Improved components, technology and packaging / cooling concepts have reduced the size (watts / cubic-inch), weight (watts / pound) and initial cost (dollars / watt) of power conversion products. When implemented properly, these support new system goals of smaller, lighter and cheaper, along with improved reliability and reduced maintenance (i.e., lower operating costs). However, as vendors strive for initial sales advantage, design for smaller/lighter/cheaper is invariably hotter, denser and less reliable, to the detriment of the primary system goals of maximum performance and reliability while minimizing total cost of ownership. This paper will set inviolable guidelines of best practice to assure optimal design in the tradeoff between smaller/lighter/cheaper and the essential goals of maximum reliability and minimum cost of ownership.