IEE Colloquium reportIEE Colloquium on Gate-turnoff thyristorsSavoy Place, London15th May 1985

Some 80 engineers occupied the lecture hall at Savoy Placeon Wednesday 15th May for the second IEE Colloquiumon Gate-turnoff thyristors (GTOs). During the day's threesessions, eleven papers were presented on a variety oftopics concerned with the design and application of GTOs.

The first session (Chairman J.M.W. Whiting, GECTraction Ltd.) was devoted to GTO manufacturers. Itopened with a review by D. Grant (InternationalRectifiers) of typical medium-to-large GTOs, includinginteresting photographs of large crystals and the com-ponents of hockey-puk packages. His remarks about theneed for careful gate circuit and snubber circuit designwere later echoed by many speakers.

E. Huang (Mullard Ltd.) then described a 120 A wire-bonded GTO which incorporated an antiparallel diodeunder the cathode bond pad, and showed that theresulting diode had the right characteristics for use in pul-sewidth modulation (PWM) AC motor control inverters.

Much larger GTOs, up to 4.5 kV at 2.5 kA peak, werethen described by M. Takeuchi (Toshiba UK Ltd.). Thepros and cons of amplifying gate structures were shown,with the conclusion that the diode separating the main andauxiliary gates should be external to the main crystal,because of the problems of achieving acceptable diode per-formance by selective lifetime killing. In a comparison ofGTOs using gold killing with those using anode shortingfor lifetime control, Mr. Takeuchi showed the advantagesof anode-shorted structures for combined minimumonstate voltage and minimum turnoff time, especially fordevices with blocking voltages greater than 1 kV.

The last paper in the morning session, by P. Taylor(Marconi Electronic Devices Ltd.), looked to the future ofthe device. The complex trade-offs between, on the onehand the turnon and onstate behaviour, and on the otherhand the turnoff and offstate behaviour, might be eased bya change towards buried-gate structures, even to the field-controlled thyristor (FCT).

There followed an energetic discussion on the causes of'undershoot' at the end of turnoff, and on the need forstandardisation of the circuit symbol and the data par-ameterisation of the GTO.

The second session (Chairman P. McLoughlin, GECIndustrial Controls Ltd.), mostly concerned with modellingand circuit techniques, opened with a contribution fromR.L. Coleman and P. Hartland (ERA Technology Ltd.) oncomputer simulation of the transient behaviour of GTOs.The eventual aim is to model GTO behaviour from knowl-edge of the manufacturer's published data, and so to assessthe stability of a device for an application before outlayingmoney.

J. Heard (GEC Power Engineering Ltd.) then gave adiscussion of a technique using capacitor discharge to turnoff a GTO, in which the storage time could be tailored byusing an applied negative gate voltage initially in excess ofthe gate-cathode reverse breakdown voltage, but withoutexcessive dissipation. This permitted more flexibility inpositioning of the gate-drive circuit, by allowing longerconnecting leads. The author pointed out a deficiency inthe data sheets of some manufacturers, which do not givefigures for maximum average gate power.

IEE PROCEEDINGS, Vol. 132, Pt. B, No. 6, NOVEMBER 1985

The third paper of the afternoon session, given by H.Gibson (GEC Power Engineering Ltd.) gave a generalsurvey of the special features required by the componentsin snubbers and antiparallel diodes around the GTO. Theproblems of choosing snubber capacitors and snubberdiodes were highlighted.

M. Fulcher (GEC Industrial Controls Ltd.), in the lastpaper of the afternoon, moved towards the main topic ofthe third session; practical applications of GTOs. Hedescribed their use in variable-frequency PWM AC motordrives, comparing them very favourably to other powersemiconductor switches. Overcurrent protection of eachGTO within its own gate-drive circuit was seen as essen-tial, but needs minimised storage times or large series reac-tors.

In the lengthy discussion that followed, topics rangedfrom comparison of GTO gate drives with transistor basedrives to techniques for a four-fold reduction of snubber-diode forward recovery voltages by 'priming' them with asmall current.

After tea, the evening session (Chairman R.W. Stokes,ERA Technology Ltd.) covered more GTO applications.T.G. Carthy (Renold PLC) gave a case history of the intro-duction of GTOs into an existing AC motor controllerproduct range, to extend to 3-phase mains inputs andhigher motor powers. He discussed the problems involvedin making the circuit usable with GTOs from more thanone manufacturer, having different characteristics.

D. Smith (TASC Drives Ltd.), in describing the suc-cessful application of GTOs in inverters for switched-reluctance (SR) motors, drew attention to the lack ofapplication information (and sometimes of data) forGTOs, creating an aura of mystique about their use whichmight discourage potential users from joining the club.

The last contribution, from D.A. Ioannidis (GEC Trac-tion Ltd.), described applications of the GTO in railwaytraction, with examples from public passenger(underground) and from goods stock (docks railways). Thedifference in design constraints imposed on each, forexample the difference between natural cooling and forced-air cooling, resulted in the choice of GTOs with similarcurrent ratings for widely different motor power ratings.

Discussion followed concerning the relative efficienciesof GTO and thyristor inverters (with the GTO having onlytwo-thirds the losses of the thyristor). Considerable interestwas shown in the SR drive.

In general, the papers were well presented and listenedto attentively by those present. The discussions were lively(one lasting almost 90 minutes). If there is one slight reser-vation on the part of this participant, it is that the twoyears since the last GTO Colloquim at the IEE might betoo short a time for enough significant advances to bemade. This is reflected in the reduced number of peopleattending compared with the previous occasion. However,the fervour of the GTO devotees made a success of theevent.

F.J. BURGUM

4185B

345