Motor control | Focus E-Mobility Engineering | November/December 2023 39 the voltage supply, the DESAT and the gate driver temperature. The parameters and thresholds for these features are configurable using a standard serial peripheral interface, which allows the device to be used with nearly any SiC MOSFET or IGBT. A standard way to evaluate a traction inverter’s power-stage switching performance is the double pulse test (DPT), which turns the SiC power switch on and off at different currents. Varying the switching times makes it possible to control and measure the SiC’s on and off waveforms over the operating conditions, allowing the efficiency and SiC overshoot, which affects reliability, to be evaluated. A strong gate drive can reduce SiC switching losses with a significant efficiency gain, depending on the traction inverter’s power level. While the choice of gate driver will dictate the overall efficiency of the power stage through the reduction of switching losses, the choice in gate driver bias supply can contribute by limiting conduction losses. Modelling with the WLTP procedure and real drive log speed and acceleration settings has shown SiC powerstage efficiency gains as high as 2%, corresponding to an additional 7 miles of range per battery, which can add up to over 1000 miles per year for an EV user. Seven miles could mean the difference between a consumer reaching a charger and becoming stranded. Higher voltages To accommodate the higher voltages of EV batteries, another gate driver has a built-in 3.75 kVrms (kV root mean square) isolator, which is higher than the 2.5 kVrms isolator in previous devices and can support power devices with a withstand voltage of up to 1200 V for 800 V and 1000 V battery packs. This provides data transfer with highvoltage isolation between the primary circuit (on the microcontroller side) and the secondary circuit (on the IGBT side). The driver boosts the CMTI (Common Mode Transient Immunity) performance at 150 V/ns or higher, providing reliable communication and increased noise immunity while meeting the high voltages and fast switching speeds required in inverter systems. The gate driver is packaged in an SOIC16 package for smaller inverter systems. For heavy-duty off-road EVs, a dual channel gate driver operates at up to 1000 V. The gate driver provides automotive-related protection functionalities and diagnostics. A galvanically isolated IGBT module integrated NTC read-out provides IGBT module temperature information. This supports a PWM switching frequency of up to 20 kHz and uses 1.5 W per channel at maximum ambient temperature. Single-phase PWM measurements PWM of the output of motor drives makes capturing stable measurements of signals challenging. Manually determining the right combination of filters and triggers to achieve stable waveforms is very difficult, yet this is a prerequisite for achieving consistent measurements. In addition to measuring the output of the drive, measurements to evaluate the performance of the drive’s input stages such as harmonics, power and power factor are also important. While exporting raw waveforms into a spreadsheet or other analysis software is possible, the process is time-consuming and requires care in designing the calculations. The measurements involve many connections to the device under test. Incorrect probing of the motor drive system and poor integrity of connections are common sources of errors in making motor drive measurements. Mechanical measurements are also key and can be made using sensors. However, it can be difficult or impossible to get measurements in engineering units of speed, acceleration or torque without custom processing and scaling. For these reasons, getting a good view of a motor drive system with an oscilloscope requires careful setup, stable waveforms and robust measurement algorithms. Vector drives/ field-oriented control The more advanced drives for AC induction motors and synchronous motors use vector drive techniques. These are more flexible and efficient than scalar drives, but also more complex. A dual-output split gate SiC driver for motor control (Courtesy of Texas Instruments)
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