26 POWER MOSFETS www.onsemi.com

Issue 1 2014 Power Electronics Europe www.power-mag.com

MOSFETs Make Step Changein Performance to Meet NewApplication RequirementsLow voltage MOSFET devices (<40 V) are used extensively in the power systems of portable electronicdevices, domestic appliances, data communication servers, medical equipment and telecom infrastructuredeployments. Exacting demands continue to be placed on the engineers involved in the design of suchMOSFETs. The article will look at the forces at work and how significant, yet potentially conflicting, technicalchallenges need to be tackled. Wharton McDaniel, Product Marketing Manager Power MOSFETs,ON Semiconductor, Phoenix, USA

Our everyday lives are now almosttotally reliant on the use of various formsof electronics equipment, however thereare major concerns about the rise inworldwide energy consumption that this isleading to - its depletion of fossil fuels, theimpact it has on the environment (in termsof carbon emissions, etc.) and theenlargement of utility bills too. Greaterconsumer awareness, the implementationof hard-hitting legislative measures (suchas EnergyStar), plus OEMs’ testingperformance roadmaps, are allcompounding the acute pressure beingplaced onto power semiconductormanufacturers. Next generation of powersystems will consist of components thatpossess a number of highly favorableattributes.

Important driversThere are two main dynamics that arecurrently responsible for defining MOSFETdevelopment. On one side you have thehigh end processing being required bysever farms. Here the power demand anddensity of the microprocessors found inracks has grown immensely - presentingelectricity consumption, real estate andthermal management obstacles that needto be overcome. Conversely, in computingit is not the processing capacity that is nowthe main concern. As computer platformshave migrated from bulky desktop PCs tolightweight, streamlined portable products,such as tablets and smartphones, this is nolonger the key selling point for customers.The criteria by which the power systemsare outlined have changed here, withbattery life and system compactness nowbeing prioritized. The switching frequenciesof both these types of power systemsmust be raised, so that smaller magnetics

and passive components can be used,with less space thus needing to beallocated.Having established what the drivers are

at the application level, how does thistranslate into the components specs?There are three main parameters whichshould be scrutinized when specifyingMOSFETs for a power system. These are:1. On resistance (RDSON) is vital when itcomes to mitigating a MOSFET’sconduction losses, so this should be aslow as possible.

2. Figure-of-Merit (FOM) - defined by RDSON

x QG (total gate charge), this is anindicator of both the switching andconduction losses of the MOSFET, so itis used as an important selection criteriawhen deciding on which component touse.

3. Switching performance - The betterswitching characteristics of the MOSFETare, the lower the switching losses willbe. With increased switching frequenciesbeing witnessed all the time, this willbecome even more crucial in thecoming years.

MOSFET design and its implicationsMOSFET usage became more common inthe 1980’s, and in the decades that havepassed since then a lot has changed interms of the way these devices arefabricated and the performance specs theycan deliver. Though it is for very differentreasons, the MOSFETs serving both of thevery disparate applications scenarios wediscussed earlier need to dissipate as littlepower as possible (when active and alsowhen inactive), so keeping RDSON low mustbe one of the fundamental goals. Supportvery high conversion efficiencies is also ahuge plus point, as is having a small die

size - as in both portable products andserver implementations board space is at apremium. The question is how to go aboutachieving this performance ‘wish list’ at thecomponent level.There are two elementary constituents

that MOSFET design can be broken downinto. In the past it was possible forcomponent manufacturers to get awaywith just doing one of these really well,then dealing with the other solely as anafterthought. Now though they need to beaddressed simultaneously, with equalemphasis. Process technology - ten years ago a

CPU generally required a current of around10 A to drive it, today it is more likely to be100 A. This has dictated an ordermagnitude reduction in RDSON simply tokeep form factors and heat dissipationlevels in check, otherwise a substantialexpansion in the size and cost of electronicequipment would have been unavoidable.Furthermore, a MOSFET’s overall switchingperformance can be greatly augmentedthrough a reduction in its capacitances.Continued development of newsemiconductor processes have allowed theMOSFET RDSON and capacitance reductionsneeded by OEMs to be realized. Thesereductions should remain scalable -keeping pace with smaller semiconductorgeometries for the foreseeable future. Theadvent of shielded-gate trench topology ishelping to keep the industry ahead of thegame, with features which minimizeswitching overshoot, complementing thelowered RDSON. With switching frequenciesrising, as already stated, overshoot isbecoming more of an issue, so thistopology’s ability to mitigate its effect ishighly advantageous.Package technology - a great deal of

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