Researches of Transmission Line Transformer Application in High-voltage Pulse Source

8/12/2019 Researches of Transmission Line Transformer Application in High-voltage Pulse Source

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Researches of Transmission Line TransformerApplication in High-voltage Pulse Source

WeiDong ZhaoInstitute of Electrical Engineering and Information

AnHui University of TechnologyMaanshan 243000, China

e-mail: [email protected]

Yong HuInstitute of Electrica Engineering and Information

AnHui University of TechnologyMaanshan 243000, China

e-mail: [email protected]

Abstract -Transmission line transformer (TLT) is a newtype of pulse transformer developed by combining theory oftransmission lines and principle of transformer, its biggestfeature is good high-frequency characteristic, it can reduce theeffects of the distributed inductance and the distributedcapacitance of the traditional transformer on the transmissionof the pulse better. This paper introduces the transmission linetransformer theoretical model and its basic principles, theconditions of the best match in the actual project and theactual situation in the waveform distortion caused byimpedance mismatch based on transmission line theory andwave catadioptric principle. The transmission line transformer

made use of the coaxial cable and the high-permeabilityFe-based amorphous magnetic material were put forward,which was apply to the output transform devices and therelated high voltage pulse experiment was carried. The resultsshow that the transmission line transformer has good responsefor the output pulse, the improvement of the rising time, thelower energy consumption and the test result is consistent withthe theoretical calculation result.

Key words Transmission Line Transformer(TLT);Transmission Line Theory; Impedance Match;Inductive-adder

I. I NTRODUCTIONPulsed power technology is by saving energy in a long

time and fast compression, then conversion, and finally theinstantaneous release of tremendous power to the load.Therefore, the application of pulsed power technology cansupply very large instantaneous power and mainly used inhigh-tech and defense industries [1,2]. How to improve the

pulse voltage and make the pulse front steeper is one of thedevelopments of modern pulsed power technology. Thewithstand voltage of the switching devices will be a

parameter limiting to improve the output voltage of the pulse source in practical applications, etc. The output of the pulse source usually uses two ways, superimposing inductedvoltage and boosting by common pulse transformer, to raisethe output voltage. Although the superimposed inducedvoltage can effectively overcome the pulse transmission

distortion because of the multi-module stack and 1:1 singleturn structure of the output transformer, its complexstructure with synchronous control system , and requiringgood consistency , therefore less used [3,4] . The traditional

pulse transformer makes use of the principle of fluxcoupling of energy delivery between primary and secondaryside, which has simple structure [5]. The high-frequencycomponent of the nanosecond level fast rising edge of pulse(the front) is very rich and possibly forms the LC oscillation

by leakage inductance exists in the coils and straycapacitance between turns, resulting in the distortion ofoutput pulse. Transmission line transformer (TLT) transfersenergy in different ways and has good frequency responsefor the high frequency pulse signal because its distributiveinductance and stray capacitance is uniformly distributed onthe whole line so that LC oscillation circuit does not exist[5-7]. Based on the above reasons, we research output oftransmission line transformer formed by a coaxial cable.This paper designed a transmission line transformer and itsapplication in high-voltage pulse source in order to analyzethe principle of transmission line transformer and therelationship between matching and the frequency response.

. T RANSMISSION LINE TRANSFORMER

Rise time and bandwidth of the traditional pulsetransformer are influenced by the transformer leakageinductance and stray capacitance between the windings. It isdifficult to transmit pulse without distortion when at highfrequency. Compared with ordinary transformer,transmission line transformer transmits energy in differentways, as shown in Fig.1. The distributed inductance andstray capacitance between wires is uniformly distributed onthe whole line. At high frequency, when inputting the signal

in the ports 1 and 2, the capacitance was delayed charging because the inductance value of the unit length varies withthe frequency increases. According to the transmission linetheory, the energy is transmitted through charging anddischarging of stray capacitance and distributed inductanceof the line.

Fig.1Transmission Line Equivalent Circuit

Therefore, a transmission line transformer is a

combination of a lumped parameter transformer and aimpedance converter of transmission line, which has simplestructure, wide frequency band, and the high frequencycharacteristics [8,9].Therefore, it is appropriately to selecttransmission line transformer to transmit pulse in order tomake the output pulse waveform no distortion and toimprove the output voltage in the small and medium-sized

pulse power source. The basic working principle oftransmission line transformer has no difference, though itcan be designed for the desired impedance transformation

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ratio according to the different topological.Fig.2 is transmission line transformer equivalent circuit

with the impedance transformation ratio of 1:4, inner core1-4 and the outer shielding line (ground) 2-3 of the coaxialtransmission line form double-line winding on the magneticcore, and ports 1 and 3 short circuit, primary for line 2-3,secondary for line 2-3 and 1-4. Since line 2-3 is not only a

part of the primary, but also a part of the secondary, and theline 2-3 is always closely with line 1-4 of the secondary.Therefore , the magnetic flux generated by the current of the

primary line 2-3 will be substantially through the secondaryloop , i.e. the leakage flux of the transformer isapproximately zero so that the leakage inductance can beignored . If the transmission line is a coaxial line , K , thecurrent coupling coefficient between the inner and the outerconductor, is equal to 1 , i.e. K = 1 , in that case, the leakageinductance of the coaxial transmission line transformer iszero, and the conclusion is strict .

Fig.2 1:4 Transmission Line Transformer Equivalent Circuit

A. Transmission Line Equation

According to Fig.2, combined with the theory oftransmission line, we can get the following equations:

1 2 1 0 0 2 0 0( )cos sinV V V l L C jZI l L C = + (1)2 1

1 2 0 0 0 0( )

cos sinV V

I I l L C j l L C Z

= +

(2)1021 )( V Z I I V g ++= (3)

2 2 LV I R= (4)l , 0 L , 0C , are respectively for the electrical length,

inductance per unit length and capacitor of transmission line,0 Z is impedance of pulse generator and Z is characteristic

impedance.Solution of the equation group, it yields

0 02

00 0 0 0 0 0 0

(1 cos )

cos 2 (1 cos ) ( )sin

g

L L

V l L C I

Z R R l L C Z l L C j Z l L C

Z

+=

+ + + +

(5)Output of power source is

2 22 0 0

22 2 20

0 0 0 0 0 0 0

(1 cos )

[ cos 2 (1 cos )] ( ) sin

g Lout L

L L

V R l LC P I R

R Z R l LC Z l LC Z l LC

Z

+= =

+ + + +

(6)The formula (5) and (6) indicates that the transmission

coefficient and efficiency of the transmission line are bothrelated to frequency, the characteristic impedance oftransmission line, load impedance and impedance of pulsesource. Therefore, the pass band and the matching betweenthe internal resistance of the pulse source and loadimpedance should be considered in order to obtain themaximum transmission efficiency.

In the low frequency

0 0 1l L C


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thyratron . Obviously, the dashed box portion constitutes the pulse source and its internal resistance (impedance), thecircuitry for CLR circuit, its impedance should be constitute

by the characteristic impedance of the LC circuit Z 0 and R g .

C L

Z =0 9

Fig.4 Schematic Diagram of the Tran smission Process

Since the turn-on resistance of hydrogen thyratron is verysmall, so it is appropriate that pulse source internalresistance (impedance) is replaced by 0 Z L C = whileignoring line resistance, and impedance matching oftransmission line transformer and pulse source only needs tomatch characteristic impedance. According to formula (9)we can obtain a matching by adjusting the magnitude of theL and C. We always minimize L in order to reduce the pulsefront. Therefore, it is feasible to reduce storage capacitorand increase the impedance of the pulse source to matchtransmission line transformer while the energy storage isenough.

Impedance of TLT is determined by both characteristicimpedance and magnetic core properties. The direct load of

pulse source is a plate-type bounded-wave simulator endedwith an absorption resistance. Usually, the absorptionresistance is selected to match the simulator impedance,which is given by

W H

Z 2

ln1203 = (10)

H and W are, respectively, the height and the width of thesimulator. Thus height H and width W can be tuned tomatch the impedance of TLT.

Next, we will analyze waveform distortion caused byimpedance mismatch. Take Z (TLT section) and Z1(simulator section) for example as seen in Fig.4, forsimplicity, we only concern the rise edge. Assuming that the

pulse waveform is an infinite bevel flat-roofed wave withlinear rising edge and is shown in Fig.5:

Fig.5 Pulse Transmission Process

Fig.6 Schematic Diagram of the Voltage on B

Assuming that (1) the wave enters into point A at t=0

with gradient at leading edge and speed of v , (2)theeffective distance from terminal of TLT to simulator's input port is 1l , (3) the amplitude of the bevel flat-roofed wave isU0, and (4) the wave arrives at point B after 1l v = ,reaches its maximum at /01 U T = , the voltage at point B(leading edge of the simulator) can then be expressed as

021

20

2U

Z Z Z

U U B +== (11)

And the amplitude of reflection wave can be written as

00 U U U f = (12)At 1 12 l v = , the reflected wave arrives at point B, adds

with the input wave, which is repeated. Distortion willemerge with mismatching impedance from Fig.6. Distortionis not included in Fig.6 for clearly visualization. Actually,the effective distance 1l is very small in transmission

process. As a result, the voltage adding steps in Fig.6 shouldhave shrunk to an infection point.

C. The Analysis of the Pulse Response of Magnetic CoreMaterial

The lowest high-order mode cutoff frequency of thetransmission line is

22

11

2

1ba

f +

13

a , b , and represent respectively an inner andouter radius for the transmission lines, dielectric constantand permeability. The formula (13) shows the frequency

band of the transmission line transformer in the lowfrequency band is cut. Similarly, the cutoff frequency can bechanged by changing a , b , and . Considering

the economic cost, changing the permeability

is oftenused as a preferred approach among these parameters, so inorder to improve low frequency performance of thetransmission line transformer. In full consideration of highfrequency transmission we use the high permeability ofFe-based crystal magnetic material and take

0 0 L L Z R Z R Z = = = as the best transmission matching.Magnetic loop ,which is made of iron-based amorphousalloy, has high-saturation magnetic induction,high-frequency response and fast pulse responsecharacteristics, excellent magnetic properties, small corelosses, good temperature stability and oil resistance, andmany other fine features, they have been widely used in

pulse transformers, magnetic switches and linear

transformers (LTD) pulsed power technology field[ 10-12 ] .

. HIGH VOLTAGE PULSE SOURCE

Fig.7 is high voltage pulse source system, principle ofwork of the entire electric circuit is: high voltage DC powersupply charges the capacitor C through the LC fundamentalseries resonance circuit. Then we can produce a pulsevoltage by controlling to switch the hydrogen thyratron onand off after the capacitor C being charged fully. Finally,the pulse voltage is output through the transmission linetransformer. The hydrogen thyratron trigger signal is

produced by the DSP to control the turn-on or turn-offthrough the drive and control circuits. The biggest constraintof the using of hydrogen thyratron in high voltage pulsesource is the output power and voltage, the currentinternational mature products of its maximum output powerand voltage can amount to 10GW/100kV which can meetthe high power pulse output needs, but at the risk of high

price. Domestic hydrogen thyratron technology at 35kV and below is more mature , stable performance , the repetitionfrequency of up to 200Hz , and relatively inexpensive.

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Therefore, the system selects 35kV hydrogen thyratron as aswitching device and uses the coaxial structure of thetransmission line transformer to improve the output voltage.

Fig.7 Schematic Diagram of the System of High Voltage Pulse Source

. EXPERIMENT AND ANANLYSIS

The load used in the experiment is parallel-plate boundedwave simulator of which the impedance is approximately75 , followed by a resistance of solution of CuSO4 about100(18 oC, under DC conditions checksum ), a pulsesource voltage is approximately 12.5kV . The outputvoltage was measured with a Tektronix P6015A voltage

probe, in which the intrinsic standoff ratio is 1:1000, andthen decayed 10 times through DS1052E oscilloscope.Cable, the special power grounding coaxial cable filled withnylon, is selected and its impedance and withstand voltageare, respectively, 50 and 50kV. The experiment with twoseries of inductive-adder was shown in Fig.8 (a), it can beseen that the leading edge of the output pulse more slowly,about 40ns. The output amplitude of the pulse should betwice of the working voltage without consideration of theloss of the line itself, and the measured voltage is 22kV,which dropped 3kV.

Fig.8 (b) showed the output pulse waveform of TLT, itcan be seen that the output pulse has a very fast fall-time of18ns and the measured amplitude is 24kV only dropped1kV, basically match with formula (9), and the drops may

be caused by circuit losses itself. Moreover, the footstep inleading edge of TLT may be caused by the impedancemismatching between the output impedance of pulse sourceand TLT characteristic impedance or the output straycapacitance and distribution inductance.

In order to get a higher output voltage, we can increasethe TLTs series and select the 1:N2 impedancetransformation ratio mode. To obtain the higher voltagetransformation ratio and better output waveform bychanging the pulse generator, impedance of parallel plate

bounded wave simulator and resistance of absorption.

(a)

b

Fig. 8 (a) Superimposed Output Pulse Waveform

(b) Output Pulse Waveform of Transmission Line Transformer

. C ONCLUSIONGH

This paper analyzes the transmission characteristics of thetransmission line transformer and the catadioptric process ofthe traveling wave, and carries out comparative experimentsof a conventional transformer inductive-addr and the outputof the transmission line transformer. The leading edge of theoutput waveform the conventional transformer is distorteddue to the presence of the leakage inductance anddistributed capacitance. Transmission line transformer is theuse of these parameters to achieve the principles of thetransmission line and the transformer principle combined,so that the output waveform and amplitude was significantly

better than the traditional transformer inductive-adderoutput mode. The results of experiment are basicallyconsistent with the theoretical analysis.R EFERENCES[1] Liu Xisan. High power pulse technology, Bei Jing National defenseindustry press, 2005[2] Han Min. Foundations of the Pulse Power Techniques, Bei J ing ThePress of Tsinghua University, 2010[3] Meng Zhipeng, Yang Shi, Yang Hanwu, et al. Response characteristics

of induction voltage adder module to square pulse. High Power La ser andParticle Beams, 2010, 22 (8): 1949-1953[4] Zhang Huang, Xia Liansheng, Li Jin. Travel wave analysis andsimulation of induction voltage adder. High Power Laser and ParticleBeams, 2007, 19 (3): 495-499[5] Wang Songsong. Investigation of Transmission Line pulse Transformer,Changsha: National University of Defense Technology, 2008[6] Rossi J 0, Smith P W. THE FREQUENCY RESPONSECHARACTERISTICS OF TRANSMISSION LINE TRANSFORMERS[J]. UK: London, 1996.[7] Pablo Gomez-Jimenez, Pablo Otero, Member, et al . Analysis andDesign Procedure of Transmission-Line Transformers [J]. IEEETRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES,2008, 56(1): 163-171.[8] Zhang Jigang. Rf ferrite broadband devices. Beijing: Science Press,1986[9] C. Strowitzki, A. Gortler and M. Baumann. Losses in MagneticSwitches [C]. Power Modulator Symposium. 2002: 214-216.[10] P. Kwapulinski, J. Rasek, Z. Stoklosa, et al. Magnetic Properties ofAmorphous and Nanocrystalline Alloys Based on Iron [J]. Journal ofMaterials Processing Technology. 2004: 735-742.[11] Meng Zhipeng , QIAN Bao-liang , Yang Hanwu , linear responsecharacteristics of the pulse transformer module [ J ] . High Power Laserand Particle Beams, 2009, 21 ( 10 ) : 1566-1570 .[12] Qiu Jian. A Thesis Submitted in Partial Fulfillment of theRequirements for the Degree of Master of Engineering, Wuhan: HuazhongUniversity of Science and Technology, 2007.

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Researches of Transmission Line Transformer Application in High-voltage Pulse Source