A Method to Determine Overload Safe Operating Area

When the load is suddenly shorted in a transistor circuit, the full D.C. rail voltage appears across the transistor and the current is limited by the gain. During this shorted load operation, the transistor is opetrating in the Forward Bias Safe Operating Region. The length of time a transistor can sustain this overload is dependent on the transistor junction temperature. This paper is a study of the variations of gain versus VCE voltage to 800 volts on a 1000 volt transistor. Also, the relationship of high voltage gain to the standard data sheet low voltage gain is investigated. Operation at high voltage and high current can result in excessive junction temperature which can cause immediate failures.. A method of determining safe operation based on the junction temperature just before a load short, using the high voltage gain and the Forward Biased Overload Safe Operating Area is presented.