The I^{2}T (Amps^{2}*Seconds) rating is defined as the single cycle surge current, I_{fsm}, 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, V_{f }(forward voltage drop), is dependent on the resistive element of the diode. V_{f} becomes proportional to current in the diode expressed by V_{f} = R_{diode} * I_{f}. The I^{2}T 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 I^{2}T 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 I^{2}T can help determine if a diode will survive a current surge.

Example: A diode has an I_{fsm} rating 100A. Will it work at 150A surge for 1uS?

Solution:

1. Calculate I^{2}T

I^{2}T = (100A)^{2} * 8.3mS = 83A^{2}S

2. Determine if I^{2}T under the new conditions is much less than the original calculation. Is the I^{2}t calculation at 150A, 1uS much less than 83A^{2}S?

Is 83A^{2}S >> (150A)^{2}*1uS?

= .023 A^{2}S

Yes, .023A^{2}S is << 83A^{2}S

The diode should be able to handle a surge of 150A for 1uS.

Comment: When approaching fast pulse times (i.e. ns range), I^{2}T using I_{fsm} is used as an upper limit. Operating I^{2}t should not exceed half of the upper limit.