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What does critical rate rise of off-state voltage dVD/dt means?

Latest Updated:02/01/2008

Question:

A cause of malfunctioning of a thyristor is critical rate rise of off-state voltage dVD/dt. What is this critical rate rise of off-state voltage dVD/dt?

Answer:

dVD/dt is an inclination of rising of the applied voltage as shown below when a surge voltage is superimposed onto the AC supply voltage waveform or when the switch is turned ON.
If this abruptly rising voltage is applied to the anode as a positive voltage and to the cathode as a negative voltage, the thyristor faultily turns ON.
For the thyristor, the maximum voltage rise rate at which the product does not change its state from the OFF state to the ON state is defined as critical rate rise of off-state voltage dVD/dt [V/us].

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