New Power Switching Technologies and the Changing Landscape of Isolated Gate Drivers

The emergence of new power switching technologies based on materials such as Silicon Carbide (SiC) and Gallium Nitride (GaN) has resulted in dramatic performance improvements beyond traditional systems based on MOSFET and IGBT technologies. Higher switching frequencies will reduce component size, thereby reducing cost, system size and weight. These are key advantages in markets such as automotive and energy. The new power switches will also drive changes to their control components, including gate drivers. This article will explore some of the key differences between GaN and SiC switches and IGBT/MOSFET, and how gate drivers will support these differences.

The emergence of new power switching technologies based on materials such as Silicon Carbide (SiC) and Gallium Nitride (GaN) has resulted in dramatic performance improvements beyond traditional systems based on MOSFET and IGBT technologies. Higher switching frequencies will reduce component size, thereby reducing cost, system size and weight. These are key advantages in markets such as automotive and energy. The new power switches will also drive changes to their control components, including gate drivers. This article will explore some of the key differences between GaN and SiC switches and IGBT/MOSFET, and how gate drivers will support these differences.

The choice of power switching technology for power delivery systems has been very simple for many years. At low voltage levels (usually below 600 V), MOSFETs are usually chosen; at high voltage levels, IGBTs are usually chosen more. This situation is under threat 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 photovoltaic inverters used in energy applications such as solar panels, as well as target markets such as automobiles. Increasing the switching speed from 20 kHz to 100 kHz can significantly reduce the weight of the transformer, thereby making the electric vehicle’s motor lighter, and also expand the range and size of inverters used in solar applications, making them more suitable for domestic application. In addition, higher operating temperatures (especially for GaN devices) and lower turn-on drive requirements also simplify the system architect’s design efforts.

Like MOSFETs/IGBTs, these new technologies (at least initially) appear to be able to meet different application needs. Until recently, GaN products were typically in the 200 V range, although these products have grown rapidly in recent years, and multiple products in the 600 V range have emerged. But this is still nowhere near the main range of SiC (closer to 1000 V), which shows that GaN has naturally replaced MOSFET devices, and SiC has replaced IGBT devices. Now that superjunction MOSFETs can bridge this gap and enable high-voltage applications up to 900 V, it is not surprising that some GaN R&D is starting to offer devices capable of handling voltages above 600 V.

However, while these advantages make GaN and SiC power switches extremely attractive to designers, the benefits are not without cost. The main price is increased cost, the price of this device is several times higher than the equivalent MOSFET/IGBT product. IGBT and MOSFET production is a well-developed and extremely accessible process, which means lower costs and more price competitiveness than its newer rivals. Currently, SiC and GaN devices are still several times more expensive than their traditional counterparts, but their price competitiveness is improving. Numerous experts and market research reports have shown that the price gap must be substantially narrowed before widespread adoption. Even if the price gap is closed, it is unlikely that the new power switches will achieve large-scale applications immediately, and even from a long-term forecast, traditional switching technologies will continue to occupy the majority of the market for some time to come.

In addition to pure cost and financial factors, technical factors also have some influence. Higher switching speeds and operating temperatures may be well suited for GaN/SiC switches, but they still present problems for peripheral IC support devices required to complete the power conversion signal chain. A typical signal chain for an isolated system is shown in Figure 1. While higher switching speeds have implications for the processor that controls the switching and the current sense system that provides the feedback loop, the remainder of this article will focus on the changes encountered by the gate driver that provides the control signal for the power switch.


Figure 1. Typical Power Conversion Signal Chain

GaN/SiC gate drivers

The gate driver can receive logic level control signals generated by the system control process and provide the drive signals required to drive the gates of the power switches. In isolated systems, they also provide isolation, separating high-voltage signals on the live side of the system from users and sensitive low-voltage circuits on the safe side. To take full advantage of the higher switching frequencies that GaN/SiC technology can provide, gate drivers must increase the frequency of their control signals. Current IGBT-based systems may switch in the tens of kHz range; emerging requirements suggest that switching frequencies of hundreds of kHz or even one to two MHz may be required. This can be confusing to system designers as they try to eliminate inductance in the signal path from the gate driver to the power switch. Minimizing trace lengths to avoid trace inductance will be critical, and close placement of gate drivers and power switches may become standard practice. Most of the recommended layout guidelines provided by GaN suppliers emphasize the importance of low-impedance traces and planes. Additionally, users will expect power switch and supporting IC suppliers to address various issues arising from packaging and gold wires.

The higher operating temperature range offered by SiC/GaN switches is also very attractive to system designers, as it gives them more freedom to increase performance without worrying about heat dissipation. While a power switch will operate at higher temperatures, the silicon-based components surrounding it will still encounter the usual temperature limitations. Because the driver must be placed next to the switch, designers who want to take advantage of the higher operating range of the new switch are faced with the problem of not exceeding the temperature limits of silicon-based components.


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

Higher switching frequencies also create common-mode transient immunity issues, which can be a serious problem for system designers. A high slew rate signal coupled across the isolation barrier in an isolated gate driver can corrupt data transfer, causing unwanted signals at the output. In conventional IGBT-based systems, gate drivers with immunity between 20 kV/μs and 30 kV/μs are sufficient against common-mode interference. However, GaN devices tend to have slew rates that exceed this limit, and a gate driver should be chosen for a robust system with a common-mode transient immunity of at least 100 kV/µs. Recently introduced products, such as the ADuM4135, which utilizes Analog Devices’ iCoupler® technology, provide up to 100 kV/µs of common-mode transient immunity for such applications. However, increasing CMTI performance tends to incur additional delays. Increased latency means increased dead time between the high-side and low-side switches, which degrades performance. This is especially true in the field of isolated gate drivers, where signals travel across the isolation barrier, typically with longer delays. However, the ADuM4135 not only provides 100 kV/µs CMTI, but its propagation delay is only 50 ns.

Of course, it’s not all bad news for gate drivers tasked with driving new power switching technologies forward. Typical IGBTs have gate charge charges in the hundreds of nC, so we typically find gate drivers providing output drive capability in the 2 A to 6 A range. GaN switches are currently available on the market with more than 10 times better gate charge charge performance, typically in the 5 nC to 7 nC range, so the gate driver drive requirements have been significantly reduced. Reducing the drive requirements of the gate driver allows the gate driver to be smaller and faster, and also reduces the need to add external buffers to enhance current output, saving space and cost.

in conclusion

Predicted for a long time, GaN and SiC devices will become new solutions in power conversion applications, and this technology has been long-awaited and now finally realized. While this technology can offer attractive advantages, they are not without cost. To provide superior performance, new switching technologies require changes to the requirements of the isolated gate drivers used and create new problems for the system designer. The advantages are obvious, and a variety of solutions to these problems have emerged. Moreover, there are already ready and viable GaN and SiC solutions on the market.

The emergence of new power switching technologies based on materials such as Silicon Carbide (SiC) and Gallium Nitride (GaN) has resulted in dramatic performance improvements beyond traditional systems based on MOSFET and IGBT technologies. Higher switching frequencies will reduce component size, thereby reducing cost, system size and weight. These are key advantages in markets such as automotive and energy. The new power switches will also drive changes to their control components, including gate drivers. This article will explore some of the key differences between GaN and SiC switches and IGBT/MOSFET, and how gate drivers will support these differences.

The emergence of new power switching technologies based on materials such as Silicon Carbide (SiC) and Gallium Nitride (GaN) has resulted in dramatic performance improvements beyond traditional systems based on MOSFET and IGBT technologies. Higher switching frequencies will reduce component size, thereby reducing cost, system size and weight. These are key advantages in markets such as automotive and energy. The new power switches will also drive changes to their control components, including gate drivers. This article will explore some of the key differences between GaN and SiC switches and IGBT/MOSFET, and how gate drivers will support these differences.

The choice of power switching technology for power delivery systems has been very simple for many years. At low voltage levels (usually below 600 V), MOSFETs are usually chosen; at high voltage levels, IGBTs are usually chosen more. This situation is under threat 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 photovoltaic inverters used in energy applications such as solar panels, as well as target markets such as automobiles. Increasing the switching speed from 20 kHz to 100 kHz can significantly reduce the weight of the transformer, thereby making the electric vehicle’s motor lighter, and also expand the range and size of inverters used in solar applications, making them more suitable for domestic application. In addition, higher operating temperatures (especially for GaN devices) and lower turn-on drive requirements also simplify the system architect’s design efforts.

Like MOSFETs/IGBTs, these new technologies (at least initially) appear to be able to meet different application needs. Until recently, GaN products were typically in the 200 V range, although these products have grown rapidly in recent years, and multiple products in the 600 V range have emerged. But this is still nowhere near the main range of SiC (closer to 1000 V), which shows that GaN has naturally replaced MOSFET devices, and SiC has replaced IGBT devices. Now that superjunction MOSFETs can bridge this gap and enable high-voltage applications up to 900 V, it is not surprising that some GaN R&D is starting to offer devices capable of handling voltages above 600 V.

However, while these advantages make GaN and SiC power switches extremely attractive to designers, the benefits are not without cost. The main price is increased cost, the price of this device is several times higher than the equivalent MOSFET/IGBT product. IGBT and MOSFET production is a well-developed and extremely accessible process, which means lower costs and more price competitiveness than its newer rivals. Currently, SiC and GaN devices are still several times more expensive than their traditional counterparts, but their price competitiveness is improving. Numerous experts and market research reports have shown that the price gap must be substantially narrowed before widespread adoption. Even if the price gap is closed, it is unlikely that the new power switches will achieve large-scale applications immediately, and even from a long-term forecast, traditional switching technologies will continue to occupy the majority of the market for some time to come.

In addition to pure cost and financial factors, technical factors also have some influence. Higher switching speeds and operating temperatures may be well suited for GaN/SiC switches, but they still present problems for peripheral IC support devices required to complete the power conversion signal chain. A typical signal chain for an isolated system is shown in Figure 1. While higher switching speeds have implications for the processor that controls the switching and the current sense system that provides the feedback loop, the remainder of this article will focus on the changes encountered by the gate driver that provides the control signal for the power switch.


Figure 1. Typical Power Conversion Signal Chain

GaN/SiC gate drivers

The gate driver can receive logic level control signals generated by the system control process and provide the drive signals required to drive the gates of the power switches. In isolated systems, they also provide isolation, separating high-voltage signals on the live side of the system from users and sensitive low-voltage circuits on the safe side. To take full advantage of the higher switching frequencies that GaN/SiC technology can provide, gate drivers must increase the frequency of their control signals. Current IGBT-based systems may switch in the tens of kHz range; emerging requirements suggest that switching frequencies of hundreds of kHz or even one to two MHz may be required. This can be confusing to system designers as they try to eliminate inductance in the signal path from the gate driver to the power switch. Minimizing trace lengths to avoid trace inductance will be critical, and close placement of gate drivers and power switches may become standard practice. Most of the recommended layout guidelines provided by GaN suppliers emphasize the importance of low-impedance traces and planes. Additionally, users will expect power switch and supporting IC suppliers to address various issues arising from packaging and gold wires.

The higher operating temperature range offered by SiC/GaN switches is also very attractive to system designers, as it gives them more freedom to increase performance without worrying about heat dissipation. While a power switch will operate at higher temperatures, the silicon-based components surrounding it will still encounter the usual temperature limitations. Because the driver must be placed next to the switch, designers who want to take advantage of the higher operating range of the new switch are faced with the problem of not exceeding the temperature limits of silicon-based components.


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

Higher switching frequencies also create common-mode transient immunity issues, which can be a serious problem for system designers. A high slew rate signal coupled across the isolation barrier in an isolated gate driver can corrupt data transfer, causing unwanted signals at the output. In conventional IGBT-based systems, gate drivers with immunity between 20 kV/μs and 30 kV/μs are sufficient against common-mode interference. However, GaN devices tend to have slew rates that exceed this limit, and a gate driver should be chosen for a robust system with a common-mode transient immunity of at least 100 kV/µs. Recently introduced products, such as the ADuM4135, which utilizes Analog Devices’ iCoupler® technology, provide up to 100 kV/µs of common-mode transient immunity for such applications. However, increasing CMTI performance tends to incur additional delays. Increased latency means increased dead time between the high-side and low-side switches, which degrades performance. This is especially true in the field of isolated gate drivers, where signals travel across the isolation barrier, typically with longer delays. However, the ADuM4135 not only provides 100 kV/µs CMTI, but its propagation delay is only 50 ns.

Of course, it’s not all bad news for gate drivers tasked with driving new power switching technologies forward. Typical IGBTs have gate charge charges in the hundreds of nC, so we typically find gate drivers providing output drive capability in the 2 A to 6 A range. GaN switches are currently available on the market with more than 10 times better gate charge charge performance, typically in the 5 nC to 7 nC range, so the gate driver drive requirements have been significantly reduced. Reducing the drive requirements of the gate driver allows the gate driver to be smaller and faster, and also reduces the need to add external buffers to enhance current output, saving space and cost.

in conclusion

Predicted for a long time, GaN and SiC devices will become new solutions in power conversion applications, and this technology has been long-awaited and now finally realized. While this technology can offer attractive advantages, they are not without cost. To provide superior performance, new switching technologies require changes to the requirements of the isolated gate drivers used and create new problems for the system designer. The advantages are obvious, and a variety of solutions to these problems have emerged. Moreover, there are already ready and viable GaN and SiC solutions on the market.

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