I was looking at the datasheet for IRF1405 and got really excited when I saw 169A @ 25C! Then I started scrolling down and it says 'package limitation current is 75A'.
Does that mean the maximum current I can get out of this MOSFET is 75A? And what does it mean when it says 'package limitation'? Are they talking about the maximum temperature that the package can handle before it melts which is why it can only take 75A?
Do some more reading of the datasheets. Devices like MOSFETs can be offered in several packages, from large TO-247 style to SMD with a small metal tab that solders to the board. So the package used determines the amount of metal area to dissipate heat. For many popular MOSFETs the change shows up as a modified part number, with the base part number the same.
This way designers know the performance specs because of past usage. Voltage, speed and gate capacitance do not change, they just have to de-rate the wattage it can dissipate if going from a TO-247/TO-220 package to a surface mount. The best way to do this is to lower the current rating of smaller packages.
It makes sense because not every application will need the large TO-247 package as a high-side switch or a powerful SMPS. On many boards point-of-use switches and local SMPS may need a fraction of the wattage the TO-247 can handle, yet the designer wants to stay with known good part numbers.
On a smaller scale the humble BS170 MOSFET is offered in a TO-92 package, and a de-rated version is offered in a SOT-23 SMD 3-pin. The base part number '170' is prepended with MMBF. The TO-92 package can be pulsed at 1200 mA 830 mW, while the SOT-23 package is limited to 800 mA 300mW.
All of this info can be on the same datasheet, but clearly defined by different packages, prefixes and suffixes, and "package limitation current".
