Thanks to their low conducting and dynamic loss of silicon carbide (SiC) devices, Infineon CoolSiCTM MOSFETs are used more and more in industrial applications like photovoltaics, fast EV charging infrastructure, energy storage systems, and motor drives. But at the same time, engineers are faced with unique design challenges. Achieving smaller form factors while maintaining the thermal performance in a power conversion system are conflicting challenges, but Infineon's innovative .XT technology with SiC offers a solution.
Fortunately, customers can take a leap forward by using the new 1200 V CoolSiCTM MOSFETs with low ON resistance (RDS(ON)). This new product implements .XT technology in TO247 packages and is available in 7 mΩ, 14 mΩ, 20 mΩ ,40mΩ. With the juction-to-case thermal resistance (RthJC) reduction from .XT technology, it is possible to output larger currents while keeping the same virtual junction temperature (Tvj) rise or the same output current capability with lower juntion temperature rise. It is also possible to balance the two to achieve both, which not only improves the system output current capability, but also extends the lifetime of the device.
How to achieve more output power with SiC MOSFETs
To make SiC MOSFETs with the same ON resistance (RDS(ON)) have more output power capability in the system, the most direct way is to reduce the thermal resistance (RthJC) of the device.
.XT technology improves the solder technology between the chip and the package baseplate through a process known as diffusion soldering. Compared with the previous standard soldering technology, the newly improved .XT connection technology reduces the thickness of the solder layer to one-fifth of the previous thickness (Figure 1). Meanwhile, diffusion soldering also greatly reduces the probability of voids, which directly benefits thermal resistance (RthJC) reduction.
Fig. 1: .XT interconnection technology for discretes
As seen in Figure 2, the maximum thermal resistance (RthJC) can be reduced by 24 percent with the same chip size when using diffusion soldering, enabling more power output or less temperature rise for the device.
Fig.2: RthJC comparison between .XT interconnection and standard interconnection
To give you a concrete example, let’s take a three-phase inverter application to see the real influnce of the thermal resistance (RthJC) reduction here. It can be seen that just by using .XT technology, you can reduce the virtual junction temperature rise (ΔTvj) by around 15 degrees Celsius. If you keep the maximum virtual junction temperature (Tvjmax) the same, output power may increased by up to 15 percent correspondingly.
Fig.3: Maximum junction temperature (Tvjmax) .XT interconnection and standard connection.
How to improve power cycling capability with .XT technology
You may be more curious how .XT technology can increase power cycling capability. We’ve seen that .XT technology can reduce the virtual junction temperature rise(ΔTvj) by 15 degrees Celsius under these conditions. According to Infineon's AN2019-05, for a repetitive junction temperature swing (∆Tvj), we can read from the PC diagram that the device can withstand a certain number cycles. If .XT technology lowers the virtual maximum junction temperature (Tvjmax), then the reduced ΔTvj will put less stress on the device - ultimately extending its lifetime and reliability in the system.
So, by using .XT technology in the new 1200 V CoolSiCTM MOSFET with low ON resistence (RDS(ON)) to reduce thermal resistance RthJC, not only can you obtain more power output, you can also get better lifetime in your system. These features can deliver a critical advantage in applications such as photovoltaics, fast EV charging infrastructure, energy storage systems, and motor drives, where performance can be increased in the same form factor.
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