A reusable planar triggered spark-gap switch batched-fabricated with PCB technology for medium-and low-voltage pulse power systems

Zhi Yang ,Ke Wang ,Peng Zhu ,,Peng Liu ,Qiu Zhang ,Cong Xu ,Hao-tian Jian ,Rui-qi Shen

a School of Chemical Engineering,Nanjing University of Science and Technology,Nanjing,210094,China

b Key Laboratory of Special Energy Materials,Ministry of Education,Nanjing,210094,China

c Micro-Nano Energetic Devices Key Laboratory,Ministry of Industry and Information Technology,Nanjing,210094,China

Keywords: Pulse power systems Printed circuit board technology Triggered spark-gap switch Planar discharge switch Electrical performance

ABSTRACT Triggered spark-gap switch is a popular discharge switch for pulse power systems.Previous studies have focused on planarizing this switch using thin film techniques in order to meet the requirements of compact size in the systems.Such switches are one-shot due to electrodes being too thin to sufficiently resist spark-erosion.Additionally,these switches did not employ any structures in securing internal gas composition,resulting in inconsistent performance under harsh atmospheres.In this work,a novel planar triggered spark-gap switch(PTS)with a hermetically sealed cavity was batched-prepared with printed circuit board(PCB)technology,to achieve reusability with low cost.The proposed PTS was inspected by micro-computed tomography to ensure PCB techniques meet the requirements of machining precision.The results from electrical experiments demonstrated that PCB PTS were consistent and reusable with lifespan over 20 times.The calculated switch voltage and circuit current were consistent with those derived from real-world measurements.Finally,PCB PTS was used to introduce hexanitrostilbene(HNS)pellets in a pulse power system to verify its performance.

1.Introduction

A discharge switch is one of the key components of pulse power systems,and it signi ficantly impacts the output of the system.Pulse power switches need to turn on at high speeds and transfer the energy stored in a capacitor to a load[1].A few kinds of switches have been developed to drive pulse power systems,including the semiconductor switch[2,3],shock-induced dielectric switch[4-6],triggered vacuum switch[7],and triggered spark-gap switch[8-11].Triggered spark-gap switch is a kind of closing switch with capacitor energy storage,and it has the advantages of hightemperature operation,leakage current,and radiation hardness[12].Pulse current,resulting from the directional migration of free electrons,flows through this switch via the arc gap between two electrodes.This conduction process can be explained by the streamer theory of breakdown with an electron avalanche in dielectric materials[13,14].Traditional spark-gap switches are large and cylindrical,so it does not facilitate easy integration with strip transmission lines.They are also expensive due to the multi-step fabrication processes.

With the development of miniaturized pulse power systems,such as exploding bridge wire(EBW),exploding foil initiator(EFI),and capacitor discharging unit(CDU)[15],pulse power switches with planar geometry,robustness,low cost,and compatibility with integrated circuit(IC)are required.Thin film technologies have recently been utilized to planarize spark-gap switches.In 2011,Baginski introduced a switch,with Kapton as dielectric material,which could be integrated into strip lines utilized in a CDU[8].Liu fabricated an in-plane switch using micro-electro-mechanical system(MEMS)techniques in 2013[9].Such triggered spark-gap switches were not reusable due to poor arc-ablation resistance.These switches did not employ any structures to secure internal gas composition,and thus,their performances were inconsistent under harsh atmospheres.

Printed circuit board(PCB)is an established technology capable of mass-producing electric circuits at low cost[16,17].In this study,PCB technique was employed in fabricating planar triggered sparkgap switch(PTS)with hermetically sealed cavity to meet the reusable requirement in pulse power systems,and then the performance of the proposed switch was characterized.Electrostatic field distribution was simulated with COMSOL Multiphysics to ensure the PCB PTS worked as intended.Secondly,four kinds of PCB PTS with hermetically sealed cavities were prepared in batches,and their three-dimensional(3D)morphologies were reproduced using micro-computed tomography(μCT)scanning technology.Next,the reusability of the PCB PTS was studied,accounting for lifetime,delay time(time from triggered break to beginning of currenttd),jitter,risetime(time from start of current to first peaktdi/dt),and peak current.The circuit current and voltage across the PTS were modeled and found to be consistent with those derived through experimentation.Finally,a CDU was used to verify the performance of PCB PTS in pulse power systems.The result showed that hexanitrostilbene(HNS)pellets were successfully detonated in this CDU.

Fig.1.Schematic of PCB PTS with hermetically sealed cavity of(a)Three electrodes located in the inner layer(top PCB is shown lifted up)(b)Speci fic structures of electrodes in PTS.

2.Design,simulation and fabrication

The most widely used PTS has three electrodes,namely,ground or common electrode(cathode),triggered electrode,and highvoltage input electrode(anode).The triggered electrode can be embedded into one of the main electrodes(cathode and anode)or placed between them.The con figuration and detailed geometry of the three electrodes in PCB PTS are presented in Fig.1.The anode was connected to the positive end of a capacitor through a pad,the cathode was soldered onto the negative terminal,and the triggered electrode was attached to a positive pulse voltage signal.All three of the electrodes were within the sealed cavity with standard atmospheric pressure,and the mean free path of a gas molecule was much smaller than the gap distance between two main electrodes(main gap).

The PTS working process can be divided into two stages according to both gaps between adjacent electrodes:(1)when pulse signal was applied to the triggered electrode,an electric field with high intensity was formed in the gap between cathode and triggered electrode(triggered gap),causing air breakdown and generating ions and electrons;and(2)electrons collided,inducing the avalanche effect between anode and cathode so that pulse current flowed into the switch with the directional movement of free electrons.The structural parameters of PCB PTS are listed in Table 1.

Table 1 Structural parameters of PCB PTS.

The electrostatic field distribution of the PCB PTS was simulated using COMSOL Multiphysics.The matching relationship between triggered voltage(UT)and speci fic triggered gap(a)was obtained.Fig.2(a)presents the contour lines of the electrostatic field gradient distributed among the triggered gap atUT=1000 V(capacitor voltageUC=0 V).Fig.2(b)illustrates the relationship between electrostatic field strength and triggered gap.The minimum electrostatic field strength in Fig.2(b)was 5.3×106V/m,which was greater than the air breakdown field intensity of 3.0×106V/m.Therefore,PTS can be reliably triggered by 1000 V in theory.

In Fig.3,PTS was prepared by joining two PCBs(both FR-4 basematerial).The bottom PCB was a single-sided copper-plated board with 1 mm thickness,and the top PCB was also a single-sided board with 2 mm thickness.The three-electrode circuit was constructed on the top layer of the bottom PCB,and three corresponding pads were placed on the top layer of the top PCB through vias to facilitate connection.A cavity (dotted rectangle in Fig.1),with size 4.0 mm(l)×3.0 mm(w)×1.5 mm(h),was drilled at the top PCB from its bottom to top layer.The hermetically sealed cavity ensured consistent performance in harsh environments through securing internal gas composition and adjusting electric field distribution.The circuit status should be detected before actual action in highvalue-added pulse power systems,in order to minimize the economic losses and safety accidents.Therefore,the three-electrode circuit was thickened with 2 oz(about 70μm thickness)Cu to make PCB PTS resistant to spark-wear,ensuring reusability to meet the online tests.Main gaps were marked at the bottom layer of the bottom PCB to distinguish PTS with different main gaps.The PCB PTS was planar,and had an overall size of 13.0 mm(l)×8.0 mm(w)×3.0 mm(h).

Fig.2.Electrostatic field gradient between cathode and triggered electrode of(a)Color plot(b)Electrostatic field intensity.

Fig.3.PCB PTS with four main gaps of(a)PCB process for fabricating PTS(b)Sample photograph.

The internal structure of the PCB PTS could not be photographed using an optical camera due to the existence of the cover board(top PCB),so its 3D morphology was produced usingμCT technique[18],as seen in Fig.4.Fig.4(a)shows the 3D morphology,including detailed geometries,as well as relative positions of the threeelectrode circuit,vias,pads,and sealed cavity.Fig.4(b)shows the top-view of the three-electrode circuit,located in the inner layer,with the structural parameters marked.These marked structural parameters were consistent with those in Table 1,therefore,the PCB PTS meet the requirements of machining accuracy.

3.Experimental setup for electrical performance

Researchers have focused on the electrical performance of pulse power switches.The test circuit for the PCB PTS was designed as seen in Fig.5.The measuring system consisted of two high-voltage differential probes,a Rogowski current coil,and a multi-channel digital storage oscilloscope.Two high-voltage probes were adopted to obtain the triggered voltage across the triggered electrode and cathode,as well as the main voltage signal between both ends of the switch.A Rogowski transducer was utilized to monitor circuit current.Both voltage signals and one current waveform were recorded on the oscilloscope.Then,the reusability of PCB PTS was studied,accounting for lifetime,delay time,jitter,risetime,peak current(Ipeak),lumped inductance(Ll),and lumped resistance(Rl)in terms of different capacitor voltages during multiple operations.

4.Results and discussion

4.1.Typical working process

To begin with,an extra 500 V was added to the simulation result to ensure that the switch can be reliably triggered since there was a delay to every triggered break.Fig.6 shows the representative conduction process in PCB PTS(d=0.9 mm)during the discharging state whenUC=1250 V andUT=1500 V(C=0.22μF).When 1500 V triggered voltage(rising edge was about 600 ns)was applied to the triggered electrode,the 1250 V between the main electrodes of PCB PTS(UPTS)was discharged,and as a result,current began flowing into PTS almost immediately,that is,td=0 ns.Subsequently,oscillating current emerged from the oscillating circuit.tdi/dt=155.1 ns andIpeak=2.10 kA,and therefore,the current rise rate was 13.5 kA/μs.

Fig.4.Scanning image of PCB PTS(d=1.5 mm)viaμCT photography of(a)3D tomography(b)Three-electrode circuit con figuration.

Fig.5.PCB PTS test circuit.

Fig.6.Conduction process in PCB PTS(d=0.9 mm).

4.2.Characterization of reusability

To ensure the switch functioned after receiving trigger command,the capacitor voltages for PCB PTS at 0.6 mm,0.9 mm,1.2 mm,and 1.5 mm were 750/1000 V,1000/1250/1500 V,1500/1750 V,and 1750 V,respectively.The operating circuit,including the switch with dynamic resistance,is regarded as anRLCcircuit,so circuit parametersLlandRlare solved according to Kirchhoff Voltage Law(KVL),as given by Eqs.(1)and(2):

whereT=t2-t1is the period of the currents,I1is the first peak current,I2is the second peak current,t1is the time corresponding to the first peak current,andt2is the time corresponding to the second peak current.

The lumped circuit parameters are listed in Table 2.Table 2 suggests that lumped inductance(Ll)was unaffected by main gaps and capacitor voltages,while lumped resistance(Rl)was affected by those factors.For the same main gaps,lumped resistance decreased with the increasing operating voltages,and for the same working voltages,lumped resistance increased with main gaps.The mean value of period,lumped inductance,and lumped resistance of the circuit were calculated as 582.3 ns,37.9 nH,and 141.6 mΩ,respectively.

Table 2 Lumped circuit parameters.

The lifespan of the PCB PTS was mainly dominated by the electrodes erosion effect,which was evaluated at different main gaps and operating voltages.Withd=0.6 mm andUC=750 V or 1000 V,PCB PTS switched reliably over 100 times.Withd=0.9 mm andUC=1000 V,PCB PTS switched reliably over 50 times.Lifetime reduced with increasing arc-ablation effect in electrodes due to increasing capacitor voltage.WhenUC=1250 V,PCB PTS switched reliably about 50 times.Trigger voltage was increased after multiple consecutive tests,and testing was reduced to about 30 times as operating voltage increased to 1500 V.Withd=1.2 mm or 1.5 mm and operating voltage of 1500 V or 1750 V,reliable switching reduced to about 20 times.

Fig.7 shows plots of delay time,risetime,and peak current of PCB PTS for different main gaps and capacitor voltages,considering electrical performance data from the reusability experiment.In Fig.7(a),the uniform electric field was unaffected by lower voltage owing to the spark-erosion resistance of thickened Cu electrodes.Therefore,the mean values oftdwere less than 10.0 ns when PCB PTS(d=0.6 mm)operated under 750 V or 1000 V,and jitters(i.e.,standard deviations)were about 0 ns.As the step-up of operation voltages,the uniformity was subjected to the electrodes erosion where existed the corona discharge and polarity effect,resulting in the switch could not work robustly.For instance,the averagetdof PCB PTS(d=0.9 mm)increased from 20.3 ns to 45.2 ns and then to 110.2 ns with the increase of voltages from 1000 V to 1250 V and then to 1500 V.Jitter also increased from 16.4 ns to 36.5 ns and then to 131.9 ns.PCB PTS(d=1.5 mm)had the maximum jitter of 163.4 ns(td=118.7 ns)when it operated under 1750 V.Delay time was contradicted with the reusability for the switch operating at higher voltage.For example,delay time of PCB PTS(d=1.2 mm)operated at 1500 V exceeded 1000.0 ns,when capacitor voltage increased to 1750 V,delay time decreased(td=106.0 ns)and lifespan was shortened.

From Fig.7(b),risetimes for PCB PTS(d=0.6 mm or 0.9 mm)with capacitor voltages below 1250 V were slightly varied,and the average was about 160.0 ns with no obvious fluctuations.PCB PTS working beyond 1500 V required longer risetimes due to its wider arc gap distance.Referring to Fig.7(c),peak current at each capacitor voltage did not fluctuate during most switching operations.The standard deviations in Fig.7(c)were all less than 50 A,which indicated that the switch could work consistently among multiple operations.PCB PTS with smaller main gaps had higher peak currents under identical capacitor voltages.For instance,the mean was 1.61 kA with 0.9 mm main gap operating at 1000 V,but 1.70 kA with 0.6 mm main gap.Similar results were derived under 1500 V and 1750 V.These results indicated that smaller main gaps increased the colliding probability of electrons,so the time required to form a stream pass was reduced and the rapid conduction increased current.

Fig.7.Electrical parameters of PCB PTS after reusability experiment of(a)Delay time(b)Risetime(c)Peak current.

In summary,the novel PCB PTS meet the reusability requirements of medium-and low-voltage systems.In addition the main gap of the PCB PTS could be widened to make it suitable for high-voltage systems.

5.Modeling of electrical parameters

The circuit current and the voltage across the main electrodes of PCB PTS(UPTS)were deduced.First,theRLCcircuit equation was combined with KVL,as given by Eq.(3):

wherei(t)is circuit current,Cis capacitance(0.22μF),UCis capacitor voltage,Rl*is the lumped resistance excluding switch resistance,Rv(t)is the time-varied resistance of PCB PTS,andLlis the lumped inductance of circuit.

Rompe-Weizel model is applicable in calculating dynamic resistance with gap distance less than 35.0 mm under standard atmospheric pressure[19,20],and it was adopted to solve forRv(t)in Eq.(4).

The differential of Eq.(5)is given by Eq.(6):

whereαis a constant for characterizing the nature of the gas,and its value is 0.08-0.1 MPa cm2s-1?V-2for air,pis the standard atmospheric pressure,anddis the main gap.In this work,αis 0.09 MPa cm2s-1?V-2,pis 0.1 MPa,anddis 0.9 mm.

We assumed that there was no resistance in the circuit=0 mΩ)besides theRv(t),sinceRv(t)was much higher than the loop resistance in the switching process. WhenUC=1250 V(C=0.22μF)andLl=37.1 nH,i(t)andRv(t)at=0 mΩ was solved by combining Eq.(3)and Eq.(4)with Eq.(6)using MATHEMATICA.Initial conditions werei(0)=0 andRv(t)→∞.The minimumRv(t)was attained at the moment corresponding toI1,and its value was 112.2 mΩ.We can estimateaccording toRllisted in Table 2(133.3 mΩ)and the minimumRv(t),and its value was 21.1 mΩ.So numerical solution ofRv(t)andi(t)were achieved by substitutingRl*=21.1 mΩinto(3)and(4).

UPTSconsists of voltage from the static loop resistanceand dynamic resistance(Rv(t)),and another voltage from the switch inductance(LPTS):

LPTScan be evaluated from the current waveforms in the measurement:

Fig.8.Measured and simulated voltages and current(UC=1250 V,UT=1500 V,C=0.22μF)in PCB PTS(d=0.9 mm).

Thus theLPTScan be calculated as 8.8 nH.UPTS(t)can be achieved by solving(3),(4),(6),and(7)with numerical methods.Experimental and simulation results ofI-tandUPTS-twere plotted in Fig.8.We could find that theI-tcurve is an oscillation decay curve,and the simulated peak current and risetime are 2.05 kA and 152.0 ns,respectively,which were found to be in line with those derived through experimentation of 2.11 kA and 148.4 ns.It was concluded that the theoretical voltage and current,based onRLCcircuit equation and dynamic resistance model,were consistent with the experimental curves,especially during the first 1/4 oscillation period that is signi ficant for pulse power systems.

6.Application of PCB PTS

To verify the performance of the proposed switch,a CDU was connected in series,including a capacitor,PCB PTS,and EFI.Referring to Fig.9,PCB PTS(d=0.9 mm)was selected due to sufficient self-breakdown voltage and the ability to transmit high current.A compact surface-mounted switch with a flat outline was bene ficial to integration with EFI.EFI was used to induce detonation in the ammunition system,which consisted of a ceramic substrate,bridge foil,flyer layer,barrel,and explosives pellet of HNS[21-23].

Fig.9.CDU consisting of PCB PTS with main gap of 0.9 mm.

The CDU was first tested with the switch in short circuit.The capacitor was charged with DC voltage and CDU was in an open state due to the withstand voltage of the switch gap.High-density pulse current(about 108A/cm2)flowed into EFI through PTS when the switch was triggered by 1500 V.Bridge foil exploded into gas and plasma with high temperature and pressure to shear and drive the flyer inside the barrel[24].High-speed flyer out of barrel impacted the HNS pellet.The experiment showed that HNS pellets with an size of 4.0 mm(Ф)×4.0 mm(H)were reliably detonated under firing conditions of 0.22μF/1350 V.As a result,a hole was created on the aluminum veri fication block on the HNS surface,as shown in the upper-right corner of Fig.9.

7.Conclusion

In conclusion,PTS with a hermetically sealed cavity were batched-prepared using PCB technology to meet the pulse power system requirements of low cost,easy integration,and reusability.Reusability under different operating conditions was characterized in terms of lifetime,delay time,jitter,risetime,and peak current.The experimental results demonstrated that(1)lifespan of PCB PTS over 20 times,(2)the mean values of delay time were all less than 120.0 ns with jitter less than 200.0 ns,(3)the average risetime was about 160.0 ns with no obvious fluctuations,(4)peak current did not fluctuate during most switching operations(the standard deviations<50 A).All the parameters indicated that PCB PTS could meet the reusability requirements of medium-and low-voltage systems.The calculated switch voltage and circuit current were calculated and found to be in line with those obtained through experiment.Finally,PCB PTS performance was veri fied using it to detonate HNS pellets in a CDU.In future studies,PCB techniques will be considered for integrating other electronic components into the switch to further increase the compactness of the pulse power systems and minimize the parasitic impedances of the circuit.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to in fluence the work reported in this paper.

Acknowledgement

We gratefully acknowledge support from the Natural Science Foundation of Jiangsu Province of China(Grant No.BK20151486).