FROM EDN EUROPE: Smaller FET targets 105A VRM spec

Using its DirectFET surface-mount package, International Rectifier is claiming a new peak in power density—power delivered per unit-board area—with a control MOSFET/synchronous-MOSFET pair of devices (Picture). The parts target next-generation-processor core-voltage requirements, as embodied in the VRM10.x VRM (voltage-regulator-module) specification) and similar low-voltage/high-current, multiphase, dc/dc-converter designs. The IRF 6608 control MOSFET has a typical on-resistance of 8.5 mΩ, and the 6618 synchronous MOSFET has a typical on-resistance of 1.7 mΩ. The company claims that this figure represents the first time a MOSFET has achieved an on-resistance of less than 2 mΩ.

The DirectFET package is an inverted copper "dish" in which the FET chip is directly mounted (see "A good fit: Power FETs find their place," EDN Europe, May 2003, pg 30). Drain termination is by direct contact from the back of the die to the metal can; source and gate terminations are contact bumps for direct soldering, coplanar to the pc board. These bumps are recessed a few microns below the can profile; this approach produces the reflow-solder profile that best resists thermal and mechanical stresses, according to IR. The new devices are in cans that are smaller than the original DirectFET, at 4.9×3.6 mm, or 16.6 mm², with a profile of just 0.7 mm. You can mount these devices on the rear of a pc board, below the profile of a normal standoff, and sandwich a layer of thermally conductive gel between the board and the case to provide direct cooling to the free space on the case of your product.

In its VRM 10.1 demonstration, with the reduced device size, IR has built a five-phase, rather than four-phase, converter, allowing a peak efficiency of less than 90% at 70A and 1.35V (converting from 12V). The design can output the full specification of 105A in the 1U outline. Price is $1.03 for the 6608 and $1.52 for the 6618 (10,000).

International Rectifier, +4420 8645 8003, www.irf.com.