The I2T (Amps2*Seconds) rating is defined as the single cycle surge current, Ifsm, multiplied by a standard pulse width of 8.3mS (sine wave is assumed).
It is used as a rule-of-thumb to gauge surge capability at different pulse widths. It works because at higher currents, Vf (forward voltage drop), is dependent on the resistive element of the diode. Vf becomes proportional to current in the diode expressed by Vf = Rdiode * If. The I2T calculation yields energy dissipated in the diode during the pulse duration. Power multiplied by pulse time gives the energy pulse. Energy dissipated during a surge current pulse is proportional to I2T and is usually the driving force behind a failure. The energy pulse causes localized heating which induces mechanical fractures or disruption of the silicon crystal structure. Calculating the maximum I2T can help determine if a diode will survive a current surge.
Example: A diode has an Ifsm rating 100A. Will it work at 150A surge for 1uS?
1. Calculate I2T
I2T = (100A)2 * 8.3mS = 83A2S
2. Determine if I2T under the new conditions is much less than the original calculation. Is the I2t calculation at 150A, 1uS much less than 83A2S?
Is 83A2S >> (150A)2*1uS?
= .023 A2S
Yes, .023A2S is << 83A2S
The diode should be able to handle a surge of 150A for 1uS.
Comment: When approaching fast pulse times (i.e. ns range), I2T using Ifsm is used as an upper limit. Operating I2t should not exceed half of the upper limit.