1 August, 1994 OCR OutputGeneva, Switzerland

C0sta Mesa, Calybrnia, 27-30. 06.1994Paper presented at the 21 st International Pawer Modulator Symposium

is required.kicker systems [1] of the Large Hadron Collider, where a 35 kV, 30 kA, 3 us risetime switchsuccessful, the switch is an interesting altemative to the gas switches for the beam dumpingpossible tum-on delay differences as well as the over-voltage protection circuit are discussed. Ifconsisting of ten in series connected 4.5 kV FHCT devices is being prepared. The effect oflosses and conduction losses are only a few Joules. At this moment the construction of a switchThe gate controlled tum-on delay time is about 600 ns. The measurements show that tum-onpractically reduced to zero. One important operation condition is a fast and powerful gate drive.70 kA at an off-state blocking voltage of 1.6 kV. The tum-off capabilities of these devices are32 kA amplitude. Recent tests achieved a di/dt of 20 kA/us with a current pulse amplitude of

Our tests have originally resulted in a di/dt of l6 kA/its for a half—sine wave current pulse ofon characteristics.with light gold doping, show that the device can be used as a closing switch with excellent turnHowever recent tests on a symmetrical 4.5 kV, 66 mm unshorted anode FHCT, manufacturedprocesses, in particular irradiation tend to slow down the tum-on characteristics of the device.by three methods, heavy metal doping, diffused anode—shorts and irradiation. All these

Normal GTO thyristors are optimised for their gate turn-off characteristics. This is achievederratically and are not subjected to electrode erosion.lifetimes. Besides that they do not need high voltage holding conditioning, cannot conductpreferable to gas switches because they operate with higher efficiencies and have longertraditionally been reserved for gas tubes. Solid state devices as these are for certain applicationsrestricted to gas switches. FHCT's are making headway in fields of application which hadfrom blocking voltages of several kilovolts. Such high di/dt capabilities have up to now beenThyristors, FHCT's) allows switching tens of kiloamps with a very high rate of current rise

The interdigitated gate structure of optimised high power GTO thyristors (Fast High Current

Abstract

WESTCODE SEMICONDUCTORS Ltd., Chippenham, U.K.M. Evans, F. Wakeman, R. Youdan

J. Bonthond, L. Ducimetiere, G.H. Schroder, E.B. Vossenberg

GTO THYRISTORS

HIGH CURRENT, HIGH DI/DT SWITCHING WITH OPTIMISED

me BN Bw

CERN-SL-94-50LHC Nm. 286lllllllllllllllllllllllllllllllllllllllllllllllllllIllllll

CERN SL/94-50 (BT)

CERN LIBRARIES, GENEVA

CERN — SL Division

EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH

must be held charged and ready to fire. The current risetime of The LHC beam dumping system operates between 450 Ge\ OCR Output12 pulsed magnets are used and 12 high power pulse generators Table l shows the most important switch requirementsleading to the dumps are fast extraction systems. For each beamroughly tangentially to the LHC ring. The two extraction lines Table 1: Switch requirementsabsorber blocks are located at the end of tunnels excavated Prefire rate < 10‘°available at all beam energies from 0.45 to 7 TeV. The beam

Total lifetime 100000 pulses89 us and safely dumped. The beam dumping system should be Repetition time typ. @ 35 kV 6 h332 MJ have to be rapidly extracted in one beam revolution of Repetition time min. @ 2.25 kV 30 sin case of emergency the beams, each with an energy of up to Charge transfer pos./neg. 89/20 mCper nucleus, using lead-ion beams. At the end of a physics run and Current pulse duration 8 uselectron beam and ion-ion collisions up to an energy of 1148 TeV

Current rate of rise/fall 12/30 kA/uscollisions, namely electron—proton events by using the LEP

Current rise/fall time 3/1.5 usmass, and also allow investigation of other types of particle

Current amplitude pos./neg. +30/-15 kAwill produce proton—proton collisions of 14 TeV in the centre-of

D.C. voltage range 2.25-35 kVconstruct a new accelerator. the Large Hadron Collider (LHC). lt

CERN, the European Particle Physics Laboratory, plans to high power switch S should operate with a very low prefire rate.Beam dumping must work with a daunting reliability. The

Introduction

(Hor. scale 1 us/div., Vert. scale 5 kA/div.)where a 35 kV, 30 kA, 3 us risetime switch is required.

Figure 2 Switch and magnet currentbeam dumping kicker systems [I] of the Large Hadron Collider.the switch is an interesting altemative to the gas switches for the

. t... .-20as the over—voltage protection circuit are discussed. If successful.-15prepared. The effect of possible tum-on delay differences as well

-10consisdng of ten in series connected 4.5 kV FHCT devices is beingonly a few Joules. At this moment the consuuction of a switch -l---2-—»3--4-\~5 » -877 97 7 710measurements show that tum-on losses and conduction losses are

The gate controlled tum-on delay time is about 600 ns. Theimportant operation condition is a fast and powerful gate drive.

10capabilities of these devices are practically reduced to zero. One S wifdi1570kA at an off-state blocking voltage of 1.6kV. The tum-off

;·· Mogwai;20achieved a di/dt of 20 kA/us with a current pulse amplitude of

a half-sine wave current pulse of 32 kA amplitude. Recent tests 25Our tests have originally resulted in a di/dt of 16 kA/us for *~;.;;;.;.:;;.; ;; .. - ..30

closing switch with excellent tum-on characteristics.with light gold doping, show that the device can be used as asymmetrical 4.5kV, 66 mm unshorted anode FHCT, manufactured Figure 1 Basic circuit diagramcharacteristics of the device. However recent tests on a Lm=2.l pH Tl=12.5 Q. 20 ns S=CX—l575Cin particular irradiation tend to slow down the tum-on C=2 pF Rs=O.7 Q Ls. Lf‘=l 50 nHdoping, diffused anode-shorts and irradiation. All these processes.characteristics. This is achieved by three methods, heavy metal

Nomial GTO thyristors are optimised for their gate turn—0f`f Df

electrode erosion.

Lmconditioning, cannot conduct erratically and are not subjected to - ° *1..1* Lf S Rslifetimes. Besides that they do not need high voltage holding

Ds I Ls A I T1because they operate with higher efficiencies and have longerthese are for certain applications preferable to gas switches 35kVtraditionally been reserved for gas tubes. Solid state devices as 2.25FHCT's are making headway in fields of application which had

shown in Fig. l and Fig. 2.capabilities have up to now been restricted to gas switches.

the first 10 ps of the current in the closing switch and magnet arefrom blocking voltages of several kilovolts. Such high di/dtThe basic circuit diagram of the prototype generator andswitching tens of kiloamps with a very high rate of current rise

GTO thyristors (Fast High Current Thynstors, FHC'I`s) allows Generator and switch requirementsThe interdigitated gate structure of optimised high power

purpose and the flat top duration should be 86 us.

Abstract 3 us must be synchronised with the gap in the beam left for this

WESTCODE SEMICONDUCTORS Ltd., Chippenham, United KingdomM. Evans. F. Wakeman. R. Youdan

CERN, Gcncva. Switzerland

J. Bomhond, L. Ducimctiérc. G.H. Schrédcr, E.B. Vosscnbcr

HIGH CURRENT, HIGH DI/DT SWITCHING WITH OPTIMISED GTO THYRISTORS

effect on the maximum di/dt. area closest to the cathode will start conducting prior to the more OCR Outputslow down the tum-on delay tgd, however it seems to have no plication effects bring the FHCT into complete conduction. Theat a clamping force of 25-35 kN. The silicon wafer thickness does and electrons into the base regions continues until charge multian overall diameter of 100 mm and compressed height of 26 mm brought into conduction by plasma spreading. lnjection of holesis built into a sealed 3 inch presspack cell housing. The FHCT has the edge of the cathode n·ernitter, the remainder of the emitter lximproved mechanical suength and high themtal capacity. The disk emission from the cathode to the anode. Turn—on is initially alonglayer is alloyed on a 4mm thick molybdenum disk to obtain Injection of hole current from the gate to cathode causes electronmain n-layer wafer, which has a thickness of 1.25 mm. The p-base The tum-on mode is very similar to that of a thyristorgate p-layer and the p-base layer of 50 pm are diffused on the

Table 2: Main data cathode-gate structurefor a 10 msec pulse. The cathode n—layer of 20 um thickness, theunshorted anode device with an action integral of l3O0xl0As A/mrntCathode current density at 30 kA 843 z

The standard symmetric GTO, type WG20045. is a 4.5 kV %Cathode/anode surface ratiofor tum-on delay and leakage current. mm`358Total surface cathodeproduced a series of 15 FHCT's of the same type. They were tested Total boundary line gate—cathode 7.lstudied in more detail at CERN. After the tests WESTCODE Total number of cathode islands 1436

values. WESTCODE tested several 4.5 kV devices which were Anode / gate p-layer thickness 70 / 50been investigated in a test circuit in order to detemiine their limit 20Cathode n·layer thickness

The FHCT’s, manufactured by WESTCODE, GB have mmMolybdenum disk thicknessdiscussed below. mm1.25Silicon thicknessto conduct extremely fast under certain conditions, which will be mm66Wafer diameterperiphery, the dynamic plasma spread at tum—on allows the deviceshort cathode-gate distances and extremely long gate—cathode Figure 3 Cathode-gate structureinterdigitation of the ring gate-cathode structure. Due to the very125 A/us [2]. This has been made possible by a high degree ofwith a maximum gate current of 50 A and with a leading edge offast current pulses of 7 kA for 2 ps at a di/dt level of 15 kA/its

However, recently it has been shown that it is possible to switchmaximum rate of rise of current during tum—on (600 A/ps).

The apparent limitation of the standard GTO thyristor is its

General

Fast High Current Thvristor€7li7IIl£

5fI »¢)idevelopment and extensive lifetime testing has been temiinated.gate turn-on thyristors. The final choice will be made when the IIEswitch types are under study: pseudo spark switches and optimiseddynamic voltage range. For this reason two other better suitedshould have 3 gaps. However this is at the cost of a too lowIn order to obtain an almost pretire free operation the thyratron schematic lateral structure of a symmetric GTO during tum-on.current pulse discharge actions is very inferior to their capabilities. Table 2 gives the main wafer data, while Fig. 4 shows theon a regular basis, whereas in this application the number of Fig. 3 shows the cathode-gate finger structure of the FHCT.1575C hollow anode thyratron. Thyr·atrons require a replacement diameter.

The prototype generator is equipped with a 2 gap EEV CX centimeters in a conventional type of thyristor of the same7 meters of gate-cathode periphery compared to about 30with about l0° fast closing, high current actions.

requirement for switch S is a prefire free operation at 35 kV DC highly interdigitated gate-cathode structure creates approximately

acceleration to 7 TeV takes about 30 minutes. The main has high forward blocking voltage and high di/dt capability. Theabout 6 hours, while the machine filling at 450 GeV and rate closing switch in fast pulse power applications. The deviceabout 3.5 mC. At 7 TeV the physics run of the LHC machine lasts This device has been opdmised for use as a low repetitioninterrupts the negative switch current at an amplitude of I5 kA is state voltage drop resulting in low tum—on and conduction losses.beam energy, the recovery charge of the series diode stack, which doped to ensure an imifonn switching-on with an acceptable oncharging voltage of 35 kV which corresponds to 7 TeV stored dissipation. In this case the device has only been lightly goldby the series diode stack Ds and critically damped by Rs. At a voltage drop of the device and consequently higher powerinductances Ls and Lf. The oscillating switch current is interrupted disadvantage of carrier lifetime control is the increased on—statethe oscillating frequency between C and the sum of the stray and irradiation, typically electron, alpha and gamma. Thevery rapidly. The rate of switch current decrease is determined by are used such as heavy metal doping, typically gold or platinumconducting. At the same time the current in switch S decreases radially uniform around the device. Many fomis of lifetime controlcapacitor voltage reverses and the diode stack Df becomes recombination of the carriers is controlled and this takes place

the switch S. After the current has reached its maximum the with controlled carrier lifetime. When the GTO current reduces.discharging the capacitor C into the inductive magnet load Lm via begin to support a blocking voltage. GTO's normally are designedstability of 5 % percent. The current pulse is generated by conduction, have to be removed quickly after which the device cannsetime of 3 ps and a flat top length of 86 us with a flat top charge carriers in the bulk of the GTO, which are due to forwardenergy. The required beam dump magnet current has a maximum which takes a very short time to open. During tuming-off theinjected proton beam energy and 7 TeV maximum stored beam A normal GTO may be considered as a closing switch

The FHCT has to profit of its maximum forward blocking OCR Outputshould limit the reverse voltage across the FHCT. Stray inductance20 % current overshoot.volmge drop. The diode has to provide a quick current by-pass andselected. The pulse shaping circuit is adjusted for approximatelyrating, fast recovery time, low leakage current and low forwardinductance, high surge current type for photoflash applications wasThe 4.5 kV antiparallel diode D is selected on high currentconfiguration. For the main electrolytic capacitor C. a loulonger decrease delay times and tum-on losses.for the IGBT's. The IGBT's are connected in a common collectorsize of the gate current is chosen such that further increase does nowhich is amplified via a driver circuit into a 15 V gate drive pulsefimctions of the gate pulse magnitude and risetime. In this case the

The gate drive unit operates with a fiber optic input pulse.time (tgt) and the tum-on energy losses (Em), are all strongconnected in parallel.minimum. The tum·on delay time (tgd), the gate controlled tum-onnecessary for protection of the switching element. two lGBT'svery important. Loop inductances in the gate trigger were kept to aadded for risetime improvement. A current limitation resistor isFHCT remain in conduction. The performance of the gate drive isFig. 7 is a capacitor discharge circuit with a pulse shaping sectionminimum level of 100 A to ensure that all the active areas of the

Basically the gate drive unit used for the test and shown inconduction time of D, the gate current was maintained at athe total length of the current oscillations, including the

Figure 7 Gate drive circuitpulse of 240 A. with a leading edge of 200 A/us was used. During

COM isovFor tum-on, a gate drive producing at 150 V a gate currentand conduction losses of the FHCT andthe antiparallel diode D. COM 15V

somwaveform across the FHCT, the gate current signal and the tum-on

Fig. 6 shows the circuit discharge current, the voltage` I/( ,, , I l rw __ . {

. (mtiFigure 5 FHCT test circuit.L N lC=l8 pF, L=195n1—1, FHCT=WG20045E6G, D=SM45CXC864 Q’_§’“$U +—i Tu- Hq v··* f·;r‘ l l t` l .

+15V

+150V

Trigger circuit

Table 3: F HCT data measured in the test circuit

Conduction losses (1-6 us Joules4. 4kv FHCT

JoulesTum-on lossesthe LHC beam dump generator.32 kAMaximum currentquarter period length of 3 us which corresponds to the risetime of

kA/us16Maximum di/dtwaveform is a damped sine-wave of about 6 oscillations, with aI •Ig3te {O I •\/anode) linductance. The 30 kA amplitude circuit discharge current

Gate controlled tum-on time tgtSM45CXC864, is mounted close to the FHCT with minimum loopns(0. l•lgm to 0.9•VmOdc) 550stray inductance. The antiparallel diode, type WESTCODE

Gate controlled tum·on delay time tgdobtained. The capacitor C is charged to 4.4 kV. L is the circuit240Gate current amplitudeFig. 5 shows the test circuit, with which results have been

Mus200Gate current rate of rise

4.4 kVTest circuit Test voltage

The date measured in the test circuit are given in Table 3.around the device.voltage transient across the F HCT.features will allow a very rapid uniform spreading of the plasmaare kept to a minimum since these give rise to a large negativeexceptionally high gate pulse together with the above mentioned

kept to a strict minimum. For this particular operation theFigure 6 FHCT test voltage and current waveformsachieved. The level of inhomogeneities in the cathode periphery is

high level of uniformity of the structure during processing isinitially tumed-on periphery is very large and it is essential that aalmost 1500, heavily interdigitated with the gate electrode, theDue to the high number of narrow cathode elements. in this casesuch that no one area of the device generates a thermal hot spot.ure is that this power dissipation is made as uniform as possible,remote areas and thus start to dissipate power. The important feat

WivFigure 4 Schematic lateral structure (turn-on mode)

electrons ——h01es A

v ` v v \ v

'§§§Yll

Research Project 2745-1. Palo Alto, California. August. 1987.200 A. The leakage inductance is smaller than 0.85 pH. The droopElectric Power Research Institute report no. EL-5331.primary voltage is 2 kV and each secondary voltage is 200 V at

[3] H. Mehta, "Series Cormection of Gate-Tumoff Thyristors"consists of one primary winding and l0 secondary windings. TheMexico. June 21-23, 1993.transformer with 40 kV isolation. The step-down transformerlntemational Pulsed Power Conference, Albuquerque, NeuThe FHCT's will be triggered via a special pulseSemiconductors AG, CH-5600, Switzerland, 9th IEEEvoltage waveforms of the slowest F HCT is shown in Fig. 9.thyristors for closing switch applications". ABBwith parallel capcitances of 100, 250 and 500 nF. The calculated

[2] E. Ramezani, A.Welleman, "High peak current, high di/dtof 300 ns. The overvoltage on this particular device was calculatedSymposium, San Diego, California, June 25-28, 1990.step was the introduction of one FI-ICT with an extra tum-on delayproposed 16 TeV collider LHC", 19th Pulse Power Modulatorthe FHCT when triggered with the gate drive unit of Fig. 7. Nextpulse generator for the beam abort system of CERN'ssimulated. The simulation is based on the voltage breakdown of

[1] G.H. Schroder, E.B. Vossenberg, "A prototype high powerequivalent switching circuit a series suing of 10 FHCT‘s wasPSPICE analogue simulation program. After development of the

Referencesthe capacitor the switching FHCT was first simulated with theaccommodated by parallel capacitors [3]. To determine the size of

a full scale 35 kV, 30 kA solid state switch starts later this year.in switching speeds and tum-on delays of the FHCT`s will be voltage protection during tum·0n. The construction and testing ofspread of 100 ns with practically no jitter was found. Differences switching speeds of the FHCT's and ensure a satisfactory overdelay times of` each device have been measured. A maximum Parallel capacitor arrangement can accommodate mismatches inWith the earlier mentioned gate drive power supply the tum—on can probably in certain cases replace gas and vacuum switches.

Figure8 shows the basic circuit diagram of the switch. minimum. For high voltage applications, series connected FHC`T'sDue to the soft tum—on, noise levels are kept to a

Figgre 8 String of 10 in series connected FHCT’sstrings of FHCT‘s.code can provide useful data for the development of high voltageBMOBMO BMOtum-on of series connected mismatched devices with the PSPICE

supplies must be optimised for each application. The simulation ofkiloamps. The life time carrier control and the gate drive powerswitches with a di/dt of 20 kA/ps and peak currents of tens ofswitching high currents. Elements of 4.5 kV can operate as closingC I C

FHCT's with large junction sizes are capable of fast

ConclusionCD I ®

switch is operational.behaviour of the FHCT will be analysed in detail as soon as the

open cage snucture with 9 rods. FHCT is being developed by WESTCODE. The switchingclamping force. The coaxial mounting fixture is realised by an An adequate PSPICE subcircuit model of the high speedThe design of the mounting fixture ensures an evenly disu·ibuted is simple to detect.clamped with a force of 25-35 kN in a coaxial mounting fixture. the voltage over the last FHCT will increase by about 10 %. whichtype WESTCODE, SM45CXC864. The stack of l0 FHCT‘s will be ln case one FHCT breaks down the switch stays operational andvoltage divider. Every FHCT is connected to an antiparallel diode, by means of the voltage on the FHCT which is on earth potential.

3 MQ, 5 watt metal film resistors are taken for the static Ari interlock will control the voltage division of the dividervoltage and to install only the 50 Q gate-cathode resistor. of the F HCT's.discussions with WESTCODE not to use a negative gate bias sufficiently long gate pulse during 6 us to avoid partial tuming—oft`and the low operation junction temperature it was decided after is better than 30% over 5 us. The FHCT string will receive athe beam dump generator and in light of the low leakage currentsDue to the very slow voltage increase of the discharge capacitor of (Hor. scale 0.5 ps/div., Vert. scale 500 V/div.)The average value was 21 uA with a standard deviation of 14 uA. time of 300 ns and protection capacitor of 100, 250 and 500 nF

Figure 9 Over-voltage of one FHCT with a delayed turn-onleakage current was 58 uA while the minimum current was l0 uA.resistance of 50 Q. 15 Samples were tested and the maximum

0 0.5 1 1.5 2 2.5 3without reverse gate bias voltage but with a gate-cathodedivider the leakage current of each FHCT was measured at 3 kV

0.5be connected in series. To determine the size of the static voltageFor the LHC beam dump proto-type switch 10 devices will

1.535 kV, 30 kA Switch

2.5 "“ C=500nFtemperature of 25°C.ensures a correct blocking capability at a FHCT junction

3.5 LH-y`-y-~—-—]__ ;..4.L`\. · , C=250nFvoltage has been omitted and a 50 Q gate-cathode resistance· c=i00¤rcircuit ensures full blocking voltage. ln this case the negative bias

nY .4_5of the device. Also the low impedance of the traditional tum—off' `

reverse biased by 2 Volts. This ensures the full blocking potentialvoltage. Normally in more Lraditional applications thc gate is