InterviewFeatured in ELEKTRONIK PRAXIS
Jan 27th, 2025
ROHM Commercializes 4th Gen IGBTsWhy the focus is not only on SiC but also IGBTs
*The content and speaker titles are accurate as of the time of the interview (January 2025)
---------- ROHM is widely recognized for SiC power devices, a reputation held by many engineers. Given this, why is ROHM involved in the development and production of IGBTs (Insulated Gate Bipolar Transistors), a power semiconductor based on silicon?
Figure 1. ROHM’s Power Devices
---------- IGBTs are power devices with a history of practical application since the 1980s When did ROHM enter the IGBT business?
Ishimatsu:
Our company’s history with IGBTs is relatively recent. We began development of IGBTs in 2010 (Fig. 2). Production started in 2012 at our Miyazaki plant (now the LAPIS Semiconductor Miyazaki Plant), later expanding to the Shiga plant in 2016. Since then, we have continuously worked to improve IGBTs, and in November 2024 announced the commercialization of our 4th gen IGBTs.
Figure 2. History of ROHM’s IGBT development.
---------- How do you envision the market segmentation between SiC power MOSFETs and IGBTs?
Ishimatsu:
While SiC power MOSFETs are not inexpensive, their adoption is advancing due to their exceptional cost-to-performance ratio. Which customers (users) are adopting them? Mainly those in the mobility sector.
For example, using SiC power MOSFETs in electric vehicle (EV) power circuits such as inverters and DC-DC converters makes it possible to extend the cruising range while reducing battery capacity by reducing volume and improving conversion efficiency. These benefits hold tremendous value in mobility applications. The growing adoption of SiC power MOSFETs in mobility applications suggests that customers find the benefits well worth the higher cost.
So what applications still require IGBTs?
They are motor-equipped devices outside the mobility sector such as industrial equipment. Fundamentally, these devices cannot be made substantially smaller even if SiC power MOSFETs are used. While improved power efficiency can be expected, the motors themselves are not composed of L (inductance) or C (capacitance), making miniaturization unfeasible. As a result, IGBTs remain advantageous even for motor-equipped devices.
Commercializing 4th Gen IGBTs
---------- 4th Gen IGBTs were commercialized in November 2024. What are the key features of these products?
Ishimatsu:
IGBT performance has been improved by cultivating two major technologies: the thinning of Si wafers and miniaturization (narrower pitch) of trench gates (Fig. 3). The 4th Gen IGBTs take these advancements even further.
Figure 3. Device structure by generation.
As the first products in the lineup, the RGA series of 1200V IGBTs comply with the AEC-Q101 automotive reliability standard, making them suitable for applications such as vehicle electric compressors, HV (PTC) heaters, and inverters for industrial equipment. A standout feature is significantly reduced power loss. For example, when applied to a 3-phase inverter, power loss can be reduced by approx. 35% compared to conventional products and by 24% vs competitor products (Fig. 4). What's more, short-circuit withstand time is an industry-leading 10us. The RGA series will also expand to include 650V models, which are scheduled for release by the end of March 2025.
Figure 4. Loss Reduction Effects of 4th Gen IGBTs.
---------- What other applications are the 4th gen IGBTs designed for?
Ishimatsu:
We plan on releasing different series of products targeting various applications (Fig. 5). More specifically, this includes the 650V-rated RGE series for industrial inverters and motor drives, the 650V/1200V RGH series for automotive onboard chargers, solar inverters, and consumer AC, and the 600V RGHL series for solar inverters, consumer AC, and uninterruptible power supplies (UPS). All of these are scheduled for commercialization in 2025.
Figure 5. IGBT product lineup (Source: ROHM)
Launching multiple series for different applications
---------- What distinguishes these 4th gen IGBT products in terms of characteristics?
Ishimatsu:
The required characteristics of IGBTs will vary depending on the application (Fig. 6). For instance, short-circuit withstand capability is highly prioritized in electric compressors for car AC. If the short-circuit withstand time is too short, the IGBT could fail even when a short-circuit current is detected, since the interrupt operation may not be completed in time. A longer short-circuit withstand time is therefore essential. Specifically, (although it depends on the design of the current interruption circuit) a short-circuit withstand time of at least 6us is often necessary. In some cases, a short-circuit withstand time of 8us or even 10us is required.
Conversely, switching loss (switching frequency) is important in applications such as EV onboard chargers, solar inverters, and consumer AC power supplies. Increasing the switching frequency makes it possible to reduce the external dimensions of power supply circuits such as inverters.
Figure 6. IGBT product roadmap.
---------- What is the typical switching frequency for power circuits such as onboard chargers?
Ishimatsu:
For power circuits such as onboard chargers, the switching frequency is often around 70kHz. In car AC, 20kHz is common, while 5kHz is typical for consumer AC.
---------- Is it possible to improve both switching frequency (switching loss) and short circuit withstand capacity at the same time?
Ishimatsu:
For IGBTs, there is a trade-off among three key characteristics: switching frequency (switching loss), short-circuit withstand time, and collector-emitter saturation voltage (VCE(sat). In other words, improving one characteristic often comes at the expense of another. To address this, it is necessary to balance and optimize these characteristics according to the specific application. In practice, while all 4th gen IGBT series are based on the same core technology, the process conditions are finely tuned to optimize the balance of these three characteristics.
Collaboration with Semikron Danfoss on IGBT Modules
---------- ROHM is also engaged in the development and commercialization of gate driver ICs for IGBTs. Does this provide a differentiating advantage for the IGBT business?
Ishimatsu:
We believe there are significant benefits to developing both IGBTs and their gate driver ICs. This capability allows us to propose the best usage methods and tailored solutions to our users (customers). Even though all IGBTs share common functions, the optimal driving method will differ for each product due to variations in characteristics such as input capacitance. If driven incorrectly, power loss and noise cannot be minimized. Offering gate driver ICs allows us to provide support for optimal drive methods that can minimize these factors.
Developing gate driver ICs also delivers significant advantages for IPMs (Intelligent Power Modules). IPMs integrate an IGBT chip and gate driver into a single package. Our IPMs incorporate gate drivers specifically optimized for IGBTs, reducing power loss while suppressing noise.
However, there are cases where power loss, noise, and surges cannot be sufficiently minimized by the gate driver design alone. In such instances, it is possible to finely adjust the IGBTs process conditions to match the gate driver. In practice, strong synergy was created through the close collaboration between the gate driver and IGBT design teams.
---------- What is an IGBT module?
Ishimatsu:
A module that integrates multiple IGBT chips into a single package. They are widely used in motor-driven equipment, inverters, and power supplies. For example, a 7-in-1 module incorporates IGBTs for the upper and lower arms of a 3-phase motor, along with an IGBT for braking.
However, we do not manufacture IGBTs modules ourselves. Instead, we supply IGBTs in wafer form to module manufacturers, who then assemble and sell them as IGBT modules. ROHM has maintained a collaborative relationship with Semikron Danfoss for over 10 years. Semikron Danfoss plans to commercialize their 10A-150A rated MiniSKiiP IGBT modules that incorporate our RGA series of 4th Gen 1200V IGBTs.
---------- We would like to know the current state of ROHM’s IGBT business. What are some key applications?
Ishimatsu:
Currently, ROHM is focused on electric compressors for car AC (Fig. 7). We hold a strong position in this market, largely because we were among the first to obtain qualification under the AEC-Q101 automotive discrete quality standard ahead of our competitors.
At the same time, we provide enhanced support from FAEs (Field Application Engineers) to expand our presence in the market. For instance, we offer services such as analyzing power loss from the IGBT, circuit, and heat sink used to determine the resulting temperature distribution.
Figure 7. Market forecast for electric compressors in xEVs.
---------- How do you see the growth rate for IGBTs progressing in the future?
Ishimatsu:
The highest growth rate is expected in automotive applications, with a projected compound annual growth rate (CAGR) of around 20%. For industrial equipment such as servo motors and inverters, a CAGR of 5-10% is anticipated. However, for consumer AC, a high growth rate is unlikely, since the inverter adoption rate has reached almost 100% in China, following Japan. Of course, this could change depending on the progress of inverter adoption in emerging markets like India, but it is likely to remain at a modest CAGR of around 1-2%.
---------- Is the IGBT market shrinking due to the expansion of the SiC power MOSFET market?
Ishimatsu:
In mobility applications, the adoption of SiC power MOSFETs is certainly expected to grow, significantly impacting the market. However, as mentioned earlier, IGBTs will remain widely used, particularly in certain automotive and industrial applications. As such, we believe that the market for IGBTs will continue to expand.
In addition, development is underway on a new ‘hybrid drive’ technology that combines SiC power MOSFETs and IGBTs. Typically, a single upper or lower arm switch is composed of multiple power devices connected in parallel – for example, 3-4 SiC power MOSFET chips. Some of these can be replaced with IGBTs, such as connecting two SiC power MOSFETs and two IGBTs in parallel. SiC power MOSFETs excel in achieving extremely low power loss in the low-current range, but at high currents the difference in power loss compared to IGBTs becomes smaller. This allows SiC power MOSFETs to handle the low-current region while IGBTs manage the high current range, improving the cost-performance ratio without increasing loss.
Going forward, we will continue to expand our lineup to support such applications while developing new, higher performance products by fundamentally redesigning device structures.
Ishimatsu:
The reason is simple – there are applications where SiC power devices (SiC power MOSFETs) are not necessary. While SiC power MOSFETs offer superior performance, they are generally more expensive than IGBTs. On the other hand, IGBTs provide relatively high performance at a lower cost. This ensures a continued market for IGBTs. Silicon power devices will not become obsolete, and SiC power MOSFETs will never be the sole option. With this in mind, we are actively developing, manufacturing, and selling IGBTs.