Iam looking at the datasheet of a dual MOSFET (IRF9358PbF.) which contains 2 drain pins and 1 source pin for each MOSFET.

What is the purpose of having 2 drains, when there is only 1 source pin?

Is it for increased current capability? Since drain current is same as source current, shouldn't the source pin be also 2 pins?

  • \$\begingroup\$ If you find the datasheet of a CPU with different 3V3 pins from ground would you ask the same question? \$\endgroup\$
    – Oldfart
    Sep 18 '19 at 5:06
  • \$\begingroup\$ The package has eight pins - may as well use them all, and it makes a nice symetrical pinout. \$\endgroup\$ Sep 18 '19 at 5:07
  • \$\begingroup\$ I assume that the MOSFET silicon die itself has the Drain and Body connected to the substrate, which is both the mechanical and electrical connection on the bottom of the die. An electrically conductive epoxy bonds the die substrate to the leadframe, while the Gate and Source connections use wire bonds from the top of the die to the leadframe. Comparable MOSFET in the D-Pack package has very large area for the Drain pin, so large that it is the entire bottom surface of the D-Pack package. The IRF9358 is a dual so I assume it has two independent MOSFET silicon die on the leadframe. \$\endgroup\$
    – MarkU
    Sep 18 '19 at 5:13

Most of the heat is conducted out through the lead frame. Typically the drain connection has the most direct connection (thermally) with the lead frame, so having multiple pins to conduct the heat out to the PCB is optimal.

Sometimes the gate and source are connected to the other pins with bonding wires (perhaps many in parallel in the case of the source, to reduce resistance), sometimes with a second clip part that goes overtop of the die, but in both cases the drain is a more direct thermal connection to the die.

Bonding wire design from here:

enter image description here

Newer clip style from here:

enter image description here

Having as much copper as possible connected to the drain pins will help dissipate power, as will having power planes as continuous as possible under the part.

Many more modern packages have better thermal performance than the SO-8, when properly mounted.

  • \$\begingroup\$ +1 But how is this answer different from the link I provided with the duplicate question? \$\endgroup\$
    – user103380
    Sep 18 '19 at 5:17
  • \$\begingroup\$ @KingDuken Updated from the 2012 answer with the clip style and internals images. \$\endgroup\$ Sep 18 '19 at 5:29

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