EP11 datasheet(39/46 Pages) POWERINT

39 / 46 page

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 (TJ ) exceeds 140

oC (typ.). This junction temperature will be higher

than the package tab temperature (TC) depending on the amount of power being

dissipated. It is good design practice to keep this junction temperature below

120

oC to guarantee continuous operation with adequate margin. For the EP11,

thermal shutdown occurs at a TOP249 tab temperature (TC) of approximately

120

oC (depending somewhat on the output power level). At 65 Watts output, the

EP11’s TOP249 tab stabilises at TC=103.6

oC in an ambient of approximately 40oC

(while in the enclosure with a 90VAC input). At 70Watts this tab temperature

stabilises at TC=111.9

oC. 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.

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