Mike SpeedIntroducing a game-changing component is a bit like golfer hitting a hole in one. Both events happen relatively infrequently, yet everyone knows there’s another one coming, but not exactly when or by whom. Even for the keenest observer its occurrence is not usually predictable.

So, under the category “look what we have here” is the appearance of a sub-1m? RDS(ON) 30V MOSFET. The new Fairchild FDMS7650 is an N-channel device that offers a Max RDS(ON) of 0.99 m? (at VGS = 10 V, ID = 36 A), the industry’s lowest ever RDS(ON) in a 5 x 6mm POWER56 package. RDS(ON) performance for competing 30V MOSFETS in 5 x 6mm packages are in the 1.4 to 1.6 m? range.

By way of review, the total drain-to-source on-state resistance RDS(ON) of a MOSFET is comprised of chip resistance and package resistance. The parameter is important for two reasons: first, because power dissipation is directly related to it and second, it determines the current handling capability of the silicon for a given application. In this case the new Fairchild MOSFET consists of an advanced package and silicon combination optimized for high density DC switch applications such as active ORing redundant power supplies for high availability telecom and server systems. FDMS7650 is ideal for ORing applications since the low RDS(ON) ensures minimal forward voltage drop in the on-state, for maximum efficiency with minimal conduction losses and better thermal performance.

Replacing Diodes in ORings
High-availability applications such as telecom and communications infrastructure systems as well as data center servers often have multiple power supplies in parallel for flexibility, redundancy or capacity reasons. Power supplies can either share the load-- achieved through careful power bus design--or have one active and the other in standby. In this way if any one supply fails the whole system will not come down as a result, since the redundant supply can pick up the load.

Redundant power architectures, in turn, are reliant on ORing solutions. An active ORing solution combines a power MOSFET and a controller IC. ORing MOSFETs ideally should have a fast dynamic response to react to power source failures as well as minimal RDS(ON). Why? Low RDS(ON) minimizes conduction losses and promotes system efficiency. Conduction losses are dependent on duty cycle, drain to source resistance and the load current through the device. Since duty cycle and load current are determined by the application, RDS(ON) is a variable that can be specified by the designer, and so should be selected to be as low as possible.

Although the discrete ORing diode solution has been used for some time and is inexpensive to implement, with the increase in power requirements for systems widely expected to continue, its principal drawback-- power dissipation loss—becomes untenable. With its extremely low RDS(ON) characteristic, an FDMS7650 MOSFET ORing is a more practical solution because it avoids the diode voltage drop, power loss and heat dissipation you get using the traditional Schottky diode with the equivalent current.

Diode ORings also can be cumbersome. Because of its low power dissipation and substantially reduced dependency on thermal management, MOSFET ORing solutions using products such as FDMS7650 can be made to be much smaller, fitting in well with the trend in today’s telecom/datacom infrastructure toward smaller module dimensions.

Saving Energy in Data Centers
Data centers have a thirst for energy. A typical data center can consume between 1MW and 20MW of electricity and, for every dollar spent on computing hardware for the facility, an estimated 50 cents more is spent on powering and cooling the hardware.

Industry studies show that measured on a global scale data centers consume approximately 180 billion kWh of electricity per year – over 1% of the world’s electricity consumption. This is equivalent to the typical yearly electricity consumption of 60 million households –over a third of the number of households in the EU. Indeed, published reports suggest that once Google’s newest data center in Dalles, Oregon is running at full capacity, as is expected by 2011, it could require as much as 103MW of power to run, which is enough to supply electricity to every home in Newcastle.

Improving the efficiency of power supplies will also have another, indirect impact on data center costs. Since the wasted electricity of inefficient power conversion manifests itself as unwanted heat, more efficiency systems will reduce energy bills for cooling the facility; studies show that a data center's cooling system can account for 40 percent or more of its energy consumption.

FDMS7650 Packaging
This N-Channel MOSFET is fabricated with Fairchild’s advanced-performance PowerTrench process technology, making it possible to achieve lower RDS(ON) and higher load currents in smaller package sizes.

Power MOSFETs in an SO-8 package have been popular for quite sometime, although in the last few years traction has been achieved by an upgraded power package called Power56. Compared with SO-8 packages, Power56 offers reduced package inductance and resistance parameters as well as reduced thermal resistance.

Improving efficiency in data centers, telecom and Communications

Power56 packages are designed to minimizes board space and maximize RDS(ON) and, with a large drain tab at the bottom for conducting heat out of the package and away from the PCB, it also exhibits improved thermal performance. Power56 package current and thermal capability is comparable to DPAK but only uses 40% of the PCB area.

This packaging technology requires mounting techniques that are slightly different from a conventional SO-8; however, Fairchild engineers report that those familiar with mounting packages with large planar areas will find the techniques similar. When designing a board for Power56, the designer should remember that the drain is the most efficient at dissipating heat. Because of the unique design of the Power56 it is possible to run copper traces from three sides of the die. For best thermal performance it is recommended that the user put as much copper on the drain as possible, while keeping the thermal path short by keeping the area around the drain tab open. Standard pick and place equipment can be used with Power56 components.