Get that edge in performance by taking advantage of surface-mount power devices. Historically, two factors have limited IMAX—the maximum sustainable current delivered by a power MOSFET: the channel on-resistance —R_DS(on) due to I²R heating, and the mounted package's net thermal impedance. The latter determines the efficiency with which heat can flow from the die.

Computing a theoretical IMAX is relatively straightforward:

where

is the sum of thermal impedances from junction to ambient; T_J(MAX) and T_A(MAX) are the maximum allowable junction temperature and the maximum rated ambient temperature, respectively; and k is a data-sheet guard-band factor.

This is straightforward analysis, but it does ignore the packaging interconnect's resistance—on the order of milliohms—held (the channel resistance of leading power devices was a large fraction of an ohm). However, innovations in power-semiconductor fabrication processes and powerdevice designs have reduced R_DS(on) to the order of milliohms as well, so power dissipation in the die no longer necessarily sets the upper current limit.

The distinction between older devices and modern power MOSFETs with ultra-low channel resistance is an important one. Understanding the mechanisms that set the current maximum gives insight into potential failure modes in devices that are subject to current overstress.

A LINK TO FUSING

Two types of current overstress in MOSFETs distinguish themselves by the durations over which they operate, as well as the failure modes to which they lead. Pulse-over-current conditions can ultimately result in fusing failures.

Metallic filaments analysed over durations significantly shorter than the thermal time constant of their surroundings generally follow an I²t fuse characteristic in vacuum or gas. That characteristic depends on the metallic's melting temperature, cross section, and resistivity. Fuse currents for metal wires embedded in solid materials, such as organic packaging compounds, shift depending on the thermal properties of the compound.

Improvements in process, device, and packaging technologies, coupled with the further miniaturisation of powerstage, circuit-board layouts enabled by those advances, reduce the total impedance in a current transient's path.

For traditional power packages, for instance the TO-220 and D²PAK, this means innovations that might increase the maximum sustainable current are likely to have little or no effect on the device's peak-pulse-current limit. In practice, the fusing-current limit of power-device bond wires and power-section pc-board traces set an application's peak-pulse-current limit.