Innovation Award Infineon AG
Direct Drive for CoolSiC
Infineon’s CoolSiC transistor is a normally on JFET device, combining the well known low ohmic performance of high voltage SiC transistors with an extraordinary level of ruggedness since no susceptive gate oxide with questions about interface quality and lifetime is used in the component. However, the device is normally on and a way to make it familiar with system requirements must be identified. For comparable wide band gap devices like earlier JFETs or today’s normally on GaN HEMTs the traditional cascode arrangement is used. This simple concept offers by a series connection of the normally on component with a normally off low voltage silicon MOSFET (the blocking voltage of the MOSFET must exceed the voltage required to block the device) and connecting the gate of the normally on transistor with the source of the MOSFET. The concept can be easily derived from the equivalent circuit of each DMOS today.
However, this concept has some disadvantages like potential dynamic avalanche stress on the MOSFET or limited controllability of the switching slopes. Thus, a modified setup was developed at Infineon, sill being based on the series connection of the two devices, but now controlling each gate separately. To enable an easy implementation a driver IC was developed to operate the setup. In this mode, the switching is no longer performed via the MOSFET gate, but directly via the JFET. The MOSFET is passive in this configuration and just acts as a safety switch for start up or failure mode in which the original cascode idea is maintained. The concept is called Direct Drive.
The idea deals with the challenge of operating a normally on device safely under modern system aspects and securing lowest losses at the same time. It addresses as well ruggedness problems of competing solutions and requirements from the application with respect to the dv/dt control in PWM operation.
The idea extends the original cascode idea in a way that their negative points are diminished. By disconnecting the gate contact of the normally on JFET from the MOSFET source we can access the main switching devices directly for easy dv/dt control. Furthermore, the MOSFET is no longer switched in each dynamic cycle and thus, no additional MOSFET losses have to be considered. Finally, the MOSFET is not driven in each cycle into avalanche what increases the ruggedness of this circuit.
The novelty of the concept is the extended cascode concept into a direct drive mode which offers a lot of additional advantages and lowest losses combined with high operational stability. Since the control philosophy is
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integrated into a corresponding driver IC the efforts on the user side is minimized, they can operate the device as they are familiar with from earlier power switches, but taking full advantage from the outstanding performance.
Kyma Technologies PVDNC AlN Templates
PVDNC AlN stands for plasma vapor deposition of nanocolumns. Kyma deposits nanocolumn AlN on silicon and sapphire substrates to create a great nucleation surface for growth of GaN devices thereupon. Device fabricators realize better (lower defect density) GaN buffer layers earlier in the buffer layer growth process. A lower defect density translates to higher thermal conductivity and presents other benefits depending on the device application.
Kyma supplies both materials and equipment for making PVDNC AlN templates, which is growing in importance in supporting GaN on sapphire based LEDs and GaN on Si power electronics.
The most difficult part of growing a GaN device on sapphire or silicon is in the initiation of the buffer layer. Kyma’s PVDNC AlN materials present the ultimate in terms of a great nucleation surface for growing GaN based devices on top of it. The PVDNC process creates a nanostructured AlN surface which is optimal to nucleate GaN growth on top. The process works on both flat and patterned substrates. PVDNC puts an important nanostructure on top of flat or microstructured substrates.
PVD was thought to be a low-tech approach to growing crystalline films. However, Kyma’s PVDNC process can create perfect nanowires of GaN.
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