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The mechanicals of Silicon Carbide

In the last few years, silicone carbide (SiC) MOSFET technologies have advanced significantly. When designing a power module with SiC, it is important to get the best out of the relatively high material cost. For example, Danfoss has equipped over 25 million cars and served over 3 gigawatts of solar power systems with high-power modules.

By Alix Paultre, Editor Power Electronics News (based on a technical paper presented by Alexander Streibel, Danfoss)


Among the biggest improvements in using wide-bandgap semiconductors is their electrical performance compared to silicon, but this is not without its own issues. As the the current flows faster, the resulting voltage overshoot gets bigger, leading to the requirement for low DC stray inductance, for example. Of course, if you lower the stray inductance and you shorten the rise and fall times of voltage and current, the loss integer gets smaller and this enables you to increase the switching frequency.

 

It is also important to pay attention to the mechanical and thermal requirements for SiC. For example, Danfoss introduced Bond Buffer technology to enable the highest power-cycling capability by addressing the CTE mismatch between aluminum bond wires and the semiconductors. By centering the dyes to the DBC, and by achieving a copper top-side connection, the company was able to remove the two most critical failure mechanisms inside the module, solder degradation below the chip, and the bond-wire liftoff.

Proper packaging and cooling is key

With SiC, the maximum allowed junction temperature increases, so that the reliability issues become a limiting factor. But proper bonding and joining technologies, along with cooling efficiency, are two main factors in SiC designs, beyond the electrical performance issues. Danfoss silicon power addressed the cooling issue with their ShowerPower cooling technology, able to cool more than 300 watts per square centimeter. The biggest benefit of ShowerPower, and it’s parallel connected channels, is surely the homogenous heat distributions on the substrate. And furthermore, the 3D structure below the baseplate makes the module very stiff. So it also gives mechanical benefits.

This demonstrates that dedicated packaging technologies can support the thermal and mechanical demands of advanced applications, with features such as double-side sintered die attach, with copper wire bond, to fulfill the power densities that are required in the markets. Another technique is to supplement the package using transfer mold technology for robust power modules that must fulfill shock and vibration requirements.

These key factors are very important, and of course cost is one of the major issues for everybody. But you can save money using silicon carbide in your system, if your application can utilize its full potential.

Package optimization

In setting up a package, we can optimize or customize the pin-out, by press-fit or solder pins. This can raise more issues, especially related to silicon carbide, because the control circuit bond wires are completely separated from the load current. Using individual gate resistors per chip allow us speed adjustments. Also DC caps can be integrated as an option.

The module shown in the figure below uses 12 planar silicon carbide Mosfets per logical switch, paired with Schottky diodes. This has been developed for a train application, but this module can also be used in a solar or medical drive application. It’s a half bridge operating at 25 kilohertz. You can see here the reduction during turn off by a factor of 5, in turn off switching loss.

Another example is a public project is called ThermoFreq. Together with Siemens, Fraunhofer, ELT, Hereos, FNK Devotec, Schuster Elektronik, and Fachhochschule Kiel, the project is developing technology that will be used for laser welding on top of the chip . It is a very simple form of spec, consisting of silicone carbide chip centered on a copper heat spreader, mounted to a base-plate through some isolation foil. This technology will enable high integration of fast switching and miniaturized power devices.

The ThermoFreq project aims to advance bonding technologies to increase module density and reliability

Looking forward

The key takeaways are that silicon carbide enables the highest compactness and efficiency for power conversion currently available, but these challenging topics must be considered: First, silicon carbide requires a cost-effective, customized low-inductive package. Second, silicone carbide requires advanced bonding and joining technologies, and third, dedicated cooling technologies.

 

Components & Devices Thermal Management Technology

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