1

Design and Evaluation of Mounting Clamps forCrowbar Application

Subhash Joshi T. G. and Vinod John

Abstract—Thyristors of press pack package is a widely usedswitch in high voltage pulse power applications. Thyristors areto be properly mounted to meet the datasheet specifications.Improper mounting results into higher electrical and thermalresistances and also adversely affects its surge current capability.Mounting method becomes important in application like crowbarwhere it demands high surge current and high di/dt ratings. Thisbecomes more critical when thyristors are series connected. Hencein application like crowbar using the pre-calibrated mountingclamps available in the market without evaluation is inappropri-ate. This paper addresses the design method for the fabricationof mounting clamps for press pack package and analyses thetorque-force relationship for a non-metallic bolt. The paper alsodetails the procedure adopted for the evaluation of the mountingclamps to ensure it meets the desired tolerance levels. Based on theformulated design method compact mounting clamps for 10kV,1kA crowbar are fabricated. Experimental results on a mountingclamp confirm the theoretical analysis.

Keywords—crowbar, mounting clamps, press pack package, nutfactor, mounting force.

I. INTRODUCTION

IN recent years, thyristors are used in many switchingapplications especially in the area of high voltage and high

power. Static transfer switch, high power phase controlledrectifiers, crowbar and traction are some of the applicationswhere thyristors are inevitable. In these applications thyristorsare used due to its higher voltage and current ratings, com-pared to other semiconductor devices. Still in many of theseapplications to meet the required voltage and current ratings,thyristors are connected in series or parallel or both.

Crowbar is a fault energy-diverting element [1] [2] builtwith series connected thyristors, connected at the output ofhigh voltage dc source as shown in Fig.1. The dc source willbe feeding power to sensitive loads, like microwave or plasmatubes. All series connected thyristors are turned-on when faultsignal receives from fault sensing elements, diverting the faultenergy through crowbar in few microseconds. When crowbar isturned-on, the dc capacitor will discharge through the crowbarwith high di/dt and high peak current, followed by a followthrough current of lower magnitude until the input circuitbreaker opens. Hence the thyristors used in crowbar applicationshould have higher surge current and di/dt ratings.

This work is supported by Department of Electronics and InformationTechnology, Govt. of India, through the project “Development of 10kV PowerSupply with Solid State Crowbar protection” sanctioned under NaMPETprogramme.

Subhash Joshi T. G. is with Development of Advanced Computing (C-DAC),Trivandrum, Kerala, India e-mail: (subhashj@cdac.in).

Vinod John is with Indian Institute of Science, Bangalore e-mail:(vjohn@ee.iisc.ernet.in).

Fig. 1. Power supply with dc capacitor (Cdc) and crowbar with voltage andcurrent measurement points.

High-Voltage (up to 1600 V), Surface-Mount, MSL Level 1, Halogen-Free

High-Voltage (up to 1600 V), Low IGT Available, Halogen-Free

12…25TTS IN TO-220 AND D2PAK

40TPS SERIES

16…25TTS IN TO-220 FULLPAK

70TPS SERIES

Fully Isolated Package, High Voltage (up to 1600 V), Halogen-Free

High Current (70 A), High Voltage (up to 1600 V)

ST230 SERIES ST1200 SERIES

ST330S SERIES DUAL P-CHANNELS

Up to 230 A, 2000 V, Metal Case with Ceramic Insulator, Hermetic Package

1650 A, 2000 V, Metal Case with Ceramic Insulator, Hermetic Package

330 A, Up to 2000 V, Metal Case with Ceramic Insulator, Hermetic Package, Stud Mounted

www.vishay.com

Vishay Intertechnology, Inc.

THYRISTORS

High-Voltage (up to 1600 V), Surface-Mount, MSL Level 1, Halogen-Free

High-Voltage (up to 1600 V), Low IGT Available, Halogen-Free

12…25TTS IN TO-220 AND D2PAK

40TPS SERIES

16…25TTS IN TO-220 FULLPAK

70TPS SERIES

Fully Isolated Package, High Voltage (up to 1600 V), Halogen-Free

High Current (70 A), High Voltage (up to 1600 V)

ST230 SERIES ST1200 SERIES

ST330S SERIES DUAL P-CHANNELS

Up to 230 A, 2000 V, Metal Case with Ceramic Insulator, Hermetic Package

1650 A, 2000 V, Metal Case with Ceramic Insulator, Hermetic Package

330 A, Up to 2000 V, Metal Case with Ceramic Insulator, Hermetic Package, Stud Mounted

www.vishay.com

Vishay Intertechnology, Inc.

THYRISTORS

(a) (b) (c)Fig. 2. Thyristor (a) Module type (b) Study type (c) press pack type.

Based on the packages available in the market, thyristorsare classified as (a) Module type (b) Stud mount type and (c)press pack type.

a) Module type: Here the silicon wafers are placed on anelectrically isolated metallic base plate and then covered usinga plastic enclosure as shown in Fig. 2(a). The heat transferwill be taken place through the metallic base plate which willbe mounted on a heatsink.

b) Stud mount type: Here the anode will be of threadedbolt used to mount on a heatsink and the cathode is of thickmetal cable, shown in Fig. 2(b), used to connect thyristor tothe remaining circuit. Here heat transfer take place primarilythrough the threaded bolt side of the thyristor.

c) Press pack type: This package is also known by dif-ferent names such as hockey puck, flat pack, disc and capsuleshown in Fig. 2(c). Here both anode and cathode have a flatcircular metallic plate, called pole faces, allowing the passageof current and provides double side cooling by mountingheatsink on both sides. Hence these type of thyristor givesexcellent cooling and can handle higher power. Availability ofthis package at higher voltage and current rating along withhigher surge current capability suits its use in high pulse powerapplications like in crowbar.

In all these packages proper mounting methods shouldbe followed to meet the specifications mentioned in thedatasheet [3]. Improper mounting will result into higher elec-trical and thermal resistances. The mounting methods requiresclose attention in application like crowbar, where current

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reaches the surge current rating in short time demandinghigh di/dt requirements. The situation becomes more criticalwhen press pack thyristors are connected in series due toaccumulation of geometric tolerances. Hence in similar ap-plications using pre-calibrated mounting clamps available inmarket without evaluation is inappropriate.

The pieces of information about various components ofmounting clamp explained in different context are availablein literatures [3]–[14]. In this paper a systematic method isformulated for the fabrication of mounting clamps for presspack package starting from the specification. It also detailsthe method to establish the torque-force relationship espe-cially needed when the bolts are fabricated with non-metallicmaterial. The design method is experimentally verified andthe mounting clamps are fabricated for 10kV , 1kA crowbarapplication which is tested for its specified voltage and currentrating. Experimental results confirm theoretical analysis.

II. MOUNTING OF PRESS PACK PACKAGE

In this package, silicon wafers are floating between thepole faces. While tightening the clamp pole faces will squeezeand makes contact with the silicon wafer [15]. The requiredamount of squeezing is specified in the datasheet as mountingforce [16]. Since electrical current flows from one mountingsurface to the other through the pole faces, the mechanicalarrangement is not only to dissipate heat energy but also toconduct electrical current. The points of concern related to themounting of press pack package are:

a) Uniform force distribution: It is important that theforce appearing on the pole faces should be uniformly dis-tributed over the entire surface. Otherwise, it leads to localmechanical stress on the silicon wafer and subsequently de-grades its performance [17]. It also results in uneven electricalresistance which leads to severe local heating, increase in on-state voltage and adversely affect its surge current as well asthermal cycling capability.

b) Surface condition: Two parameters of interest are (a)flatness defined as measure of the net variation of surfaceand (b) roughness defined as a measure of micro-structureof the surface [4]. In most of the applications flatness androughness should be kept less than 10µm [18]. If a goodflatness and roughness are not maintained the area of contactwill be incomplete and this results in local heating.

c) Maintaining force for longer period: Using bellevillewashers along with the nut helps in maintaining the set valueof force for a longer period of time [19].

Higher clamping force will damage silicon wafer. Estimatingthe force directly with mechanical tightening tools are notfeasible. But if the torque at which mounting bolt to betightened to achieve the required force can be estimated thenthe torque wrench can be used to estimate the force.

III. ESTIMATION OF TORQUE FOR THE REQUIRED FORCE

The applied force on the pole face of the thyristor can bemeasured by using load cell placed in between the mountingclamp and the pole face. By measuring the electrical outputfrom load cell the applied force can be estimated [5]. But

Fig. 3. Various force and torque appearing on the mounting clamp measuredusing a load cell.

TABLE I. K FACTOR FOR VARIOUS BOLT NUT CONDITIONS

Bolt Condition K factorNon-plated, black finish (dry) 0.20− 0.30

Zinc-plated 0.17− 0.22

Lubricated 0.12− 0.16

Cadmium-plated 0.11− 0.15

in this arrangement the load cell will become a part of theelectrical assembly. In this paper the load cell is used toestablish the relationship between the torque applied on themounting bolt and the force appearing on the pole face of thethyristor as shown in Fig.3.

If N number of bolts are used for mounting the thyristor andif the centre of thyristor pole faces coincide with the geometricmean of the bolt arrangement, then the force appearing on thethyristor pole face is given by,

FThy = NFbolt (1)

where, Fbolt is the tensile force appearing in one of the boltwhile tightening with a torque of Tbolt.

Literatures [6]–[8] reported that during tightening roughly90% of input energy is lost in overcoming the mating frictionunder the head, nut and mating threads and only remaining10% of input energy is converted into tensile force in the bolt.To take care this loss in energy a term called Nut factor (K)is introduced in the relation between the applied torque andthe tensile force created in the bolt and is given by [8],

Tbolt = KdFbolt (2)

where d is the nominal diameter of the bolt.Using (2) in (1) the relation between the applied torque on

the bolt and mounting force on thyristor pole face is given by,

Tbolt =Kd

NFThy (3)

A variety of factors which influences the friction betweenthe thread of bolt and nut influences the value of Nut factor(K). Some of the factors influencing are [8],

1) Shape of the threads2) Friction of the nut against the surface it rotates3) Type of material and plating used in bolt and nut4) Presence and type of washers used5) Type of lubricant used

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Considering the above factors, values of K for stainless steelmaterial at different conditions are given in Table I [6] wherevariation of K at each condition is between 15% to 20%.From thyristor datasheets it can be observed that the maximumallowable variation in mounting force is ±10% [9] whilesome manufacturer allow upto ±20% [16]. When accuracyof torque wrench is considered along with variation in K,the set value of mounting force can be outside the datasheetspecified limit. This error can be minimized if K is estimatedand validated with the fabricated mounting clamps, bolt andnut instead of choosing a typical K from a list such as Table I.Experimental evaluation of K becomes inevitable if the boltand nut are made of different materials like insulating plasticsand composites.

IV. MOUNTING CLAMP FOR PRESS PACK PACKAGE

A. Component for uniform force distributionIt is important that Fbolt appearing in the individual bolt

during tightening of its nut must be transformed as a homoge-neous compressive force on the pole face of the thyristor. Thisis achieved in two steps. In the first step Fbolt is transformedinto a single point compressive force, FThy, and in the secondstep FThy is uniformly distributed over the pole face.

a) Single point force: Keeping the pole face in contactwith the mounting plate at a single point Fbolt appearing in allthe bolts can be transformed to FThy. Single point contact canbe achieved by using a conical cup arrangement between poleface and mounting plate as shown in Fig. 4(a). Ideally thissingle point should coincide with the centre of the pole face.From the study conducted by thyristor manufacturers [20] themaximum deviation allowable for the position of FThy canbe within 2mm from the centre of the pole face. Hence thepointed tip of the conical cup can be made flat keeping themaximum radius of the flat portion within 2mm. The radiusof the flat surface appearing on the other side of the conicalcup will be equal to the radius of the thyristor pole face.

b) Uniform distribution of FThy: Since FThy is appliedat a single point on the conical cup, the flat surface on theother side of the cone will always see a uniformly distributedforce as shown in Fig. 4(a). If θ is the angle between FThy andthe slanted surface of the cone and Fs is the force acting alongthe slanted surface of the cone, then various force appearingin the cone are given by,

Fs =FThy

cos(θ/2)(4)

Fh = FThy tan(θ/2) (5)Fv = FThy (6)

From (4) and (5) as θ increases both Fs and Fh increases.If these forces are large then it may leads to deformation tothe material. If θ is very small then the height of the cone willbe large and handling such a structure will be difficult. Anappropriate value for θ can be fixed by choosing Fh equal toFv . Substituting, (6) into (5) gives θ equal to 900. For a 900

cone its height should be equal to half of its base diameterwhich is same as the diameter of pole face of the thyristor

Fig. 4. Mounting clamp (a) various parts and force on the conical cup (b)two parts of conical cup.

used.Since height of the 900 cone is decided by the pole face di-

ameter, for larger cone height it is split into two (a) upper coneand (b) lower cone as shown in Fig. 4(b). The upper cone isloosely mated with the mounting plate to allow the remainingparts within the thyristor stack to adapt the inherently presentnon-parallelisms in the heatsink and thyristor surfaces [20].

B. Selection of bolt-nut combinationThe manufacturers of bolt and nut will provide the recom-

mended clamp force, Fc, for each bolt [6]. If the bolt is madeof insulators like Glass epoxy G-10/FR4 or Vetresit R© [11] thenwe need to compute Fc for the selected bolt. Tensile area inmm2 for a given bolt can be computed approximately as [12],

At = K1[d− (K2 × p)]2 (7)

where, d is the nominal diameter of bolt in mm and p is thethread pitch in mm. For a standard bolt [12] the value of K1

and K2 are 0.7854 and 0.9382 respectively.Clamp force will be approximately 65% of the maximum

allowable tensile force, Ft, of the bolt [13]. By multiplyingthe tensile strength, Rm in N/mm2, specified in the materialdatasheet with At of the bolt gives its Ft. Therefore, using (7)clamp force of the bolt is given by,

Fc = K3[d− (K2 × p)]2Rm (8)

where, K3 = 0.65K1.Shear strength of the nut and bolt material determines the

strength of the thread when male threads pulls the femalethreads or vice versa. Since shear force depends on the cross-sectional area taking part in shearing, by increasing the lengthof engagement between bolt and nut, shear cross-sectional areacan be increased adequately and shearing can be avoided [8].

In long thyristor stack with more than 2 thyristors andtheir heat sinks, it may be difficult to obtain good mechanicalstability with mounting clamp having 2 bolts. In such caseit is recommended to use mechanical clamp having atleast 4bolts. In high voltage applications, these bolts can be madeof insulating materials, like Vetresit R©, Glass epoxy G-10/FR4.This makes the stack compact compared to stack with metallicbolt since metallic bolt requires more air creepage distance.

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(a)

(b)

Fig. 5. Deflection of mounting plate (a) supported by 900 cone (b) simplysupported at two edge.

C. Design of mounting plateIn most of the high voltage applications more than two

thyristors are connected in series result into mounting clampwith four bolts. Hence the analysis to find the deflection in themounting plate is carried out for four bolt mounting clamp.If the maximum allowable mounting force for the thyristor isFThy then each bolt will experience a tensile force of FThy/4.Various forces acting on the mounting plate with 900 coneand its maximum deflection, ωmax, is shown in Fig. 5(a). Thedotted line passing through the centre of the mounting platerepresents the line joining the corners of the plate when it is notdeflected. This is equivalent to simply supported plate at twosides and applying a force FThy at the centre of the plate, asshown in Fig. 5(b). To find the worst case deflection, simplysupported plate is considered in this analysis and the platematerial is assumed to be linear, homogeneous and isotropic.Using Navier Method [14], the maximum deflection for aconcentrated load (FThy) applied at the centre of a simplysupported rectangular plate is given by,

ωmax = αFThya

2

D(9)

where, α is a numerical factor, a is the width of the mountingplate and D is flexural rigidity of the plate given by,

D =Et3

12 (1− ν2)(10)

where, E and ν are the Young’s modulus and Poisson’s ratioof the material and t is the thickness of mounting plate.The numerical factor α is given by,

α =1

2π3

∞∑m=1

1

m3

(tanh

mπb

2a− mπb/(2a)

cosh2mπb2a

)(11)

From (11), α is a function of b/a and its value is computedfor various values of b/a in [14]. For b/a equal to 1, value

of α is 0.0116. Hence, the thickness of the mounting plate,t, is chosen for minimum deflection to the mounting plate,computed by (9), due to tensile force in the bolt. The width,a, and length, b, of the mounting plate (b ≥ a) is decided bythe clearance required between thyristor and the bolt.

V. FABRICATION OF MOUNTING CLAMP

The dc voltage rating of crowbar is 10kV and is realizedwith 6 thyristors (5STP03X6500 from ABB) connected inseries [16]. The pole face diameter of the chosen thyristoris 34mm. The minimum, typical and maximum values ofmounting force is 8kN , 10kN and 12kN respectively. Toimprove the electrical conductivity all the metallic parts arefabricated with copper of 99.86% purity. The surface electricalconductivity is improved by surface coating the fabricatedcopper parts with silver. Since silver tarnishes over time nickelcoating is applied over the silver coating.

A. 900 Conical cupSince pole face diameter is 34mm the height of 900 cone is

chosen as 18mm (rounded the required 17mm to the next evennumber). The diameter of one of the flat face of cone is keptto 4mm where as the diameter of second flat face is same asthat of pole face of 34mm. Cone height is measured betweenthe upper flat and lower flat region of the cone. For betteradaptation of non-parallelism exists in heatsink and thyristorsurface, the cone is split into two portion.

a) Upper cone: Shown in Fig. 6(a) with a height of 6mm,1/3rd of the total height of cone. The diameter of the surfacemating with mounting plate and the surface mating with lowercone is 4mm and 18mm respectively. Using a pin, shown inFig. 6(a), upper cone is located with the mounting plate.

b) Lower cone: Shown in Fig. 6(a) with a height of12mm. Its upper face diameter equal to 18mm and the lowerface mating with thyristor pole face have diameter equal to34mm. Lower cone is mated with the upper cone as well asthyristor using a locator pin.

B. Mounting barThe width, a, and length, b, of the mounting plate shown

in Fig. 6(b) is chosen to be the same and equal to 110mm tomeet the required clearance from the thyristor. The thickness,t, of the mounting bar is chosen as 10mm. The maximummounting force, FThy, of the selected thyristor is 12kN . Forthe copper material E and ν is chosen as 120GPa and 0.35respectively. Using (10) flexural rigidity of the mounting plateis given by 11.396× 103 Nm.

Since b/a is equal to 1, α is given by 0.0116. Using (9)maximum deflection in the mounting plate is 0.1478 mm.Comparing ωmax with a or b of the mounting plate showsthat it is approximately 0.14%, which is an acceptable design.

C. Selection of nut and boltTo make the crowbar compact by reducing the creepage

distance, the bolt and nut are made of insulating materials. Ma-terials used for the fabrication of bolt and nut are Vetresit312 R©

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Lower cone Upper cone

(a) (b)Fig. 6. (a) Top and bottom view of upper and lower conical cup with locatingpin (b) Top and bottom view of mounting plate.

and Vetresit300 R© respectively, shown in Fig. 7. From thedata provided by the manufacturer the tensile strength, Rm,of the bolt along machine direction and cross direction are350N/mm2 and 100N/mm2 respectively. Rm for nut in ma-chine and cross direction are same and equal to 220N/mm2.For the chosen bolt d and p are 10mm and 1.5mm respec-tively. From (8) maximum allowable Fc is 3.769 kN . Byconsidering a safety factor of 80% the maximum allowabletensile force in the bolt (Fbolt,max) equal to,

Fbolt,max = 0.8× Fc = 3.015 kN (12)

Considering the maximum mounting force of thyristor, 12kN ,and four bolt mounting arrangement, each bolt will experienceFbolt of 3kN , which is below the computed Fbolt,max. Here theshear area between the bolt and nut are improved by choosingthe length of the nut equal to two times its diameter.

VI. EXPERIMENTAL EVALUATION

A. Estimation of Nut factor (K)Load cell [5] of 2000lbf with accuracy of 1% is used to

estimate K. In the experimental setup shown in Fig. 7, thyristoris replaced with load cell and four numbers of M10 bolt withd of 10mm made of Vetresit312 R© are used. The experimentis repeated eight times with a pre-settable torque wrench set to4.5Nm and the output voltage from load cell for each iterationis recorded given in Table II. The force acting on the load cellis computed from its output voltage, considering 4V equal to8896N (2000lbf ). The computed force and the estimated Kusing (3) for each iteration is given in Table II. The minimumand maximum value of K from Table II are 0.3699 and 0.4110respectively. By averaging eight values of K given in Table II,the average value of K is given by 0.3875. In a four boltmounting clamp with d of 10mm and K of 0.3875, a mountingforce of 10kN can be achieved with 9.68Nm torque per bolt.

B. Evaluation of uniform force distributionUniform force distribution can be confirmed with the help

of pressure measuring film from Fujifilm [21]. When pressureis applied on the film a red patches appears. The density ofthe red colour depends on the amount of pressure applied.

← Load cell

Fig. 7. Experimental setup for evaluation of K showing the load cell anduse of Vetresit R© bolt and nut.

TABLE II. VALIDATION OF FORCE USING TORQUE WRENCH

Sl. No. Load cell output Equivalent Nut factor(Volt) force (N ) (K)

1 2.010 4470 0.4026

2 1.969 4379 0.4110

3 2.076 4617 0.3899

4 2.126 4728 0.3807

5 2.040 4537 0.3967

6 2.188 4866 0.3699

7 2.199 4890 0.3681

8 2.121 4717 0.3816

Average 0.3875

The area where larger pressure is applied will become moredenser in red colour and the area where red colour is lighterthe amount of pressure applied is low. Suitable film should beselected based on the nominal pressure applied on the film. Fora clamp force is 10kN and pole piece diameter is 34mm, thepressure applied is 11 N/mm2. For this pressure Low Pressurefilm (LW) is chosen for the validation of uniform pressuredistribution. The experimental setup is same as shown in Fig. 7except load cell is replaced with pressure measuring film cutto the desired dimension. A torque of 9.68Nm is applied withtorque wrench to the four bolt-nut arrangements. Conical cupused in the experiment are fabricated using Lathe machine aswell as Computer Numerical Control (CNC) machine. Fig. 8(a)

(a) (b)Fig. 8. Red patch on pressure film when conical cup is fabricated with (a)Lathe (b) CNC machine.

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Fig. 9. Mechanical assembly of 10kV, 1kA Solid State Crowbar.

Fig. 10. Voltage, Vcrowbar (2kV/div.) and current, Icrowbar (200A/div.)waveform during triggering of crowbar.

and Fig. 8(b) shows the photograph of red patches appearedon the film when conical cup is fabricated with lathe machineand CNC machine respectively. In Fig. 8(a) the pressure isapplied only at the centre portion of the thyristor where as inFig. 8(b) the pressure is uniformly distributed. This also showsthe importance of surface flatness and roughness required forthe uniform pressure distribution. During fabrication of conicalcup the surface flatness and roughness are specified as 10µm.The complete assembly of thyristor stack and the crowbar isshown in Fig. 9. Voltage, 9.878kV and current, 995.6A duringtriggering of the crowbar are shown in Fig. 10.

VII. CONCLUSION

Thyristors are to be mounted properly to meet the specifi-cations mentioned in the datasheet. Improper mounting resultinto higher electrical and thermal resistances and adverselyaffect its surge current carrying capability. In a crowbar whereit operate at high di/dt and surge current rating proper mount-ing becomes essential and it becomes critical when thyristorsare series connected. The paper discusses the mechanicaldesign method for the fabrication of mounting clamps for presspack package thyristor. The paper also detailed the method toestablish the torque-force relationship especially needed whenthe bolt is fabricated with non-metallic material. A detailedprocedure for the evaluation of mounting clamps is explainedin the paper and established the importance of surface flatness

for uniform force distribution. Based on the formulated designmethod a compact mounting clamp is designed and fabricatedfor 10kV , 1kA crowbar. Electrical performance of crowbar istested and it meets the system requirements.

ACKNOWLEDGMENT

The authors thank Dinesh Kumar - Model Shop, Sakeerand Rahul at CDAC, Trivandrum for their contribution to theexperimental activity.

REFERENCES

[1] J. Jensen and W. Merz, “Light triggered thyristor crowbar for klystronprotection application,” Proceedings of the Particle Accelerator Con-ference (PAC), vol. 2, pp. 749–751, 2003.

[2] Y. Srinivas, M. Kushwah, S. Kulkarni, and et.al., “Results of 10-Joulewire-burn test performed on 70kV rail-gap crowbar protection systemfor high power klystrons and gyrotron,” 19th Symposium on FusionEngineering, pp. 91–94, 2002.

[3] “Clamping Of Power Semiconductors,” Dynex semiconductor, Appli-cation Note AN4839, 2000.

[4] “Application note for device mounting instructions,” Westcode, Appli-cation Note 07AN01, Jan, 2010.

[5] Measurement Specialities, “FC23 Compression load cell,”Available at: http://www.meas-spec.com/downloads/FC23.pdf,[Accessed: 9th Sept. 2015].

[6] Fastenal Engineering & Design Support, “Bolted jointdesign,” Available at: https://www.fastenal.com/content/feds/pdf/Article - Bolted Joint Design.pdf, [Accessed: 9th Sept. 2015].

[7] William Tools Co., “Relation formula between bolt and tighten-ing torque,” Available at: http://www.wtools.com.tw/Relation-Formula.shtml, [Accessed: 9th Sept. 2015].

[8] Fastenal Industrial and construction supplies, “Technical referenceguide,” Available at: http://www.fastenal.com/content/documents/FastenalTechnicalReferenceGuide.pdf, [Accessed: 9th Sept. 2015].

[9] “Data Sheet-User’s Guide,” Dynex semiconductor, Application NoteAN4839, July, 2002.

[10] J. Waldmeyer and B. Backlund, “Design of RC Snubbers for PhaseControl Applications,” ABB Document 5SYA2020-01, Feb. 2001.

[11] W. Liebl, “Laminate Technologies,” ABB, Feb, 2014.[12] The engineering toolbox, “Stress in bolts,” Available at: http://www.

engineeringtoolbox.com/bolt-threads-stress-d 856.html, [Accessed: 9thSept. 2015].

[13] Garrett D. Euler, “Metric bolt strength,” Available at: http://euler9.tripod.com/bolt-database/22.html, [Accessed: 9th Sept. 2015].

[14] Timoshenko S and WoinowskyKrieger S, Theory of plates and shells.McGraw-Hill Book Company, 1976.

[15] G. A. Sites, “Proper Installation of SCRs will Extend Life,” AMETEKHDR Power Systems, Application Note 1019, Dec, 2012.

[16] ABB, “Datasheet of thyristor,” Tech. Rep. 5STP 03X6500 5SYA1003-07, Jun, 2012.

[17] A. SCHWEIZER, “Data Sheet-User’s Guide,” ABB SemiconductorsAG.

[18] “Discretes Explanations Thyristors / Diodes,” SEMIKRON, Applica-tion Note 1389.

[19] Solon Manufacturing Co., “Belleville springs keep joints tight,”Available at: http://www.solonmfg.com/springs/features.cfm,[Accessed: 9th Sept. 2015].

[20] B. Backlund and T. Schweizer, “Recommendations regarding me-chanical clamping of Press-pack High Power Semiconductors,” ABBSwitzerland Ltd, Application Note 5SYA2036-03, June, 2006.

[21] Fujifilm, “Prescale,” Available at: http://www.spare.it/prescale-fuji/Fuji-Prescale-Film-Brochure.pdf, [Accessed: 9th Sept. 2015].