How to Stay Safe

This gives rise to a pertinent question: what if the application at hand can't accommodate a four and a half pound (2kg) heat sink and a liquid nitrogen bath? What's the part's current carrying capacity under slightly more practical conditions? It depends on what those conditions are, exactly; while the character of the part itself does set the general scale of what's achievable, the portion of this potential that can be utilized is largely a question of thermal management, subject to the user's application constraints and design choices. Some of the major considerations are are discussed below, and their effects on maximum permissible drain current are shown in the following table.

Max junction temperature: From a reliability standpoint, operating a device at it's rated Absolute Maximum Junction Temperature (175°C in this case) is not good practice. Aside from degradations in device characteristics that typically attend elevated temperature operation, higher maximum allowed temperatures usually translate into larger temperature changes as a system is turned on & off, experiences changes in load, etc. The resultant mechanical stresses within a device due to thermal expansion effects shorten service lives and increase the likelihood of random failures, to an increasing degree as the magnitude and rate of the temperature changes involved increase. Depending on other design considerations, allowing a 25°C margin to maximum rated junction temperature might be a reasonable to aggressive design choice. Because it also happens to be convenient for purposes of consulting the datasheet's figure 4, a maximum junction temperature of 150°C (25°C less than the rated maximum of 175°C) will be used in some of the scenarios tabulated below.

Maximum ambient temperature: The temperature difference between an electronic device's innards and and the environment in which it's operating is the thing that drives heat transfer from one to the other; all else being equal, smaller temperature differences translate to less heat transfer and lower operational limits. Since a designer often doesn't have direct control over the operating environment, this is frequently established via a design specification applied to the end product. A figure around 40°C (104°F) is common for office equipment, consumer goods, and similar items designed to be co-located with people. The "ambient" temperature for thermal modeling purposes however refers to the environment in which the modeled system exists; if that's the inside of a poorly ventilated outer enclosure for example, the effective "ambient" temperature may well be higher. To be generous, the following scenarios assume that this is not the case, and assume a maximum 40°C ambient temperature.

Thermal interface: A broader discussion of thermal interface materials and their usage is available here, but one of the overarching themes is that if one needs a thermal interface to be electrically insulating also, there's a thermal resistance penalty to be paid. The datasheet for the transistor being modeled suggests 0.5°C/W is a reasonable figure when an electrically non-insulating smear of thermal grease is used. In comparison, a reasonable figure for an electrically-insulating interface (discussed in part 1) is closer to 2.9°C/W. Both of these values are used in different scenarios in the following table.

Device package type/mechanical design: Up to this point, the TO-220-packaged variant of the IRL3713 transistor has been assumed, in conjunction with the use of some specified heat sink. Other heat sinks than those mentioned are available with different thermal characteristics, all of which are application-dependent to some degree. The transistor itself is also available in a surface-mountable D2PAK package, which offers a significant tradeoff between ease and cost of manufacture against thermal performance. In addition to the heat sink used for example in part 1, the surface mount option as characterized by the manufacturer (total junction-ambient thermal resistance =40°C/W) is included in the following scenarios.