AN5381

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AN5381-2 February 2014 LN31349 Author: Colin Rout The non-repetitive surge current ITSM and the I2t value define the limit of the electrical stress in the forward direction of a thyristor provided that it is triggered with sufficient gate current. These characteristics of the semiconductor are used to design short circuit protection, namely fuses or circuit breakers. By definition, this level of stress does not destroy the thyristors or diodes. If a thyristor becomes short circuit in the forward direction and a current flows which is greater than the surge current limit, destruction of the encapsulation will not normally occur until this current is substantially greater than the surge current. This is because the thyristor is effectively triggered on by the fault current and normal injection over a large area of the silicon takes place. Fig 1. Fault current flowing back through a failed thyristor in a 3 phase bridge which shorts out two phases of the supply. If the thyristor becomes defective in the reverse blocking state, a short circuit current can flow in the reverse direction. The cathode area that remains undamaged does not take part in carrying the current. A small edge around the failure melts and an arc develops in the case. The intense heat generated by the arc will lead to either cracking of the ceramic case through thermal shock or melting of the metal flanges of the encapsulation. Hot plasma then escapes through the breach into the enclosure. In high power installations where strong magnetic fields exist, an equipment short circuit or even burn down of the equipment may be the consequence. Fig 2. A failed device in a parallel application can experience a current equal to the full forward current through all parallel paths. The case non-rupture current rating is the value of the half

devices to have a specified value of non-case rupture rating and that it has no control of the variations that are observed. Any values of case non-rupture current are only indications of observed results to date and cannot be construed as ratings values or guarantees. Testing devices has shown that there are three main places where the hermeticity of the device is breached. At currents just above the rupture current the electric arc that is struck through the ionised gas inside the enclosure can burn through the thin copper of the flanges. Fig 4. Thyrisor after test showing hole burnt in copper flange At higher current levels the ceramic can crack due to thermal shock. Fig 5. Thyristor after test showing cracked ceramic due to thermal shock Dynex has designed arc shields which contain the arc and keep it away from the ceramic and the flange. Fig 6. Schematic diagram of arc shield design With the ceramic and flange shielded from the arc, the cold weld that joins the upper and lower flanges together is the weakest point and is pulled apart by the build up of pressure inside the ceramic. The resulting non case- rupture rating is substantially improved over that for devices without the arc shields. All these failure modes would emit hot ionised gas into an equipment cubicle. Arc Shield

be degraded to below 200V. Only then were the basic units encapsulated in the ceramic housings. Again, to try and produce the worst case scenario, the failure point was aligned with the gate tube in the ceramic which might be deemed to be the weakest point.

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