A Novel All Solid-State Sub-Microsecond Pulse Generator for

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IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 41, NO. 3, MARCH 2013

A Novel All Solid-State Sub-Microsecond Pulse Generator for Dielectric Barrier Discharges Junfeng Rao, Kefu Liu, and Jian Qiu

Abstract—This paper presents the design for a novel all solidstate sub-microsecond pulse generator for dielectric barrier discharges (DBDs). This generator consists of a MARX generator, Blumlein transmission lines (BTLs) and one magnetic switch (MS). As a power supply, the all solid-state MARX generator is capable of outputting nanosecond-pulses with a voltage peak of up to 20 KV. The key elements are BTLs which are charged by the MARX generator. Due to the unmatched impedance of DBD load, energy stored in BTLs begins to oscillate through DBD load after the MS turns on. The violent oscillation lasts until all the energy is consumed. During the violent oscillation, over ten discharges are excited in 5 μs under a single-shot condition. Thus, extremely intense plasma can be produced due to the accumulation effect. The alternating-current decaying voltage over the MS has a demagnetization effect, and DC reset circuit can therefore be spared. Experiments with matched resistor load were also carried out, and rectangular pulses with voltage up to 20 kV and duration of 220 ns were obtained. The ratio of the energy consumed by the resistor from the energy stored in the BTLs is 84.9%. The DBD images under a single shot and 100 Hz are presented. Index Terms—All solid-state, dielectric barrier discharge (DBD), magnetic switch (MS), MARX, sub-microsecond pulse.

I. I NTRODUCTION

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VER THE last two decades non-thermal plasma at atmospheric pressure has been studied for a variety of industrial applications such as industrial ozone generation, surface modification of polymers, thin films deposition, pollution control, excitation of CO lasers, excimer lamps, and most recently, large-area flat plasma-display panels. Dielectric barrier discharge (DBD) is a typical approach for generating large-volume non-thermal plasma at atmospheric pressure in an economic and reliable way. In general, DBD is excited by an alternatingcurrent (AC) periodic voltage of several kV at 50 Hz to several kHz. The amplitude of the discharge current is usually on the order of tens to hundreds of milliamperes. However, AC excited DBD suffers from many disadvantages, such as low efficiency and local overheating resulting from microdischarges. In recent years, pulsed high voltage has been used to excite DBD with a better efficiency, and this has been experimentally confirmed by many investigations [1], [2]. In particular, nanosecond-pulse exManuscript received May 31, 2012; revised September 13, 2012 and November 9, 2012; accepted November 14, 2012. Date of current version March 7, 2013. This work is supported by the Chinese National Nature Science Foundation under the Grant 50837004 and 11075041. The authors are with the Institute of Electrical Light Sources, Fudan University, Shanghai 200433, China (e-mail: raojunfeng1985@163.com; kfliu@ fudan.edu.cn; jqiu@fudan.edu.cn). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TPS.2012.2228885

cited DBD generates more powerful non-thermal plasma with the amplitude of the discharge current over 10 A. Nevertheless, the energy efficiency is restricted by two main factors. First, in most pulsed DBDs, only one or two discharges at the rising or falling edges are excited per voltage pulse, which means only a minor part of energy in each pulse is consumed to produce plasma, while most energy at the flat top period is wasted on other elements. Second, the active particles produced in each discharge dissipate before the formation of next discharge since the time interval between them is much longer than the discharge period (usually less than 100 ns). To maintain stable operation, even a resistor in parallel with DBD load is required in certain cases [3]. All these factors contribute to a relatively low efficiency of pulsed DBD. Generally speaking, there are two fundamentally different methods for improving DBD efficiency. One is to increase the number of discharges during each voltage pulse. Another is to make full use of the remnant [4], [5] produced in former discharges since it is favorable for DBD. Both methods indicate that continuous and sharp oscillation with fast rising and falling edges is expected. The discharge characteristic of Blumlein transmission lines (BTLs) with unmatched load can meet all these requirements [6]. In addition, a high-voltage pulsedpower supply and a high-power switch are also expected. High-voltage pulsed power supply with repetitive rate has been widely utilized in industry and military applications such as materials surface treatment, pollution control, and highpower microwave weapons. Gas switches are utilized in many conventional pulsed power sources due to their high rated voltage and current. However, they all suffer from short lifetime, low pulse repetition frequency (PRF), high maintenance costs, and complex control apparatus. With the continuous improvement of power semiconductors in switching speed, rated voltage and current, employment of solid-state switches in pulse modulators began to prevail around 20 years ago. All solid-state pulse generators have many advantages such as long lifetime, high PRF, compactness and stability of output parameters, etc. Various all solid-state pulse generators based on MARX have been designed in our group, and much experience has been accumulated [7]–[10]. Some MARX generators can output unipolar pulses with voltage amplitude of up to 60 kV at 500 Hz [7]. Others can output 2 kV bipolar pulses with rise time of 37 ns at 21 kHz. In this paper, an all solid-state sub-microsecond pulse generator for DBD is proposed based on MARX and magnetic switch (MS). This generator is able to excite continuous DBD in a few microseconds under a single shot. Experiments with matched resistor load are also carried out to make comparisons.

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RAO et al.: NOVEL ALL SOLID-STATE SUB-MICROSECOND PULSE GENERATOR FOR DIELECTRIC BARRIER DISCHARGES

Fig. 1.

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Schematic circuit of the pulse generator.

Fig. 3. Gate driver circuits for IGBTs in MARX. Fig. 2.

Schematic of all solid-state MARX.

Generator introduction and loads description are presented in Section II. Section III gives experimental results and discussion. Main conclusions are summarized in Section IV. II. G ENERATOR I NTRODUCTION AND L OADS D ESCRIPTION The schematic circuit of this generator is shown in Fig. 1. This generator consists of an all solid-state MARX generator (the dashed rectangle part), an inductor, BTLs rather than a capacitor, and one MS. MARX charges BTLs to 20 kV through the inductor Ls in 1 μs. Then, the MS is saturated, and BTLs begin to discharge to the load. The inductor Ls has two functions here. First, it limits the charging current to avoid overcurrent breakdown to MARX. Second, it suppresses the impact and interference to MARX when BTLs are discharged to load. Each part is introduced below. 1) MARX Generator: When gas switches in traditional MARX generators are replaced by semiconductor switches, many parameters of MARX generator (such as PRF, lifetime, compactness and stability) are considerably improved. In this paper, insulated gate bipolar translators (IGBTs) are used as main switches of the MARX generator which consists of 24 stages. The simplified structure of the proposed MARX with three stages is shown in Fig. 2. Via the inductor L and all diodes, capacitors Ci in parallel are charged to U0 (usually under 1 kV) by a DC source. During the discharge period, all IGBTs are turned on, and all diodes are reverse biased. All capacitors discharge to the load in series as indicated by the red arrowed line, and a pulse with the voltage amplitude of nU0 is obtained at the load (where n is the number of the stages contained by MARX) [11], [12]. An important advantage of all solid-state MARX is that if the IGBT in one stage fails to turn on, other stages will not be influenced. Suppose that Q2 (in Fig. 2) failed to turn on; then, D22 would not be reverse biased, and the discharge current would flow through C1, Q1, D22, C3, Q3, and the load. This means that the MARX generator would still work even though the voltage amplitude of the output pulse would decrease from nU0 to (n − i)U0 , where i means the number of IGBTs that failed to turn on. The rating voltage of IGBTs and capacitors is 1.2 kV, and the operating voltage will be no higher than 900 V. Then, a pulse with maximal voltage ampli-

Fig. 4. Different output waveforms. (a) Different voltage amplitudes. (b) Different pulse widths.

tude of 21 kV could be obtained at the load. To improve the turn-on speed of IGBTs, self-supplied high-speed gate drivers IXDN414PI (IXYS Corporation) with a common primary signal are utilized in each stage as shown in Fig. 3 [13]. Coaxial cables are employed as both primary and secondary windings of these transformers to shield EMI signals [9]. Every four stages share one common transformer with four secondary windings to make it more compact for the generator. Waveforms of different pulses outputted by this MARX generator with a 500 Ω resistor are given out in Fig. 4. Waveforms of voltage with amplitudes varying from 6 kV to 20 kV are presented in Fig. 4(a). In Fig. 4(b) are waveforms of voltage with full width at half maximum varying from 700 ns to 3 μs. These waveforms prove the flexibility of outputting various pulses by all solid-state MARX generators, which is extremely difficult to achieve by means of traditional MARX generators using gas switches. Another important advantage of all solidstate MARX is that it is capable of outputting high-power pulse (over 1 MW) in only a few hundred nanoseconds. When it is used together with MS, the voltage-second product of MS is considerably decreased, which enables a significant reduction of the core volume, saturate inductance, and thus the eddy current loss. The rise time of output pulse of the MARX generator increases from 43 ns to 560 ns when the load current increases from 20 A to 200 A. 2) BTLs: BTLs serve as energy storage devices which can output rectangular pulses on matched loads. In this generator, they are composed of two coaxial transmission lines with the same length and type. Here, SYV50 transmission lines with characteristic impedance of 50 Ωare selected. The length of

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TABLE I M AJOR PARAMETERS OF THE M AGNETIC C ORE

each line is set to 20 m, which gives a delay time constant of 110 ns. Thus, pulse width of 220 ns could be expected on matched resistor load. Obviously, the pulse width could be adjusted by changing the length of transmission lines. The equivalent capacitance of each line is calculated to be 2.11 nF with its known relative dielectric constant (εr = 2.3), inner diameter, and outer diameter. There are two reasons for using BTLs rather than a capacitor. First, BTLs can output continuous bipolar nanosecond pulse over DBDs loads due to their unmatched impedance, which excites continuous DBD. When a capacitor is used, only one unipolar nanosecond pulse could be applied to the loads. Second, when MS is used as the main discharge switch of BTLs, no extra resistor in parallel with DBD load is required. However, in some cases, it is indispensable if capacitors are used as energy storage component [1], [3]. This resistor frequently results in extra loss and low efficiency of a generator. 3) MS: To realize the long life, low cost, and high frequency of the DBD generator, an MS is utilized to replace the traditional hydrogen thyratrons. In contrast to other solid-state switches like IGBT, MS is superior in many aspects such as lifetime, rated voltage, and current, and no need for driver circuits. The design procedure of MS has been introduced in many literatures [14], [15]. In our experiment, MS should saturate after the charging of BTLs. The saturated inductance should be as low as possible in order to obtain a fast rising pulse. Finally, nano-crystalline alloy core was selected due to its large magnetic flux density and high permeability. The major parameters of the core are listed in Table I. The abbreviation OD, ID, and H, respectively, stand for outer diameter, inner diameter, and height. The winding number was designed to be 16. Two 16-turn windings were wound in conjugation to minimize the saturated inductance. The voltage-second product was measured as 8 mV • s. To make full use of the magnetic core, a DC reset circuit was employed to reset MS when necessary [16]. The reset DC current is 1 A. It should be pointed out that the load current can only pass through MS in the direction that it saturates. 4) Loads Description: Experiments were carried out with matched resistor and DBD loads. The matched resistor load is a custom-made 100 Ω and 100 Watt resistor with low inductance. Fig. 5 sketches the cross sections of two different DBD reactors and the equivalent circuit of DBD reactor. The normal DBD reactor comprises two parallel electrodes separated by a gap as shown in Fig. 5(a). Both electrodes are made of aluminum plates with a diameter of 74 mm. As can be observed, rounded profiles have been employed at the edges of both electrodes in order to avoid local electric field enhancements due to the sharp edge effects. The dielectric covering the cathode is glass with a relative dielectric constant of εr = 3.7. The diameter of the

Fig. 5. (a) Normal DBD reactor. (b) Staggered arranged DBD reactor. (c) The equivalent circuit of DBD reactor.

Fig. 6.

Waveforms with matched resistor load.

glass plane is 90 mm, and the height is 1.1 mm. The distance of the gap between the electrodes is set to 2 mm. The HV signal is applied on the anode, while the cathode is grounded. The capacitances of the air gap and the dielectric layer are calculated to be 27.5 pF and 95.8 pF, respectively. The DBD image could be easily captured when using the normal DBD reactor. For the staggered arranged DBD reactor, it is quite convenient to adjust its scale, and thus the capacitance which will be explained later. III. E XPERIMENTAL R ESULTS AND D ISCUSSION Experiments with matched resistor load and DBD loads were carried out for comparison. Experimental results and discussion are presented in following sections. A. Matched Resistor Load Waveforms of voltage with 100 Ω resistor load have been presented in Fig. 6. The black line represents the voltage over the 100 Ω resistor. The waveform of current across the resistor load is not given because it is almost the same as the voltage waveform. The red line and the blue line represent the voltages at both ends of the BTLs as marked in Fig. 1. During the period of t0 ≤ t < t1 , the transmission lines are charged by the MARX generator which results in a 5 kV prepulse over the resistor load. The charging process results in a voltage over the MS and ends when the MS saturates and turns on. According to the wave processes analysis [6], a negative rectangular pulse with pulse width of 220 ns is formed at the matched resistor load after 110 ns. The voltage amplitude is 20 kV, and both the rising and falling time are 60 ns. After the rectangular pulse, there is a slight oscillation. This is because


Fig. 7.

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Waveforms with DBD load.

MARX is not completely turned off after the MS is saturated, and more energy is fed into the resistor load through the BTLs. According to measured voltages at both ends of BTLs (U1 and U3 in Fig. 6), the energy E0 stored in the BTLS could be calculated. The average voltage in the BTLs is 21 kV since the peak values of U1 and U3 are 22 kV and 20 kV, respectively. Thus, E0 is calculated to be 0.9305 J according to (1) 1 2 . E0 = 2 × C0 Uave 2

(1)

In which, C0 is the capacitance of each transmission line, and it is 2.11 nF as stated before, Uave is the average voltage in the BTLs. Energy E1 consumed on the resistor during the discharge process (time interval from t1 to t2 ) can be obtained using (2). Waveform of the voltage over the resistor load is input into Origin software, and integration operation is carried out. E1 is calculated to be 0.7903 J. This indicates that the ratio of the energy consumed by the resistor load into the total energy stored in the BTLs is about 84.9%. It should be emphasized that this value is for reference only because it is very difficult to obtain the precise value of E0 . The lost energy mainly dissipates on BTLs and MS t2 E1 =

U2 dt. R

(2)

t1

B. DBD Loads The DBD reactor can be treated as two series-connected capacitors until the air gap is broken down, which is determined by its physical structure shown in Fig. 5. Usually, applied voltage over the DBD reactor exceeds its breakdown voltage so that non-thermal plasma can be excited after the breakdown of the air gap. The operating principles are the same as those of matched resistor load. Waveforms of voltage and current with DBD load under a single shot were presented in Fig. 7. As shown in Fig. 7, the voltage oscillates violently as expected. There are two reasons for the violent oscillation. First, the discharge loop is typical C-L-C circuit taking the DBD reactor as a capacitor load. Second, due to the low resistance in the loop, only a few dozens millijoules of energy are consumed during each discharge, whereas the energy stored in BTLs reaches

Fig. 8. Waveforms with staggered arranged DBD reactor: (a) 140 pF DBD reactor; (b) 690 pF DBD reactor.

about 0.85 J. The oscillation lasts until all the energy stored in the BTLs is consumed. Therefore, the oscillation would last longer if more energy is stored in the BTLs or less energy is consumed during each discharge. Experiments were carried out using the staggered arranged DBD reactor in Fig. 5(b) to prove this point. Both the cathode and anode are covered by glass tubes, and they are arranged in staggered pattern. The scale of the reactor could be easily adjusted by changing the number of parallel-connected electrodes. The capacitance of the DBD reactor was measured by digital universal LCR meters. The capacitance of the DBD reactor is changed from 140 pF to 690 pF for comparison. Experiments were carried out under the same condition, and relative waveforms are presented in Fig. 8. Judging from the decaying amplitude of the voltage over DBD load, less energy is consumed during each discharge for the smaller scaled DBD reactor [Fig. 8(a)], and the oscillation also lasts longer. It should be pointed out that DBD is only excited when the voltage over the reactor is high enough, which means that there is only displacement current but no DBD current at the end of the oscillation process as shown in Fig. 8(b). The main difference between DBD current and displacement current is that the period of the former is usually shorter than that of the latter. Compared with common unipolar nanosecond-pulse DBD generator, three main advantages of this generator are listed as follows: 1) There are over ten discharges in a few microseconds. Typically, there are only one or two DBD discharges at the rising or falling edges for each voltage pulse. This limits the energy efficiency of pulsed DBD generators.

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Fig. 9. Waveforms of voltages over MS and current through MS.

However, the results in this paper point to a different conclusion. Judging from the current waveform in Fig. 8(b), there are over ten discharges in 5 μs. In particular, there is more than one discharge at each falling edge due to large dv/dt of the oscillation. This indicates that most energy is fed into the DBD reactor to produce non-thermal plasma. Meanwhile, multi-discharges in such a short time imply that the remnant in one discharge is not fully dissipated before the formation of the next microdischarge. This also increases the plasma density in that it usually takes a few microseconds for positive ions to move from one electrode to another [4]. Due to this accumulation effect, the total efficiency of the pulse generator is considerably improved. 2) As stated before, no extra resistor in parallel is required for DBD load, and most energy stored in the BTLs is directly fed into the DBD reactor. 3) No reset circuit for the MS is required. Relative waveforms with MS are presented in Fig. 9. Waveform of voltage over MS is shown in black and current through MS in blue. It has been proved both experimentally and theoretically that the AC decaying amplitude voltage over the MS has demagnetization effect [17]. Because it provides an alternating magnetic field whose amplitude is slowly reduced to zero. The DC reset circuit can be spared in this case, which improves the total efficiency of the generator. It also shows that there is only unipolar current passing through MS, and the amplitude attenuates gradually. In addition, this generator also has all the merits of all solidstate generators such as good stability, long lifetime, compactness, and high PRF. C. DBD Image A Nikon D90 camera was used to photograph the DBD image with the normal DBD reactor. Exposure time was set to 1 s and ISO 1000 to capture the discharge image when the generator worked under a single-shot condition. Fig. 10 shows the DBD images under single-shot and 100 Hz operations. It is observed that obvious filaments perpendicular to the electrodes are randomly distributed in the air gap. For an isolated microdischarge, the lateral extension of the surface charge


Fig. 10. DBD images under single shot (top, exposure time: 1 s, ISO: 1000) and 100 Hz operation (bottom, exposure time: 0.2 s, ISO: 800).

(bottom) is considerably larger than the channel diameter. Charge deposited over this area of the dielectric layer leads to quenching of the filamentary discharge. The extension of surface discharge determines the capacitive coupling across the dielectric barrier and the area of reduced field after termination of the microdischarge. Undisturbed high-field outside the influence of this area causes other microdischarges. As long as the external voltage keeps rising, subsequent microdischarges will therefore preferentially strike at other locations outside this region. As a result, the dielectric serves a dual purpose. It limits the amount of charge and energy imparted to an individual microdischarge and, at the same time, distributes the microdischarges over the entire electrode area [18]. IV. C ONCLUSION A novel all solid-state sub-microsecond pulse generator for DBD is designed. It can generate nanosecond rectangular pulses with matched resistor load at the energy efficiency of 84.9%. With a normal DBD reactor load, over ten discharges in a few microseconds were excited under a single-shot condition. More DBDs can be excited if more energy is stored in the BTLs or less energy is consumed during each discharge. Compared with common unipolar nanosecond pulsed DBD generators, there are many advantages such as multi-discharges, demagnetization effect, and no need of extra resistor. All these advantages considerably improve the total efficiency of the generator. The plasma produced here is also much more intense than the usual one owing to the accumulation effect. Highfrequency operation and long lifetime can be easily achieved for this all solid-state pulse generator, which indicates great potential for use in various industrial applications. Further work will be focused on improvement of the discharge uniformity and high-frequency operation. R EFERENCES [1] J. M. W. A. Ganguly, “Comparison of high-voltage AC and pulsed operation of a surface dielectric barrier discharge,” J. Phys. D, Appl. Phys., vol. 39, no. 20, pp. 4400–4406, Oct. 2006. [2] R. P. M. A. Falconer, “Visible and VUV images of dielectric barrier discharges in Xe,” J. Phys. D, Appl. Phys., vol. 34, no. 23, pp. 3378–3382, Dec. 2001. [3] S. Tao, Z. Dongdong, Y. Yang, Z. Cheng, W. Jue, Y. Ping, and Z. Yuanxiang, “A compact repetitive unipolar nanosecond-pulse generator for dielectric barrier discharge application,” IEEE Trans. Plasma Sci., vol. 38, no. 7, pp. 1651–1655, Jul. 2010.


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Junfeng Rao was born in Hubei, China in 1985. He received the B.S. degree in electrical and electronic engineering from Huazhong University of Science and Technology, Hubei, China, in 2008. Currently, he is working toward the Ph.D. degree in physical electronics in Fudan University, Shanghai, China.

Kefu Liu was born in Anhui, China, in 1963. He received the B.S. and Ph.D. degrees in electrical and electronic engineering from Huazhong University of Science and Technology, Hubei, China, in 1987 and 1999, respectively. After 1987, he was with the Institute of Plasma Physics, Chinese Academic of Science, Hefei, Anhui, China. In 1998, he returned to Huazhong University of Science and Technology. Currently, he is a Professor in Fudan University, Shanghai, China. His research interests include pulsed power and plasma technology and applications.

Jian Qiu was born in Hubei, China, in 1982. He received the B.S. and M.S. degrees in electrical and electronic engineering from Huazhong University of Science and Technology, Hubei, China, in 2004 and 2007, respectively. Currently, he is working toward the Ph.D. degree in physical electronics in Fudan University, Shanghai, China.