Bottomless MOSFETs cut heat and resistance

A novel "Bottomless" MOSFET package combines the thermal performance of the much larger TO-263 (D2-PAK) with very low on-resistance in a standard SO-8 package outline.

Fairchild Semiconductor has a new family of 18 N-channel mosfets housed in the industry's first package that combines thermal performance of the much larger TO-263 (D2-PAK) with very low on-resistance in a standard SO-8 package outline.

Fairchild's advanced Bottomless packaging offers: better reliability and efficiency due to reduced die temperature for a given application; higher efficiency due to lower package resistance; higher current capability per given PCB area; reduced switching times due to lower package inductance and lower gate resistance; and standard SO-8 package dimensions, footprint and compatible pinout.

The 18 new products using Bottomless technology include two 150V and two 200V products for 48V telecomms DC/DC applications; six 40V products designed for use in the primary-side of low input voltage single transistor forward DC/DC convertors, two transistor forward convertors, and half bridge convertors; and four 20V and four 30V products for low-side switch applications in computing and synchronous rectifier isolated DC/DC convertor applications.

The novel package is constructed using the solder-bump technology that Fairchild pioneered with its mosfet BGAs.

A conventional SO-8 package is constructed using wire-bond technology.

Wireless versions of the SO-8 reduce the package resistance and inductance but do not significantly improve thermal performance.

The Bottomless package accomplishes both goals simultaneously by eliminating the wire-bonds and allowing the PCB heat sink to be in direct contact with the solderable backside of the mosfet die (drain connection).

This new package reduces the junction-to-case thermal resistance below 0.5C/W, a dramatic improvement from the junction-to-lead thermal resistance of 25C/W found in conventional SO-8 packages.

The Bottomless SO-8 even has a lower thermal resistance than the D2-pak, yet its footprint is only 30mm2, compared with 155mm2 for the D2-pak.

It also enables designers to achieve the performance they are looking for in only 20% of the PCB surface area required for the D2-pak.

Thermal resistance is further improved by providing heat conduction from both the drain contact on the bottom of the package and the source leads, which are thermally well coupled to the mosfet source.

The Fairchild Bottomless package is offered in the standard 3-source SO-8 footprint to provide greatly improved thermal handling capabilities, while maintaining compatibility with the SO-8 pin out assignments.

Fairchild also offers the Bottomless SO-8 package in a unique seven-source pinout that provides the lowest package inductance and resistance contribution combined with the highest current handling available - for any standard outline SO-8 in the industry.

This seven-source solution provides up to 1mohm lower on-resistance than the standard three-source SO-8.

Available now are four 20V products in the Bottomless package featuring on-resistance capabilities as low as 3.5mohm.

They include two three-source pinout products, the FDS6064N3 and FDS6162N3, and two seven-source pinout versions, the FDS6064N7 and FDS6162N7.

Also being announced are four 30V mosfets with on-resistances as low as 5.5mohm: the three-source pinout versions, the FDS7066N3 and FDS7064N, and the seven-source versions, the FDS7066N7 and FDS7064N7.

Both the 20 and 30V Bottomless SO-8 products are aimed at low-side switch applications in notebook, desktop computer, and synchronous rectifier isolated DC/DC convertor applications.

Also available now are two 150V products, the FDS2070N3 and FDS2070N7, and two 200V products, the FDS2170N3 and FDS2170N7.

All four products are targeted at the primary-side of DC/DC convertors.

Simultaneously delivering high efficiency, extremely low on-resistance and low gate charge, these PowerTrench mosfets are the natural choice for demanding 48V telecomms DC/DC applications.

The 150-200V products are designed specifically to improve the overall efficiency of DC/DC convertors using either synchronous or conventional switching PWM controllers.

Also being introduced are six 40V products, the three-source FDS4070N3, FDS4072N3, and FDS4080N3, the seven-source FDS4070N7 and FDS4072N7 and the FDS4080N7, capable of achieving on-resistance as low as 7mohm.

These products are designed for use in the primary-side of low-input-voltage single-transistor forward DC/DC convertors, two-transistor forward convertors, and half-bridge convertors.

Samples and production quantities are available now, with lead times of 8 weeks or more for larger orders.

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