SiC Technology for Motor Control
In solutions where active cooling to manage semiconductor losses is an important element for performance and reliability, a reduction of losses by up to 80% can be a game-changer. Last month, Infineon Technologies introduced the new silicon carbide (SiC) based CoolSiC™ MOSFETs with .XT interconnection technology in a 1,200V optimized D²PAK-7 SMD package. This combination enables passive cooling in power density critical motor drive segments like servo drives, thus supporting the robotics and automation industry in implementing maintenance-free and fanless motor inverters.
In automation, fanless solutions enable new design opportunities, driven by the fact that they save costs and effort on maintenance and material. Infineon’s CoolSiC trench MOSFET chip solution with .XT interconnect technology offers attractive thermal capabilities in a small form factor. The resulting small system size makes it ideal for drive integration in a robotic arm.
Motor control, in particular the frequency-controlled drive, is a technology that has developed very rapidly in recent years due to the massive use of motors in various application contexts and the possibility to save massive amounts of energy. Since their introduction, frame base power modules for motor control have been a considerable revolution for different aspects in application fields that are particularly sensitive to cost, size, and performance. The industrial automation sector is certainly the most emblematic.
A servo drive is the motor activation element in many automized production equipment like robots, transport belts, etc. The ohmic conduction losses and fully controllable switching transients of SiC MOSFETs offer a perfect match for their load profile.
The use of SiC devices in motor control and electrical power control applications, in general, is a real breakthrough thanks to features such as energy savings, size reduction, integration opportunities, and reliability. Among other options, it is now possible to use in the inverter circuit the optimum switching frequency for the connected motor which leads to important advantages in motor design.
The new CoolSiC MOSFET SMD devices have a short-circuit withstand time of 3 µs and are rated from 30 mΩ up to 350 mΩ. This meets the requirements of servomotors.
“The chip technology inside this product is our well known first generation of trench based silicon carbide MOSFET. So, we can offer very low switching losses and an exclusive three-microsecond short-circuit withstand time. Due to a sufficiently high threshold voltage and a favorable device capacitance ratio, a zero volt turn off option is enabled in those products, which significantly simplifies the driving circuitry. As expected from silicon carbide MOSFETs we have a robust body diode integrated into this component, which can be used for hard commutation,” said Dr. Peter Friedrichs, Senior Director SiC at Infineon Technologies.
The SMD package allows for very simple assembly by a fully automated line. Due to the low losses compared to IGBT solutions, Infineon highlighted that this type of transistor technology enables fanless cooling of the power semiconductors. It fits the long year dream of designers of motor drive inverters since it naturally reduces field maintenance considerably.
Why are servo drives so interesting in combination with SiC MOSFET features? In those drives, we can see periods of strong acceleration and braking where high power or torque and, therefore, high current is needed, while during nominal 90% operation low current is required (Figure 1).
Figure 1: conduction loss reduction in all operating modes (Source: Infineon)
This specific driving cycle in combination with the linear output characteristic similar to the ones in silicon carbide MOSFET to much lower losses compared to IGBT’s in the low torque operation mode.
“We can, at least at lower temperatures and in partial load mode, outperform the conduction losses with respect to the IGBT. During braking when we have a reverse output operation, the same situation due to the internal body diode, we are also again able to reduce the conduction losses significantly. So, in all operating modes, you can lower static losses. The same holds for the switching losses. Those of course are heavily influenced by the switching speed. But even in the low dV/dt range of 5 to 10 volts per nanosecond which is typical in several drives, the total switching losses can be reduced by 60% compared to today’s IGBTs, predominantly due to the negligible Qrr, elimination of tail currents and temperature-independent switching behavior,” said Friedrichs.
Figure 2: CoolSiC Mosfet in comparison with IGBT (Source: Infineon)
The use of this type of component in different drives provides higher power density. Compared to an IGBT with a similar rating, a higher current can be achieved with the same form factor, depending on the type of power chosen for the CoolSiC, while still maintaining a constant junction temperature that is significantly lower in the case of a silicon carbide MOSFET (around 40-60 K) than in the case of an IGBT (105 K). A SiC MOSFET allows higher currents to be driven without a fan for a given device size.
“This would be possible thanks to CoolSiC trench MOSFET Chip and .XT interconnection technology. It combines excellent thermal capabilities with a small package form factor. The 1200 V optimized SMD version offers > 6mm creepage and clearance distances with an extra source pin for advanced switching,” said Friedrichs.
Classical interconnection technology for discrete devices in molded packages is based on a copper lead frame where the chip is placed, and a solder material is added to connect the chip to the lead frame. Thermally the solder layer limits an effective heat removal from the chip to the underlying lead frame, mainly in the case of SiC which has a similar thermal conductivity like the copper used for the lead frame. The new interconnection uses a special back metallization which can directly react with the copper lead frame. Thus, no material is added between the chip and the lead frame, resulting in a significant reduction of the Rth.
“We believe that, with this new technology, we can offer an alternative to today’s classical 3phase bridge solutions for several drives segments, which are either realized already today by discrete devices or even in frame-based power modules. However, all of them need heat sinks. Now, with this new process, an optimized thermal behavior, and a powerful chip technology inside, we believe that very smart and compact designs based on SMD components without classical and bulky heat sinks are possible. Our reference design demonstrates how this technology can be used in a real application,” said Friedrichs.
Silicon carbide allows operating at higher density power levels. In many power electronics-based applications, such as industrial motor control units, requirements like space, weight, and efficiency play an increasingly important role. With advances in the SiC ecosystem, many solutions will benefit from a reduction of the overall losses, a reduction in cost and size of the drive.