GaN is Ready For Digital Power Control


It’s interesting how the term “ready” has so many different meanings. Having a house full of daughters, “ready” means getting ready to be ready; we won’t leave for another 30 minutes. On an airplane, “ready” means put away your cellphones; eventually, we might take off.

We have heard spokespersons from our industry announce that “GaN is ready for prime time.” This announcement seems to imply that GaN is ready for a broad audience, a group of users or a wide number of applications. This also suggests that GaN is has matured to the point that it should not be considered a questionable technology. I’ll leave it up to you to decide what’s true.

So what do I mean when I say that “GaN is ready for digital power control”? One way to test this is to look at how GaN-enabled power supplies are being developed. In many cases, power designers use digital control to demonstrate a GaN application. This may be because of the flexibility of digital control, which allows designers to accurately control the switching waveforms. It could also be that digital control can provide multiple control loops and protection circuits that can manage any GaN shortcomings.

To me, “GaN is ready for digital power control” means much of what I mentioned above, but it also means that digital control is ready for GaN. For digital power control to be ready for GaN, it needs the time-base resolution, sampling resolution and calculation horsepower for the higher switching frequencies, narrower duty cycles and precise dead-time control. Figures 1 and 2 show the rise and fall time for a silicon (Si) MOSFET and a GaN MOSFET. The figures show that the dead time is different by a factor of two, with the Si MOSFET being slower. Additionally, the GaN MOSFET rise and fall is more linear. These attributes make finer edge control highly desirable.

GaN enables switching frequencies to be increased without paying a penalty. This benefit allows for smaller passive elements in the power stage as well as a faster transient response. In order to have the required control of these higher frequencies, however, the control circuit must be faster. For example, the sampling and converting time needs to be fast enough to not limit duty-cycle width or phase delay. In addition, the calculation for the next control effort needs to be fast enough to not limit the switching speeds. For today’s greater-than-1MHz switching supplies, completing sampling and conversion in a few 100ns is necessary. The calculation delay must also be in this same range.

Fortunately, we have digital power controllers that have had this capability for years. Not every digital power controller can meet these needs, but at least power designers have options.

So is GaN ready for digital power control? The answer is more that digital power control is ready for GaN. So while GaN continues its development and finds homes in high-density and high-performance power solutions, we don’t have to wait for controllers to be developed to take advantage of what GaN brings to the industry. So that’s what “ready” means: it means “now.”

What are your thoughts on this topic?

 

Figure 1: Resonant LLC Si MOSFET dead time

 

Figure 2: LLC GaN MOSFET dead time

 

Additional Resources:

-LMG5200 half bridge power stage

-GaN FET module performance advantage versus silicon.

-A comprehensive methodology to qualify the reliability of GaN products.

-Advancing power supply solutions through the promise of GaN.

 

  • Very handy article to evaluate this new technology.  Great approach to summarising the issues, thank you. Confused by the graphs though: at the first transition Vgs falls and then Vds rises - makes sense, the device is turning off. Why then at the next transition does Vds fall before Vgs rises? Isn't the cause and effect back to front? What are the SEC measurements? And finally, are the red dashed lines in the wrong place? Should they not be measuring the time between a Vgs transition and a Vds transition, if as the article suggests the point is the dead time?