alternate MOSFET


Joined Aug 16, 2010
For MOSFETs you need to consider the application before selecting an alternate. If the MOSFET is used in a switching application (as in a switching power supply) then you need to be concerned about the various capacitances because they will greatly affect the AC switching loss and the efficiency of the power supply. Rather than looking at all the capacitances, an easier parameter to check is the gate charge Qg. This is often used as a figure of merit when selecting MOSFETS for switching applications. Just make sure the one you select has a close or lower rating than the one you're replacing.

The Ids (drain current) specification is not really all that useful. The limitation in a MOSFET as in all semiconductors is temperature. When designing with MOSFETs you would calculate the power disipated by the MOSFET (both DC conduction loss, and AC switching loss) then multiply this power by the thermal resistance of the MOSFET (deg C / watt) to determine the temperature rise of the part. You then add this to your maximum operating temperature and make sure it doesn't exceed the max junction temperature given on the MOSFET datasheet (typically 125 or 150C). A good rule of thumb is to always stay below 100C. If your calculations exceed this limit then you need a heatsink. So, when selecting an alternate MOSFET for an existing design, make sure the thermal resistance is comparable between the old and new part. Most of the time it will be close because this is mostly a function of the device package. Measure the temperature of the old MOSFET when the circuit is running, and compare this to a measurement of the new MOSFET to make sure all is good.