The benefits of using SiC are best experienced when used in applications that leverage the material’s qualities
BY STUART HODGE
Cree Power Devices
Durham, NC
www.cree.com/products/power.asp
In satisfying the requirements for an application, sometimes a device based on silicon (Si) is the right choice; sometimes it’s one based on silicon carbide (SiC). The question for many engineers is which material is the best choice for the project they are currently specifying.
SiC’s higher breakdown field is ideal for high-voltage applications.
The evolution of silicon as the material used for the majority of semiconductor devices happened for many reasons, including the ease of manufacture of the raw material as well as the resulting power devices and high-density ICs. In low-voltage high-current applications Si creates highly efficient devices, but in high-voltage applications it loses some luster and other materials begin to shine.
Materials matter
Lately, new compound semiconductor materials are challenging Si’s dominance in many markets. In addition to SiC, such materials include gallium arsenide (GaAs), gallium nitride (GaN), and diamond.
Of these, GaAs and SiC are the most mature, but GaAs has limited capabilities for high-voltage applications. SiC products exist today, including RF amplifiers, 300 to 1,200-V Schottky diodes for power applications, and LED emitters. JFETs, MOSFETs, BJTs, and PiN diodes for high voltage are also in development.
SiC’s much higher breakdown field (10 times that of Si) is ideal for high-voltage applications, and SiC Schottky diodes reduce power losses and generate much less EMI than equivalent Si parts. Lower loss and lower EMI translate into value through component reduction and energy efficiency, and make SiC an attractive solution.
SiC excels in applications requiring high voltages and high-speed switching. Because of the high breakdown field of SiC, the material allows the creation of high-voltage diodes that would normally only be created at low voltages with Si.
While Si exceeds the performance of SiC at lower voltages, it has to do with inherent characteristics in the semiconductor itself such as a much lower forward-voltage drop at lower voltages. Si Schottky diodes at low voltages can achieve levels of less than half a volt forward drop, which cannot be achieved with SiC diodes.
We all know the benefit of Si Schottky diodes at low voltages; those same benefits are realized in SiC Schottky diodes at high voltages. In any design, there are applications for both low- and high-voltage components. When specifying high-voltage components, the choices available with the associated performance will dictate the final choice.
Criteria
When choosing between silicon and silicon carbide devices, there are three considerations an engineer should evaluate first:
1. How much diode do I need for this particular application? In the past, engineers often chose large Si PiN diodes for applications like PFC based on factors like cooling requirements and managing reverse-recovery currents. The industry’s perceptions of the high costs of SiC are often a result of not understanding how to properly specify SiC diodes. In PFC applications, a device rating of around 150 W per ampere of diode rating will provide an optimal tradeoff of performance and cost. For example, this would translate to a 4-A SiC Schottky in a 600-W supply.
2. What are my current limitations? SiC Schottky diodes have an operational saturation point. Reaching that point limits the surge capability of the diode, so engineers must pay attention to how the power supply operates in startup modes because high-surge currents through the diode will kill the diode. This is not a concern in Si because of much higher surge ratings. For designers, careful attention to the current limit set point is important and will enable use of the smallest diode possible.
3. Do I have cost limitations? Despite misconceptions in the marketplace, SiC diodes can drive down the overall costs of a design. Their performance allows for fewer and smaller components to be used in conjunction with them, requiring fewer parts and less labor. For example, SiC Schottky diodes allow design engineers to re-evaluate their choices for EMI filter components, to eliminate the need for snubbers across the MOSFETs and diodes because of the fast switching speed, and reduce the size and number of MOSFETs that are used in these circuits because the losses are significantly lower.
Cost
In the past, cost was a major, if not the major, consideration for an engineer in specifying Si or SiC for a particular application. Now, however, concerns about energy usage, as well as advancements in SiC technology, have made payback so significant that SiC is now a cost-effective solution for appropriate applications.
For example, a 750-W power supply that is operational 24 hours a day, 365 days a year, in a location where there is an electricity cost of $0.10/kWh, will generate yearly energy costs of $657. Improving that power supply’s efficiency by just 1% will save $6.57 annually. In today’s world, it’s no longer just a matter of cost versus complexity; it’s a matter of increasing energy efficiency long term.
Electricity usage in the United States is expected to grow by 20% in the next ten years, accounting for 39% of energy usage. SiC technology can benefit all segments of electricity usage—HV diodes and MOSFETs for heating and cooling, IT equipment and motion applications, and high-brightness LEDs for lighting. SiC devices can help meet efficiency goals while lowering the overall cost of a system. A 2% efficiency improvement in PFC applications equals a 24% reduction in loss.
Where energy efficiency (and energy cost) is a concern, the lower switching loss of SiC devices makes them an ideal choice. SiC devices allow you to raise the operating frequency and reduce the overall size of converters; this can be shown to reduce overall system cost and create an energy-efficiency payback.
The demand for energy efficiency is expected to continue to climb in the years to come, increasing the importance of energy efficiency as a top consideration of design engineers specifying parts in the power industry. Ongoing advancements in SiC technology are targeted to make it a cost-effective energy-efficient solution for many of these high-voltage applications. ■
For more information on SiC diodes, visit http://electronicproducts-com-develop.go-vip.net/ discrete.asp.
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