Power semiconductor ratings under transient and intermittent loads

Methods for rating semiconductor power rectifiers and controlled rectifiers under steady-state conditions are well established and accepted. These techniques assume uniform temperature across the entire semiconductor junction area and limit this temperature under different load and ambient conditions to a predetermined level consistent with good performance and high reliability.1,2Supplementary methods for determining junction temperature under transient and intermittent loads are necessary for such applications as motor-starting and lamp-dimming controls,3 and for calculations involving other types of overload duty on semiconductor equipment. This paper discusses methods of handling these intermittent duty types of applications. Examples illustrate suggested techniques for determining the peak junction temperature under various kinds of intermittent loading.