High Current, High DI/DT Switching With Optimised GTO Thyristors

Abstract

The interdigitated gate structure of optimised high power GTO thyristors (Fast High Current Thyristors, FHCT’s) allows switching tens of kiloamps with a very high rate of current rise from blocking voltages of several kilovolts. Such high di/dt capabilities have up to now been restricted to gas switches. FHCT’s are making headway in fields of application which had traditionally been reserved for gas tubes. Solid state devices as these are for certain applications preferable to gas switches because they operate with higher efficiencies and have longer lifetimes. Besides that they do not need high voltage holding conditioning, cannot conduct erratically and are not subjected to electrode erosion. Normal GTO thyristors are optimised for their gate tum-off characteristics. This is achieved by three methods, heavy metal doping, diffused anode-shorts and irradiation. All these processes, in particular irradiation tend to slow down the turn-on characteristics of the device. However recent tests on a symmetrical 4.5kV, 66 mm unshorted anode FHCT, manufactured with light gold doping, show that the device can be used as a closing switch with excellent turn-on characteristics. Our tests have originally resulted in a dildt of 16 Wps for a half-sine wave current pulse of 32 kA amplitude. Recent tests achieved a di/dt of 2 0 W p s with a current pulse amplitude of 70 kA at an off-state blocking voltage of 1.6 kV. The tum-off capabilities of these devices are practically reduced to zero. One important operation condition is a fast and powerful gate drive. The gate controlled tum-on delay time is about 60011s. The measurements show that tum-on losses and conduction losses are only a few Joules. At this moment the construction of a switch consisting of ten in series connected 4.5 kV FHCT devices is being prepared. The effect of possible turn-on delay differences as well as the over-voltage protection circuit are discussed. If successful, the switch is an interesting alternative to the gas switches for the beam dumping kicker systems [ I ] of the Large Hadron Collider, where a 35 kV, 30 kA, 3 ps risetime switch is required.