What should we pay attention to when using wide bandgap power devices on the car?

This article is reproduced from the Electronic Engineering Album

Author: Liu Yuwei, Deputy Chief Analyst, Electronic Engineering Special Issue

Recently, the latest report released by management consulting firm McKinsey stated that the European automobile industry needs to adjust its strategy as soon as possible to narrow the gap with Chinese competitors. Among the seven strategies devised by McKinsey to reshape the competitiveness of the European automobile industry, the new Chinese market strategy was emphasized, saying that China will still be the largest automobile market in the future. For upstream companies, it is very important to gain competitiveness in China. important.

Changes in electric vehicle sales of major European car companies (solid black line) and emerging forces in China and the United States (solid blue line) (unit: thousand units, source: McKinsey)

Judging from the export data in the first half of the year, Chinese electric vehicle manufacturers have made a good start in Europe. According to automotive consultancy Inovev, 8% of new electric cars sold in Europe so far this year are made by Chinese brands, up from 6% last year and 4% in 2021.

According to Allianz research, at least 11 new Chinese-made electric vehicles will be launched in Europe by 2025.

Under the charge of BYD, NIO, MG and other vanguards, Europeans' stereotypes about Chinese cars have been greatly improved. Nowadays, Chinese-made electric vehicles are popular all over the world. How does Europe, the former hegemon of fuel vehicles, view this trend?

Who is SET?

During college, Frank majored in radio frequency and communication electronic engineering. After studying in Germany and Scotland, he started his career in the aerospace field. During this period, he worked as an engineer and developed many electronic projects, and then in 2001 he founded SET GmbH ("SET"), a company focusing on the development of aerospace and defense test systems. In 2007, when electric vehicles began to gain prominence, SET entered the field of power semiconductor reliability testing and became a professional testing company.

“From that point on we had a direct focus and really only focused on two things: one was hardware-in-the-Loop (HIL) testing in aerospace, and the other was power semiconductors. In Power Semiconductors "Our main focus is reliability testing and end-of-line testing for high-voltage applications and high-power applications, including electric vehicles, aviation equipment and anything related to power semiconductors," Frank said.

SET has been deeply involved in the power semiconductor industry for many years and now has a roundtable forum organization called "Power Semiconductor Reliability Round Table" in Europe and the United States. This expert group invites more than 70 test experts from more than 50 industry leading companies in 14 countries. It organizes roundtable discussions on power semiconductors twice a year and invests a lot in wide bandgap, power semiconductors, and reliability testing. The energy and research have become a source of public knowledge for reliability testing of wide bandgap devices.

Frank said that in this roundtable organization, SET can use its own know-how to provide some feedback to the industry or the automotive upstream and downstream industries, such as what are the current testing challenges and so on. "We are members of the European Center for Power Electronics (ECPE), and AQG-324 is the standard we participate in providing for the automotive industry. Its scope of application includes power electronic modules and equivalent special designs based on discrete devices. I am also one of the initiators and drafting chairmen. one."

It can be seen that SET has a certain say in the formulation of standards for the entire automotive power semiconductor testing industry, so how to transmit these to the entire downstream automotive supply chain and automotive semiconductor reliability testing standards? It requires some strong alliance.

What difference will it make after SET joins NI?

As a leader in automated testing, measurement tools and control solutions, NI's ADG business also includes aerospace. The years of cooperation between NI and SET also began with ADG's semi-physical simulation. In the past few years, NI and many partners have launched a series of semiconductor test systems such as STS, entering specific vertical fields and taking a step closer to the industry from a platform basis.

However, NI has not really entered the field of power semiconductor testing. This is because NI's products have not been able to meet the requirements of power semiconductor testing in the past, so for many years they have cooperated with companies like SET by providing platforms such as PXI.

On March 6, 2023, NI officially announced the acquisition of SET. Official news stated that after SET joins NI, they will jointly shorten the time to market of key and highly differentiated solutions, and use power electronic materials such as silicon carbide (SiC) and gallium nitride (GaN) as entry points to accelerate the transition from semiconductors to NI. to automotive supply chain integration.

In the past, SET's business model with its partners was one-way communication, such as purchasing hardware from NI, then purchasing the system from a system integrator, and then doing integrated development. NI does not have much hardware support in the field of power semiconductors, while SET has strong know-how in the field of power semiconductors. Most of them are self-developed hardware. The acquisition of SET will allow NI to complete the entire semiconductor testing by strengthening power semiconductors. industrial chain.

The merger of NI and SET has brought more and more efficient two-way communication. "Since we became NI, providing solutions for the power semiconductor industry has been a focus of NI's semiconductor strategy. This also means that we bring technology and expertise in power supply, high voltage applications, reliability and power semiconductor test from SET to Arrived in NI." Frank explained the changes brought about by joining NI.

SET will provide special solutions and continue to use NI's software and hardware platforms on some platforms. "In the past, NI served as a supplier to provide hardware, but now we are a family and can form a more complete research team to interact. SET can feed back the industry's needs and hardware design to NI, and jointly communicate about the next product design in the industry. ” he said.

According to reports, SET focuses on chip testing to the end of the production line, discrete devices and modules. NI already has a dedicated inverter test system. The integration of the two can open up the entire test supply chain from chips to modules, combine all test data from chip level to inverter, and expand the business to high-power applications. Testing of inverters and batteries.

In addition, NI's great advantage lies in software, including not only LabVIEW and TestStand in the traditional sense, but also data analysis software. SET's solution can be combined with NI software to form a larger industrial chain.

How to define static testing and dynamic testing?

In fact, the silicon carbide technology that is popular today appeared more than 40 years ago, but it has never been applied on a large scale because it faces many challenges.

Frank believes that the failure mode of silicon carbide is different from the failure mode of silicon-based semiconductors in the past, so the testing method must be changed. In view of this, SET proposed a dynamic test for the field of power semiconductors to find more material or component failure modes to solve industry challenges. Now they provide test systems for both silicon-based and wide-bandgap power semiconductors, especially for automotive industry applications, "because electric vehicle giants like BYD are really driving this market. "

So for power device reliability testing, what is static testing and what is dynamic testing?

For example, static gate testing follows the AQG-324 standard by applying a voltage to the gates of multiple devices for thousands of hours and monitoring basic leakage current parameters. However, the disadvantage of static testing is that it cannot feedback the real situation, because there is no constant excitation under real working conditions, so dynamic testing is required to try to make the device as close to the actual working conditions as possible.

Dynamic gate tests include DGS, dynamic HTGB, DRB, dynamic H3TRB or high-temperature DGB, etc., which will apply high dv/dt or PWM signals to the gate that are more in line with real application characteristics, stimulating different operating modes, thereby seeing the original The real working behavior of the devices in the equipment, and monitor all voltage and current changes in real time during the process.

"In SET's solution, we can accurately control each data voltage and current, and discover device defects under this parameter in real time. Instead of like the past solution, we can only find out when the device burns out," Frank said , even when the device is burned, the dynamic test solution can record all parameters of the entire change process afterwards, making it easier for engineers to conduct subsequent failure analysis.

However, dynamic testing also faces some challenges. During testing, there are often as many as 960 devices under test (DUTs) that require dynamic excitation, current control and permanent monitoring. How to control so many devices at one time is a challenge. SET has different solutions for this. Even if one of the devices is broken down, it will not affect the other devices, and the current will continue to be controlled within a limit to avoid burning the device.

Challenges Facing Wide Bandgap Semiconductor Testing

Testing of silicon-based semiconductors has been around the world for over 50 years, and people know exactly what IGBTs do and how other silicon-based power devices behave. When wide bandgap semiconductors such as silicon carbide were first introduced, the industry once believed that we could adopt the same testing standards as silicon and "copy and paste" them for silicon carbide applications.

However, in the past four years, SET and the industry have discovered that the failure modes on silicon-based semiconductors do not apply to wide-bandgap semiconductors such as silicon carbide, so the large-scale application of wide-bandgap semiconductors is still full of uncertainty.

For example, the threshold voltage (Vth) drift in silicon-based semiconductors is not so obvious, but there are many obvious threshold voltage drift phenomena in the silicon carbide field, which will affect device performance. In some static tests, the drift of the entire threshold voltage cannot be observed, but after dynamic testing, the impact of the drift of the entire gate threshold voltage on the entire device can be clearly seen.

Threshold voltage drift or when the power device is switching, the internal on-resistance Rds(ON) will actually have a great impact on the device performance. Interestingly, basically every vendor's silicon carbide device supply is affected by this effect, as SET is seeing it on all devices tested, with varying degrees of severity.

"Recently, we have found in testing many devices that this effect is not short-term, but a long-term evolution process." Frank said that this effect lasts throughout the entire life cycle of electric vehicles, so after the wide bandgap power devices are installed in the vehicle, As time goes by, the threshold voltage drift will seriously affect the switching efficiency and battery life of the entire electric vehicle. This result cannot be discovered through short-term static testing in the laboratory, but requires some long-term dynamic testing. .

Threshold voltage drift is actually only one of the challenges currently encountered by silicon carbide devices. In addition, there are many pain points. For example, the switching frequency of silicon carbide is much faster than IGBT or other silicon-based chips. A high switching frequency means a high voltage rise slope, and dv/dt will also change greatly, which will induce other defects. Therefore, SET hopes to introduce the special physical behavior of this new field and new technology into test standards and equipment, so that device failure characteristics can be discovered at an early stage.

About AQG test standards

When it comes to the test standards for automotive-grade chips, the first thing most people may think of is AEC-Q101, which defines the test standards for automotive-grade chips. It is formulated by the AEC committee and has basically completed all standardization work by JEDEC. But while car manufacturers are using AEC-Q101, they are also using AQG-324.

Compared with AEC-Q101, AQG-324 is a purer qualification guideline for the automotive industry (Automotive Qualification Guideline). It was released by the ECPE European Power Electronics Research Network in April 2018. It was formerly LV324 and was prepared by representatives of manufacturers in the automotive industry. , including Audi, BMW, Daimler, Porsche, Volkswagen, etc. In theory, the AQG standard only applies to cars and modules, not discrete devices, but it can also be used for discrete devices.

As of the latest version in March 2021, AQG-324 has updated some test procedures for silicon carbide power modules. However, Frank pointed out that JEDEC has always had a lot of exchanges with people from Chinese standardization agencies, especially when formulating standards in the automotive field such as AEC-Q101. "As the setter of the AQG standard, we have also been trying to bring the level of standardization closer to commercialization. Therefore, we have conducted a lot of communication with all players in the market to avoid the emergence of different standards."

This situation does exist. Standards in the electronic and electrical field are often different in different regions. One for the United States, one for the European Union, and one for China... If such different standards are used to test automotive grade power devices, the cost of time and money for manufacturers will be very high.

Frank said that AQG is the European standard in the field of vehicle certification. The communication with the American standards organization has been relatively close, and most American car manufacturers will refer to it. "It is true that manufacturers in China, Japan or other regions will have some standards of their own, but we are also maintaining close communication. For example, in China, we will have discussions with local standardization agencies every six months, hoping to reach a consensus and unify the standards. "

Fuel vehicle companies are losing their traditional advantages

The car is actually a whole, and power semiconductors are one part of it. The transition from semiconductors to automotive supply chains means that some tests must be conducted from the perspective of the entire machine, which involves tripartite cooperation between NI, OEMs, and semiconductor manufacturers.

In the past, the coupling between automobile manufacturers and the automotive chip supply chain was not so tight. After mass production, a chip can often have many different application scenarios, and the entire vehicle is just one of them.

"Today's rise of silicon carbide is not the only driving force for electric vehicles. It is just due to the interesting effects that occur when more people start to own electric vehicles." Frank said that the car is a very complex system. In the past, the main technology was Internal combustion engines, "so for a start-up car company, building a car is a very cumbersome thing because you need to have experience with internal combustion engines."

Now that new energy vehicles have become a trend, many of the advantages of traditional automakers in the past may not necessarily be advantages now. Because these fuel vehicle companies have lost their knowledge advantages in professional fields due to the disappearance of internal combustion engines, they urgently need to establish a knowledge system in new professional fields.

"The core of today's new energy vehicles is, first, the electronic control software of the vehicle, and the second is the three electricity (motor, electric drive, electronic control). These emerging technologies require more know-how. All the difficulties boil down to - You need to have a reliable electronic transmission system. " Frank believes that this is why all of a sudden, OEMs are starting to make their own chips. "BYD is a perfect example. They have really integrated silicon carbide and silicon technology into their automotive strategy. "

Similarly, traditional car manufacturers such as Volkswagen and BMW are also aware of this problem. In this new era, car manufacturers need to become experts in the semiconductor field and need to have a very in-depth understanding of their own technology.

NI happens to be a manufacturer that can provide car manufacturers with HIL test system solutions from chip level to transmission system, as well as supporting technology for the entire supply chain. In the past, NI and SET cooperated to connect the entire power semiconductor industry chain from chip testing, power testing to power HIL testing, connecting the entire link of automotive power devices, bringing greater application scenarios.

After BUization, NI hopes to achieve front-to-back coverage in this field. Because in addition to the field of power semiconductors, NI also has a large part of the business in chip testing, including battery and cell testing in the "three electricity", and battery simulation with power or scenarios (Battery In the Loop, BIL).

The future of wide bandgap semiconductors in automobiles

In the past, power devices in the automotive industry were dominated by silicon-based MOSFETs or IGBTs. Although many newly developed models are slowly beginning to embrace silicon carbide, it is still not popular. It can be said to be a transitional period for hybrid solutions of silicon carbide and IGBTs. .

However, as the cost of silicon carbide gradually decreases, Frank believes that future automotive power semiconductors will be dominated by silicon carbide . "When we look at the new applications we are seeing now, we will no longer see IGBTs. The new generation of car manufacturers led by BYD and Tesla will lead the way, and others will follow. The future will be silicon carbide - —Especially in 800V applications, the efficiency of silicon carbide is unmatched by IGBT.”

In addition to silicon carbide, in the next 5 to 7 years, another wide bandgap material, gallium nitride (GaN), will also appear in large quantities in automotive applications. Therefore, Frank believes that there will be a large number of inverters combining silicon carbide and gallium nitride in the next 10 years , “because the advantage of gallium nitride is that the price is lower, and this combination is more cost-effective. I am very sure that silicon devices will be used in Disappeared in automotive inverters, such as IGBT.

Gallium nitride is cheaper, switches faster than silicon carbide, and has been widely used in the field of communication radio frequencies. The reason why it has not been widely used in automotive applications is that it matures later than silicon carbide technology. In addition, the effective withstand voltage of gallium nitride is relatively low, and the thermal conductivity is also lower than that of silicon carbide.

"But this does not prevent us from using it, because this problem can be overcome with larger chip sizes. The real problem is that there is no suitable architecture for inverter applications." Frank said when talking about gallium nitride applications in automobiles Speaking of, the special thing about gallium nitride is that it is permanently on, cannot be turned off after power is turned off, and is not as neutral as MOSFET, which brings increased power consumption and uncontrollable safety hazards.

“Given the large amount of energy in a car battery, permanent control of the power device is required. We cannot directly control the switching of its (gallium nitride) conduction now. It needs to be done with additional drivers, protection and algorithm design. This will be done in "But that will change over time and we will see different inverter architectures once the right architecture and the right application are available for gallium nitride," he added. You will see gallium nitride really rise in automotive inverter technology.”

Currently, gallium nitride devices already dominate charging stations. Generally speaking, silicon carbide and silicon carbide are two different technical routes. In the field of automotive transformers, many chip manufacturers have also begun to design new gallium nitride architectures.

Different strategies for serving chip customers and car companies

Compared with major test and measurement equipment manufacturers such as Keysight and Tektronix, SET is not so well-known. So, we are also testing power semiconductors. What is the difference between the positioning of SET and these manufacturers?

Frank said that SET focuses on providing complete test solutions for reliability testing, rather than equipment. Another difference is that the test and measurement giants focus not just on power semiconductors, but on multiple areas of all other industries. As a solution provider, SET only focuses on power semiconductors and is very vertically involved in this field. Including reliability tests such as high-voltage applications, drift effects in high-current applications, wide bandgap defects, and line-end tests.

In addition to chip manufacturers, car manufacturer OEMs are also the target customers of SET. The fault mechanisms of complete machine OEMs are different, so the mechanisms for serving chip manufacturers are also very different:

Both require failure analysis of the entire device, and different application scenarios will have different failure analysis models;

OEMs often do not "put their eggs in the same basket" - they are not bound to a specific supplier. Different models may have different silicon carbide device suppliers, and they need to judge the performance of different suppliers' products, or It is the failure characteristics under different scenarios;

More and more OEMs are beginning to build their own entire component supply chain - including silicon carbide devices and modules. BYD, for example, can more efficiently introduce and develop self-developed power devices through its self-built supply chain combined with its own use scenarios and know-how.

The current division of labor in this industry chain is that some manufacturers only supply silicon carbide chips, while others supply silicon carbide modules. Silicon carbide chips are supplied to module factories, and module factories supply them to OEMs. In the future, it is possible that OEMs will directly purchase silicon carbide chips and then make their own modules.

“These situations will continue to happen now and in the future because efficiency really depends on the integration of the chip and the inverter. More and more inverter and OEM companies are becoming chip experts or even becoming their own chip suppliers. Frank said, "Measurement equipment alone is not enough at this time. You also need to use know-how from manufacturers like NI to understand the effects of physical failure and learn and master these effects."

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