22-Jan-01 EPR-00011 - 70 W Adapter
Page 39 of 46
Power Integrations Inc
Tel: +1 408 414 9200 Fax: +1 408 414 9201
www: http://www.powerint.com
13.3 Heat Spreading and Enclosure Surface Temperature:
The enclosures of adapters are generally limited to an absolute maximum external
surface temperature (defined by safety approval agencies such as U.L.). For higher
power adapters, it is often necessary to use an internal heat spreader to evenly
distribute the internally generated heat across the inside of the enclosure’s outside walls.
This will help eliminate “hot spots”. The “heat spreader” is generally nothing more than
an additional foil wrap (or sheets of copper or aluminum) between the converter and the
outside enclosure walls. The caveat is that these heat spreaders must generally be
electrically insulated from the heat sinks to provide safety isolation. This electrical
insulation invariably contributes significant thermal impedance. Even with heat
spreaders, care must be taken to avoid crowding the heat generating components
together more than is necessary.
13.4 Component Temperature:
The maximum operating temperature of the power devices within the converter may be
limited by various considerations depending on the type of component.
- TOPSwitch-GX :
The TOPSwitch-GX is thermally protected by its internal thermal shutdown
feature. This feature prohibits the device from operation when the internal junction
temperature (T
J
) exceeds 140
o
C (typ.). This junction temperature will be higher
than the package tab temperature (T
C
) depending on the amount of power being
dissipated. It is good design practice to keep this junction temperature below
120
o
C to guarantee continuous operation with adequate margin. For the EP11,
thermal shutdown occurs at a TOP249 tab temperature (T
C
) of approximately
120
o
C (depending somewhat on the output power level). At 65 Watts output, the
EP11’s TOP249 tab stabilises at T
C
=103.6
o
C in an ambient of approximately 40
o
C
(while in the enclosure with a 90VAC input). At 70Watts this tab temperature
stabilises at T
C
=111.9
o
C. Care must be taken with any package design to make
sure that adequate margin remains at the maximum output power and ambient
temperature. One way to test this is to stabilise the supply at maximum power in
the desired environment and subsequently increase either the power or the
ambient temperature in small increments (waiting for the internal temperatures to
stabilise each time) until shutdown occurs. This allows the designer to determine
how much margin there is for thermal shutdown.
