Addressing thermal management of GaN semiconductors

There will always be innovation barriers which make it quite hard and complicated to use novel technologies and materials like GaN. But  let’s assume that these obstacles can be overcome. Which brings us to the next question, which is “What are the major challenges using this GaN power semiconductors?” For an application engineer, what makes it so hard and complicated to use these technologies in a real system? 

By Alix Paultre, Editor Power Electronics News (based on a technical paper presented  by Dr. Marvin Tannhaeuser, Siemens)

In power electronics one has to master a quite wide range of different engineering disciplines, from the power semiconductor, to the electrical connection, to the mechanical connection, and then to the thermal design. Each power semiconductor needs an appropriate gate driver circuit and auxiliary circuits like power supply, isolators, protection circuits and so on. In addition, we also need measurement circuits to finally control the whole power electronics system. One also has to integrate these low-level power electronics systems to some kind of embedded platforms, which realize the digital control concepts also for complex multilevel circuits or other topologies.

Of course you have to address these topics in each power electronics system, especially for grid applications. One needs to consider from the beginning the whole concept, and for component design, like inductor design, but also for the thermal design. Another major element in each power electronics system is always the power inductor, its own engineering field where you need some fundamental knowledge about electromagnetic materials as well core and winding design, and their related issues.

Thermal design

Thermal design should always be considered in combination with the whole system concept, the application, and whether or not you can use active or passive cooling methods. Bringing gallium nitride into that game changes the whole system. If you just replace a silicon MOSFET with another GaN device, you will see a small change in system performance in an already given design. But if you use all the performance advantages and their very high switching speed, you will see novel and new challenges which we have to handle in a complete system design.

Because GaN devices enable a very fast and very efficient switching, you can increase the frequency of the power circuit, which reduces the size of the inductors as well. At that point, you need power semiconductor packages with very low stray inductance to address these high switching speeds, and the control algorithm needs to address these challenges as well.

There needs to be improvements in SMT packaging as well, to use the high-switching speed capabilities to design power circuits and power cells with very low stray inductance. Another benefit is that a discrete SMT package can take better advantage of automated manufacturing processes (Figure 1).

On the other hand, there needs to be new approaches for the thermal management of these devices. Therefore one should have a closer look to the PCB, not as an element for electrical connection, but more as an element for thermal management, for heat spreading and dissipation.

In a steady-state equivalent circuit of the PCB one can consider two different thermal paths. One is the local thermal path directly under the device put through the PCB into the thermal heating. Another thermal pass, which is always available, is a kind of spreading resistance into the PCB in combination with the distributed vertical resistance, and resistance from PCB surface to ambient. All these thermal elements can be influenced by different design formations from the PCB. For example, the amount of copper on the surface, the copper thickness in the inner layers, or the substrate material, the sheer density, and of course the whole geometry of the layout (Figure 2).

Figure 3 compares different setups to compare overall thermal performance from the device path to the heat sink, which is the thermal resistance through the PCB. The first setup reaches 5.8 kelvin per watt, and in the second thermal path, the spreading thermal path, without any kind of local thermal VIAs under the device path. In the second setup it’s  8.2 kelvin per watt, but if you see with all the standard versions like 70 or 105 micrometer of copper thickness, you can decrease the thermal resistance to approximately 60% of the initial value.

Setup III is a more realistic case where both thermal paths were combined, with local vias also spreading into the PCB. Using 70 micrometers as the copper thickness for the PCB, one can reach a value of 3.5 kelvin per watt as a thermal resistance through the PCB. In addition to PCB design setup, different cooling methods can be used to address the application needs. In general, to handle these losses from the package, a major element must be the PCB design. The thermal spreading within the PCB is as important as the use of the local thermal vias, and can reduce the thermal resistance of the PCB by nearly 40%. An optimized thermal PCB designs also depends on the cooling method. The use of a heat sink and/or fans are normally defined by the boundaries of the addressed application.

In the case of GaN, one can use a cheap and easy cooling method due to its high efficiency and performance. A GaN converter prototype was built to show the feasibility of that thermal concept (Figure 4). The prototype targeted DC/AC conversion, with a three-phase connection on one side, and a DC connection on the other. The prototype was tested at 175 kilohertz, and no additional fans or heat sinks were used for thermal management, just passively by the PCB’s four copper layers, at a standard 70 micrometers of copper layer thickness, and standard FR4 material. The input voltage was 700 volts, and the output voltage was a little more than 230 volts.

The prototype exhibited a very high efficiency of 98.5%, which shows the excellent performance of GaN devices The two pictures at the top of Figure 4 show the power cell at the power semiconductor, in a little bit more in detail. At this operation it was 1 kilowatt. The transistor reaches a maximum temperature at the case of just 57 degrees, which is a very good value considering the lack of heat sinks and fans, and a power of one kilowatt.

Looking forward

GaN power semiconductors enable a very high switching speed and very efficient switching. Therefore, the power electronic systems can become more efficient, but can also work with higher switching frequency, which allows you to use smaller passive components. In addition, GaN power semiconductors can also enable multilevel converters with much higher system efficiencies than existing comparable solutions. GaN will lead us to smaller and lighter converter systems with a much higher level of integration.

Considering the increasing demand of power electronics in the growing markets for renewables, electromobility, factory automation, and other spaces, one could address all these markets in the future with smaller, lighter and cheaper systems, which are ready for automated high-volume manufacturing.

Components & Devices Thermal Management Technology

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