Servo drives are ubiquitous and used in a variety of applications. Among them are robotics, CNC machinery, or industrial automation. All of them share the need for function-specific, high-quality servo motors, dedicated controllers, and motor drivers. In a traditional approach, designers place the motor driver and the 3-phase inverters in a separate control cabinet, apart from the motor at the load side. They then connect the cabinet and the load side via a 3-phase AC cable delivering the pulse-width modulated (PWM) power, and add a control feedback cable. The latter carries the motor information required for the closed loop control, like position, velocity, and acceleration.
Figure 1: Traditional implementation of a servo application
This approach has a couple of limitations: one is the total cable length. It can’t be too long, as reflections can cause a motor winding overvoltage. Another one is the necessity to shield the AC cable to reduce EMI (electromagnetic interference) noise. This shielding is quite expensive. And lastly, a lengthy control cable might downgrade the system response because of its time delay. This matters, as the data rate on this line is quite high. The drive must receive new information about the motor position in an interval of some 10 microseconds to a few milliseconds, so any delay can be harmful.
Integration reduces cabling requirements
Designers can avoid all these issues by integrating the 3-phase inverter with the motor at the load side. This means that only the rectifier stays in the control cabinet. Doing so has several benefits: A DC cable delivers the power to the inverter, removing the restriction on the length of the cable. Power is transmitted as DC and not AC, removing the AC cable limit influencing the motor control performance. Cables may even be cheaper, as shielding is no longer required, and as no reactive current is present.
A communication line is also necessary. Over it, the central control sends the commands to the inverter and the motor, and receives the process monitoring information. The data rate here is much lower, with update frequencies ranging from some tens of milliseconds to seconds. This also reduces the complexity of the application.
Figure 2: Integrating the 3-phase inverter with the motor reduces system complexity
But this solution is not without challenges. Typical motor drives use IGBTs (Insulated Gate Bipolar Transistors), which have a similar or even larger size than the motor itself, enlarging the system at the load side considerably. This is undesirable because there are usually massive size constraints on the load side.
SiC MOSFETs enable fanless drives
Wide bandgap (WBG) devices come to the rescue here. They have lower losses than IGBTs, something which helps to minimize the physical size of the drive significantly, shrinking it from the size of an A4 sheet of paper to a diameter of about 11 cm. The low losses result in part from the very small drain-source on-resistance (RDS(on)) and the excellent reverse recovery behavior of the SiC MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors) during the transition of the body diode from the conducting to the blocking state. Here, the reverse recovery time (Trr) and reverse recovery current (Irr) values are so small that the energy loss (Err) becomes negligible. All of this also gives a much better thermal performance.
If designers choose a suitable device, for example, our CoolSiC™ MOSFETs in a D2PAK package, the system can be cooled passively, with no need for a forced airflow. Reflow soldering the MOSFETs to an IMS (Insulated Metal Substrate) printed-circuit board (PCB) makes it possible to mount them to the back cover of the motor. The rear plate will then act as a heat sink for the motor and the inverter. This enables a design which is maintenance-free over the entire lifetime of the servo drive.
Figure 3: Block diagram of the reference design showing the inverter and the driver
Head start through reference designs
Together with our partners Jingchuan and Maxsine, we developed a reference design for this motor drive integration. It comprises two boards: one gate driver board and one power board, and it includes the driver circuit and the three-phase inverter, making it ready for integration with a servo motor. The driver circuit is based on the 1EDI20I12MH EiceDRIVER™ with a miller clamp function and the power board is built around 1200 V/45 mΩ CoolSiC™ MOSFETs.
Figure 4: The two PCBs of the reference design demonstrate the possibilities
The PCB design data, including the schematics, bills of material, layout files, and an Altium Design file is available from the tools page of the reference design.
While the reference design comes with a motor, its control is not the main focus of the design. For an easy evaluation of the electrical performance of the system, engineers can connect the reference design to an iMOTION™ MADK EVAL-M1-101T design kit powered by an IMC101T-T038 iMOTION™ Motor Control IC.
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