Overlapping Pads

Tim Wescott wrote:

I have two problems that I need to solve, both involving high-current

circuits:

 

First, I'm going to be laying down some FETs in SO-8 packages, and I

want a good solid connection from the four drain pins to a surrounding

polygon, both for current flow and to provide a thermal path for heat

sinking.  For the same part I want the same solid connection to the

source pins, although I don't need a polygon.  Connections that allow

good wide traces (up to 200 mils would be nice) without stomping on

the gate connection would be nice.

 

Second, I'll be putting in some current sense resistors.  While I'm

going cheap and using 2-terminal resistors, I'd like to make a pseudo

4- terminal device by laying the part out with four pads, but then

connecting pairs of pads with a polygon or some such so that it looks

and solders like a regular resistor, but so that I can have my nice

high- current connections going to it with honkin' big traces, and

still have some thin traces coming off of the pads -- independent of

the big traces -- in a "pseudo Kelvin" connection.

 

Has anyone solved this problem before?  Got any suggestions on how to

proceed?

Hi Tim

Mock up an image of what you are attempting. That would make a potentially

long reply more targetted.

Warren

--

Viewed / responded via the newsgroup at

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Tim Wescott wrote:

I have two problems that I need to solve, both involving high-current

circuits:

 

First, I'm going to be laying down some FETs in SO-8 packages, and I

want a good solid connection from the four drain pins to a surrounding

polygon, both for current flow and to provide a thermal path for heat

sinking.  For the same part I want the same solid connection to the

source pins, although I don't need a polygon.  Connections that allow

good wide traces (up to 200 mils would be nice) without stomping on

the gate connection would be nice.

 

Second, I'll be putting in some current sense resistors.  While I'm

going cheap and using 2-terminal resistors, I'd like to make a pseudo

4- terminal device by laying the part out with four pads, but then

connecting pairs of pads with a polygon or some such so that it looks

and solders like a regular resistor, but so that I can have my nice

high- current connections going to it with honkin' big traces, and

still have some thin traces coming off of the pads -- independent of

the big traces -- in a "pseudo Kelvin" connection.

 

Has anyone solved this problem before?  Got any suggestions on how to

proceed?

Hi Tim

Mock up an image of what you are attempting. That would make a potentially

long reply more targetted.

Warren

--

Viewed / responded via the newsgroup at

news.cadsoft.de

On Tue, 07 Feb 2012 11:01:29 +1300, Warren Brayshaw wrote:

Tim

See this document

http://www.irf.com/technical-info/appnotes/an-1154.pdf

 

Do you want to achieve the land pattens they have for their SO8 Fig 4

Pretty much, yes.

--

My liberal friends think I'm a conservative kook.

My conservative friends think I'm a liberal kook.

Why am I not happy that they have found common ground?

Tim Wescott, Communications, Control, Circuits & Software

http://www.wescottdesign.com

Tim

See this document

http://www.irf.com/technical-info/appnotes/an-1154.pdf

Do you want to achieve the land pattens they have for their SO8

Fig 4

Warren

--

Viewed / responded via the newsgroup at

news.cadsoft.de

On Tue, 07 Feb 2012 07:56:59 +1300, Warren Brayshaw wrote:

Tim Wescott wrote:

>> I have two problems that I need to solve, both involving high-current

>> circuits:

>>

>> First, I'm going to be laying down some FETs in SO-8 packages, and I

>> want a good solid connection from the four drain pins to a surrounding

>> polygon, both for current flow and to provide a thermal path for heat

>> sinking.  For the same part I want the same solid connection to the

>> source pins, although I don't need a polygon.  Connections that allow

>> good wide traces (up to 200 mils would be nice) without stomping on the

>> gate connection would be nice.

>>

>> Second, I'll be putting in some current sense resistors.  While I'm

>> going cheap and using 2-terminal resistors, I'd like to make a pseudo

>> 4- terminal device by laying the part out with four pads, but then

>> connecting pairs of pads with a polygon or some such so that it looks

>> and solders like a regular resistor, but so that I can have my nice

>> high- current connections going to it with honkin' big traces, and

>> still have some thin traces coming off of the pads -- independent of

>> the big traces -- in a "pseudo Kelvin" connection.

>>

>> Has anyone solved this problem before?  Got any suggestions on how to

>> proceed?

 

Hi Tim

 

Mock up an image of what you are attempting. That would make a

potentially long reply more targetted.

 

Warren

SO-8 case (view in fixed-width):

   _________

  /         \________

  \-|-|-|-|-/

   .-------.

   '-------'

  /-|-|-|.|

      thick source trace to wherever

R2512 case:

                 _____            _____

                /     \          /     \

            ____|   .----

.   |______

                \_____/\        /\_____/

                      thin traces to

                      voltage amplifier

--

My liberal friends think I'm a conservative kook.

My conservative friends think I'm a liberal kook.

Why am I not happy that they have found common ground?

Tim Wescott, Communications, Control, Circuits & Software

http://www.wescottdesign.com