Novel MOSFETs boost convertor efficiency

Two new 30V MOSFETs exhibit the benefits of improved fast-switching technology, especially when applied, respectively, to notebook computers and POL (point of load) convertor designs.

Fairchild reckons its advanced-performance PowerTrench mosfet process technology yields exceptionally low values for Miller charge (Qgd), on-resistance and total gate charge (Qg), and improves gate-drain charge to gate-source charge (Qgd:Qgs) ratio - enhancements that result in superior switching performance and thermal efficiencies when used in synchronous buck applications.

The benefits of low Miller charge are reduced switching losses and reduced "dead time" for improved regulation.

A favourable Qgd:Qgs ratio offers immunity against false gate triggering that result in shoot-through currents between switching mosfets when employed as a synchronous rectifier.

Low on-resistance is critical in minimising conduction losses and low total gate charge reduces the power consumed to turn the mosfet on and off each cycle.

Two new 30V mosfets from Fairchild, the FDS6294 and FDS7288N3, exhibit the benefits of this improved fast-switching technology, especially when applied, respectively, to notebook computers and POL (point of load) convertor designs.

With its very low Miller charge, the new FDS6294 provides excellent performance in high-frequency narrow-duty-cycle switching applications.

Very fast switching is essential to high-efficiency synchronous buck power supply designs used in notebook computers.

In this application, the typically large input-to-output voltage ratio and large voltage swing across the high-side mosfet often leads to excessive switching losses if the device cannot turn on and off quickly.

The extremely low Miller charge value of 3nC (typical) of the FDS6294 minimises this transition time, and in conjunction with a low on-resistance of 11.3mohm maximum, allows the device to offer superior performance to existing solutions.

The FDS6294 is available in an SO-8 package.

In addition to possessing the superior performance attributes of low Miller charge, low on-resistance, low total gate charge, and a Qgd:Qgs ratio less than 1, the new FDS7288N3 is packaged in a SO-8 FLMP (flip-chip in a leaded moulded package).

This advanced wireless packaging technology provides extremely low junction-to-case thermal resistance, low electrical resistance and low package inductance.

Low junction-to-case thermal resistance is achieved by extracting heat directly from the under side of the package, which allows this device to perform much more efficiently than conventionally packaged mosfets.

The lower electrical package resistance reduces static losses while the reduction in package inductance reduces the switching losses as well.

The FDS7288N3's excellent thermal characteristics are ideally suited for high current density DC/DC power supply applications commonly found in POL (point of load) convertors used in computer servers, telecommunications equipment and Internet hubs and routers.

The FDS7288N3 and FDS6294, coupled with the FAN5093 two-phase controller and driver combination, provide a complete POL solution; and the FDS7096N3 and FDS7288N3, in combination with the FAN5009 driver and FAN5019 controller, form a highly efficient system solution for VRM server applications.

The FDS7288N3 is a lead-free (Pb-free) product that meets or exceeds the requirements of the joint IPC/JEDEC standard J-STD-020B and is compliant with the European Union requirements, which will take effect in 2005.

David Grey, Fairchild's Technical Marketing Manager, said: "Fairchild's new fast-switching mosfet silicon technology, coupled with innovative packaging, substantially improves the performance of the upper side switch and the synchronous rectification function over our previous generation mosfet products".

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