7004 Datasheet, PDF (11/12 Pages) Bourns Electronic Solutions – Network Interface Device

and copper. For example, we pack fiberglass in our walls to insulate our homes. Hardly the
kind of material we would choose to conduct large amounts of heat away from our power
devices. Copper is the primary means of conducting heat away from the source and into the
air stream. Probably the biggest mistakes made in laying out PCBs for power devices are the
removal of copper in the vicinity of the power devices, and the reliance on inner layers for inter-
connecting the power devices. When possible, leave as much copper around the power
devices, and interconnect devices using topside copper. This allows the generated heat to
spread laterally and to be removed into the air stream passing over the board. Thicker copper
is better, and with most of the manufacturing technology currently in use, thermal reliefs are
not required, nor should they be used.
Whenever possible, leave the plane areas under and around the power devices, and the inner
layers and backside of the board intact. This helps with the heat spread and power dissipation.
For optimal thermal control, add an area of large diameter vias around the power device that
connect into the inner and back layers of the board. If possible, allow these vias to fill with
solder to form a thermal pipe between all the layers. Avoid vias for power path interconnects,
especially the commonly used micro-vias. If vias must be used to run power between layers,
use a minimum of one via per amp of average current – two vias per amp are preferred.
Another consideration is the use of thermal pads for conduction of heat to the chassis.
Manufacturers such as Bergquist, Avid, and Thermalloy, produce thick,
compliant silicone pads that are loaded with thermally conductive materials. If the power
devices can be mounted on the back side of a PCB, and a thermally conductive pad sand-
wiched between the PCB and a steel chassis, a great deal of heat will be very effectively
removed. Surface mount heatsinks are also available from some of the previously mentioned
manufacturers which drastically improve the PCB’s ability to transfer heat to the airstream.
In all cases, airflow across the PCB is very important. Even a small amount of air movement
will make a big difference in the device temperatures. Try to avoid putting the power devices in
the airflow shadow of tall components such as a microprocessor or the power supply’s own
input or output capacitors.
Reference:
Marty Brown, "What Everyone Should Know About Switching Power Supplies"
Rudolf Severns, Gordon Bloom, "Modern DC-DC Switchmode Power Converter
Circuits"
©2001 Fairchild Semiconductor Corporation
11
Rev. A, June 2001