如果您拆开任何一辆插电式混合动力汽车，您会发现在电池与电机的连接位置会有一个大小如同饮料六联包一般大的黑色盒子，这就是我们今天的主角“功率逆变器”，其作用是将电池组的高压直流电转换成控制牵引电机的交流脉冲电。

DC-AC (直流-交流) 逆变器是一种快速反应的硅晶半导体转换装置，其功能类似于燃油汽车的发动机管理系统。它能够将驾驶员发出的指令转换成频率在10Hz~10kHz的频率脉冲宽度调制驱动信号发送给牵引电机。

但是由于电动牵引功率需要全部通过功率逆变器转换，在此过程中事必会产生了能量损耗，并直接导致在纯电动模式下的续航里程缩短。

对于提高下一代插电式电动汽车的续航里程来说，提升高效逆变器技术至关重要，其重要性仅次于提升电池功率密度。

不仅电动车和混合动力车需要性能优良的逆变器，其他领域也同样需要。高效的逆变器技术也在工业电机、消费者电子产品、电器设备和数据中心，以及光伏和风能系统等领域发挥重要作用。

这也是全球的电子和材料研究人员都在努力进行半导体的研发工作，希望实现比常规硅材料逆变器更为卓越的性能（如交互损失更小、热效率更高，系统成本更低等等）。甚至连谷歌也在研究这个课题，他们去年举办了一个有奖竞赛——小盒挑战（Little Box Challenge），旨在推动“绿色能源”的应用，最佳逆变器的设计者将获得100万美元的奖励。详细信息见https://www.littleboxchallenge.com/。

这项研究的目标是研发宽带隙半导体（WBG）。物理学家认为，WBG具有相对较大的量子能量范围，并且在这个范围内不存在电子态。与价带顶部到导带底部的硅材料相比，WBG具有更大的电子能隙。在实际应用中，电子能隙是指从用于导电的特殊半导体材料中释放电子需要的总能量。

举例来说，带隙更宽的半导体的优势包括可以承受更高的应用电场或电压，也可以在更高温度、更大的功率密度，以及更高频率工况下运行。

近期美国能源部向通用汽车拨款399万美元，并向德尔福拨款246万美元，这些经费将用于支持一系列为期三年的研发项目，与两个公司分摊项目运行成本，为插电式混合动力车研发基于WGB半导体的高效能、高性价比的集成功率逆变器模块。

《汽车工程杂志》之前曾报道过丰田正在进行中的研究工作，这项工作旨在研发更高效的碳化硅汽车动力电子模块。详细内容见http://articles.sae.org/13244/。

更小、更轻薄的逆变器

Pete Savagian现任通用的电力传动和系统工程总监，是通用“先锋EV-1”项目中一位经验丰富的成员。据他介绍，目前插电式混合动力车所使用的逆变器依靠的是一种基于硅材料的功率晶体管技术，这种技术专为工业应用而研发，已有超过25年的历史。他解释道，这些绝缘栅双极型晶体管（IGBT）通常为汽车专用，能效较高，并具备良好的快速交换性能。但是，如果要扩展插电式混合动力车的纯电动续航里程，那么硅材料的性能就不能满足要求了。

Savagian还提到，两种新型的WBG半导体——碳化硅和氮化镓，有望满足上述要求，因为这两种材料“在运行状态下的能源效率是硅材料的三到十倍，而在关闭状态时效率甚至更高。当操作指令的信号频率达到10000 Hz时，有效降低能源损失是非常重要的。”

宽带隙逆变器技术“拥有晶体管材料的性能，可以在更高的温度下运行，能源损失比硅材料动力电子设备更小”，德尔福的电子控制先进工程总监A.J. Lasley称，“能效的提升，意味着可以实现更长的续航里程。”

Lasley指出，宽带隙材料，尤其是碳化硅半导体，已在业内推广多年。近期也有许多美国能源部支持的项目上马，旨在将插电式混合动力车逆变器技术的提升到新的高度。

Lasley表示：“新材料有望将逆变器的尺寸缩小30%，并减少20%到30%的能源损失。”

通用的Savagian称，新型WBG半导体比“现有的逆变器半导体更节省材料，而所有相关的支持性设备，比如电气连接器件、制冷系统、换热器，以及外壳和底盘架构等，也都可以做得更小。”

Savagian预计，运行效能的提升和材料的节约，可以使功率逆变器单元成本更为低廉。他指出，逆变器的成本通常占电气传动系统（包括电动机和减速系统）总成本的40%左右。

Savagian和Lasley都强调，WBG半导体能够为插电式混合动力车带来的最大好处之一是，工程师可以直接将功率逆变器集成到传动系统中。

 “功率逆变器尺寸的缩小，意味着装配和封装可以更牢固，” Savagian表示。“工程师们也可以将装置集成到传动单元，以节省空间、减轻重量。例如可以不再使用电缆线，这样组装也会更容易。”

比硅的性能更优良

专家表示，氮化镓和碳化硅有相似的带隙特点，而后者技术更为成熟。但碳化硅芯片制造“非常昂贵，而氮化镓则有望实现低成本制造，因为它与底层基板材料的兼容性更好，” 北卡罗来纳州立大学的功率半导体研究中心主任Jayant Baliga解释道。Baliga是动力电子学领域的先锋人士，他在通用电气公司任职期间发明了IGBT，并实现了该装置的商业化应用。

Baliga领导的NCSU中心负责进行“美国能源（Power America）项目”的研发工作，该项目也称为 “下一代动力电子国家制造创新研究所”（Next Generation Power Electronics National Manufacturing Innovation Institute）。这项研发工作为期五年，经费共计1.4亿美元。美国能源部于2015年1月正式开始推进这一项目，目的是为了“降低WBG半导体的成本，提高其相对于硅材料的竞争力”。如Baliga所言，该项目的目标是促进实现从硅材料芯片制造向碳化硅芯片制造的转型。

美国能源部先进制造办公室的高级WBG专家Anant Agarwal表示，预计五年内，使用新型半导体材料的高效动力电子设备价格，将降至与硅材料常规设备相同的水平。

 “美国能源”项目的成员来自十几家企业、以及七所大学和实验室，包括ABB、阿肯色动力电子国际公司（Arkansas Power Electronics International）、科锐（Cree）、德尔福、约翰·迪尔公司（John Deere）、Monolith半导体公司、Qorvo、东芝、Transphorm、United Silicon Carbide碳化硅公司、伟肯（VACON ）和X-FAB公司等。

除了NCSU之外，该项目的学术机构和实验室合作伙伴还包括亚利桑那州立大学、弗罗里达州立大学、美国国家可再生能源实验室、美国海军研究实验室、加州大学圣塔芭芭拉分校，以及弗吉尼亚理工学院暨州立大学等。

作者：Steven Ashley
来源：SAE《汽车工程杂志》
翻译：SAE上海办公室

Plug-in vehicles await better power electronics

Inside every plug-in vehicle there’s a black box the size of a six-pack cooler that connects the battery to the electric motor. It’s called the power inverter. This crucial, but often overlooked component converts the battery pack’s high-voltage direct current (DC) into alternating current (AC) pulses that control the traction motor.

A DC-to-AC inverter, basically a fast-acting silicon semiconductor switch, functions something like an Engine Management System does in a internal-combustion power plant. It feeds the driver’s commands to the traction motor in the form of pulse-width-modulated drive signals at frequencies that can range from 10 Hz to 10 kHz.

Because all electric traction power passes through the inverter, any efficiency losses that occur within cut directly into a plug-in vehicle’s battery-only driving range.

In fact, more efficient inverter technology ranks second in importance only to more power-dense batteries for extending battery-only range in next-generation plug-ins.

Improved electric and hybrid vehicles are not alone in their need for better inverters. High-efficiency inverter technology would also greatly benefit industrial motors, consumer electronics, appliances and data centers as well as photovoltaic and wind energy systems.

It’s no surprise then that electronics and materials researchers worldwide are working to develop improved semiconductors that could deliver inverter performance that is superior to conventional silicon—including fewer switching losses, greater thermal efficiency and importantly, reduced system costs. EvenGoogle is working on this issue, having established last year a prize competition—The Little Box Challenge—that will award $1 million to the developer of the best inverter design for "green energy" applications; see https://www.littleboxchallenge.com/.

The goal of this research is to develop what are called wide bandgap (WBG) semiconductors. To physicists, WBG materials exhibit a relatively large quantum energy range in which no electron states can exist—a bigger electron energy gap compared to silicon between the top of the valence band and the bottom of the conduction band. In practice, it refers to the amount of energy that is needed to release electrons from a particular semiconductor material for conduction.

Semiconductors with wider bandgaps can, for example, withstand higher applied electric fields, or voltages, as well as operate at higher temperatures, power densities and frequencies.

In the automotive sector, the U.S. Department of Energy recently awarded research grants to General Motors ($3.99 million) and Delphi ($2.46 million) to support three-year, cost-shared projects to develop high-efficiency, cost-competitive integrated power inverter modules based on WGB semiconductors for plug-in vehicles.

Automotive Engineering previously reported on Toyota’s continuing research efforts to develop more efficient automotive power electronics modules using silicon carbide; see http://articles.sae.org/13244/.

Smaller, lighter inverters

Inverters in current plug-ins rely on silicon-based power transistor technology that was developed for industrial applications over the last 25 years, said Pete Savagian, GM's General Director for Electric Drives and Systems Engineering and a veteran of the company’s pioneering EV-1 program. These insulated-gate bipolar transistors (IGBTs), often tuned for automotive use, combine good efficiency and fast switching, he explained, but expanding plug-ins’ battery-only driving range means moving beyond silicon.

Two emerging WBG semiconductors, silicon carbide and gallium nitride, are expected to fill that role, Savagian continued, because they “can bring three to ten times better energy efficiency when they're turned on and especially, when they're turned off. And when you’re switching at rates of 10,000 Hz, reducing losses becomes important.”

Wide-bandgap inverter technology "plays upon the ability of the transistor material to run at higher temperature and with fewer losses than silicon-based power electronics," explained A.J. Lasley, Director of Electronic Controls Advanced Engineering at Delphi in Indianapolis. “The improved efficiency can directly translate into longer range.”

He noted that wide-bandgap materials, particularly silicon carbide semiconductors, have been trying to push into industry for many years, with the recent DoE-supported projects aiming to "push the limit" in plug-in inverter technology.

"The new materials offer great potential for allowing us to reduce the size of inverters by as much as 30% and cut energy losses by 20% to 30%,” Lasley said.

According to GM's Savagian, the new WBG semiconductors would allow “using less semiconductor material in inverters than we do now. The resulting smaller footprint means that everything else can shrink as well, including all the support equipment—electrical connectors, cooling system, heat exchanger, and the housing and chassis structures.”

Such physical and operational downsizing should in addition yield significantly cheaper power inverter units, Savagian predicted. He noted that the inverter typically accounts for about 40% of the total cost of an electric drive train, which includes an electric motor and a gear reduction system.

Both Savagian and Lasley stressed that one of the principal benefits of WBG semiconductors to plug-in vehicles is that they would enable engineers to integrate power inverters directly into the transmission systems.

“Their smaller size means that the mounting and packaging can be more rigid and robust," Savagian observed. "It also would enable engineers to incorporate the devices into the transmission units, saving space and weight. You could, for instance, get rid of the electrical cables, which makes assembly easier.”

Beyond silicon

Experts note that gallium nitride has similar bandgap characteristics to silicon carbide, which is a more mature technology. But silicon carbide chip fabrication "is very expensive, while gallium nitride offers the possibility of lower-cost manufacturing because of it is more compatible with the underlying substrate materials,” said Jayant Baliga, Director of the Power Semiconductor Research Center at North Carolina State University. Baliga, a pioneer in power electronics, invented and commercialized IGBT devices when he worked at General Electric.

Baliga’s NCSU center is taking the lead in the Power America program, also known as the Next Generation Power Electronics National Manufacturing Innovation Institute. This is a five-year, $140-million R&D effort established in January 2015 by the DoE “to drive WBG semiconductor costs to make them more competitive with silicon materials.” In the case of silicon carbide, the researchers are attempting to adapt existing silicon chip foundries to silicon carbide chip fabrication, Baliga noted.

Anant Agarwal, the senior WBG expert at the DoE’s Advanced Manufacturing Office, has said he expects that highly efficient power electronic devices using the new semiconductors will be able to achieve price parity with traditional silicon-based devices within about five years.

Power America’s members comprise a dozen companies as well as seven universities and laboratories, including ABB, Arkansas Power Electronics International, Cree, Delphi, John Deere, Monolith Semiconductor, Qorvo, Toshiba,Transphorm, United Silicon Carbide, VACON and X-FAB.

Besides NCSU, the program’s academic and lab partners are Arizona State University, Florida State University, the National Renewable Energy Laboratory, theU.S. Naval Research Laboratory, the University of California, Santa Barbara andVirginia Polytechnic Institute and State University.

Author: Steven Ashley
Source: SAE Automotive Engineering Magazine