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H DEC 1067 /G&L SIMPLE THYRISTOR CIRCUITS TO PULSE-FIRE IGNITRONS FOR EP-RR 16 CAPACITOR DISCHARGE H DEC 1067 /G&l C. F. VANCE March, 1967 Department of Engineering Physics Research School of Physical Sciences THE AUSTRALIAN NATIONAL UNIVERSITY ra, A.C.T., Australia. HANCOCK TJ163.A87 EP-RR16. f T.J 1 63* 1924138 . A87 F. P - R R 1 6 A.N.U. LIBRARY This book was published by ANU Press between 1965–1991. This republication is part of the digitisation project being carried out by Scholarly Information Services/Library and ANU Press. This project aims to make past scholarly works published by The Australian National University available to a global audience under its open-access policy. SIMPLE THYRISTOR CIRCUITS TO PULSE-FIRE IGNITRONS FOR CAPACITOR DISCHARGE H DEC 1967 by C. F. VANCE M arch. 1967 Publication EP-RR 16 Department of Engineering Physics Research School of Physical Sciences THE AUSTRALIAN NATIONAL UNIVERSITY C anberra, A.C.T. A ustralia S UMMARY Simple thyristor circuits have been used for firing capacitor-discharge ignitrons in a plasma physics * experiment. Experience of over 18, 000 discharges shows that the problem of semiconductor failure in an environ­ ment of high voltage transients can be overcome by atten­ tion to screening and filtering. Speed limitations of present-day thyristors are discussed and found acceptable for medium-speed plasma experiments. *(By October 1967, 48, 000 trouble-free discharges have been obtained). ii 1 . INTRODUCTION The use of thyristors in place of thyratrons has been advertised by a number of manufacturers for some time, including their use for firing ignitrons. However, de­ tailed information on this application is elusive and it has been necessary to assemble fragments of information from many sources and from local experiments to arrive at the designs described herein. No comprehensive discussion of design principles is attempted; it is not sug­ gested even that the designs are optimum, but they have survived exposure to an environ­ ment of high voltage, high current transients which plagued some earlier workers in the field. The first units developed were General Purpose Firing Modules (section 2). These fire ignitrons, the cathodes of which may be as much as 10 kV from earth, through insulated pulse transformers. The primary circuits are ’’earthy” and the inputs are deliberately attenuated. Thus large input pulses of 80 volts or more can be used in long trigger pulse cables, and false firing due to interference eliminated. To drive these firing modules, trigger pulse amplifiers (section 3) are used, these boost 6 volt step outputs from a digital tim er3 * to 90V pulses in 70 ohm cable. Finally, for the special application of firing a clamp ignitron without bringing voltage measuring leads outside the capacitor room, a self-contained Clamp Trigger Unit (section 4) was installed. It is believed that double screening of this unit has been an essential aspect in the survival of its semiconductors. 2. GENERAL PURPOSE FIRING MODULES When ignitrons are used for capacitor discharge,1 it often proves inconvenient to earth their cathodes. This has in the past led to two alternative solutions: firing units direct coupled to the ignitron, 2 having high-voltage insulated mains supply transformer and input trigger pulse connection; or earthy firing circuits with high-voltage insulation on output pulse transformers. The former is intrinsically capable of faster rising igni­ tor current and may be considered essential if the main ignitron current rise time is very short. The latter method gives an ignitor current whose initial rate of rise is limited by the pulse transformer leakage inductance which, for single layer windings, is roughly proportional to the insulation thickness. Where the slower firing pulse is acceptable, the pulse transformer option brings two main advantages: the earthed primary circuits can share a charging circuit and the transformer ratio is adaptable to suit the optimum working voltage of the chosen form of primary switching. 3 In the installation used on the toroidal plasma physics experiment at the Australian National University, it was decided to try thyristors instead of thyratrons for discharging the capacitor into the pulse transformer (Figure 1). After hearing of thyristor breakdown due to failure to observe limiting rates of current turn-on, it was encouraging to find one thyristor type designed for pulse duty. 5 (In two years no other GENERAL PURPOSE FIRING MODULE 2 UR 70 560 K Y 40216 On back piugqmg panels : Shunt Monitor, 6Vln70n/ fo r 600 A Fig.I I gnitron Firing M odule GENERAL PURPOSE FIRING MODULES 3 supplier has quoted us an equivalent.) By plotting the turn-on current-time curve on logarithmic scales and doing the same (on tracing paper) with typical waveforms, such as a half sine-wave and a critically damped waveform, it is very easy to find the short­ est permissible time to a given peak current. For the type 40216 thyristor working at 600A peak, the minimum rise time is 8 y sec for a half sine waveform and 10 y sec for a critically damped waveform. The latter was selected because of the 5V reverse igni­ tor voltage limit quoted for most ignitrons. As a compromise between rated firing conditions for BK24, BK178 and GL7703, secondary output for the units was chosen as 1. 8kV open circuit, 200A short circuit. All three types have been operated at capacitor bank voltages up to 9 kV, without a known failure to fire. The diodes (Figure 1) do not conduct during normal operation, as the circuit is somewhat over-damped. Their function is to protect the thyristors against reverse breakdown in case a unit is triggered with its secondary open-circuited. The capacitors are an extended foil type sold for photo-flash duty. It was found that ordinary rectangu­ lar cased paper capacitors gave only half output, which implies an internal inductance about equal to the transformer leakage inductance (3 u H, referred to primary). The transformer construction is described in the Appendix. The input attenuation shown in Figure 1 was chosen to ensure firing of thy­ ristors of the manufacturer's minimum sensitivity.'^ Since the current batch proved to be about three times as sensitive as this, the 5 ft between gate and cathode has been further shunted so that the average unit requires about 60V to fire. A high voltage level in the trigger cables is necessary to swamp interference. Although the rise time of ignitor current delivered by these units is 10 y sec, tests have shown that the time delay from the application of an 80V trigger pulse to the start of main anode current in a GL7703, discharging capacitors from 9kV, is only 2 y sec. 3. TRIGGER PULSE AMPLIFIERS (Figure 2) These are necessary because the basic timing signals are positive-going steps of only 6V, derived from a digital timer. ^ The poor signal to noise ratio, in a labora­ tory with currents (in a discharge plasma) rising at a few times 10^ amp/sec, necessi­ tates shunt capacitors on the input (470 pF) and output (0.033 y F). In addition the 0. 47 y F capacitors, which are discharged to produce the output, are not charged until after interference from operation of A. C. contactors is over. Finally, the lieft pre­ set potentiometers compensate for varying sensitivity of the 2SF105 thyristors. These are of a type normally used for "mains duty", but all but one out of seven proved accep­ table for fast pulse duty. This one (since replaced) turned out to require a lower voltage, but longer duration, input to fire. A typical 2SF105 in this circuit has a delay of less than a microsecond followed by a current rise time (10% to 90%) of about one microsecond. Output pulses from some of these units are used to trigger the firing modules (above), and other output pulses are used to trigger oscilloscopes. TRIGGER PULSE AMPLIFIERS 4 470 K 2 S F 10^ Output 90V peak info 70 n malched cable. 2 2 K 470 K ■AAAA Fig.2 Trigger Pulse Amplifier 4. CLAMP TRIGGER UNIT 5 The economical use of energy storage capacitors calls for the use of a clamp switch to limit the voltage reversal. In the present experiment the clamp switch is a BK178 ignitron, with a "holding” anode. Rather than take a capacitor voltage signal out to trigger the "earthy" input of a general purpose firing module, it was considered worth while making a self-contained unit to mount directly below the ignitron and use its holding anode supply. There is a dearth of information about time delays of ignitrons at low ignitor voltages so tests were made which showed: a. On first applying ignitor voltage, a current flows which is substantially what one would calculate from the "cold" ignitor resistance (a few tens of ohms) and the applied voltage. b. This continues almost constant for a time which depends strongly on applied voltage, after which the ignitor current rapidly rises, at a rate determined by the external circuit, and the holding anode current begins within a fraction of a microsecond. In one BK178, 200V gave a "current pause" lasting about 500 y sec, 300V gave 50 y sec and at 400V this delay was only 2 y sec. In the present application the voltage on the main capacitor bank goes through zero at the rate of only 5 volt/ y sec, so that 400 V supply for ignitor firing is acceptable. In the clamp trigger unit (Figure 3), this is obtained from the same insulated mains transformer as the holding anode supply, using a doubler circuit to charge a 20 y F electrolytic capacitor.
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