Dr. Adam Barkley, VP of Power Technology Development at Wolfspeed, talks to Alistair Winning from PSD about advantages of the company's Gen 4 SiC devices
RDS(on) is perhaps usually the most important metric that is highlighted by companies when they talk about their wide bandgap products. Other figures of merit are also important, but every manufacturer that I can think of specifically highlights low RDS(on) as a major plus point. And that’s no real surprise, as the lower RDS(on) figure means less power is dissipated as heat when the transistor is conducting current, bringing higher efficiency. That made it quite surprising when I met with Dr. Adam Barkley, VP of Power Technology Development at Wolfspeed, to talk about the company’s new Gen 4 SiC technology when claimed that RDS(on) was not the company’s primary concern when developing the technology.
He explained, “this is the first time that we've taken the mindset shift from specifically reducing RDS(on), although we actually did that too, with around a 22% reduction in high temperature RDS(on). We balanced that smaller reduction with improvements in switching losses and particularly the cleanliness of the waveforms. These changes included a pretty big improvement in body diode reverse recovery. That means a Gen 4 device can be dropped in to replace a Gen 3 device and the waveforms instantly become cleaner, and those smoother waveforms minimize common-mode voltage and radiated emissions, making EMI filter design much easier, and bringing efficiency improvement through lower resistance and lower switching losses. Reliability is also improved with extra voltage headroom from drain to source.”
However, according to Barkley, their full potential is realized when designers leverage the cleaner switching characteristics to push performance boundaries. The softer body diode in the Gen 4 devices dramatically improves EMI performance in comparison to the Gen 3 devices’ "snappy" body diode. He continued, “as the switching is so much cleaner, the gate resistance can be reduced, even down to zero ohms, intermittently up to 185oC operating temperature, and depending on the application, there can be between 3X to 6X reduction in turn-on switching loss. Low switching frequency applications, such as electric vehicle traction applications, switch between 8 and 12kHz and it's mostly conduction loss dominated, and so the high temperature RDS(on) of the Gen 4 devices is a big improvement there. For higher frequency applications, for example totem pole power factor correction stages for industrial power supplies, higher frequencies bring more energy dense solutions. For those applications where energy efficiency is really key, the more efficient you can be, the easier and cheaper it is to incorporate thermal management. Another big application area that we were seeing and then we targeted here was energy storage systems, like uninterruptible power supplies, where, again, you need to process power twice.”
Reliability and performance have been assisted with an improved capacitance ratio - the ratio between the gate to drain capacitance, and the gate to source capacitance. This is very important for high speed switching applications. When there is a high drain to source voltage, that capacitively couples into the gate voltage. Then, there is a risk of the gate voltage rising above the threshold creating a parasitic self turn on or shoot through event, which is effectively a short circuit. The improvement in this ration in the Gen 4 devices means that the resistance to this type of effect is dramatically reduced.
The new MOSFETs can achieve switching speeds of 100 V/ns and 10 A/ns, representing a two to three-fold improvement over previous generations. They also support up to 2.3μs withstand time without compromising RDS(on) performance to ensure safe shutdown during faults. The devices are available in advanced packages that are tailored to minimize parasitic inductance in power, gate, and common-source loops, enhancing efficiency, reducing switching losses, and enabling the use of lower-voltage-rated SiC devices, with less derating required. Advanced thermal design and compact layouts support high-power applications, thermal control, and higher switching frequencies.
PDF
