Leading-edge SiC/GaN power converter drivers

ADI Isolated Gate Drivers, Power Controllers, and Processors for Next-Generation Power Converters

Stefano Gallinaro Analog Devices

Introduction

The power converter market is evolving rapidly and will continue to evolve rapidly, moving from simple cost-effective designs to a broader and more sustainable innovation model. New challenges are constantly emerging, such as producing smaller and more efficient power converters that can be used in small servo drives or integrated into distributed energy storage unit power converters. This also means managing higher powers with higher operating voltages without increasing weight and size, such as in applications such as solar string inverters and electric vehicle traction motors.

New high-efficiency, ultra-fast power converters based on wide-bandgap (WBG) semiconductors such as silicon carbide (SiC) and gallium nitride (GaN) have begun to make inroads in a variety of innovative markets and applications—applications such as solar PV inverters, energy storage, and vehicle electrification (such as chargers and traction motor inverters). To fully exploit the new power conversion technologies, a complete IC ecosystem must be implemented in the converter design, from the nearest chip to the power switch and gate driver. The requirements for isolated gate drivers have begun to change from previous silicon IGBT drivers. For SiC and GaN MOSFETs, high CMTI >100 kV/μs, wide gate voltage swings, fast rise/fall times, and ultra-low propagation delays are required. ADI's ADuM4135 isolated gate drivers have all the necessary technical features in a 16-lead wide-body SOIC package. In conjunction with the ADSP-CM419F high-end mixed-signal control processor, they can manage the high-speed, complex, multi-layer control loops of the next generation of high-density power converters based on SiC/GaN.

Fig. 1. Power converter market forecast for 2021.

The power converter market is growing at a CAGR of more than 6.5% and is expected to reach $80 billion by 2021. Currently, traditional inverters and converters based on silicon IGBTs dominate the market (accounting for more than 70%), mainly due to motor drive applications in factory production lines and first-generation wind and solar inverters.

New technological advances in power switching have begun to bring third-generation SiC MOSFETs and first- and second-generation GaN MOSFETs to the market. After being limited to a few niche power applications for a while, WBG technology has begun to be used in a variety of applications such as battery-based energy storage applications, electric vehicle chargers, traction motors, solar photovoltaic inverters, etc. Its price has dropped rapidly due to the expansion of new markets, which has led to its entry into other markets that were originally price-sensitive. Mass production has further reduced prices, and this trend will continue. The popularity of WBG semiconductors is a great example of a technology (and overall economic) cycle.

The main applications driving the popularity of SiC/GaN power switches are solar photovoltaic inverters, electric vehicle chargers and energy storage converters. Here, the added value of ultra-fast, small and efficient power switches is exploited, bringing ultra-high switching frequencies and outstanding efficiency targets of more than 99% to the market. In order to achieve these goals, designers face new challenges to reduce the weight and size of power converters (i.e., increase power density).

Of course, these issues cannot be solved overnight. Advances and innovations are needed in all the relevant processes. One example of this is the technical bottlenecks associated with the implementation of high voltage power electronic systems. From an architectural perspective, high voltage (HV) systems are an option, but certain semiconductor technologies have long hindered this choice. Now, the advent of wide bandgap semiconductors has brought light to this problem, making high voltage systems a more viable option and worthy of consideration. The standard for solar string inverters is 1500 VDC, while 1000 VDC and soon 2000 VDC will become the standard for energy storage converters (battery-based) and electric vehicle chargers.

In fact, the move to high-voltage systems compatible with WBG semiconductors is very interesting for three reasons: First, high voltage means low current, which means less total copper used in the system, which in turn directly affects system cost reduction. Second, the reduced resistive losses of wide-bandgap technologies (enabled by high voltage) mean higher efficiency and can also reduce the size and necessity of cooling systems. Finally, at the subsystem level, they allow engineers to move from baseboard power module-based designs to discrete designs or lightweight power module-based designs. This implies the use of compatible PCBs and smaller wires rather than bus bars and heavier wires.

In summary, if the core design goal is to reduce weight and/or cost or improve performance, high voltage systems are worthwhile. Therefore, for secondary applications, 1.7 kV and 3.3 kV SiC MOSFET high breakdown voltages have become the standard, while 1.2 kV SiC MOSFETs are the mainstream power switches for the new generation of second and third level applications.

From an engineering perspective, SiC/GaN offer clear advantages. First, the inherently superior dV/dt switching performance of WGB semiconductors means very low switching losses. This enables high switching frequencies (50 kHz to 500 kHz for SiC and over 1 MHz for GaN), which in turn helps reduce the size of the magnets while increasing power density. Inductance, size and weight can be reduced by more than 70%, while the number of capacitors can also be reduced, resulting in a converter that is only one-fifth the size and weight of a conventional converter. The amount of passive components and mechanical parts (including heat sinks) can be saved by about 40%, with the added value being in the control electronics IC.

Another major advantage of these technologies is their high tolerance to high junction temperatures. This tolerance helps increase power density and reduce thermal issues.

Other features of SiC/GaN switches that help reduce losses include: no need for any recovery of the diode (reduced rectification losses), low Rds(on) (which reduces conduction), high voltage operation mode, etc.

These advantages allow innovative power electronics topologies to be designed and implemented for new applications. SiC/GaN power switches are very useful in the design of resonant circuits (such as LLC or PRC), bridge topologies (phase-shifted full bridge) or bridgeless power factor correction (PFC). This is due to their high switching frequency, high efficiency (thanks to zero voltage switching and zero current switching) and the resulting high power density.

SiC-/GaN power transistors can realize multi-level power conversion and full bidirectional operation mode, while silicon IGBTs are subject to some limitations due to the inverter operation mode.

Bidirectional operation is increasingly becoming a mandatory requirement in applications such as energy storage, where power flows to or from a battery to a load or grid. Designing high-power converters in compact packages opens the door to distributed energy storage systems where battery charging accuracy is possible.

To realize the many advantages of SiC/GaN-based designs, we should face the various technical challenges associated with them. We can divide these challenges into three categories: driving the switches, the correct choice of combined power supplies, and the correct control of the power converter loop.

When driving SiC MOSFETs, engineers need to consider new issues, such as negative bias (for gate drivers) and the accuracy of the drive voltage (even more important for GaN). Such errors should be avoided as much as possible because they may affect the entire system.

ADI iCoupler® isolated gate drivers overcome the limitations of optocoupler-based and high voltage gate drivers. Optocouplers are slow, power hungry, difficult to integrate with other functions, and their performance degrades over time. In contrast, iCoupler digital isolators, an alternative to optocouplers, combine high bandwidth on-chip transformers with sophisticated CMOS circuitry to provide designers with improved reliability, size, power consumption, speed, timing accuracy, and ease of use. iCoupler technology was introduced a decade ago to address the limitations of optocouplers. ADI’s digital isolators achieve thousands of volts of isolation using a low stress thick film polyimide insulation layer that can be integrated with standard silicon ICs to form monolithic systems in single, multi-channel, and bidirectional configurations: 20 μm to 30 μm polyimide insulation layer with greater than 5 kV rms withstand.

Figure 2. iCoupler transformer coil on polyimide insulation.

The most representative ICs in ADI's gate driver portfolio are the ADuM4135 (high-end isolated gate driver for SiC MOSFET) and the ADuM4121 (fast, compact solution for high-density SiC and GaN designs). Using ADI's proven iCoupler technology, the ADuM4135 isolated gate driver brings several key advantages to high-voltage, high-switching rate applications. The ADuM4135 is the best choice for driving SiC/GaN MOS because it has excellent propagation delay (less than 50 ns), channel matching time less than 5 ns, common-mode transient immunity (CMTI) of more than 100 kV/μs, and supports a full-life working voltage of up to 1500 VDC in a single package.

Figure 3. ADuM4135 evaluation board.

Figure 4. ADuM4135 block diagram.

ADuM4135采用16引脚宽体SOIC封装，包含米勒箝位，以便栅极电压低于2 V时实现稳健的SiC/GaN MOS或IGBT单轨电源关断。输出侧可以由单电源或双电源供电。去饱和检测电路集成在ADuM4135上，提供高压短路开关工作保护。去饱和保护包含降低噪声干扰的功能，比如在开关动作之后提供300 ns的屏蔽时间，用来屏蔽初始导通时产生的电压尖峰。内部500 µA电流源有助于降低整体器件数量，如需提高抗噪水平，内部消隐开关也支持使用外部电流源。考虑到IGBT通用阈值水平，副边UVLO设置为11 V。ADI公司

iCoupler chip-scale transformers also provide isolated communication of control information between the high-voltage and low-voltage sides of the chip. Chip status information can be read from dedicated outputs. When a fault occurs on the secondary side of the device, a reset operation can be controlled on the primary side.