Selection and application of DC-DC converter bricks

Geof Potter, Vice President Technical Marketing, Semiconductor Circuits Inc. -July 03, 2012

1/32nd bricks have the potential to dominate a major portion of the isolated, board mounted market as they mature as production devices and show steady improvement in efficiency. Basically, smaller bricks cost less than larger ones, so the most cost effective power solution is one that leaves the least unused power capacity on the table when the final system is running.

 A cost-conscious system designer wants the smallest brick that will provide the desired interface between power source and electronic load package, and that is not always the smallest brick that advertizes suitable wattage or output amperage. We will look at several other factors.

 

Market Observation

Over the past two decades, power levels once available only from larger size modules have migrated downward. Once-upon-time, 150 regulated output watts were available only from Full Brick modules. Smaller sizes didn’t exist. As conversion efficiency improved (FET switches got better, among other reasons) 150 watts migrated down to within 1/8th size capability. Full Bricks migrated up to 900 watts, or more, for the same reasons. But, because user power needs have remained relatively fixed in terms of watts / sq.in., due in part to relatively fixed limits on circuit board heat dissipation, typical circuit board power needs are supplied by smaller and smaller bricks. Continuous cost improvement !                                                        

A significant step in system cost / watt improvement was made by centralizing input-output isolation in multi-voltage systems and providing voltage regulation locally in the most cost effective format yet devised the ubiquitous buck regulator.  

Figure 1 shows relative footprints of standard brick modules along with typical power dissipation numbers. Power loss numbers are necessarily approximate due to variations in conversion efficiency, general construction and physical mounting parameters. They represent expected loss at maximum rated output power with naturally circulating 20°C air (no forced airflow).

From a dc-dc converter perspective, cooling fans, to increase airflow rates, are necessary when air temperature increases above about 25°C, or when more dissipation from the module (if available) is required or a combination of both. Small modules, like the 1/32nd brick, tend to reach their maximum specified output current before they reach thermal limitation (de-rating) as long as they are mounted on reasonably conductive host circuit boards. That means there is untapped capacity in those module sizes that can be made available by design evolution.

 

Figure 1

 Adoption of a physical standard, like the new DOSA 1/32nd brick, puts competing products on a relatively level plane for comparison, but there are differences in performance and application cost reflecting what each manufacturer thinks is important in their target markets. This article is aimed at assisting users who may not be thoroughly familiar with “brick” technology and nuances involved in using these remarkable devices.

What is a brick?

A brick, in this context, is a  DC-DC conversion module with standardized physical characteristics (footprint primarily) intended for service as a power management system component. Thermal management, isolation, EMI, power control, sequencing, failure protection and in some cases operational monitoring are balanced to optimize a total power solution. A brick provides voltage level shifting and regulation. It also provides some level of ripple and noise reduction but not usually to a satisfactory degree without additional external components.

Bricks need external system components to operate at their full potential. Designers can manage system cost impact imposed by bricks of differing size and manufacturer by evaluating input and output filter requirements and thermal characteristics.

Cost effective application of open frame converter modules depends on selection of a module size that will reliably supply sufficient regulated power without a lot of unused output capacity.

Although that seems obvious there are subtleties that may not be apparent, particularly the need for planning heat transfer from the module to its environment during operation. This is particularly true for the smaller size (1/8th, 1/16th, and 1/32nd ) bricks. Those modules are given power output ratings based on circuit design margins, heat transfer to moving air (with or without attached heat spreaders) and heat flow through input/output pins into a host circuit board. Absent a thermally conductive host PCB, or attached heat spreader, standard bricks are used with reduced output ratings, in enhanced airflow or benign temperature environments.


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