Why Three Parameters Define Semiconductor Protection

I2t, cut-off current, and arc voltage are the three defining characteristics of a high-speed DC fuse -- yet they are frequently misunderstood or ignored in favor of simpler voltage/current ratings. For designers protecting expensive semiconductor devices in EV inverters, DC-DC converters, and PCS systems, these parameters are the difference between reliable protection and costly field failures.

What Is I-squared-t?

I2t (pronounced 'eye-squared-tee') is the energy let-through by a fuse during a fault -- the time integral of the square of the fault current over the clearing time. Two components are reported. Pre-arcing I2t (I2t pre) is the energy required to melt the fuse element during the pre-arcing phase. Total clearing I2t (I2t cl) is the total energy from fault initiation to final circuit interruption, including arc energy. For semiconductor protection, pre-arcing I2t is the more critical value: it represents the energy delivered to the protected device before the fuse even begins to open.

170M18XX I2t Data at 1000 Vdc, 50 kA

An IGBT module with a 50,000 A2s I2t rating requires fuses with total clearing I2t below this threshold at maximum fault current. Note that I2t increases rapidly with fault current -- verify coordination at maximum available fault current, not nominal current.

Cut-Off Current (Peak Let-Through)

During a short-circuit fault, the DC fault current rises exponentially from zero (unlike AC which can be interrupted at a zero crossing). A fuse does not open instantaneously -- it requires time to melt the element and extinguish the arc. The cut-off current curve shows the maximum instantaneous current the fuse allows as a function of the prospective (available) fault current. At very high prospective currents, the fuse's impedance builds rapidly, limiting the actual let-through peak to a fraction of the theoretical maximum.

Practical example: at a prospective DC fault current of 20 kA, a 160A 170M18XX fuse may limit the actual let-through peak to approximately 4-6 kA. Without the fuse, the battery protection circuit would see the full 20 kA peak, destroying the MOSFETs or IGBTs in the DC-DC converter. The cut-off current curves published by Bussmann show this relationship at multiple time constants (tc=1 ms, 3 ms, 10 ms).

Arc Voltage (Peak Arc Voltage During Clearing)

When the fuse opens, the arc voltage is the voltage that develops across the fuse terminals during the arcing phase. This voltage is applied to the load -- in addition to the normal circuit voltage -- and must be controlled for two reasons. First, IGBTs and MOSFETs have maximum blocking voltage ratings. If the fuse arc voltage exceeds the device's VCETO or VDS(max), the device may fail even after the fuse clears the fault. Second, higher arc voltages stress cable insulation, connectors, and downstream equipment beyond their rated limits.

Bussmann datasheets include arc voltage curves (UL = f(Eg, RMS)) showing peak arc voltage as a function of applied working voltage. For the 170M series at 1000 Vdc, arc voltage is typically held below approximately 2000V -- well within the withstand rating of 1200V-rated IGBT modules commonly used in PCS converters.

How to Use Bussmann Datasheet Curves

Bussmann publishes three curve types for each fuse series. Time-current curve (nominal melt): X-axis is RMS prospective current, Y-axis is virtual pre-arcing time. Use this to determine if the fuse will carry your maximum operating current without opening during normal operation. Cut-off current curve: X-axis is prospective fault current in kA, Y-axis is peak cut-off current in kA, with multiple curves for different time constants. This is essential for semiconductor coordination. I2t vs. prospective current curve: shows pre-arcing and total clearing I2t as functions of fault current level, used for energy-based coordination. Always plot your system fault current trajectory on all three curves before finalizing fuse selection.