a new soft switching current-fedconverter for high...
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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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