Proceeding of International Conference on Electrical Machines and Systems 2007, Oct. 8---11, Seoul, Korea

A New Soft switching Current-fed Converter forHigh Voltage High Power Applications

Bahador Fani 1, Majid Delshad2 ,Ghazanfar Shahgholian'

'Islarnic Azad University ,Najafabad Branch ,Isfahan, Iran21slamic Azad University, Khorasgan Branch, Isfahan, Iran

delshad@khuisf.ac. ir

Abstract- In this paper a new soft switching current-fedconverter for high voltage high power applications is presented.This converter utilizes parasitic components of high voltagetransformers as resonant elements and employs fixed frequencyphase shift PWM control to implement soft switching condition soimplementation of control circuit is easy and cheap. Every sixswitches turn on at zero current condition (ZCS) and it has highefficiency. The detailed steady state analysis of the converter ispresented by computer simulation and analytical method. Finallythe experimental results certificate analysis and simulation resultsof this converter.

I. INTRODUCTION

Fig. 1. The proposed ZCS-PWM converter

The overlap of S4 with Ss and S6 must be long enough toallow Ss and S6current to reach zero

A. ModeI: to < t ~ t1

Operation begins when Sl,S4,SS,S6 are on. Since resonantcapacitor voltage (VCr) is equal to nV% and is directlyapplied to the resonant inductance (Lr), the current through S5and S6 is reduced linearly and thus S5 and S6 can tum off atZCS. During this mode, energy is transfer to the output. ModeI end when the current in S5 and S6 reaches zero.

iLJ-(to) = I in ( 1)

nViLr (t) = -_o_(t - to) + iLr (to) (2)

ml.;

(4)

(3)

lint1 -to =--

nvO

ml.;

High voltage de-de converters are widely used inelectronically equipments such as x-ray generators, RFgeneration, traveling wave tube, etc. However, the design ofhigh voltage de-de converters is problematic due to large tumratio of the transformer which exacerbates the transformers nonidealistic components. Leakage inductance cause undesirablevoltage spikes and winding capacitance may result in currentspikes and slow rise times. These no idealities can greatlyincrease switching and snubber losses and reduced converterefficiency and reliability [1],[2],[3],[4].

A new soft switching current-fed converter shown in fig 1 isproposed which utilizes parasitic components of the highvoltage transformers as resonant elements. Voltage multiplierin secondary side is used to reduce turns ratio and also peakvoltage on the rectifying diodes. The third leg is placed toreduce the second leg current and thus has same gate signals ofswitches as the third legs. Since this converter operates at fixedfrequency, the implementation of its controller circuit issample. A lead_lag controller is used in the converter. Thisconverter has ten modes of operation during a switchingperiod.

(5)

II. MODES OF OPERATION

The steady state operation of this converter is explainedconsidering all circuit components except the transformer areideal. The primary to secondary turns ratio is defined as nandthe multiplier gain is m.

B. Modell: t1 ~ t < t2

With S. and S4 both on, the input inductor stores energy andno energy is transferred from the input to the load. The desiredenergy transfer from input to output determines the interval ofthis mode.

iLJ-(t) = a

vCI- (t) = ~ vam

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C. Mode III: 32 ttt << Mode III begins at t2 when S2 and S3 is turned on. S1 current

is transferred to S2 and S3, in a resonant fashion. Specifically, by allowing inductor current to resonate to -Iin ,S1 current goes to zero which provides ZCS for S1 and concluding this mode.

))(()( 200

ttSinmZnV

ti OLr −

−= ω (6)

rrr

r

OCr

CLCLZ

ttCosVmnV

1

))((

00

20

==

−=

ω

ω

(7)

Overlap of S1 with S4 must be long enough to allow S1 current to reach zero.

)()(0

0123

Vmn

ZISintt in−=− (8)

D. Mode IV: 43 ttt << During this mode resonant capacitor Cr1 discharges linearly

to –nVo/m and Cr2 charges to + nVo/m.

inLr

Or

inOCr

Iti

ttCosVmntt

CIV

mntV

−=

−+−−=−=

)(

))(()()( 34034 ω (9)

The time interval t4–t3 is obtained as following

in

romI

CosCnVtt

))(1()( 34

γ+=− (10)

E. Mode V: 54 ttt <<

During this mode energy is transferred from input to output. The iLr and VCr equations are given by

oCr

inLr

VmntV

Iti

−=

−=

)(

)( (11)

The converter operation modes VI through X are symmetric with respect to the first five modes as illustrated in figures 2 and 3.

sw1

sw2

sw3

sw4

sw5

sw6

t1 t2 t3 t4 t5 t6 t7 t8 t9 t10 Fig2-Gate signals for inverter switches

t 1 t 4 t 5 t 6 t 9

I o

0

Vcr

iLr

t1 t2 t3 t4 t5 t6 t7 t8 t9 t10

t1 t2 t3 t4 t5 t6 t7 t8 t9 t10

Fig 3-Main theoretical waveform

III. STEADY-STATE ANALYSIS

In the steady-state analysis (previous section), three mode durations (I,III,IV) that are fixed were obtained. Two additional equations are needed to solve for two variable duration modes. The first relation is obtain by averaging the output current and the second relation is obtained by summing the time durations in a half switching period as shown in equations (12) and (13).

2

)()(*5. 4501

S

ininO T

ttnInIttI

−+−= (12)

)45()34()23()12()01(2

ttttttttttST−+−+−+−+−= (13)

The steady state control curves of figure 4 is obtained by MATLAB using the following definitions

O

Sn

loadn

in

On f

ffZ

RQmVVM ===

0 (14)

Fig 4-Steady-state control curves, M versus B.

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The curves are presented to show the qualitative relationships between load, gain, switching and resonant frequencies. Increasing B corresponds to a gain increase and is analogous to increasing D in a boost converter. Also, it is clear that an decrease in load results in load regulation range decrease for constant fs and Z0. Notice that this decrease is nonlinear (load doubling yields maximum gain decrease from 80 to 50). A more significant decrease in regulation range comes from reducing frequency resonant.

IV. MULTIPLIER VOLTAGE CIRCUIT

The high frequency inverter-fed multistage voltage multiplier is only composed of diodes and capacitors [6]. The multiplier also provides rectification and produces a high dc voltage from a high frequency AC voltage source. In comparison to a simple rectifier, the introduction of this multiplier would greatly reduce the number of secondary turns causing reduction in leakage inductance, stray capacitance and diode voltage ratings. The multiplier is a symmetrical Cockroft Walton for which the output voltage (VO), voltage drop )( V∆ and ripple )( Vδ are given by

max.2 VnVo = (15)

)31

41

61.(

.23 nnn

cfiV L ++=∆ (16)

2.

.n

cfiV l=δ (17)

V. DESIGN EXAMPLE AND SIMULATION RESULTS

For a typical TWT load the converter input voltage can be considered 800V and output voltage 50KV. From the developed equations the following parameter value are evaluated.

FCrKHzfnFCn

HLrnmHL

SO

in

µ

µ

102010061

50615

2

1

====

===(18)

The following figures refer to main waveforms of converter

Fig5-Converter waveforms are output voltage ripple, resonant capacitor voltage, and resonant inductor current respectively

Fig6-These figures are input current ripple, current and voltage of S4, current and voltage of S5 respectively

such as VO, iLr, VCr1, ilin ,… As illustrated in this figures S3

and S4 switches turn on and turn off at ZCS.

VI. EXPERIMENTAL RESAULTS

To certificate the simulation results and soft switching in all switches an experimental example implemented so that it’s results are shown in blow figures. figure 7 shows the soft switching in S4 and figure 8 shows the waveform of resonant voltage capacitor. figure 9 is the waveform of input inductance current .

As seen, experimental results of converter very close to simulation results

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Fig 9 - The waveform of input inductance current

REFERENCES [1] S .D Johnson “Comparison of Resonant Topology in High Voltage

DC Application”IEEE Trans on Aerospace and Electronic Systems pp263-274 May1988

[2] B.S.Jacobson and R.A.Dipema,”Fixed Frequency Resonant Converter for High Voltage High Density Applications” PESC94pp357-363

[3] J.A.Pamilio and J.B.Pagan ,”Resonant High Voltage Source Working at Resonance for Pulse Laser,”PESC96,pp1627-1635

[4] V.Garia ,”An Optimized DC TO DC Converter Topology for High Voltage Pulse Load Application,”PESC94,pp1413-1421

[5] C. Iannello, S. Luo, and I. Batarseh, “A full bridge ZCS PWM converter For high voltage, high power applications,” in Proc. IEEE PESC’00 Conf., 2000, p. 28_5.

[6] J.Sun,M.Nakaoka ,”Series Resonant High Voltage PFM DC_DC Converter with Voltage Multiplier based A Two Step Frequency Switching Control for Medical Use Xray Power Generator,”PIEMC2000 pp596-601

Fig7-The soft switching in S4

Fig 8 - The waveform of resonant voltage capacitor

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