The power converter market is currently evolving rapidly and will continue to do so, moving from simple, cost-effective designs to a broader and more sustainable innovation model. New challenges are constantly emerging, such as producing smaller, more efficient power converters for use in small servo drives or integrated into distributed energy storage units. This also means managing higher power with higher operating voltages without increasing weight and size, for 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) are gaining traction in a variety of innovative markets and applications, including solar photovoltaic inverters, energy storage, and vehicle electrification (such as chargers and traction motor inverters). To fully leverage these new power conversion technologies, a complete IC ecosystem must be implemented in the converter design, from the latest chips to power switches and gate drivers. The requirements for isolated gate drivers are evolving, differing from previous silicon IGBT drivers. For SiC and GaN MOSFETs, high CMTI (CMTI > 100kV/μs), wide gate voltage swing, fast rise/fall times, and ultra-low propagation delay are required. Analog Devices' ADuM4135 isolated gate driver possesses all the necessary technical characteristics in a 16-pin wide-body SOIC package. Combined with the ADSP-CM419F high-end mixed-signal control processor, they can manage the high-speed, complex, multi-layered control loops of next-generation high-density power converters based on SiC/GaN.
The power converter market is growing at a CAGR of over 6.5%, and its size is expected to reach $80 billion by 2021. Currently, traditional inverters and converters based on silicon IGBTs dominate the market (accounting for over 70%), primarily due to motor drive applications in factory production lines and first-generation wind and solar inverters.
Technological advancements in power switching have begun to bring third-generation SiC MOSFETs, as well as first- and second-generation GaN MOSFETs, to market. After a period of being confined to niche power applications, WBG technology is now being used in a variety of applications, such as battery-based energy storage, electric vehicle chargers, traction motors, and solar photovoltaic inverters. Benefiting from the expansion into new markets, its price has fallen rapidly, which in turn has driven its entry into other price-sensitive markets. Mass production has further reduced prices, and this trend is expected to continue. The widespread adoption of WBG semiconductors is a prime example of a technological (and economic) cycle.
The main applications driving the widespread adoption of SiC/GaN power switches include solar photovoltaic inverters, electric vehicle chargers, and energy storage converters. These applications leverage the added value of ultrafast, small, and efficient power switches, bringing ultra-high switching frequencies and outstanding efficiency targets exceeding 99% to the market. To achieve these goals, designers face new challenges, requiring them to reduce the weight and size of power converters (i.e., increase power density).
Of course, these problems cannot be solved overnight. Advances and innovations are needed across all relevant processes. One such example is the technological bottleneck associated with the application of high-voltage power electronics systems. Architecturally, high-voltage (HV) systems are an option, but certain semiconductor technologies have long hindered this choice. Now, the advent of wide-bandgap semiconductors offers a glimmer of hope in solving this problem, making high-voltage systems a more feasible and worthwhile option. The standard for solar string inverters is 1500VDC, while 1000VDC, and soon 2000VDC, will become the standard for energy storage converters (based on batteries) and electric vehicle chargers.
In fact, the shift to high-voltage systems compatible with WBG semiconductors is quite interesting for three reasons: First, high voltage means low current, which in turn means a reduction in the total amount of copper used in the system, directly impacting system cost reduction. Second, wide bandgap technology (achieved through high voltage) reduces resistive losses, resulting in higher efficiency and allowing for a smaller cooling system, reducing its necessity. Finally, at the subsystem level, they allow engineers to move from substrate-based power module designs to discrete or lightweight power module-based designs. This implies the use of compatible PCBs and smaller wiring, rather than busbars and heavier wiring.
In summary, high-voltage systems are worthwhile if the core design objective is to reduce weight and/or cost or improve performance. Therefore, 1.7kV and 3.3kV SiC MOSFETs with high breakdown voltages have become standard for secondary applications, while 1.2kV SiC MOSFETs are the mainstream power switches for next-generation secondary and tertiary applications.
From an engineering perspective, SiC/GaN offers significant advantages. First, WGB semiconductors inherently possess superior dV/dt switching performance, meaning extremely low switching losses. This enables high switching frequencies (50kHz to 500kHz for SiC, and over 1MHz for GaN), resulting in reduced magnet size and increased power density. Inductance, size, and weight can be reduced by more than 70%, while the number of capacitors can also be reduced, making the final converter only one-fifth the size and weight of a conventional converter. The use of passive components and mechanical parts (including heat sinks) can be reduced by approximately 40%, with added value found in the control electronics IC.
Another major advantage of these technologies is their exceptional tolerance to high junction temperatures. This tolerance helps to increase power density and reduce heat dissipation issues.
Other features of SiC/GaN switches that help reduce losses include: no diode recovery required (reduced rectification losses), low Rds(on) (reduced conductivity), and high-voltage operating mode.
These advantages enable the design and implementation of innovative power electronic topologies for novel applications. SiC/GaN power switches are particularly useful in the design of resonant circuits (such as LLC or PRC), bridging topologies (phase-shifted full bridge), or bridgeless power factor correction (PFC). This is because they feature high switching frequencies, high efficiency (thanks to zero-voltage switching and zero-current switching), and the resulting high power density.
SiC-/GaN power transistors can achieve multi-stage power conversion and fully bidirectional operation, while silicon IGBTs are somewhat limited by their inverter operation mode.
In applications where power flows to or from the battery to the load or grid (such as energy storage), bidirectional operation is increasingly becoming a mandatory requirement. Designing high-power converters with compact packages has made it possible for distributed energy storage systems where battery charging accuracy can be significantly improved.
To realize the many advantages of SiC/GaN-based designs, we must confront the various technical challenges associated with them. These challenges can be categorized into three main types: switch driving, proper selection of the combined power supply, and proper control of the power converter loop.
In SiC MOSFET driving, engineers need to consider new issues, such as the accuracy of negative bias (for the gate driver) and drive voltage (which is even more critical for GaN). Such errors should be avoided as much as possible, as they can affect the entire system.
ADI's iCoupler® isolated gate drivers overcome the limitations of optocoupler-based and high-voltage gate drivers. Optocouplers are slow, power-intensive, difficult to integrate with other functions, and their performance degrades over time. In contrast, iCoupler digital isolators, an alternative to optocouplers, integrate high-bandwidth on-chip transformers and fine CMOS circuitry, improving reliability, size, power consumption, speed, timing accuracy, and ease of use for designers. iCoupler technology was introduced a decade ago to address the limitations of optocouplers. ADI's digital isolators utilize low-stress, thick-film polyimide insulation to achieve thousands of volts of isolation, allowing integration with standard silicon ICs to form monolithic systems in single-channel, multi-channel, and bidirectional configurations: 20μm to 30μm polyimide insulation with a withstand capability greater than 5kVrms.
The most representative ICs in Analog Devices' gate driver portfolio are the ADuM4135 (a high-end isolated gate driver for SiC MOSFETs) and the ADuM4121 (a fast, compact solution for high-density SiC and GaN designs). Utilizing Analog Devices' proven iCoupler technology, the ADuM4135 isolated gate driver offers several key advantages for high-voltage, high-switching-rate applications. The ADuM4135 is the optimal choice for driving SiC/GaN MOSFETs due to its excellent propagation delay (less than 50ns), channel matching time (less than 5ns), common-mode transient immunity (CMTI) exceeding 100kV/μs, single-package design, and support for a full-lifetime operating voltage of up to 1500VDC.
The ADuM4135 is housed in a 16-pin wide-body SOIC package and includes Miller clamps for robust single-rail power-off of SiC/GaNMOS or IGBTs when the gate voltage is below 2V. The output can be powered by a single or dual supply. Desaturation detection circuitry is integrated on the ADuM4135, providing high-voltage short-circuit switching protection. Desaturation protection includes noise reduction features, such as a 300ns shielding time after switching action to shield against voltage spikes during initial turn-on. An internal 500µA current source helps reduce overall device count; an internal blanking switch also supports external current sources for improved noise immunity. Considering the common IGBT threshold levels, the secondary-side UVLO is set to 11V. The ADI iCoupler chip-level transformer also provides isolated communication for control information between the high- and low-voltage sides of the chip. Chip status information can be read from a dedicated output. Reset operations can be controlled on the primary side in case of a secondary-side fault.
For more compact and simpler topologies (e.g., GaN-based half-bridges), the new ADuM4121 isolated gate driver is the optimal solution. This solution is also based on ADIiCoupler digital isolation technology, with a propagation delay of only 38ns, the lowest in its class, supporting the highest switching frequencies. The ADuM4121 provides 5kVrms isolation in a narrow-body 8-pin SOIC package.
A key aspect associated with driving SiC/GaN switches is their need to operate under high voltage and high frequency conditions. Under these conditions, the use of capacitive or inductive parasitic components is simply not permitted. Designs must be meticulously crafted, requiring extreme care in PCB routing and layout definition. This is a significant but essential challenge to avoid all EMI and noise issues. WGB Semiconductor designs demand the use of high-voltage and high-frequency passive components (magnets and capacitors). The challenges in scaling, designing, and manufacturing these devices cannot be underestimated. However, technological advancements in these areas will undoubtedly increase the ease of access to these devices in the future.
As mentioned earlier, WBG semiconductors are particularly effective at achieving high-efficiency, high-density topologies, especially resonant topologies. However, these topologies are highly complex, and their control is a challenge in itself. For example, adjusting a resonant topology requires inputting a large number of parameters (input voltage, input current, output voltage, etc.), and adding frequency modulation and phase modulation (ultra-high frequency) further complicates the engineer's work. The selection of digital components (DSPs, ADCs, etc.) is also crucial.
System control units (typically a combination of MCU, DSP, or FPGA) must be able to operate multiple high-speed control loops in parallel and manage safety features. They must provide redundancy and a large number of independent PWM signals, ADCs, and I/O. ADI's ADSP-CM419F allows designers to manage high-power, high-density, hybrid switching, multi-layer power conversion systems simultaneously using a mixed-signal dual-core processor.
The ADSP-CM419F processor is based on the ARM® Cortex®-M4 processor core, with a floating-point unit operating at up to 240MHz and an integrated ARM® Cortex-M0 processor core operating at up to 100MHz. This allows for dual-core safety redundancy on a single chip. The ARM Cortex-M4 main processor integrates 160kBS RAM with ECC, 1MB Flash memory with ECC, accelerators, and peripherals optimized for power converter control (such as 24 independent PWMs), as well as an analog module consisting of two 16-bit SAR-type ADCs, one 14-bit Cortex-M0 ADC, and one 12-bit DAC. The ADSP-CM419F operates on a single power supply, generating its internal voltage source using an internal regulator and an external adjustment transistor. It is packaged in a 210-pin BGA package.
Analog Devices (ADI) has partnered with Watt & Well to develop a range of high-end power converters based on SiC MOSFETs. The first project in this collaboration involves designing a high-voltage, high-current evaluation board for ADI's isolated gate drivers. High power specifications (such as 1200V, 100A, switching frequencies above 250kHz, and a reliable, robust design) allow customers to comprehensively evaluate ADI's IC portfolio for driving SiC and GaN MOSFETs.
Figure 9 shows the main components of the power switch driver, from the LT3999 DC-DC transformer driver that generates a positive gate voltage level, to the REF19x (or LT1121x) high-efficiency linear regulator that generates a negative gate voltage level, and then to the ADuM4135 isolated gate driver. The main controller is represented by the ADSP-CM419F processor, which can be embedded in the circuit board or connected to a high-frequency cable to generate PWM signals for the isolated gate driver.
Providing high-performance drive circuitry presents challenges beyond simply employing the best isolated gate drivers on the market. A unique aspect of ADI's solutions lies in their ability to offer readily available, complete system-level designs, a result of the integration of ADI devices with Linear Technology (now part of ADI). The combination of dedicated power supplies and stable overshoot/undershoot free reference voltage sources is essential for applications operating at frequencies exceeding 250kHz. Initially, PCB layout schemes, schematics, and user manuals are provided to strategic customers, and then published on the ADI website by the end of the year.
Analog Devices (ADI) and Watt & Well have collaborated in this high-end design field, organically combining ADI's extensive knowledge at the silicon and system levels with Watt & Well's expertise to create robust, highly reliable applications that can easily handle the demanding requirements of high switching frequencies, high power densities, and high-temperature environments. Through this collaboration, ADI can provide customers with fully feasible solutions, helping them achieve leading-edge new designs in a short time, thereby improving competitiveness and reliability.
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