In my previous column (“Applications Drive Component Power Designs,” Sept. 11, 2008, p. 18, ED Online 19486), I contended that the application drives the selection of a specific power component—or it even forces the creation of a custom component. But, it must be admitted, there is another side of the coin.
Many price-sensitive engineers see value in changing their architecture to accept a component that is readily available from many manufacturers because of the benefits of low price and product honed to near perfection by intense competition. There are no more popular power components than those with 48-V inputs.
The telecom industry brought a lot of commodity bricks to the 48-V realm as a voltage settled on in the early 1980s. Other markets, such as data communications, followed that in a migration to 48 V slowly gathering momentum in that industry, which was booming in the late 1990s, as well as the emerging distributed power architecture of the time. So anybody making 48-V converters then had a sweet business.
If designers can gain cost advantages by changing whatever input voltage is native to the market or application, they’re likely to consider it. That’s probably what happened as the intermediate bus architecture (IBA) emerged. Surely it was a consideration in the selection of 48 V for the first VI chip.
48 V EVERYWHERE
Now, everything in the blade server market is 48 V. Much medical equipment now has gone to 48 V. Even the 42-V automotive initiative could take advantage of many 48-V power products if that ever takes hold, and at some level it has, but it’s not a mass-produced product yet.
What’s more, many ac front ends put out 48 V now, reinforcing the trend to 48 V. A search on front ends shows that it’s the popular voltage now, to bring it down to 48 V and distribute it from there. Virtually any power designer that wants to use a standard front end that is also at the leading edge will select one at 48 V.
As soon as you get 48 V out of your ac front end, it doesn’t matter what the application is. You’re in the 48-V distribution business. So immediately, there’s a whole laundry list of applications that can be plugged into the wall. You name it. That could be anything, even consumer. You can put bricks downstream, you can do intermediate bus downstream, you can do factorized power. It’s your call.
Some ac front ends, of course, deliver high voltage, and that’s relatively straightforward with a boost converter. But most power architects don’t want to be distributing high voltage if it can be avoided, because it’s dangerous, safety approvals are difficult to get, and there are liabilities. They try to get highvoltage ac down to some safe voltage in one package.
The highest safe voltage is 60 V, but of course the dc-dc converter input range is typically 36 to 75—a nominal 48 V— leaving some headroom from the absolute maximum. At this level, safety approvals can readily be gained, so a 48-V input is a perfect call. And it’s been done for years, so it’s a natural.
An ac front end additionally could deliver a lower voltage. But from a system performance standpoint, busing low voltages around usually means higher current. As current increases, the amount of copper increases, the infrastructure of busing the voltage around becomes larger and more costly, and I2R losses increase.
Consequently, system efficiency is also impacted by the interconnect. But for most systems, 48 V represents probably the best tradeoff for bus voltages in regards to being able to bus large currents effectively without incurring significant losses that would occur at 12 V or lower. It’s a safe voltage, easily accessible for serviceability.
SOME RESTRICTIONS MAY APPLY
But 48-V input converters aren’t for everybody. Some market segments require a certain operating voltage because of the way the industry works. As an example, railway applications specify input voltages of 72 or 110 V. That’s the way the trains are built. It’s the same with the military, which requires input voltages of 28, 155, or 270 V. In such market segments, a change of input voltage would require massive and impractical architectural change. They’re not going to do that any day soon.
Finally, the main driver for this distribution voltage, however, is cost. And when you peel back the onion, other clear advantages are present. There have to be. It can’t be a wash performance-wise. There have to be efficiency, safety, and performance benefits.
The 48-V input converter is a competitive marketplace, so a great deal of engineering effort has gone into designing these products in a commodity world, where manufacturers have battled each other on cost for decades—not just years, decades.
So the fact that there’s been so much engineering talent put toward the development of 48-V converters means there’s a large selection, and they’ve been cost-reduced many times over. It’s the best of both worlds—a well-engineered product at a low cost.
Tom Curatolo holds a BSEE from Worcester Polytechnic Institute, Worcester, mass. look for more Power Design columns from him in print and online.