Author:
John Garcia, Director Applications Marketing, Motion Control, Battery-Powered Systems, at Infineon Technologies
Date
01/31/2022
PDF
Click image to enlarge
Introduction
As discussed in previous installments in this series, integrating power electronics co-located or in close proximity to motors is an important design goal in many battery-powered applications. Faced with increasing functionality and power requirements, as well as the size and weight constraints in applications such as power tools and robotics, system designers find themselves seeking out and relying on technological advances to meet these significant challenges.
For motor drive and control specifically, increased switching frequency and implementation of field-oriented control (FOC) can help system designers overcome limitations associated with heat dissipation and bulk capacitance. This, in turn, results in increased system-level efficiency and a resultant reduction in thermal rise on both the motor and associated inverter design. It is in this area that advances in wide bandgap (WBG) semiconductors such as GaN enable a leap forward to overcome these significant system-level challenges.
Boosting efficiency with GaN HEMT switches and optimized gate drivers
Traditional motor drive designs featuring silicon (Si) MOSFETs present a demanding tradeoff between switching frequency and maximum output power due to the high switching losses, even in an optimized design. The move to wide bandgap (WBG) semiconductors such as GaN greatly alleviates this problem. With significant reductions in rise/fall times and associated reductions in switching losses, the latest generation of 100 V [1] enhancement-mode (e-mode) GaN HEMTs and GaN-optimized gate drivers enable a leap forward in motor drive applications.
The fast switching speed of GaN devices gives power electronics designers the freedom to liberally choose a higher switching frequency, looking at the end-to-end efficiency and total size of the design rather than only at the inverter efficiency. The impact of these improvements is most effectively highlighted when a FOC algorithm is implemented [1], as the higher frequency allows for a more accurate approximation of pure sinusoids while minimizing torque ripple and associated vibrations. Figure 1 shows the phase current waveform at 20 and 100 kHz switching frequencies, which are overlaid for comparison. The reduction in current ripple at 100 kHz compared to 20kHz provides a cleaner fundamental current waveform and less reactive current, leading to a reduced temperature at the motor winding.
Click image to enlarge
Figure 1.Phase current waveforms compared at 20 kHz and 100 kHz
An important aspect of this improvement is provided by the inherent lack of a body diode in GaN technology. This, in turn, eliminates the need to account for reverse recovery currents and yields the ability to significantly reduce the amount of dead time in the inverter. By doing so, the generation of sinusoidal currents is smoother, thus reducing the overall harmonic distortion generated and yielding a more efficient system. Figure 2 shows the phase current at an identical modulation index, operating at 100 kHz. Operation at 50 ns not only provides less distortion during zero-crossing but overall increased phase current.
Click image to enlarge
Figure 2. Influence of deadtime reduction in generating sinusoidal currents
In addition, the lower current ripple at the motor, in turn, has a positive impact by reducing winding and core loss while lowering the motor temperature. Since switching loss scales linearly with the switching frequency, using a Si MOSFET would be unfavorable in terms of system efficiency at increased frequencies. As seen in Figure 3, inverter efficiency can be improved with GaN switches (compared to silicon) at higher switching frequencies.
Click image to enlarge
Figure 3. Efficiency comparison of Infineon’s CoolGaN™ (GaN HEMTs) vs. OptiMOS™ 5 (Si) switches at 100 kHz switching frequencies
As illustrated, the benefit of higher switching frequency is more apparent at light load due to a relative reduction of RMS phase current; however, the benefits are very noticeable when looking at the winding temperature over the load range. In thermally constrained applications such as cobots, this provides a significant advantage.
Additional improvements enabled by advanced GaN technology implementations
Increased switching frequencies afford the overall system advantages at multiple levels. In addition to improvements in torque ripple which result in smoother motor operation (due to reduced harmonic content associated with a cleaner sinusoid), improved efficiency, and reduced motor temperature, it is also possible to achieve significant reductions in overall inverter size when GaN is implemented. This is realizable due to the reduction in necessary DC-link capacitance in the system.
A key consideration for required capacitance in these systems is its ability to suppress the ripple current from the motor inverter. Multilayer ceramic capacitors (MLCC) are inherently superior to larger, bulkier electrolytic capacitors in this respect; however, they are also much lower in capacitance than their electrolytic counterparts. The following equation illustrates the relationship between switching frequency and required DC-link capacitance:
Click image to enlarge
By comparing the requirements between systems switching at 20 kHz and 100 kHz at equivalent 11 A RMS current operating conditions, the resultant DC-link capacitance requirements are as follows:
Note that in order to meet lifetime requirements, the capacitance needs to be overprovisioned to reply to current specifications. Given the significant reduction in required capacitance, a distributed MLCC network can be realizable for a given inverter design, yielding a significant reduction in the size of the design. An additional consideration of using distributed capacitance (as in the MLCC case) is that parasitic inductance is reduced, thereby improving EMI behavior. In a final, application-specific design, this capacitor bank can be further optimized for the selected switching frequency.
A resultant side by side comparison is shown in Figure 4 below:
Click image to enlarge
Figure 4. Side-by-side comparison of motor drive implementation using GaN and Silicon power stages
Greater efficiency yields greater flexibility
The impact of a GaN-based design on an inverter in a motor control system can certainly be significant in terms of overall efficiency improvements; however, this also increases design flexibility. By creating a more power-efficient system, a designer can use that design headroom in a multitude of ways. In a battery-powered system, the increase in efficiency can be used to extend the operating time between required battery recharge; however, that increase can also be used to deliver more power to the load (i.e., increased torque capability of the motor) while maintaining the same run time on an individual charge. Alternatively, the increase in efficiency can translate to the use of a smaller battery, thus allowing for a decrease in the weight and/or form factor of the system.
Outlook
Moving forward, new wide-bandgap devices will establish the foundation of higher switching frequency drives, helping improve system performance by improving efficiency and accuracy while reducing size and weight. The increased switching frequency capability of GaN technology enables the use of lower inductance motors which are smaller, lighter, and faster, which, when coupled with the enhancements discussed in this article, yield a truly game-changing scenario in battery-powered motor control applications.
With an ever-growing portfolio of micro-controllers, gate drivers, power switches (GaN, SiC, and Si), and sensor technologies, Infineon is committed to providing world-class system-level motor drive and control solutions for battery-powered, consumer- and high-powered industrial applications.
To find out more, make sure to browse Infineon’s offering for battery-powered motor drive solutions here and GaN technology product solutions here.
References:
[1] M. Wattenberg, E. A. Jones and J. Sanchez, “A Low-Profile GaN-Based Integrated Motor Drive for 48V FOC Applications,” PCIM Europe digital days 2021; International Exhibition and Conference for Power Electronics, Intelligent Motion, Renewable Energy and Energy Management, 2021, pp. 1-8.
[1] Coming soon! To be released in Q4, CY 2022
