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Driving SiC/GaN power converters

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By Stefano Gallinaro, Analog Devices, and Imad Owaineh, WATT&WELL

The power converter market continues to evolve fast, moving from simple performance/cost designs to broader and more sustainable innovations. However, new challenges are proliferating, including the need to make smaller yet more efficient power converters that can serve smaller servo drives or be integrated into distributed energy storage units. This also means having higher working voltage to manage higher power without increasing weight and dimensions, like in solar string inverters and electric vehicle traction motors.

New high-efficiency, ultrafast power converters based on wideband gap (WBG) semiconductors such as silicon carbide (SiC) and gallium nitride (GaN) MOSFETs are starting to expand in various markets and applications. But to fully harness these, a complete ecosystem of ICs must be implemented in converter designs, starting from the closest chip to the power switch, or gate driver. Requirements for isolated gate drivers are changing from past silicon IGBT drivers. For SiC and GaN MOSFETs crucial are high CMTI > 100kV/μs, wide gate-voltage swing, fast rise and fall times and a very low propagation delay.

Figure 1: Power converter market forecast for the year 2021

Fast Growth Market

The power converter market is growing with a CAGR of over 6.5%, expected to reach $80bn by 2021; see Figure 1. Today, traditional inverters and converters based on silicon IGBTs make up most of the market (> 70%), primarily due to electrical motor drive applications in factory production lines and to the first generation of wind and solar inverters.

New technology improvements in power switches are bringing the third generation of SiC MOSFETs and the first and second generation of GaN MOSFETs to market. After being restricted to some niche power applications many years, WBG technologies are being quickly adopted in various applications, such as battery-based energy storage, electrical vehicle chargers, traction motors and solar PV inverters. This expansion into newer markets has contributed to a rapid price reduction, which in turn has enabled entry into other markets where price had initially been a factor. Mass production has further decreased the price, a trend that’s likely to continue.

Leading Applications

The added values of ultrafast, small and efficient power switches are exploited to bring to market extremely high switching frequencies and outstanding efficiency targets of over 99%. To achieve such results, designers are facing new challenges by cutting and reducing the weight and footprint – that is, increasing the power density – of the power converters.

Of course, solving all these issues can’t be done at once; many advances and innovations in related engineering processes are necessary. One such example is the technological bottleneck related to using high-voltage power electronics systems. Moving toward high voltage (HV) systems is an architectural choice, but for a long time it was hindered by some semiconductor technologies. This has been remedied today with wideband gap semiconductors, which have made HV technology easier to adopt. The standard for solar string inverters is 1500VDC, whereas 1000VDC and, soon, 2000VDC will be standard for energy storage converters (battery-based) and electric vehicle chargers.

Indeed, moving toward HV systems compatible with WBG semiconductors is interesting for three main reasons: first, high voltage implies lower currents, which means that the total copper in the system will be reduced, which will reduce the entire system cost. Second, wide gap technologies (enabled by HV) generate less resistive losses, which in turn present better efficiency and reduce the size and need for a cooling system. Finally, on a subsystem level, they will enable engineers to move from baseplate power module based designs to discrete-based or lighter power module based designs, which means using compatible PCBs and smaller wires instead of bus bars and heavier wires.

To conclude, high voltage systems are worthwhile if reducing weight and cost are core objectives for the design. The high breakdown voltage of 1.7kV and 3.3kV SiC MOSFETs is thus becoming a standard for second-level applications, while 1.2kV SiC MOSFETs are becoming mainstream power switches for next-generation second- and third-level applications.

Engineering Advantages

From an engineering point of view, the advantages of SiC/GaN are considerable. To begin with, the considerable dV/dt switching (inherent to WGB semiconductors) has very low losses per switch. This makes a high switching frequency (50-500kHz for SiC, or > 1MHz for GaN) realistic, which decreases magnetics volume while increasing power density.

A reduction of over 70% in inductor value, size and weight, together with smaller capacitors, can bring the converter design to a fifth of the size and weight of a traditional one. Savings in passive components and mechanics (including heat sinks) are about 40%, with the added value of on-chip integration.

Another huge advantage of these technologies is their tolerance to high thermal junction temperatures, which increases power density, whilst limiting cooling problems.

Other characteristics of SiC/GaN switches that help reduce losses are the absence of any recovery on the diodes (fewer losses on rectification), and low Rds(on) (which generates less conduction) losses, in addition to their HV operation.

SiC/GaN power switches are very beneficial in designing resonant circuits (such as LLC or PRC), bridge topologies (phase-shift full bridge) or bridgeless power factor correction (PFC, due to their high switching frequencies, high efficiencies (thanks to zero voltage switching and zero current switching) and, consequently, high-power densities.

Working Modes

SiC and GaN power transistors enable multilevel power conversion stages and full bidirectional working modes, where silicon IGBTs have limitations because of reverse operation.

The need of bidirectional working modes is increasingly becoming mandatory in applications like energy storage, where power flows to and from batteries to loads or the grid. The possibility of designing high-power converters with compact housing also allows a distributed energy storage system, where battery charging accuracy can be higher.

In order to achieve the many advantages obtained by SiC-/GaN-based designs, we should confront the various technical challenges associated with them; challenges that can be divided into three major categories: driving the switch, selecting the proper combo power supply and controlling the power converter loops in a proper way.

By driving SiC MOSFETs the engineer is confronted with new considerations such as negative bias (needed for the gate driver) and the accuracy of the driving voltage, which is even more important for GaN. There is little tolerance for such imprecisions, and it could affect the whole system.

A critical aspect related to driving SiC/GaN switches is their operation under high voltage and high frequency conditions, under which no capacitive or inductive parasitic elements should be tolerated at all. The designs are to be finely tuned, and special care should be taken when routing the boards and defining their layout. This is a considerable yet necessary challenge to avoid EMI and noise issues.

WBG semiconductor design requires high-voltage and high-frequency passive components – magnetics and capacitances. The challenges of sizing, designing and manufacturing these devices should never be underestimated. Technology, however, keeps improving in these domains as well, and the possibilities provided by WGB semiconductors will inevitably make their procurement easier in the future.

Complex Topologies

As previously discussed, WBG semiconductors are especially effective for implementing high-efficiency, high-density topologies, and resonant topologies in particular. Yet these topologies are quite complex, and their control represents a challenge of its own. For example, the number of inputs needed to regulate a resonant topology (input voltage, input current, output voltage, etc.), combined with frequency and phase modulation (on very high frequencies), does not make the engineer’s task any easier. The choice of digital components (DSP, ADC, etc.) is of utmost importance, as well.

The system control units (generally a combination of MCUs, DSPs and FPGAs) must have the capabilities of running multiple high-speed control loops in parallel and be able to manage safety features. They have to provide redundancy and a large number of independent PWM signals, ADCs and I/Os.

Analog Devices has engaged with power electronics company WATT&WELL to develop a range of high-end SiC MOSFET-based power converters. ADI brings its know-how at silicon and system level to the collaboration, whereas WATT&WELL is well-known for its robust and reliable applications where high switching frequencies and power densities for high-temperature environments.

The collaboration is starting with the design of high-voltage, high-current evaluation boards for ADI isolated gate drivers. High-power specifications, such as 1200V, 100A, > 250kHz switching frequencies with reliable and robust design, permit customers to completely evaluate the ADI set of ICs driving SiC and GaN MOSFETs.

Figure 2: A simplified block diagram of the isolated gate driver board

Figure 2 shows the main components inside power switch drivers; from the LT3999 DC-DC transformer driver generating a positive gate voltage level to the REF19x (or LT1121x) high-efficiency linear-regulator creating the negative gate voltage level, to the ADuM4135 isolated gate driver (Figure 3). The main controller is represented by the ADSP-CM419F processor, which can be embedded in boards or connected with high-frequency cabling, and which generates the PWM signals for the isolated gate drivers. The is easily incorporated into complete system-level designs.

Figure 3: Isolated gate driver board

ADI’s iCoupler Technology

Analog Devices iCoupler isolated gate drivers overcome the limitations of optocoupler- and high voltage-based gate drivers. Optocouplers are slow, power-hungry and difficult to integrate with other functions, and they degrade over time.

The iCoupler technology was created ten years ago to address the limitations of optocouplers. ADI’s digital isolators use low stress, thick film polyimide insulation to achieve thousands of volts of isolation that can be monolithically integrated with standard silicon ICs. These can be fabricated in single-channel, multichannel and bidirectional configurations: 20-30μm of polyimide insulation withstands > 5kVrms; see Figure 4.

Figure 4: iCoupler transformer coils on polyimide insulation

ADI’s iCoupler technology is used in the ADuM4135 isolated gate driver for SiC MOSFETs for driving SiC/GaN MOSFETs; see Figure 5. It offers a propagation delay of better than 50ns with channel-to-channel matching of less than 5ns, common-mode transient immunity (CMTI) of better than > 100kV/μs, and the capability to support lifetime working voltages of up to 1500VDC in a single package.

Figure 5: ADuM4135 block diagram

The ADI iCoupler technology is also used in the new ADuM4121 (Figure 6) isolated gate driver, which is a more fitting solution for compact and simpler topologies, such as GaN-based half bridges, for example. Its propagation delay of 38ns is the lowest in class, allowing for very high switching frequencies and common-mode transient immunity of 150kV/μs.

Figure 6: ADuM4121 block diagram

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