October 26, 2020

New power switching technology and the ever-changing landscape of isolated gate drivers

Maurice Moroney Marketing Manager Analog Devices

The emergence of new power switching technologies based on materials such as silicon carbide (SiC) and gallium nitride (GaN) has led to a significant increase in performance, surpassing traditional systems based on MOSFET and IGBT technology. Higher switching frequencies will reduce component size, thereby reducing cost, system size and weight; these are major advantages in markets such as automotive and energy. The new power switch will also cause changes to its control components, including the gate driver. This article will explore some of the major differences between GaN and SiC switches and IGBT/MOSFETs, and how gate drivers will support these differences.

The choice of power switching technology for power output systems has been very simple for many years. At low voltage levels (typically below 600 V), MOSFETs are typically selected; at high voltage levels, IGBTs are often more selective. This situation is threatened with the advent of new power switching technologies in the form of gallium nitride and silicon carbide.

These new switching technologies offer several distinct advantages in terms of performance. Higher switching frequencies reduce system size and weight, which is important for PV inverters used in energy applications such as solar panels and target markets such as automobiles. Increased switching speed from 20 kHz to 100 kHz significantly reduces transformer weight, making electric motors lighter and expanding the range of inverters used in solar applications, reducing their size and making them more suitable Domestic application. In addition, higher operating temperatures (especially GaN devices) and lower turn-on drive requirements simplify the design of system architects.

Like MOSFETs/IGBTs, these new technologies (at least in the initial stages) seem to be able to meet different application needs. Until recently, GaN products were typically still in the 200 V range, although in recent years these products have grown rapidly and there are a variety of products in the 600 V range. But this is still far behind the main range of SiC (close to 1000 V), which indicates that GaN has naturally replaced MOSFET devices, while SiC has replaced IGBT devices. Since super-junction MOSFETs can bridge this gap and achieve high-voltage applications up to 900 V, it is not surprising that some GaN R&Ds are beginning to offer devices that can handle applications with voltages above 600 V.

However, while these advantages make GaN and SiC power switches attractive to designers, this benefit is not without cost. The main price is the cost increase, and the price of this device is several times higher than the equivalent MOSFET/IGBT products. IGBT and MOSFET production is a well-developed and easily mastered process, which means lower cost and higher price competitiveness than its new competitors. At present, the price of SiC and GaN devices is still several times higher than its traditional counterparts, but its price competitiveness is increasing. Many experts and market research reports have shown that the price gap must be significantly reduced before widespread adoption. Even if the price gap is narrowed, the new power switch is unlikely to achieve large-scale applications immediately. Even in the long-term forecast, traditional switching technology will continue to occupy most of the market for some time to come.

In addition to pure cost and financial factors, technical factors will have some impact. Higher switching speeds and operating temperatures may be ideal for GaN/SiC switches, but they still pose problems for the peripheral IC support devices needed to complete the power conversion signal chain. A typical signal chain for an isolation system is shown in Figure 1. While higher switching speeds can affect the processor that controls the conversion and the current sensing system that provides the feedback loop, the rest of this article will focus on the variations encountered by the gate drivers that provide control signals for the power switches.

Figure 1. Typical power conversion signal chain

GaN/SiC gate driver

The gate driver receives the logic level control signals generated by the system control process and provides the drive signals required to drive the power switch gates. In isolation systems, they also provide isolation, separating the high-voltage signal on the live side of the system from the user on the safe side and the sensitive low-voltage circuitry. In order to take full advantage of the ability of GaN/SiC technology to provide higher switching frequencies, gate drivers must increase the frequency of their control signals.

Current IGBT-based systems may switch over tens of kHz; emerging requirements indicate that switching frequencies of hundreds of kHz, or even one to two MHz, may be required. This can be a problem for system designers as they attempt to eliminate the inductance in the signal path from the gate driver to the power switch. Minimizing trace lengths to avoid trace inductance will be critical, and the close placement of gate drivers and power switches may be standard practice. Most of the recommended layout guidelines provided by GaN suppliers emphasize the importance of low impedance traces and planes. In addition, users will want power switch and support IC suppliers to address the various issues caused by packaging and gold wires.

The higher operating temperature range offered by SiC/GaN switches is also very attractive to system designers because it gives them more freedom to boost performance without worrying about thermal issues. Although the power switch will operate at higher temperatures, the surrounding silicon components will still experience conventional temperature limitations. Because the drive must be placed next to the switch, designers who want to take advantage of the higher operating range of the new switch are facing the problem that the temperature cannot exceed the temperature limit of the silicon component.

Figure 2. Propagation Delay and CMTI Performance for a Typical Gate Driver

Higher switching frequencies also create common-mode transient immunity issues, which is a very serious problem for system designers. High-voltage slew rate signals coupled on the isolation barrier in an isolated gate driver can disrupt data transmission, resulting in unwanted signals at the output. In conventional IGBT-based systems, gate drivers with immunity levels between 20 kV/μs and 30 kV/μs are sufficient to resist common mode interference. However, GaN devices tend to have slew rates that exceed this limit, and gate drivers are selected for robust systems with a common mode transient immunity of at least 100 kV/μs. Recently introduced products, such as the ADuM4135, use Analog Devices' iCouplerTM technology to provide common-mode transient immunity up to 100 kV/μs to handle such applications. However, increasing CMTI performance often creates additional delays. An increase in latency means an increase in dead time between the high-end and low-side switches, which reduces performance. This is especially true in the field of isolated gate drivers, where signals are transmitted over the isolation barrier and typically have longer delays. However, the ADuM4135 not only provides a 100 kV/μs CMTI, but also has a propagation delay of only 50 ns.

Of course, it is not entirely bad news for the gate driver that is responsible for pushing the new power switching technology forward. Typical IGBTs have a gate charge of up to hundreds of nC, so we typically find gate drivers that provide output drive capability from 2 A to 6 A. At present, the gate charge performance of GaN switches provided on the market has increased by more than 10 times, usually in the range of 5 nC to 7 nC, so the driving requirements of the gate driver have been significantly reduced. Reducing the drive requirements of the gate driver allows the gate driver to be smaller and faster, and it also reduces the need to add an external buffer to enhance current output, saving space and cost.

in conclusion

It has long been predicted that GaN and SiC devices will become new solutions in power conversion applications, which have long been expected and are now finally realized. While this technology offers attractive advantages, they are not without cost. To provide outstanding performance, the new switching technology requires changes to the requirements of the isolated gate drivers used and introduces new problems to system designers. The advantages are obvious, and a variety of solutions to these problems have emerged. Moreover, there are already available and viable GaN and SiC solutions on the market.

About the Author

Maurice Moroney is the Marketing Manager for Analog Devices' isolated power conversion portfolio and is responsible for isolated gate drivers and voltage/current sensing in motor control, automotive and energy applications. His previous positions involved marketing/application, with a focus on the consumer, industrial and automotive industries. Maurice holds a bachelor's degree in electrical engineering (2000) and a master's degree in business administration (2014) from the University of Limerick, Ireland.

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