Blackburn David L. Semiconductor Measurement Technology: Safe Operating Area Limits for Power Transistors

National Bureau of Standards, 1977. — 14 p.This is a script of a videotape presentation which addresses deficiencies of I present methods for measuring and specifying the power dissipation capabilities and safe operating area (SOA) limits for power transistors. These deficiencies result from not including adequately the effects of significant temperature nonuniformities which occur, primarily for high-voltage, low-current conditions. Firstly, the specified thermal resistance of a transistor can be applied strictly only when the junction temperature is relatively uniform, otherwise the power capability of the device can be grossly overestimated. Secondly, the use of the standard method for i measuring junction temperature can lead to a serious underestimate of the peak junction temperature if the temperature distribution is not uniform. Finally, the traditional SOA limits do not consider the phenomenon of thermal instability and the hot spots that ensue. As a result, devices can operate with potentially dangerous hot spots within operating limits that traditionally have been considered to be safe. It is proposed that the SOA limits be revised to exclude operating conditions where thermal instability and hot spots can occur. A method for detecting the onset of thermal instability and triggering a circuit to turn-off the transistor under test is described. This approach can be used to develop the revised SOA limits in a nondestructive way.