power mosfet mounting

[zotdoc]

I would like to make a pwm power mosfet circuit for an electric wheelchair base. Each of the two motors draw about 35 amps, but Probably much more when first starting. Should I construct some sort of socket for the mosfet source and drain pins? It doesn't seem like it would be possible to make a trace wide enough to handle this amount of current on a pc board. Are there any ready made mosfet sockets and if so, does anyone know a good source?

[MrAl]

Hi there,


You should realize that when a wire (or in this case a pc board trace)
is very short it does not need to be as wide as when it is long.
This means you should be able to design a pc card that can handle
the 35 amps and this can be used to mount the transistor.

You can also 'stagger' the leads sometimes to allow making the three
pads bigger. For example, with a TO-220 package the leads come
straight when you buy the device, but you can bend them any way
you want to. This allows the center lead to be bent out away from the
package farther than the other two leads. This changes the hole
pattern from a straight line to a triangular shape:

Code: Select all

instead of lead holes like this:
   o
   o
   o

use something like this:
       o
   o         
       o

The center lead here ends up being much longer than the two
outer leads and this allows the pc clad pads to be made much
larger.

The current handling can be calculated based on the circular mils
of the trace, which can be the same as a short wire that would
be able to handle this current. The leads should be bent over
on the clad side of the board so that they come into contact with
the clad for at least 1/8 inch and well soldered.

If you need help calculating the circular mils needed for the trace
width or something like that just yell.

[zotdoc]

Thanks Mr.Ai, I was looking at the size of the wires to the motors and did not think about how short the distances from thr mosfet leads were. In the to 220 pkg I'm using, the spec sheet has pin 2 and the screw plate both listed as the drain. Do you know if this means I could cut off pin 2 and screw the mosfet to a bar of aluminum, and then screw the drain wire to the aluminum to get a really big pad for lead 3 (the source)? Will this work or would there be a problem with this set up?

[gerty]

It's a common practice to use the tab in place of the middle pin (if they are common). Also, like Mr Al said, offsetting the pin holes will give you more space for a larger pad . Are you going to parallel the mosfets for higher current capabilities?