Protection arrangement for a switching device

A switching arrangement as claimed in claim 8, in which the thyristor is a pressure-packaged asymmetrical thyristor.

A high-voltage bridge arrangement for the connection of a load to a power source, in which each arm of the bridge comprises a switching arrangement comprising a switching device and a thyristor, said switching device having a pair of main current-carrying terminals and a direction of current flow for current flowing through said switching device during normal operation of said switching device, said thyristor having a pair of main current-carrying terminals and a direction of forward current flow through the thyristor, said pairs of main current-carrying terminals being connected in parallel such that said switching-device current-flow direction is the same as said thyristor forward current-flow direction, the arrangement being such that the thyristor fails when voltage conditions across the switching device become potentially damaging to said switching device, and presents an irreversible low impedance across said switching device.

In a switching arrangement comprising a switching device and a thyristor, said switching device having a pair of main current-carrying terminals and a direction of current flow for current flowing through said switching device during normal operation of said switching device, said thyristor having a pair of main current-carrying terminals and a direction of forward current flow through the thyristor, said switching device and said thyristor having respective rated reverse breakdown voltages, said pairs of main current-carrying terminals being connected in parallel such that said switching-device current-flow direction is the same as said thyristor forward current-flow direction, a method for the protection of said switching device against a damaging reverse overvoltage, comprising the steps of:

• (a) in the presence of a level of reverse voltage which is greater than the rated reverse breakdown voltage of the thyristor but less than that of the switching device, allowing the thyristor to enter its reverse breakdown mode, and
• (b) allowing the thyristor to fail and to enter an irreversible low-impedance state due to the reverse breakdown mode, the reverse voltage across the switching device being thereby clamped to a safe low level.

In a switching arrangement comprising a switching device and a thyristor, said switching device having a pair of main current-carrying terminals and a direction of current flow for current flowing through said switching device during normal operation of said switching device, said thyristor having a pair of main current-carrying terminals and a direction of forward current flow through the thyristor, said pairs of main current-carrying terminals being connected in parallel such that said switching-device current-flow direction is the same as said thyristor forward current-flow direction, a method for the protection of a switching device against a damaging rate of voltage increase (dV/dt), comprising the steps of:

• (a) in the presence of a rate of voltage increase which is greater than a dV/dt turn-on rating of the thyristor but less than that which would damage the switching device, allowing the thyristor to turn on due to the excessive dV/dt;
• (b) allowing the thyristor to experience a damaging rate of current increase or a damaging thermal overload, and
• (c) allowing the thyristor to fail and to enter an irreversible low-impedance state due to the damaging rate of current increase or thermal overload, the voltage across the switching device being thereby clamped to a safe low level.