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Gallium nitride high-electron-mobility transistors (HEMTs) have emerged as a promising technology for motor drive applications due to their unique properties. In motor drive systems, GaN HEMTs can enhance efficiency, reduce losses and enable higher-power-density designs. By switching faster and more efficiently, GaN HEMTs facilitate smoother motor operation, leading to improved overall system performance.
This article, based on a lecture1 given at the APEC 2024 event held last February in Florida, focuses on the overall cost savings that the usage of GaN HEMTs brings to motor drive designs.
Key challenges
In motor drive applications, the utilization of GaN power ICs presents a promising avenue for reducing overall system costs. This shift entails significant changes, including the elimination of heatsinks, facilitated by higher integration and streamlined automated assembly processes. By leveraging GaN power ICs, not only can costs be minimized, but efficiency can also be markedly improved. This translates to reduced energy expenditures and enhancements in performance ratings and labeling.
However, it’s essential to acknowledge that while legacy solutions employing silicon switches are entrenched within the industry and often perceived as more robust, it’s worth noting that not every application necessitates the pursuit of higher power density.
It is now a widely held view among semiconductor manufacturers that power components based on GaN will not completely replace those based on silicon but will be a complementary solution to them. Therefore, a nuanced understanding of the specific requirements and tradeoffs involved is paramount in determining the optimal approach for each motor drive application.
The key challenges and benefits deriving from using GaN power devices in motor drive designs are summarized in Figure 1. The left column lists the key features that can be implemented using GaN. This wide-bandgap (WBG) semiconductor offers advantages like high electron mobility, high breakdown voltage and low on-resistance compared with traditional silicon-based transistors. Additionally, GaN HEMTs’ reduced size and high-frequency operation capability make them suitable for compact and lightweight motor drive solutions, particularly in applications in which space and weight constraints are critical.
The center column specifies how each feature can positively impact the design of a motor drive solution, while the right column shows the real benefit arising from the implementation of this feature in real-world applications.
In the following paragraphs, we will compare a “legacy” solution based on silicon power devices with a WBG solution featuring GaN power devices.
‘Legacy’ solution
The silicon-based solution we will consider is a motor control board for a washing machine. The board, shown in Figure 2, achieves a peak output power of 600 W, operating at a switching frequency of 8 kHz. The power devices selected for this design are intelligent power modules (IPMs) with three half-bridges and bridge rectifiers. All these types of power devices are connected to a large heatsink (see Figure 2), providing the required cooling.
Note that the board is enclosed in a plastic case that, while protected from the environmental agents, has the limitation of restricting the airflow.
Figure 3 shows the block diagram of this solution and the generous size of the heatsink. The adopted topology is quite straightforward and there is no active power-factor-correction stage. The circuit includes a DC-link capacitor (220 μF), a 3-mH common-mode choke with large size to protect from the IGBT switching noise and a small differential-mode choke suitable for the relatively slow switching speed.
As shown in Figure 2 and Figure 3, the heatsink has a large size (128 × 39 × 25 mm, 89 grams), covering the length of the board. Featuring a complex layout, it provides a case-to-ambient thermal resistance of about 2.4 K/W.
GaN-based solution
Unlike the previous one, this solution has no heatsink. The cooling is instead achieved through copper areas integrated into the printed-circuit board (PCB), which efficiently dissipate heat to maintain optimal operating temperatures.
Despite its functionality, the device boasts a minimalist design with a limited number of external components, contributing to its compact and sleek form factor. Utilizing predominantly surface-mount devices, the unit demonstrates advanced manufacturing techniques and a focus on space efficiency.
Within its compact frame, the device houses essential power stage components, including a rectifier, an 82-μF DC-link capacitor for stable power delivery, three GaN power ICs configured in a half-bridge configuration to ensure efficient power conversion, and a precise current-sense component for accurate monitoring and control of electrical currents. This streamlined configuration emphasizes performance and reliability, making it ideal for various applications in which space and efficiency are paramount.
The GaN power ICs used are the GaNSense half-bridge ICs from Navitas Semiconductor (part numbers NV624xx and NV6269x). These devices feature full integration of a half-bridge circuit, including control, drive, power and protection, with integrated level-shifter and bootstrap. Providing ESD protection up to 2 kV, the IC can support a switching frequency of up to 2 MHz.
The half-bridge offers a continued drain-source voltage of 650 V, typical RDS(on) in the range of 70–275 mΩ and output power (depending on the design of the thermal management) from 200 to 600 W. The wide high-side and low-side source cooling pads integrated into the device are provided for this purpose.
The GaNSense-based option ensures a high, stable and consistent performance, allowing for a reduction in design margins. This is facilitated by low propagation delay, enhancing control-loop efficiency. The gate drive conditions are carefully controlled, contributing to outstanding reliability.
Additionally, the switching speed is adjustable to manage electromagnetic interference. The system boasts a significantly reduced component count, resulting in smaller size and lower costs, thereby enabling the integration of motors into inverters.
The lossless current sensing eliminates the need for shunt resistors, leading to improvements in cost, size, reliability and performance. Moreover, the system features fast and precise overcurrent protection, enhancing overall robustness. It includes overtemperature turn-off functionality, further bolstering system resilience. Unlike the IPM-based solution, which has a typical signal latency of 1–2 µs through the noise filter, comparator and gate driver, GaNSense can turn off the power switch in less than 100 ns if an overcurrent condition is detected.
A comparison between the GaN-based and silicon-based solutions demonstrates that the first one drastically reduces the total losses for different operating conditions. Moreover, for the GaN implementation, a higher thermal resistance of 12 K/W is sufficient.
Some small precautions must be taken by the designer when defining the PCB layout (see Figure 4) to take advantage of the thermal pads offered by the component, thus eliminating the need for an external heatsink. The main guidelines are the following (more information can be found in the component datasheet):
- Place many vias
- Use large and low-inductance traces
- Provide wide connections to shunts
- To improve cooling, use both sides of the PCB
- Avoid placing components on the bottom side of the PCB
- Place high-side components close to the output node
Reference
1Hesener, A. “Reducing System Cost with GaN HEMTs in Motor Drive Applications” Navitas Semiconductor, Applied Power Electronics Conference (APEC) 2024.
The post GaN HEMTs Achieve Cost Reduction in Motor Drive Applications appeared first on Power Electronics News.
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