/div) and is (2Ndiv); 2 peddiv.

Efficiency [%]

’:E===-

0 0 36 50 62 75 86 92 97 106

P, Wl Fig. 13 -Experimental: efficiency versus output power

VII. CONCLUSION

A new Family of PWM Soft -Single- Switched Converter has been presented . These converters with only a single switch can operate in a scft switching way in a wide load range without high voltage and current stresses.

The experimental result,s obtained validate the proposed converter family.

ACKNOVdL,EDGEMENT

The authors gratefully aicknowledge ‘MORTON INPEC for the ferrite cores support.

REFIZRENCES

[ 11 LEE, F.C. “High Frequency ‘Quasi-Resonant Converter Technologies”, proceedmg of the BEE, vol. 76 N. 4, April 1988.

[Z] BARBI, I.; BOLACEL, J.C.; MARTINS D.C.; LIBANO F.B. “Buck Quasi-Resonant Converter Operating at Constant Frequency: Analysis, Desi@ and Experimentation”, IEEE-PESC ’89, record, pp. 873-880.

[3] HUA G.; LEU C.S.; LEE F.C. “Novel Zero-Voltage Transition PWM Converter”, IEEE-PESC ’92, Ramrd, pp. 55-61.

[4] VILELA, M.;COELHO, E.; VIEIRA, J.; FREITAS, L.C.; FARIAS, V. “A family of PWM Soil-Switching Converters with low stresses of Voltage and Current”, IEEE-AFEC’96, rewr’d, pp 299-304.

[ 5 ] LAMBERT, J.; VIEIRA, J.; FREITAS, L.C.; VILELA, M. and FARIAS, V. “A Boost PWM Sofi-Singlt>S~itched Con~&er Without Stresse~ of Voltage and Current” IEEE-APEC ‘96, record, pp 469-474.

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