Barely a year ago, most intermediate-bus architecture (IBA) implementations distributed power to point-of-load converters (POLs) at 12 V. There was some talk of using lower intermediate-bus voltages because load voltages had been shrinking, and POL switching losses increase when their output voltage is significantly less than their input voltage. Still, most IBA implementations ran at 12 V.
The picture began to change six to eight months ago, and power-supply makers began designing for lower intermediate-bus voltages. Bob Cantrell of Power-One says a majority of customers designing IBA supplies using the company's quarter- and eighth-brick fixed-ratio converters now choose versions with a 5:1 step-down ratio.
This produces 9.6 V from a regulated 48-V front end, rather than 12 V from the old 4:1 ratio. Supplies may still generate 12 V on a separate bus exclusively for 12-V components, Cantrell adds, but 9.6 V is becoming the new IBA standard. No one is sure whether the intermediate bus can be reduced even lower, as conduction losses on the board dominate the power budget as bus voltage levels shrink further.
Power-One's first 5:1 fixed-ratio converter product was a 27-A eighth-brick, the SQT48T27096, introduced late in 2004. Its latest is a 46-A quarter-brick, the QTS48T46096 (see the figure). Peak efficiency is 97.3% near full load. In real-life applications, the converter delivers 95% at 70°C with just 200 linear feet per minute cooling airflow. Customers also have been asking for operation at 70°C rather than 55°C, Cantrell says.
The QTS48T46096 is proving popular in power supplies for telephone central office (CO) equipment as well. That's because of its wide input-voltage range, which hasn't been commonly available in other fixed-ratio dc-dc converters. Typically, these converters have been limited to 48 V ±5 V.
The difference is important because battery backup is common in the CO environment. Although it doesn't meet full telco specs, the QTS48T46096 has an input range from 36 to 55 V. This accommodates the 54.5-V battery float voltage of the typical CO rectifier, in addition to the battery string's 38 V in low-voltage dropout output mode. That's sufficient for most new CO installations, which are being installed in India and China.
The QTS48T46096 converter costs $40 each in quantities of 1000 units.