Linear Power MOSFETs App Note - IXYS Corporation

1 Linear Power MOSFETs Basic and ApplicationsAbdus Sattar, Vladimir Tsukanov, IXYS CorporationIXAN0068 Applications like electronic loads, Linear regulators or Class A amplifiers operate in thelinear region of the Power MOSFET, which requires high Power dissipation capabilityand extended Forward Bias Safe Operating Area (FBSOA) characteristics. Such modeofoperation differs from the usualway of using Power MOSFET,inwhichit functions likean on-off switch in switched-mode applications. In Linear mode,thePower MOSFET issubjected to high thermal stress due tothe simultaneous occurrenceofhigh drain voltageand currentresulting in high Power dissipation. When the thermo-electrical stress exceedssome critical limit, thermalhot spots occur in the silicon causing the device to fail[1].

2 Figure 1: The Output Characteristics of an N-Channel PowerMOSFETF igure 1 shows atypical output characteristic of an N-Channel Power MOSFETin whichthe different modes of operation are the Cut-off region, the gate-sourcevoltage (Vgs) is less than the gate-threshold voltage (Vgs(th)) and the device is an open-circuit or the Ohmic region, the device acts as a resistor with almosta constant on-resistanceRDS (on)and is equal the Linear -mode of operation, the deviceoperates in the Current-Saturated region where the drain current (Ids) is a function of thegate-source voltage (Vgs) and defined by:)()()(2)(thgsgsfsthgsgsdsVVgVVKI Equation(1)whereK is a parameter depending on the temperature and device geometry and gfsis thecurrent gain or transconductance.

3 When the drain voltage (VDs) is increased, the positivedrain potential opposes the gate voltage bias and reduces the surface potential in thechannel. The channel inversion layer charge decreases with increasing Vdsand ultimately,it becomes zero when the drain voltage equals to)()(thgsgsVV . This point is called the channel pinch-off point where the drain current becomes saturated[2]. Linear Power MOSFETs Basic and ApplicationsAbdus Sattar, Vladimir Tsukanov, IXYS CorporationIXAN0068 The FBSOA is a datasheet figure of merit that defines themaximum allowedoperatingpoints. Figure 2 shows a typical FBSOA characteristic for an N-Channel PowerMOSFET. It is bounded by the maximumdrain-to -sourcevoltage VDSS, maximumconduction currentIDMand constant powerdissipationlinesfor various pulse this figure, the set of the curves shows a DC line and four single pulse operating lines,10ms, 1ms, 100 s and 25 s.

4 The top of each line is truncated to limit the maximum drain current and is bounded by a positive slope line defined by theRds (on)of the right hand side of each line is terminatedat therated drain-to -sourcevoltage limit(Vdss). Each line has a negative slope andis determined by the maximum allowed powerdissipation of the device Pd:Pd= [TJ (max) TC] / ZthJC= VDSIDE quation(2)whereZthJCis the junction-to -case transient thermal impedance andTJ (max)is themaximum allowed junction temperature of the 2:Typical FBSOA graph for an N-Channel Power MOSFETT hese theoreticalconstant Power curves are derived from calculation with assumption ofessentially uniform junction temperature across thePower MOSFET die.

5 Thisassumption is not always valid, especiallyfor a large die MOSFETs . Firstly,the edge ofaMOSFET diesolderedto the mounting tabof a Power package has generally lowertemperature compared to the center of the die, the result of lateral heat flow. Secondly,material imperfections (die attach voids, thermal grease cavities, etc.) may cause localdecrease of thermal conductivity, increase of local temperature. Thirdly, fluctuationsin dopantconcentrations and gate oxide thickness and fixed charge will causefluctuations of local threshold voltage andthe current gain (gfs) of MOSFET cells, whichwillalsoaffect local temperature of the temperaturevariationsare mostlyLinear Power MOSFETs Basic and ApplicationsAbdus Sattar, Vladimir Tsukanov, IXYS CorporationIXAN0068harmless incase of switched mode operation;however, th ese can trigger catastrophicfailure in Linear mode operation with pulse duration longer than time required for a heattransfer from the junction to the heat sink.

6 Modern Power MOSFETs optimized for aswitch-mode applications were found to have limited capability to operate in the right-side bottom corner of the FBSOA graph, the area to the right oftheElectro-ThermalInstabilityboundaryi n Figure (ETI) can be understood as a result of positive feedbackmechanism onthe surface of the Power MOSFET forced into Linear mode of operation:-There is a local increase of junction temperature;-This causes local decrease of Vgs(th)(MOSFET threshold voltage has negativetemperature coefficient);-This causes increase of local current density, Jds~ (Vgs- Vgs(th))2;-Increase of local current density causes increase of local Power dissipation andfurther increase of local ontheduration ofthepower pulse, heat transfer conditions and features of thedesign of MOSFET cells, theETI may cause a concentration of all the MOSFET currentinto current filament and formation of a hot spot.

7 This normally causes MOSFET cellsin thatspecified areas to loose gate controlandturns ontheparasitic BJT withconsequentdestruction of the 3:theFBSOA graph forthe IXYS Linear Power MOSFETIXTK22N100 LIXYS Corporation has developed a Power MOSFET structure and process that providesan extended FBSOA capabilityby suppressingthepositive feedback of Electro ThermalInstability. The design ofthesenew MOSFET shas a non-uniform distribution oftransistor cells, as well as havingcells withdifferent threshold voltages[3]. Everytransistor cell is designed with a ballast resistor at thesourceto limititscurrent[4]. TheLinear Power MOSFETs Basic and ApplicationsAbdus Sattar, Vladimir Tsukanov, IXYS CorporationIXAN0068parasitic BJT of each cell is heavilyby -passedso that itwillnot turn-on underextremeelectrical stressconditions.

8 In addition,the thermal response ofeach Power MOSFET istested to assureno solder a result, IXYSnow offersa family of PowerMOSFETs with extended FBSOA suitable for reliable operation in Linear of these MOSFETs contain guaranteed FBSOA , Figure 3showsthe FBSOA graph for IXYS Linear Power MOSFETIXTK22N100 LwithitstestedDC operation pointmarked. A list ofcurrentlyavailable Power MOSFETs with extendedFBSOA capability is shown in Table 1: N-Channel Power MOSFETs with extended FBSOA from IXYSPart NoVdss(V)Id(A)RthJCoC /WTheSOAS pecificationPower (W), Tc = 90oCPackage @ Vds= 400V, Id= @ Vds= 400V, Id= @ Vds= 400V, Id= @ Vds= 400V, Id= @ Vds= 400V, Id= @ Vds= 400V,Id= @ Vds= 600V,Id= @ Vds= 800V,Id= @ Vds= 800V,Id= , @ Vds= 800V,Id= @ Vds= 600V,Id= @ Vds= 600V,Id= on equation (2), a single Power MOSFET such as IXTK22N100L with voltagerating of 1000V provides a Power rating of 700W.

9 This Power ratingis normally usedinthe circuit design forswitch-mode operation but notforlinear applications. For linearoperation,IXYS providesa Safe Operating Area (SOA) ratingthat isobtainedunder astrict DC operation condition such as240W at VDS= 800V, ID= and TC= 90oC Power MOSFETs Basic and ApplicationsAbdus Sattar, Vladimir Tsukanov, IXYS CorporationIXAN0068 Application ExamplesActive Load:Consider a 2A/600V regulated Power supply that needs to be tested with an active loadcomprised ofmultiplePower MOSFET soperating in parallel. In this application, PowerMOSFETs should have a rating of 1000V for BVDSSand be capable of dissipating themaximum output Power plus a safety margin that includes the over-current IXTK22N100 Lpower MOSFET has voltage rating of 1000V anda powerdissipationratingof 700 Wwhen used for switched mode operation.

10 But this Power ratingcannot be used when computing its thermal limitationsin Linear applications. Forthiscase, one must useits SOA ratingof240W at VDS= 800V, ID= and TC= 20% safety margin, this reducesits allowable Power dissipationto maximum output Power for the Power supply is 1200W. Assume another 20% safetymargin in the Power in order to include over-current protectionso that the multipleMOSFETs must be able to dissipate 1440W. As it can be seen, a single MOSFET doesnot havethe capability to dissipate the total Power . Thus a load bank with multiple PowerMOSFETs connected in parallel is needed to carry the total Power . The number ofMOSFETs required to connect in parallel for this application is 1440/192 = 8.