Electronic Design

Tightly Packaged MOSFETs Shrink Voltage-Regulator Modules

What causes a performance boost in International Rectifier's SO-8-compatible devices and also allows a power MOSFET to be housed in a package nearly 50% smaller? The answer is the inclusion of the latest generation of silicon in IR's DirectFET package. Consequently, IR now can offer a control and synchronous MOSFET chip set that shrinks the board size of VRM10.x-compliant synchronous buck converters.

The IRF6608 control MOSFET comes in the new DirectFET "S" package, which has the same outline as a standard Micro-8. But while the Micro-8 offers a high thermal resistance (both junction-to-case and junction-to-pc board), the S package takes advantage of DirectFET's double-sided cooling to achieve a typical 1°C/W junction-to-pc board. The thermal resistance from junction-to-case is 3°C/W.

Using the latest generation of silicon in the S package, the 6608 achieves a typical RDS(ON) of 8.5 mΩ at 10 V. Meanwhile, the same silicon enables the IRF6618, a synchronous MOSFET housed in the original S0-8-compatible DirectFET package, to achieve an RDS(ON) of 1.7 mΩ typical, or 2.2 mΩ max (see the table).

In voltage-regulator modules (VRMs) developed to power advanced CPUs, the 6608's reduced size allows the control and sync MOSFETs to be placed side by side in one row, rather than two. That tight spacing enabled IR to develop a VRM10.x-compliant five-phase synchronous buck converter on a 0.98-in. tall demo board. The low package height makes the VRM compatible with a 1U outline without having to use a right-angle board connector.

Moreover, this VRM10.x demo satisfies actual operating requirements, producing 105 A at a 1.3-V output, while working in an ambient of 45°C with 400 LFM of forced-air cooling. Efficiency for this design is better than 88% from a 40- to over 100-A output.

The IRF6608/IRF6618 cost $1.03 and $1.52 each, respectively, in lots of 10,000.

International Rectifier

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