Thyristor Turn-Off Time Trade-Offs

Turn-off time in thyristor devices is one of the most important dynamic characteristics to be considered in the design of commutation circuits for high frequency power conversion systems. Either for reducing the size of the commutating components, or for obtaining maximum operating frequency, a short turn-off time is desirable. The turn-off process amounts essentially to the removal of the excess charges which are stored in the various regions of the device structure during forward conduction. Forced commutation, in which the current is forced in the reverse direction through the structure of the device, provides a means to speed up the device recovery by removing a portion of the stored charges. The circuit parameters affect the rate of charge removal, and they must be adjusted to provide enough time interval to allow total device recovery. Reducing the device turn-off time can be achieved by adjusting some of the physical parameters of the device structure. However, a trade-off is always involved since changing these parameters also affects other important device characteristics. This paper describes the turn-off process in thyristor devices. The effects of device parameters on the device turn-off time are examined in regard to other important electrical characteristics.