Sr. Expert Power Modules, Fraunhofer IZM
SIC POWER MODULE Low inductance fully ceramic SiC power module for high-temperature automotive applications
The developed fully ceramic SiC power module offers excellent themal performance and high reliability in high temperature automotive inverter applications. Superlative switching performance compared to conventional wire-bonded power modules reduces switching losses and therefore allows range extension of electric vehicles, and consequently lower system costs.
Low Temperature Cofired Ceramic (LTCC) is up to now rarely used in power electronics. But its unique physical properties perfectly match Si, SiN and SiC. Furthermore LTCC makes possible a three-dimensional multilayer wiring power module structure offering low switching loss, excellent thermal performance and high reliability in high temperature applications.
The depth of the cavities is matched to the height of the used SiC MOSFETs (Figure 1.1). Thus it is possible to sinter SiC MOSFETs into the cavities. After sintering the MOSFETs into the cavities the remaining space between the SiC MOSFET and the LTCC is filled with underfill to ensure functional isolation (Figure 1.2).
Then a backside contact of the chip with good thermal performance can be realized by sintering or soldering the LTCC with chips to a SI3N4-AMB substrate (Figure 1.3). SiN substrate provides the best mechanical robustness compared to other ceramic substrates as well as excellent thermal conductivity and is a reasonable choice for applications that require high reliability and high power density. The materials within the resulting fully ceramic assembly are well matched in terms of coefficient of thermal expansion (CTE). Therefore high reliability at high operation temperature is expected.
Soldering of the Si3N4 bottom side to the 3D-printed Aluminum heat sink (Figure 2.1) completes the thermal path. But this heatsink is special: as shown in Figure 3 it has an extremely short thermal path from cooler surface to coolant. There is only 0.5 mm 3D printed Aluminum material between cooler surface and coolant.
The overall thermal resistance from SiC MOSFET junction to coolant was measured at 0.8K/W/per chip. Such a ceramic assembly is capable of working at much higher temperatures than 150°C in, for example, a motor inverter. It is also possible to build thermal vias in the LTCC in order to realize double sided cooling.
But in our case we use another advantage of LTCC: It is possible to place SMD components directly on the top side surface. Thus we placed a DC link RC snubber and non-isolated gate driver circuit directly on the LTCC. Components of both are also capable of operating at 150°C (Figure 2.2). As a result we have a full switching cell inside the power module. The power module fulfils the two key electromagnetic conditions required for fast and oscillation-free switching. The first condition is low primary DC link inductance The yellow loop in Figure 4.
The switching cell loop crosses both of the semiconductors, the primary dc link capacitor and the damping resistors.The damped DC link on top of the LTCC builds an ultra flat 30mm wide switching cell loop. As a result, the switching cell inductance is less than 1nH. Further DC link resistors are connected with thermal vias through the LTCC directly to the SiN substrate. Thus they absorb most of the resonant circuit energy within the switching cell in the switching moment, resulting in reduced thermal stresses in the semiconductors.
The second condition required for fast and oscillation-free switching is low gate inductance. A non-isolated gate driver circuit placed directly on the power module - close to the semiconductors - reduces the gate inductance (green loop in Figure 4) to less than 5nH. This ensures switching free of parasitic turn-on and reduces oscillations.
Integration of such a core into the system is not trivial. Not only XYZ adjustment, but also basic insulation and low inductance high-current connections are required. A plastic frame (Figure 2.3) filled with standard silicone adjusts position of the ceramic assembly respective to the PCB and ensures basic isolation.
Low inductance high-current connectors are realized as 0.3mm flat multi-contact spring connectors. These reduce the stray inductance of the connection between the 200µF secondary DC link foil capacitor on the PCB and the RC snubber on top of the power module. Low inductance of the connection is crucial for fast switching performance. As a result we have only 8.4 nH of secondary DC link inductance, shown as blue loop in Figure 4.
Switching performance comparison
The nearly perfect low inductance system integration is evident in the switching forms shown in Figure 5.
Here we see a comparison of the switching behavior between two conventional wire-bonded power modules and the LTCC based power module. Conventional power modules switch on significantly more slowly and turn off with obvious oscillations.
The absence of the voltage overshoot and oscillations enables an increase of the allowable DC link voltage from 600 to 750V.
To summarize, the LTCC with cavities enables us to build a fully ceramic CTE matched assembly suited for high operation temperature. High thermal performance is achieved by an ultra-short thermal path from junction through the soldering layers, silicone nitride ceramics and 0.5mm thin 3D printed aluminum heat sink.
Ultra low DC link inductance, thanks to the DC link RC snubber placed directly on the power module, and low inductance system integration result in fast and oscillation-free switching of the power module. Low gate impedance ensures no parasitic turn-on and low turn-on losses. The power delivered by the LTCC based power module is high enough to power a 150kW motor inverter. This power module sets the standard in terms of clean high speed switching, combining it with low thermal resistance and high operating temperature. Both ceramic substrates (LTCC and SiN-AMB) are products of Hitachi Metals Ltd.