Hello Everyone,

So I have one thing that is bugging me about MOSFETs. So I have used them on numerous projects and mainly as a through hole component. I finally have my toaster oven setup and supplies to transition to the surface mount world as a hobbyist (I have done self etching but just upgrading now for smaller boards).

For NMOS,
Why is the drain pad so large and the source still a pin and how are they able to conduct the same? I know that there is an on resistance so there is a little voltage drop but for the most part the conduction path has to be the same for the drain and the source. So what is going on inside?

 

It is for heatsinking. When you lay out a pcb for a surface mount power mosfet, you should have a large copper area for the big tab to solder to.

 

The source terminal is normally connected to the MOSFET substrate and the substrate is in contact with the large tab to best conduct away any heat generated in the transistor. That's why the source is the large tab. Here's a description of the MOSFET to help. Notice the MOSFET cross-section in the third diagram down on the right of the page.