UCC28911 - Output Power Question

Hello Mr. Martinez,

Your concern is well placed, and as one can gather from the Device Comparison Table, the issue is a matter of thermal management. The UCC28911 incorporates an internal 6.3ohm (at 25C) MOSFET and is packaged in a SOIC-7 case. The three GND leads conduct most of the heat from the device to the external ambient through the pcb copper area for this GND. In most flyback designs, conduction losses are highest at the lowest input line voltage. As the table notes indicate, certain thermal constraints are necessary to dissipate this heat out of a small, enclosed adapter shell to avoid excess junction temperature. While not impossible, it is usually uneconomical to meet these thermal requirements for the UCC28911 at 10W in an adapter. An open-frame environment, on the other hand, offers better opportunities to remove more heat with lower thermal impedances, although there are limits to this, too.

The power limitations for the UCC2891x devices in the Comparison Table are derived from the thermal constraints listed in the applicable notes in conjunction with the Thermal Information table of section 8.5 in the datasheet. They are meant as a guideline for typical applications and are not absolute limits. As long as your thermal design can maintain junction temperature below 125C under all operating conditions it should be possible to stretch the output power to 10W or higher (until some other limit is reached, such as peak current), regardless of enclosure. I wish that I could give you a straight Yes or No answer, but one must examine one's specific design freedoms, restrictions and economics involved to achieve this, to separate an academic argument from a practicable implementation.

If your adapter size target makes it unviable to use the UCC28911, the UCC28704 can offer higher efficiency with flexibility to optimize the choice of MOSFET and pcb layout. Depending on your target market, certain agency minimum efficiency requirements may apply, to which a UCC28911 design may not be able to comply without also using a synchronous rectifier. Especially in a small volume, the higher efficiency obtained with the UCC28704 can reduce the internal temperature stress on all of the other parts within the enclosure, or alternatively can allow use of a smaller volume for the same temperature rise.

Like all commercial work, even 10-W adapter design involves serious consideration and trade-offs of target specifications, component selection, transformer design, thermal, mechanical, and environmental constraints. To move from dabbling to design, consider research into TI's technical papers and design aids. In particular, I recommend to read "Under the Hood of Flyback SMPS Designs" which can be downloaded from TI's website. Type in the literature number "SLUP261" into the search box at the top of the webpage, and a link to this article should come up. I wish to emphasize that proper transformer design is a major aspect of SMPS designs (flyback or otherwise) and additional detailed design information is also available for this from TI with "SLUP132", the "TI-Unitrode Magnetics Design Handbook".

Regards,
Ulrich