AN5001

Although the straight line model is basically simple, variations in definition can lead to significant differences in calculated powers.

Different manufacturers of power semiconductors have defined V_T0 and r_T in different ways. Here are 4 variations:
1) As fig. 1, where the line is the tangent to the V_TM vs I_T curve at the average current.
2) As fig. 2, where a chord is drawn through I_T(AV) and 3xI_T(AV).
This variation is the one used by Dynex for the calculation of data sheet power losses and current ratings. The definition is commonly used for thyristors. For rectifier diodes a chord through 3xI_T(AV) and 5xI_T(AV) sometimes gives a better result.
3) A variation of 2 which uses two straight lines instead of one to approximate to the true curve. In this version the lines and 4 give adequate accuracy up to 3 x I_T(AV).

For improved accuracy a mathematical model is needed which approximates better to the true curve.