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The composition and use of an improved current rating system for high-current high-frequency thyristors is described. Commonly accepted methods for rating thyristors at low-frequency wide-pulsewidth conditions were not adequate for this task. To fill this void, a new system was developed that incorporates a combination of simultaneously measured high-frequency parameter data with a computer program based on thyristor models using device parameters measured at low frequency. A complete description of the high-frequency simultaneous parameter data is given, including the direct measurement of peak junction temperature, turnoff time, which is used as an indirect check on the junction temperature measurement, and dissipated power. A description of the computer models, including those for instantaneous active area, charge modulation, and thermal impedance as a function of area, is also given. These models generally require the input of empirical parameters, the value of which can be measured at low repetition rate. These include the steady state and dynamic forward drop for the charge modulation model, the plasma spreading velocity for the instantaneous active area model, and the steady state dc thermal resistance for the thermal impedance model. A direct comparison between computer predictions and actual performance for a particular device is presented as a check on rating system performance. When the computer correctly predicts device performance for specific cases, it can be used to derive ratings for any arbitrary current waveform.