RE: [SI-LIST] : GND plane heat dissipation

Tim Parks (
Fri, 24 Jul 1998 09:09:13 +0100

The transistor used in the Max712 switching circuit is a FZT949 in an
SOT-223 package (high current transistor). It is functioning correctly in
circuit but the body gets so hot in contact with the PCB that the board
temperature in places is 50-60 degrees celsius and higher. I'm concerned
about the effect the high surface board temperature on our digital circuit
and a rubber keymat that lays atop the PCB.

We have a six layer board. Two layers are dedicated to GND and Power plane
for the entire board. It may be possible to a allocate quite a large copper
area in one of the 2 remaining internal layers to an Analog GND plane for
the switching section. As I imagine this is a 'noisey' GND return that
should be seperate from the Digital plane.

This is to be a hand-held consumer product in a very small case, so large
open space for air flow, or fan ventilation is'nt an option open to us.

thanks for you support


At 04:08 PM 7/23/98 -0400, Andrew Ingraham wrote:
>Is this high a temperature consistent with how much power you expected
>the pass transistor to dissipate? Is it functioning correctly?
>If you were to isolate the ground plane, the area around the transistor
>would get even hotter. Should it be heat-sinked? How is the airflow?
>Andy Ingraham

Tim Parks
Technical Manager
Ferrotec Ltd
Unit T9
Maple Avenue
Stillorgan Industrial Park
Stillorgan Tel: 00-353-1-2952529
Co. Dublin Fax: 00-353-1-2953625