Electronic Design

DC-DC Footprints On The Sands Of Time

The footprint of a dc-dc converter is, to me, an interesting topic. There is a certain amount of history to it that’s kind of fascinating. Also, it really plays heavily into the decisions power designers make about what solution they’re going to use and whether, for example, another vendor is actually a true second source. Over time, the dc-dc converter footprint has, in general, experienced some changes. You might also say it has changed the industry to some degree.

Designers might not even think about the footprint of a dc-dc converter until they get into the design details. Then suddenly, they realize that the design comes down to the footprint. The footprint of the dc-dc converter on the board is so important that you’ve got to think about it up front, rather than after the fact. And, the footprint of a dc-dc converter has a history.

In The Beginning...

Without naming names, one company started the brick industry. The modular dc-dc converter brick was introduced in 1984. The new building block approach simplified power system design and reduced the time it took to bring a product to market. Other modular converters seem to have been introduced briefly around the same time, but none of the others survived.

So being the first player, this company naturally defined the footprint. From there, some twists and turns took place. Understandably, competitors tried to capitalize on the brick concept, some of them down to the original footprint. After a period of competitive contention without resolution, a number of competitors developed their own footprints. Many of them allowed others to copy theirs so they could be used as second sources to one another, ultimately resulting in a standard footprint, more or less, for the industry.

Some customers that wanted to use the original module would do an offset; that is, they’d have one set of plated-through holes that would fit the original or a competitor’s module and a second set of holes offset that connected to the previous set. Now, that may or may not be advantageous, but that’s one of the things that also came about in accommodating the different footprints of the day.

Then, so-called interposer boards were created that accommodated the original module, and the interposer pins fit the footprint of the competitive module. Or vice versa.

Some suppliers did copy the original footprint later on, but by then, there was much more momentum toward scaling down footprint size. Hence the fact that new, smaller bricks were being developed, new opportunities arose to create footprints for each new size.

By the way, it’s probably worth noting that Vicor was the first company with the half-size brick as well as the quarter-size brick. But again, most suppliers went their own way when it came to the footprint. That’s probably unfortunate for the customers, when you think about it, because they’d have to examine all of the tradeoffs for the best fit in an application.

Alliances have come about that facilitate the formation of second sources. But these alliances have their differences as well. Although customers might like to be able to specify a module without regard for vendor, that’s a condition that, in reality, is not often practical.

Some suppliers have gone beyond the quarter brick. Eighth bricks and sixteenth bricks are becoming increasingly popular. Designers supporting the alliances share these footprints as well. Such open architectures take away some of the complexity for the customer, allowing second sourcing for any brick, half brick, quarter brick, eighth brick, or sixteenth brick.

Footprint: More Than Its Size

Designers most often see the footprint as simply the physical area that the dc-dc converter takes up on the printed-circuit board (PCB). Yet when all is said and done, it’s much more than that. It is a physical interface that presents challenges and opportunities for the designer. However, it introduces electrical and thermal interfaces that require design consideration as well.

For example, most designers who use a brick think about the footprint and its impact, whether they’re going to solder it to the board, whether it’s going to be socketed. Surface-mount bricks have also come about that change the mounting and the thermal management of the design.

One twist on that, however, is some of the pin functionality may not be the same from one supplier to another, or the enable/disable may not work the same. So even if it’s the same footprint, sometimes it may not be a drop-in candidate.

One of the finer points of a module pin connection to a PCB, the interconnect, relates to how the inductance and impedance will affect performance, which is part of the reason for choosing surface mounting over through-hole mounting. Interleaving the copper planes from the output of the converter to the load can be advantageous for reducing the inductance. The copper planes also can be used as heatsinks.

But, of course, whether inductance even matters at all depends on the application. Certainly when you’re powering processors and you’ve got to fight the inductance to provide performance, it’s an issue. If it’s not as critical to an application, though, it really doesn’t matter all that much. Again, it all comes back to the application. In fact, it always comes down to that. There’s no getting around it. That’s just the way the world works.

A Final Word

Footprints will continue to shift and change over time. If they don’t, just like the “footprints on the sands of time” of Henry Wadsworth Longfellow, they’ll get blown away. Sorry, Henry.

"Lives of great men remind us

We can make our lives sublime,

And, departing, leave behind us

Footprints on the sands of time."

—Henry Wadsworth Longfellow

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