System solutions for battery-powered applications: Part 2 of 4 Editorial Series sponsored by Infineon; High power density and thermal management for compact low-voltage (LV) motor drive/control designs

Francesca Pastorelli, Senior Specialist Product Management, Power Discretes, at Infineon Technologies


How power MOSFET package innovation can overcome thermal challenges

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The challenge

Heat management is one of the biggest challenges for motor control and drive designs as it affects the efficiency and lifetime of the solution. In battery-powered applications, more sophisticated cooling mechanisms are adopted, requiring smaller heatsinks as well as different approaches other than PCB cooling. Power MOSFET package innovation is moving towards these new industry´s needs by developing products overcoming thermal challenges.

Current state-of-the-art solutions

Today most of the power MOSFET packages available in the market are bottom-side cooling ranging from Through Hole Device (THD) or Surface Mount Device (SMD). In THD (like TO-220), the pin of the components is inserted into the through-holes of the PCB, giving the possibility to spread the heat directly to the heatsink. On the other side, Surface Mount Devices (SMD), such as SuperSO8 5x6, are mounted directly on the PCB. Most innovation is now happening within the SMD packages, particularly with dual- and top-side cooling capabilities. Dual side cooling solutions help by adding another thermal path on the top via the heatsink leading to higher power density compared to the bottom side cooling one. Furthermore, in case of no heat spreading via the PCB, Top-side cooling helps to promote the heat directly to the heatsink reaching 20 percent better thermal resistance between junction and ambient (RthJA).

Package innovations leading towards better thermal designs

From 12 V to 48 V power tools, thermal performance is one of the key application requirements in particular for high power applications. Infineon offers a wide package portfolio with innovative solutions ranging from a small footprint allowing higher power density (such as Source-Down 3.3x3.3 mm2) to high current solutions for more robust designs (like the TOLx family).

Source-down package concept

Source-down is a groundbreaking approach to package innovation. This new concept takes the silicon die inside the MOSFET package and flips it upside down. The source potential is now connected to the lead frame instead of the drain potential (Figure 1).

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Figure 1.The internal construction of a PQFN 3.3x3.3 mm Drain-Down (DD) power MOSFET (left) and a PQFN 3.3x3.3 mm Source-Down (SD) power MOSFET (right)


The electrical connection of the drain (from the top of the die) can be achieved with a very large copper clip. Furthermore, two footprints are available: standard gate (Figure 2b) and center gate (Figure 2c) to allow better paralleling. The electrical connection of drain, source and gate are still in the same location, making a drop-in replacement very easy.

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Figure 2. Standard PQFN 3.3x3.3 mm (a), the new Source-Down PQFN 3.3x3.x mm (b), and the new Source-Down Center-Gate (CG) PQFN 3.3x3.3 mm (c) package footprints


This package allows numerous benefits such as a significant reduction of on-state resistance (RDS(on)), better thermal resistance (decrease of RthJC of 22 percent), and optimized layout possibilities.

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Table 1. Comparison between Source-Down 3.3x3.3 PQFN, Drain-Down 3.3x3.3 PQFN and SuperSO8 5x6


As shown in Table 1, the main advantage of Source-Down 3.3x3.3 mm2 comes from the roughly 30 percent improvement in RDS(on) and thermal management compared to the Drain-Down package. In addition, the Source-Down 3.3x3.3 mm2 package allows a 65 percent smaller footprint compared to the SuperSO8 5x6 mm2 package, which brings to more compact designs.

Concerning heat management, the Source-Down concept offers multiple benefits to enable cooler designs. Certainly, RDS(on) significantly impacts the thermal as lower losses lead to decreased device temperature. Most improvement comes from the active trenches located onto the copper lead frame. The heat generated in those trenches can be conducted directly into the PCB over the lead frame. Furthermore, the Source-Down 3.3x3.3 mm2 will be available with the great features of double-side cooling (DSC) [1] in both standard and Center-Gate configuration.

The TOLx package family (TOLL, TOLG, TOLT)

Besides the well-established TO-Leadless package, Infineon has introduced two new packages, TOLG (TO-Leaded with Gullwing Geometry) and TOLT (TO-Leaded Top-Side cooling), addressing different customer´s needs with the excellent key features of low RDS(on) and high current capability.

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Figure 3. TO-Leadless (a), TO-Leaded with Gullwing Geometry (b), and TO-Leaded with Top-side cooling (c)


TOLG (TO-Leaded with gullwing geometry)

With the same footprint and electrical performances of TOLL, the great advantage of TOLG comes from the flexibility of the leads, which is a key feature of the D2PAK 7-pin. However, compared to D2PAK 7-pin, TOLG allows roughly 60 percent board space reduction resulting in the perfect solution for remarkably compact designs.

The coefficient of thermal expansion (CTE) describes the tendency of the material to change its shape in response to a change in temperature. During MOSFET operation, the silicon die is heating up and transferring heat to the PCB. Depending on the PCB core material, different levels of PCB heat expansion will occur. In the case of FR4 and Cu-IMS board, this expansion/deformation is not severe since the PCB is flexible and has the same CTE as the MOSFET package. However, in the case of the Al-IMS board, this deformation creates mechanical stress on the solder layer between the PCB and the MOSFET. Over time, this temperature cycling is causing cracking of the solder joints and eventually leading to a failure.

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Figure 4.TO-Leadless (TOLL) warpage in FR4 and IMS board, correlated with CTE mismatch


The advantage of TOLG is particularly visible during Temperature Cycling on Board (TCoB), which is a second-level reliability test in which solder joints between the package and PCB are tested. Thanks to the flexibility of the gullwing leads, the new package shows excellent solder joint reliability on the Al-IMS board. This demonstrates two times better TCoB behavior compared to the standard requirement (IPC9701).

TOLT (TO-Leaded with Top-side cooling)

TOLT is the first top-side cooling package within the OptiMOS™ portfolio. Constructed with its lead frame flipped to position exposed metal on the top side, the package contains multiple gullwing leads on each side for high current carrying drain and source connections. In addition, the exposed pad is tin-free.

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Figure 5.TO-Leaded with Top-side cooling (TOLT) package structure


With a flipped lead frame, heat passes from the exposed metal top side directly to the heatsink through the insulating material. In the case of a bottom-side cooling package like TOLL, the heat is dissipated via the PCB to the heatsink, and power losses occur due to PCB thermal resistance. The new TOLT package achieves 20 percent better RthJA and 50 percent improved RthJC.

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Figure 6.Top- vs. bottom-side cooling heat dissipation


Comparing TOLL vs. TOLT in a three-phase BLDC motor drive inverter, TOLT can achieve a 25°C lower junction temperature compared to bottom side cooling designs.

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Figure 7.TOLL bottom-side cooling thermal resistance


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Table 2.TOLL vs. TOLT in BLDC motor drive inverter


For maximum junction temperature of 150°C, the current handling capability of TOLT with top-side cooling is 72 ARMS compared to 53 ARMS for TOLL with bottom-side cooling (using on the same size heatsink in both cases). This represents a 36 percent improvement for TOLT. Thanks to the better cooling, TOLT has improved power dissipation on the FR4 board due to lower RDS(on) at reduced Tj, resulting in smaller heatsink requirements or the possibility of handling higher output power.

A unique portfolio of heat management solutions

Infineon´s ever-growing portfolio of power MOSFETs provides numerous choices to motor control/drives and battery-powered application designers to overcome thermal challenges from small footprints such as Source-Down 3.3x3.3 mm2 – bringing the design to higher power density – to more robust solutions like the TOLx family.

To find out more, make sure to browse Infineon’s offerings for motor control and drive solutions and our latest innovative package families, such as TOLx and Source-Down.

Infineon Technologies

[1] A portfolio extension with DSC (Double Side-Cooling) products will be released in 2022.