SiC And Industrial Servos: A Perfect Match

The automation engineers of the 1960s would look with envy at the servo technology in use today. Small, precise, and, above all, electric, they are a reflection of the compactness of the semiconductor control, sensor, and power technology we have available today. Today’s biggest challenge remains the cabling between the servo and its controller. Notoriously expensive, due to having to withstand the high currents from both the motor and the control signals, cables are a significant source of electromagnetic interference (EMI). Furthermore, reflected waves caused by impedance mismatches are often a source of trouble, resulting in damaging stress on motor winding insulation. Ideally, combining drive and controller into the servo motor would resolve a lot of the challenges.

Currently, silicon IGBTs are the mainstay of the servo drive circuitry. With their excellent high-voltage performance, manufacturers have steadily reduced the impact of losses and parasitics over the years. Furthermore, packaging technology has helped to reduce the volume of circuitry. However, due to the 200% or even 300% overload conditions these drive systems have to handle, passively cooled and integrated IGBT-based servo motors remain unattainable.

Solving the servo drive challenge

Thanks to the introduction of wide bandgap (WBG) silicon carbide (SiC) MOSFETs, designers now have a new tool to apply to servo drive designs. Offering operation at higher temperatures and improved endurance over IGBTs, coupled with lower switching losses and higher drain-source voltages, they are an excellent match for this application (figure 1). The SiC MOSFET can also conduct from source to drain through the channel at a very low resistance, allowing the use of energy-saving synchronous rectification techniques.

Fig. 1: IGBTs suffer from significant energy losses due to Q_rr, which worsens at high temperatures compared to SiC MOSFETs.

The move to SiC also delivers a raft of other benefits. The losses that occur are much less temperature-dependent, with little difference between room temperature and operation at 175°C. Electromagnetic compatibility (EMC) is easier to achieve as dv/dt can be controlled via the gate resistor, R_G. The door is also opened to higher switching frequencies. This allows space-consuming magnetic components to be shrunk and enables the servo to respond faster to dynamic load changes. Compared to an IGBT-based design, designers can either lower the operating temperature by up to 40% or deliver 65% more power at a similar operating temperature.

Keeping thermal challenges in check is made simpler thanks to today’s metal-core printed circuit boards (MCPCB) in integrated servo designs, coupled with low-loss auxiliary circuitry and thermal conductivity epoxy resin. Thermal simulations show that, when using a toothed back cover of 300 cm², the top side of an integrated, SiC-based design reaches just 113°C, while the back stayed under 80°C (figure 2).

Fig. 2: Finite-element thermal analysis of a compact, fully integrated SiC-based servo.

A fully integrated SiC-based servo

Demonstrating the actual effectiveness of SiC in servo drives is a stacked, three-board evaluation system using the IMBG120R030M1H, 1200 V/30 mΩ CoolSiC MOSFET (figure 3). The power board is placed closest to the case. The compact PG-TO263-7 packages used contributes to the compact design and low weight. This package also features a Kelvin source pin that can be used to deliver a threefold reduction in EON losses, as well as being qualified according to JEDEC 47/20/22 for use in industrial applications. Also helping to mitigate thermal challenges is the use of Infineon’s .XT interconnection technology. Its diffusion-soldering method provides a 25% improvement in thermal resistance over the soldering process used in alternative packaging.

Fig. 3: Integrated servo motor (left) and compact, SiC-based driver board (right).

The next board in the stack houses the Infineon 1EDI20I12MH EiceDRIVER that offers a typical peak current of up to 6 A, matching the demands of the 1200 V SiC MOSFETs used. Galvanically isolated, using coreless transformers, they also integrate Miller clamps to protect against parasitic turn-on.

Providing the control on the final board is the XMC4800, a 144 MHz ARM Cortex-M4 industrial microcontroller. With 2048 kBytes of flash and up to 352 kBytes of SRAM, coupled with its DSP and MAC instructions, it is well-dimensioned to deal with the challenges of three-phase motor control algorithms and their digital feedback loops. The integration of low-latency communications busses, such as an EtherCAT slave, also simplify system integration. Rotor position is acquired using an anisotropic magneto-resistive (AMR) sensor, the TLE5109. With its integrated temperature compensation, it contributes to the servo’s high levels of precision.

Operating from a 600 V DC supply, the evaluation-integrated servo motor has proven to be reliable under test conditions that accelerate the servo between ± 1500 RPM over slow (150 ms) and fast (50 ms) cycles (figure 4).

Fig. 4: Slow and fast cycle (left/right) acceleration/deceleration testing for the integrated-servo evaluation design.

The availability of SiC for servo drive developers allows engineers to, at last, integrate motor and drive systems into a single solution. Coupled with compact and highly integrated 32-bit XMC microcontrollers and magnetic sensors, as well as thermally optimized MOSFET packaging, this could finally signal the end of cabling in servo systems.

To learn more about how Infineon’s SiC MOSFET portfolio and solutions can help your power design, download the whitepaper.