Power Management


Part Package PCB Footprint


Single Side Cooled on FR4 RthJA


RDC(on) ID


(Single side cooling) @ Tj


= 105°C, TA Current Density Ratio of current density


Dual Sided Cooled RthJA


RDC(on) @ Tj ID


Figure 3.Comparison of thermal resistances for different semiconductor power packages


types of packages, the lead frame and wirebonds used in the package add a significant distance to the path that the electrical current has to flow along and therefore make a large contribution to the RDS(on). When the wirebonds and leadframe removed (in the case of DirectFET) the DFPR is reduced to a value of less than half of equivalent plastic power package. The removal of this barrier ultimately means that a lower area of silicon is needed for a given Rds(on), and opening up the possibility of a system level cost saving.


Cooling down


From equation 4 its can be seen that as RthJA tends to zero the current density will increase as the heat generated in the junction is extracted and dissipated into the ambient more easily. Figure 3 shows the two thermal routes that make up the RthJA for different package types, Rth(J-C)top the thermal


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route through the top of the package, and Rth(J-PCB) through the bottom of the package to the PCB. Power packages like the D2Pak and copper strap PQFN have excellent junction to PCB thermal paths, but like all the other traditional packages they are far less effective at allowing the heat generated to flow out through the top of the package, indeed they were not designed with top side cooling in mind. However when both top and bottom thermal paths are utilized dramatic reductions in RthJA can be made, as can be seen on the DirectFET package type.


Drawing the factors or space, RDS(on) and RthJA together, Table 1 makes a side by side comparison of a large die, low RDS(on) D2pak product with a counterpart Automotive DirectFET product using equation 4. The table summarises the improvement in current density.


Current Density Ratio of current density


(Dual side cooling DF only) = 105°C, TA = 25°C


= 25°C


AUIRFS3004-7P D2Pak-7P 170 mm2


40 °C/W 1.24 mΩ


40.13 A


0.24 A/mm2 1


12.5 °C/W 1.24 mΩ


40.13 A


0.24 A/mm2 1


AUIRF7739L2


Large Can DirectFET 64 mm2


40 °C/W 0.9 mΩ


46.29 A


0.73 A/mm2 3.1


40 °C/W 0.9 mΩ


82.81 A


1.3 A/mm2 5.5


Table 1: Comparison of Current Density for a D2Pak-7P and a Large Can DirectFET with different cooling arrangements


By taking two high performance 40V power MOSFETs which are typically used in automotive applications Table 1 shows how the simple construction of package like DirectFET, the elimination of wirebonds and leadframe allows RDS(on) PCB footprint and RthJA to be minimised. This results in current density to be increased by over 3 times when compared with a traditional plastic D2Pak-7P, even in an application where dual sided cooling is not used. The ability to drive more current through a smaller space, at higher efficiency and increase power density is going to become of greater importance as the electrification of the automobile unfolds. The replacement of fuel tanks


and spark plugs of the past with batteries, IGBTs and MOSFETs of today will not happen by default. The new power electronic drivetrains will not only have to meet but exceed the performance of the traditional internal combustion engine powered solution. With such demanding goals the use of next generation power semiconductor power packages which interfere less with the operation of the semiconductor will be key to ensure next generation efficiency, power density and performance goals are met.


International Rectifier | www.irf.com Benjamin Jackson is Product Manager, Automotive MOSFETs International Rectifier


Components in Electronics July/August 29


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