Out of This World Efficiency With Silicon Carbide

Despite very rare appearances in crashed meteorites as a natural mineral from outer space – where apparently it is not so rare - silicon carbide (SiC) is better known as a compound substance discovered by accident in the 1890’s by an American inventor called Edward Goodrich Acheson, who had previously left Thomas Edison’s team of incandescent light pioneers to pursue the creation of artificial diamonds. And this is where he noticed some shiny blue crystals appearing when he heated a mixture of clay and coke in an iron bowl with a carbon arc light. Many patents later, his super-hard crystalline silicon and carbon compound first found its way into our lives as an abrasive in products like sandpaper, grinding and cutting tools, and later into bulletproof vests, car brakes and rocket engines, LED’s (since as early as 1907 in the world’s first LED, would you believe)… and power semiconductors.

Why silicon carbide for power semiconductors? The main reason is its wide energy band gap, which determines how much energy is needed to allow electrons to jump between energy bands on the SiC material, making it carry current. This wide band gap, around three electron volts, means that heat, radiation and other external factors do not have such a disruptive influence on performance.

So Silicon carbide is a material that exceeds silicon in characteristics such as permissible operating temperature and radiation exposure, as well as also possessing beneficial properties in terms of insulation breakdown field strength for use at high voltages; high electron velocity meaning it can be used at higher frequencies; and high thermal conductivity for heat dissipation, giving it great potential for use in power devices.

Or put more simply, more efficiency and lower losses at higher temperatures in smaller designs. So why isn’t silicon carbide everywhere, then? Well, give it a little more time – crystal defect issues that have hampered commercialisation in some applications continue to be resolved, manufacturing efficiency improves, and at Renesas Electronics, silicon carbide schottky barrier diodes have been in production for some time. SiC Power MOSFET’s and IGBT’s have faced additional challenges in the  SiC and silicon dioxide interface, but again, these problems are being widely researched and the situation improves daily, and while SiC-MOSFET’s continue to be developed, Renesas hybrid devices are already available, combining the ease of use of conventional silicon MOSFET’s with massive on-resistance improvements leading to much higher efficiency. As well as offering around 26% efficiency increases, our hybrid IGBT’s, embedding SiC diode into the IGBT package, also saves about 50% of the PCB space traditionally required when reduced thermal losses resulting in smaller heat sinks is also considered.

Aside from improvements in crystal production yield and process efficiency across a number of SiC component suppliers, market factors also have a part to play in the uptake of silicon carbide power technology, particularly with regard to efficiency. There is an incredibly strong demand for more efficient power conversion in applications such as air conditioners and solar power arrays where efficiency in power switching and inverter circuits is being dictated by legislation as well as consumer attitudes.

With this in mind, Renesas Electronics developed silicon carbide schottky barrier diodes (SBD) for use in power conversion applications like these to provide faster switching speeds and lower voltage operation.

The first benefit you see from a SiC SBD like the Renesas RJS6005TDPP is in the switching speed, and the resulting 40% reduction in switching power losses when compared to traditional products.

When a diode switches from on to off once the prescribed forward current has been through it, reverse current flow occurs due to a small number of carriers accumulating in the junction. The amount of time needed to get back to the prescribed current value after switching from on to off is reverse recovery time.

In our SiC SBD, we have a reverse recovery time of just 15 nanoseconds, which is around 40% faster than a silicon equivalent. (Note that this is a standard value measured at IF=15A, di/dt = 300A/us). In turn, this faster switching speed is also reducing power losses by 40 to 60% compared to silicon products. You also have the benefit of a much simpler EMI control circuit design, further reducing cost, PCB space and time to market.

The reduced switching loss manifested in silicon carbide devices is also offering further efficiency increases by affording the designer the option to run at a higher frequency. On the flipside, using the diode with an IGBT and reducing the frequency will further reduce thermal losses, giving an opportunity to further reduce the size of your heat sinks.

We also talked about voltage reduction, and this is equally impressive when comparing SiC to silicon. SiC SBD’s, such as the Renesas RJS6005TDPP, have a forward voltage rating of just 1.5V, lower than that of existing silicon fast trigger diodes. Additionally, the temperature dependency on this characteristic is small, meaning that a stable forward voltage can be obtained even under high temperature operation, in turn meaning that smaller heat dispersion measures can be taken, meaning less system cost again.

If you’re convinced by incorporating more efficiency and lower losses at higher temperature in your design, and considering a move from silicon to silicon carbide diodes, you might be wondering about the physical differences. Well, the Renesas RJS6005TDPP uses a package equivalent to the industry standard TO-220, and it’s also also pin compatible, making it an easy replacement for conventional diodes. A line-up of up to 30A is available from Renesas, with a voltage tolerance of 600V now and 1200V on the way in the near future.

Figure 1 shows devices available today from Renesas.

Figure 1: Silicon carbide diode (RJS60 series) line-up

This is just the start. According to MarketsandMarkets, the SiC semiconductor device market is set to grow at around 38% compound annual growth rate between 2012 and 2022. So whilst silicon carbide might only account for around 1% of power devices today, all the stars are aligning for a meteoric rise.