Turn-on reliability breakthrough in light triggered thyristors

A 5 kV light triggered thyristor (termed the EPRI/GE 13) has been developed (fabricated on production equipment) which, for the first time, is more reliable in turn-on than the best electrically triggered thyristor at that voltage rating. Despite requiring a typical incident turn-on photo-energy of only one nanojoule (compared to the millijoule gate energy of electrically triggered devices), the new 5 kV LTT has more than double the di/dt capability of its electrically triggered counterpart fabricated on the same production line with the same basic P-N-P starting point. The secret of this success is the use of an advanced form of controlled turn-on by which lateral resistance is built into the P-base between the gate amplifying stage of the thyristor and the main thyristor region. So rugged is this device that it has this same di/dt advantage even when triggered by very weak gates. One such gate was achieved lowering the light energy to about 0.4 nJ at which point the device would barely turn on at 2000V. It was with this weak gate that half the di/dt test was done. This ruggedness under weak gate conditions proved the key to another valuable device characteristic, that of turning on safety when its dv/dt rating of 5 kV/µs to 4.9 kV was exceeded. Unlike most thyristors whose di/dt rating applied to turn-on at perhaps, 60% of its rated blocking voltage, this device could be turned on repetitively up to 4.5 kV and was life tested at 4 kV, 80% of its blocking voltage.