如果您拆開任何一輛插電式混合動(dòng)力汽車，您會(huì)發(fā)現(xiàn)在電池與電機(jī)的連接位置會(huì)有一個(gè)大小如同飲料六聯(lián)包一般大的黑色盒子，這就是我們今天的主角“功率逆變器”，其作用是將電池組的高壓直流電轉(zhuǎn)換成控制牽引電機(jī)的交流脈沖電。
DC-AC (直流-交流) 逆變器是一種快速反應(yīng)的硅晶半導(dǎo)體轉(zhuǎn)換裝置，其功能類似于燃油汽車的發(fā)動(dòng)機(jī)管理系統(tǒng)。它能夠?qū)Ⅰ{駛員發(fā)出的指令轉(zhuǎn)換成頻率在10Hz~10kHz的頻率脈沖寬度調(diào)制驅(qū)動(dòng)信號(hào)發(fā)送給牽引電機(jī)。
但是由于電動(dòng)牽引功率需要全部通過功率逆變器轉(zhuǎn)換，在此過程中事必會(huì)產(chǎn)生了能量損耗，并直接導(dǎo)致在純電動(dòng)模式下的續(xù)航里程縮短。
對(duì)于提高下一代插電式電動(dòng)汽車的續(xù)航里程來說，提升高效逆變器技術(shù)至關(guān)重要，其重要性僅次于提升電池功率密度。
不僅電動(dòng)車和混合動(dòng)力車需要性能優(yōu)良的逆變器，其他領(lǐng)域也同樣需要。高效的逆變器技術(shù)也在工業(yè)電機(jī)、消費(fèi)者電子產(chǎn)品、電器設(shè)備和數(shù)據(jù)中心，以及光伏和風(fēng)能系統(tǒng)等領(lǐng)域發(fā)揮重要作用。
這也是全球的電子和材料研究人員都在努力進(jìn)行半導(dǎo)體的研發(fā)工作，希望實(shí)現(xiàn)比常規(guī)硅材料逆變器更為卓越的性能（如交互損失更小、熱效率更高，系統(tǒng)成本更低等等）。甚至連谷歌也在研究這個(gè)課題，他們?nèi)ツ昱e辦了一個(gè)有獎(jiǎng)競(jìng)賽——小盒挑戰(zhàn)（Little Box Challenge），旨在推動(dòng)“綠色能源”的應(yīng)用，最佳逆變器的設(shè)計(jì)者將獲得100萬美元的獎(jiǎng)勵(lì)。詳細(xì)信息見https://www.littleboxchallenge.com/。
這項(xiàng)研究的目標(biāo)是研發(fā)寬帶隙半導(dǎo)體（WBG）。物理學(xué)家認(rèn)為，WBG具有相對(duì)較大的量子能量范圍，并且在這個(gè)范圍內(nèi)不存在電子態(tài)。與價(jià)帶頂部到導(dǎo)帶底部的硅材料相比，WBG具有更大的電子能隙。在實(shí)際應(yīng)用中，電子能隙是指從用于導(dǎo)電的特殊半導(dǎo)體材料中釋放電子需要的總能量。
舉例來說，帶隙更寬的半導(dǎo)體的優(yōu)勢(shì)包括可以承受更高的應(yīng)用電場(chǎng)或電壓，也可以在更高溫度、更大的功率密度，以及更高頻率工況下運(yùn)行。
近期美國(guó)能源部向通用汽車撥款399萬美元，并向德爾福撥款246萬美元，這些經(jīng)費(fèi)將用于支持一系列為期三年的研發(fā)項(xiàng)目，與兩個(gè)公司分?jǐn)傢?xiàng)目運(yùn)行成本，為插電式混合動(dòng)力車研發(fā)基于WGB半導(dǎo)體的高效能、高性價(jià)比的集成功率逆變器模塊。
《汽車工程雜志》之前曾報(bào)道過豐田正在進(jìn)行中的研究工作，這項(xiàng)工作旨在研發(fā)更高效的碳化硅汽車動(dòng)力電子模塊。詳細(xì)內(nèi)容見http://articles.sae.org/13244/。
更小、更輕薄的逆變器
Pete Savagian現(xiàn)任通用的電力傳動(dòng)和系統(tǒng)工程總監(jiān)，是通用“先鋒EV-1”項(xiàng)目中一位經(jīng)驗(yàn)豐富的成員。據(jù)他介紹，目前插電式混合動(dòng)力車所使用的逆變器依靠的是一種基于硅材料的功率晶體管技術(shù)，這種技術(shù)專為工業(yè)應(yīng)用而研發(fā)，已有超過25年的歷史。他解釋道，這些絕緣柵雙極型晶體管（IGBT）通常為汽車專用，能效較高，并具備良好的快速交換性能。但是，如果要擴(kuò)展插電式混合動(dòng)力車的純電動(dòng)續(xù)航里程，那么硅材料的性能就不能滿足要求了。
Savagian還提到，兩種新型的WBG半導(dǎo)體——碳化硅和氮化鎵，有望滿足上述要求，因?yàn)檫@兩種材料“在運(yùn)行狀態(tài)下的能源效率是硅材料的三到十倍，而在關(guān)閉狀態(tài)時(shí)效率甚至更高。當(dāng)操作指令的信號(hào)頻率達(dá)到10000 Hz時(shí)，有效降低能源損失是非常重要的。”
寬帶隙逆變器技術(shù)“擁有晶體管材料的性能，可以在更高的溫度下運(yùn)行，能源損失比硅材料動(dòng)力電子設(shè)備更小”，德爾福的電子控制先進(jìn)工程總監(jiān)A.J. Lasley稱，“能效的提升，意味著可以實(shí)現(xiàn)更長(zhǎng)的續(xù)航里程。”
Lasley指出，寬帶隙材料，尤其是碳化硅半導(dǎo)體，已在業(yè)內(nèi)推廣多年。近期也有許多美國(guó)能源部支持的項(xiàng)目上馬，旨在將插電式混合動(dòng)力車逆變器技術(shù)的提升到新的高度。
Lasley表示：“新材料有望將逆變器的尺寸縮小30%，并減少20%到30%的能源損失。”
通用的Savagian稱，新型WBG半導(dǎo)體比“現(xiàn)有的逆變器半導(dǎo)體更節(jié)省材料，而所有相關(guān)的支持性設(shè)備，比如電氣連接器件、制冷系統(tǒng)、換熱器，以及外殼和底盤架構(gòu)等，也都可以做得更小。”
Savagian預(yù)計(jì)，運(yùn)行效能的提升和材料的節(jié)約，可以使功率逆變器單元成本更為低廉。他指出，逆變器的成本通常占電氣傳動(dòng)系統(tǒng)（包括電動(dòng)機(jī)和減速系統(tǒng)）總成本的40%左右。
Savagian和Lasley都強(qiáng)調(diào)，WBG半導(dǎo)體能夠?yàn)椴咫娛交旌蟿?dòng)力車帶來的最大好處之一是，工程師可以直接將功率逆變器集成到傳動(dòng)系統(tǒng)中。
“功率逆變器尺寸的縮小，意味著裝配和封裝可以更牢固，” Savagian表示。“工程師們也可以將裝置集成到傳動(dòng)單元，以節(jié)省空間、減輕重量。例如可以不再使用電纜線，這樣組裝也會(huì)更容易。”
比硅的性能更優(yōu)良
專家表示，氮化鎵和碳化硅有相似的帶隙特點(diǎn)，而后者技術(shù)更為成熟。但碳化硅芯片制造“非常昂貴，而氮化鎵則有望實(shí)現(xiàn)低成本制造，因?yàn)樗c底層基板材料的兼容性更好，” 北卡羅來納州立大學(xué)的功率半導(dǎo)體研究中心主任Jayant Baliga解釋道。Baliga是動(dòng)力電子學(xué)領(lǐng)域的先鋒人士，他在通用電氣公司任職期間發(fā)明了IGBT，并實(shí)現(xiàn)了該裝置的商業(yè)化應(yīng)用。
Baliga領(lǐng)導(dǎo)的NCSU中心負(fù)責(zé)進(jìn)行“美國(guó)能源（Power America）項(xiàng)目”的研發(fā)工作，該項(xiàng)目也稱為 “下一代動(dòng)力電子國(guó)家制造創(chuàng)新研究所”（Next Generation Power Electronics National Manufacturing Innovation Institute）。這項(xiàng)研發(fā)工作為期五年，經(jīng)費(fèi)共計(jì)1.4億美元。美國(guó)能源部于2015年1月正式開始推進(jìn)這一項(xiàng)目，目的是為了“降低WBG半導(dǎo)體的成本，提高其相對(duì)于硅材料的競(jìng)爭(zhēng)力”。如Baliga所言，該項(xiàng)目的目標(biāo)是促進(jìn)實(shí)現(xiàn)從硅材料芯片制造向碳化硅芯片制造的轉(zhuǎn)型。
美國(guó)能源部先進(jìn)制造辦公室的高級(jí)WBG專家Anant Agarwal表示，預(yù)計(jì)五年內(nèi)，使用新型半導(dǎo)體材料的高效動(dòng)力電子設(shè)備價(jià)格，將降至與硅材料常規(guī)設(shè)備相同的水平。
“美國(guó)能源”項(xiàng)目的成員來自十幾家企業(yè)、以及七所大學(xué)和實(shí)驗(yàn)室，包括ABB、阿肯色動(dòng)力電子國(guó)際公司（Arkansas Power Electronics International）、科銳（Cree）、德爾福、約翰·迪爾公司（John Deere）、Monolith半導(dǎo)體公司、Qorvo、東芝、Transphorm、United Silicon Carbide碳化硅公司、偉肯（VACON ）和X-FAB公司等。
除了NCSU之外，該項(xiàng)目的學(xué)術(shù)機(jī)構(gòu)和實(shí)驗(yàn)室合作伙伴還包括亞利桑那州立大學(xué)、弗羅里達(dá)州立大學(xué)、美國(guó)國(guó)家可再生能源實(shí)驗(yàn)室、美國(guó)海軍研究實(shí)驗(yàn)室、加州大學(xué)圣塔芭芭拉分校，以及弗吉尼亞理工學(xué)院暨州立大學(xué)等。
作者：Steven Ashley
來源：SAE《汽車工程雜志》
翻譯：SAE上海辦公室
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
2016款雪佛蘭沃藍(lán)達(dá)搭載了通用汽車的第二代Voltec驅(qū)動(dòng)單元，其內(nèi)部集成了一個(gè)小尺寸直流-交流的功率逆變器，使用的是常規(guī)的硅材料半導(dǎo)體。
2016款雪佛蘭沃藍(lán)達(dá)使用一個(gè)硅材料芯片功率模塊，對(duì)直流-交流逆變器進(jìn)行了集成，功率模塊由德爾福公司提供。
2015年初，科銳集團(tuán)（Cree）推出了業(yè)內(nèi)首款碳化硅900V MOSFET動(dòng)力電子裝置?？其J總部位于北卡羅來納州。
功率逆變器用半導(dǎo)體二極管和晶體管將直流電轉(zhuǎn)為交流電，實(shí)現(xiàn)對(duì)牽引電機(jī)的控制。