Power package solution for space constrained applications

16 January 2015

The DirectFET was originally introduced by International Rectifier as the next step in power MOSFET packaging.

This new package eliminated the lead-frame, bond wires, and mould compound found in traditional power packages by attaching the MOSFET die directly to the metal can via an electrically conductive, thermally enhanced epoxy (glue).  

The benefit to the new package approach when compared to a traditional power package has led to increased system efficiency, power density, and reliability as well as a reduction in parasitic ringing.  With the introduction of newer generation devices and the Large-Can; automotive and battery powered hand tools are beginning to benefit from of this package as well.

Power package 

The elimination of the lead-frame, bond wires, and mold compound has led to:

Maximised power density
• low thermal resistance
• removes heat away from the PCB

Minimised conduction losses
• low package resistance 
• minimised PCB conduction losses

Reduced parasitic ringing
• Low package inductance
• Reduced EMI
• Improved switching speed

Compatible with high volume manufacturing processes

Low profile (0.7mm), small footprint (63.7mm2)

Reliability (0.9ppm)
• No lead frame, no wire bonds, no molding compound

Environmentally friendly
• Lead and bromide free
• 100% RoHS 6/6 compliant

These benefits lead to a high density, efficient, reliable, low noise solution for power system power designers.  One of the limitations of the DirectFET has been the availability of a package capable of handling very large die typically found in D2PAK packages.  Previous to the Large-Can DirectFET, only Small and Medium Can devices were available.  Figure 2 shows a comparison between the Small, Medium, and Large-Can DirectFET packages.

With the introduction of the Large-Can DirectFET, devices like the IRF7739L1 are now achievable.  This is a 40 V, 0.7 m? device with a continuous drain current rating of 270 A @ TC=25°C which surpasses D2Pak devices on the market today.

Versus surface mount packages

In high power density, space constrained applications PCB area and package height become critical design factors.  One of the advantages the Large-Can DirectFET has over the D2Pak is a reduction in PCB area and package height.  Figure 3 shows PCB area, volume, and height comparison between two traditional wire-bonded surface mount power packages, the DPak and D2Pak, versus the Large-Can DirectFET.

Given that the Large-Can DirectFET and DPak occupy approximately the same board area; a reduction in RDS(on) of 1.2m?, and increase in current rating of 180A can be realised by switching to the Large-Can DirectFET.  There is also a reduction in height, from 2.3mm to 0.7mm, which equates to a 1.6mm savings.

A comparison between the Large-Can DirectFET and D2Pak shows a PCB area reduction from 170mm2 to 63.7mm2, and package height reduction from 4.5mm to 0.7mm.  Couple this with a reduction in RDS(on) and increase in current (0.3m? and 90A for max die size 40V devices) and you have a solution for power dense, space constrained applications.

It should be noted that the IRF7739 (DirectFET) and IRFS7430 (D2Pak) are both maximum die size devices and are approximately the same (die) size.  From this we can see that the die-free package resistance (DFPR) of the DirectFET is ~0.3m? lower than that of the D2Pak.  This becomes a significant factor when looking at sub-1m? MOSFETs.

Another advantage of the DirectFET is the removal of the bond wires.  Bond wires lead to unwanted package parasitics in addition to resistance.  Figure 4 shows the die-free package inductance (DFPI) for many common MOSFET packages, including the DirectFET.  Clearly the DirectFET (0.4nH @ 5MHz) shows a major improvement when compared to the DPak (2.2nH @ 5MHz) and D2Pak (4.9nH @ 5 MHz).  This equates to a reduction in switching losses, ringing, and EMI.

RoHS 6/6 Compliance

One of the biggest challenges facing the semiconductor industry today is fully complying with the RoHS (Restriction of Hazardous Substances) directive which is aimed at removing certain hazardous substances within electrical and electronic equipment.  The semiconductor industry has been RoHS 5/6 compliant by having a Pb-free package for a number of years now.  The challenge has been to have Pb-free solder which is used within the package for bonding the die to the lead-frame.  Meeting this requirement means you are RoHS 6/6 compliant.  While many standard packages are already compliant, i.e., SO-8, TSOP-6, SOT-23, many of the traditional power packages, i.e., DPAK, D2PAK, PQFN, … are not.  International Rectifier has already identified a number of solutions to solve this problem and is under way in implementing them; however, for designs being implemented today, the DirectFET is already RoHS 6/6 compliant.  

The DirectFET package uses an epoxy (glue) for die attach material.  This epoxy is optimised for both thermal and electrical conductivity, comparable to solder, yet contains no Pb.  

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