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Elimination of power conversion trade-offs by switching to 1700V SiC MOSFETs

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Xuning Zhang and Kevin Speer

Microchip Technology

High-voltage power system designers have strived to meet customers' needs for continuous innovation when using silicon MOSFETs and IGBTs. Desired reliability often cannot be achieved without sacrificing efficiency, nor can silicon-based solutions meet today's demanding size, weight and cost requirements. However, with the advent of high-voltage silicon carbide (SiC) MOSFETs, designers now have the opportunity to improve performance while solving all other challenges.

Today's 1700V SiC products build on the success of the 650V to 1200V SiC power devices that have become increasingly adopted over the past 20 years. Technology has already enabled significant advances in final equipment; and now, with 1700V power devices, it is extending the myriad advantages of SiC technology to new end-market segments such as commercial and heavy-duty electric vehicles, light rail traction and auxiliary power systems, energy renewables and industrial drives, among others.

Designers can maximize the benefits offered by 1700 V SiC MOSFETs with the proper power package and gate driving. This increases its advantages over current silicon solutions in the widest possible range of power levels.

Benefits at lower power levels

The benefits of 1700 V SiC MOSFETs begin at power levels as low as tens or hundreds of watts. SiC technology is the ideal solution for the Auxiliary Power Supply (AuxPS) used in virtually all power electronics systems. Without an AuxPS, there is no way to power the door controllers, sense and control circuitry, or cooling fans. Due to its critical functions, reliability is the top priority for AuxPS applications.

One of the ways that 1700 V SiC MOSFETs help mitigate AuxPS faults is through their high breakdown voltage, lower turn-on specific resistance, and fast switching speed. Taken together, these attributes allow for a more simplified circuit design using the single-switch flyback topology (see Figure 1). By comparison, silicon-based solutions either have too low a voltage rating for this topology (requiring a two-switch architecture and doubling the risk of failures), or they sacrifice performance for voltage rating. Also, they are not available from enough vendors and are more expensive than SiC devices.

flyback topology
Figure 1. Shown above is the ubiquitous auxiliary power supply, using the wide input single switch flyback topology.

By enabling single-switch flyback topology, 1700V SiC MOSFETs make it easy for today's low-power isolated switching power supplies to support diverse input and output requirements. They can accept a high voltage DC input (300V to 1000V) and output a low voltage source (5V to 48V). The single switch flyback topology improves simplicity and reduces the number of components and associated overall cost.

In addition to its higher reliability, less complex control scheme, fewer components, and lower cost, an AuxPS using 1700V SiC MOSFETs can also be more compact. The area-normalized on-state resistance, also called specific on-resistance (Ron,sp), , of SiC MOSFETs is a fraction of that of silicon MOSFETs. This means smaller packages can be used for smaller dies, and conduction losses are reduced, which can ultimately result in reducing (or eliminating) the size and expense of heat sinks. SiC MOSFETs also have lower switching losses, allowing the size, weight, and cost of transformers to be reduced with increasing switching frequency.

Figure 2 shows the degree of efficiency improvement of the different SiC devices available as a function of output power. With today's most efficient devices, system designers can even implement passive cooling, meaning no heat sink is needed.

SiC and a MOS device
Figure 2: Comparison of efficiency vs. power output for various SiC options and a silicon high-voltage MOS device.

Benefits grow as power processed increases

The impact of the faster and more efficient switching of SiC technology increases as the power processed increases. Stepping up the power scale to tens or hundreds of kilowatts (kW), there are many applications for SiC technology. Figure 3 shows a multi-kW three-phase inverter (75 kW in this example) and its topology. It can be found in electric vehicle traction, electric vehicle chargers, solar inverters, UPSs, and motor drives, among others.

three phase inverter
Figure 3. In order, the key priorities of the multi-kW three-phase inverter shown above (including functional sections and topology) are efficiency, reliability, and power density (reduction in size and weight).

Figure 4 compares the efficiency of this inverter design using 1700 V power modules in a low inductance package with that of other power semiconductors. The SiC module demonstrated a maximum efficiency of 99,4% at 10 kHz. Even when the switching frequency was tripled to 30 kHz, the SiC module still offered higher efficiency than silicon IGBTs. This allows heavy and expensive filter components to be reduced to only one third of their original size.

SiC solutions
Figure 4. The efficiency of SiC solutions is compared to that of silicon IGBTs at switching frequencies of 10 kHz and 30 kHz.

In general, MOSFETs reduce switching losses by an average of 80% compared to silicon IGBTs, allowing converters to increase switching frequency while reducing the size, weight, and cost of converters. bulky and expensive transformers. The conduction losses of SiC MOSFETs and silicon IGBTs are similar under heavy loads, but it is more important to consider the so-called "light load" conditions in which many applications spend most of their lifetime. Among them are solar inverters located under a shade structure or on cloudy days; wind turbine converters running on calm days; and train doors that only open/close periodically thanks to transportation Auxiliary Power Units (APUs). SiC MOSFETs have lower conduction losses compared to silicon IGBTs in these use cases, complementing their low switching losses. The combination of lower conduction and switching losses allows designers to reduce or eliminate heat sink or other thermal management measures.

Similar to lower power AuxPS applications, SiC MOSFETs used in this higher power range improve reliability by allowing designers to use a more simplified circuit topology and control scheme. This, in turn, reduces the number of components and the associated costs. In these applications, the higher power delivery needs of medium power converters require the use of a higher DC bus voltage, typically between 1000V and 1300V. To maximize efficiency, designers using silicon transistors at these high DC link voltages have traditionally had to choose between a few complex three-level circuit architectures. Examples are the Neutral Point Clamped (NPC) diode circuit, the ANPC (active NPC) circuit, and the T-type circuit. This changes with the 1700 V SiC MOSFETs, which allow designers to use the two levels with half the number of devices and much more streamlined control. For example, a system that previously used silicon IGBTs in a three-level circuit topology could use half (or less) as many 1700V SiC MOSFET modules in a more reliable two-level topology.

Figure 5 shows how far designers can reduce the total number of parts in NPC, ANPC, and T-type circuits with SiC technology. Without taking into account the advantages of paralleling multiple units in each switch position, the various circuit architectures used with IGBTs have 4 to 6 times more components than a SiC solution. As the number of units is reduced, the number of door controllers is also reduced and the control scheme is simplified.

SiC technology
Figure 5. SiC technology increases efficiency and power density while improving reliability thanks to the possibility of using simpler two-level topologies. This allows a 75 kW three-phase inverter to be built with as few as two units per phase section plus two controllers, as shown in the NPC, ANPC, and T-type circuit examples above.

Moving to Megawatt Application Scale

Megawatt-scale applications range from Solid State Transformers (SSTs) and medium voltage DC distribution systems to Traction Power Units (TPUs) in commercial and heavy-duty vehicles. Other applications are central solar inverters and offshore wind converters, as well as power conversion systems on board ships. Figure 6 shows an example of a multilevel modular converter.

Multilevel Modular Converter
Figure 6. Multilevel modular converter.

In applications within this multi-megawatt power range, a solid-state transformer converter like the one shown above uses multiple levels of series-connected power cells to meet voltage requirements. Each cell can be a half bridge or a full bridge. Some designers even opt for three-tier architectures. The use of modular solutions based on a basic unit cell improves scalability and minimizes maintenance. Sometimes referred to as power electronics blocks or submodules, these unit cells are configured as cascaded H-bridge converters or modular multilevel converters (MMCs).

To implement these unit cells, designers have historically used 1200V to 1700V silicon IGBTs. When replaced by 1700V SiC MOSFETs at the unit cell level, the same effect as described in lower power applications occurs: better power controllability and electrical performance. The lower switching losses of the 1700 V SiC MOSFETs allow the switching frequency to be increased. The size of each unit cell is drastically reduced, and the high blocking voltage of 1700V reduces the number of unit cells needed for the same DC link voltage. Ultimately, this increases system reliability by reducing the number of cells, while reducing cost by using fewer active switches and door controllers. For example, when using a 1700V SiC solution in a solid state transformer running on a 10kV medium voltage distribution line, the number of cells connected in series can be reduced by 30% compared to those connected in series. they use silicon alternatives.

Importance of Power Encapsulation and Proper Gate Driving

Since SiC MOSFETs can switch high power levels at very high speeds, there are side effects that need to be mitigated, such as noise and electromagnetic interference (EMI), as well as limited short-circuit withstand time and overvoltage caused by inductance. parasitic and overheating. The typical medium power converter puts out hundreds of amps across a 1000V – 1300V bus in less than a microsecond.

Microchip has SiC MOSFET module packaging options that significantly reduce parasitic inductance. These include half-bridge packages with as little as < 2,9 nanohenrys (nH) stray inductance, maximizing current, switching frequency, and efficiency (see Figure 7). These package types also offer higher power density and a compact form factor, allowing fewer modules to be paralleled for complete systems, helping to further reduce equipment size.

sic encapsulation
Figure 7: Designers have many packaging options with today's SiC modules, including half-bridge options with parasitic inductance as low as < 2,9 nH, as shown above.

In addition to minimizing package inductance and optimizing system design, designers can also use a new gate driving method specifically designed to mitigate the side effects of SiC MOSFETs' faster switching speeds. Today's fast-acting, intelligent, configurable digital gate controllers reduce drain-source overvoltages (VDS) by up to 80% compared to the traditional analog approach and reduce switching losses by up to 50%. They also reduce time to market by up to six months and provide new increased switching capabilities.

These capabilities allow designers to explore settings and reuse them for different door controller parameters, such as door switch profiles, critical system monitors, and controller interface settings. They can quickly tune door controllers to support many different applications without any hardware modifications, reducing development time from evaluation to production. They can also change control parameters throughout the design process, and change switching profiles in the field as needed and/or if the SiC MOSFETs degrade.

Current SiC MOSFET offerings should also be part of a comprehensive SiC ecosystem that provides a direct path from evaluation to production. This includes customizable module options as well as digital door controllers that allow users to optimize system performance and reduce time to market with the click of a mouse. Other elements of the ecosystem include reference module adapter plates, an SP6LI low inductance power module, mounting hardware and connectors for thermistor and DC voltage, as well as a programming kit for configurable software. Complementary discrete products complete the ecosystem.

A sequence of benefits

In a continuum of power conversion applications, from watts to megawatts, high-voltage SiC MOSFETs are driving designers beyond the compromises of silicon solutions to drive innovation in power conversion system development. They increase reliability while reducing cost, while simultaneously decreasing the size and weight of the most efficient power systems and converters. When used with smart digital gate driving, 1700V SiC MOSFETs offer their highest possible value. Microchip offers a broad portfolio of rugged and reliable SiC components in chip, discrete, and power module forms, as well as digital gate control solutions, enabling the designer to adopt SiC with ease, speed, and confidence.

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