power supplies for servers and storage in data centers. And other applications ranging from inverters for electric vehicles to solar panels and wind. Every point where you are converting electricity.”
According to Mishra, the company is already supplying test converter modules to customers in the areas of computer servers, photovoltaic inverters, and motor drives for building systems, and expects to have products for sale in the next 12 to 16 months. “The last [market] to bear fruit will be hybrid cars and electric vehicles, because the design cycle in the automotive market is so long,” he says. The company plans to demonstrate its technologies two weeks from now at the Applied Power Electronics Conference in Forth Worth, TX, the power electronics industry’s biggest annual event.
If Transphorm can help makers of power supplies shave even a few percentage points off efficiency losses, it will be the second huge win in Mishra’s career. He’s already famous as one of the pioneer researchers in applications of gallium nitride for the LED lighting industry; Nitres, the Santa Barbara company he started in 1995 to commercialize that work, was acquired by Durham, NC-based LED manufacturer Cree in 2000 for $300 million, and is now operated as Cree’s Santa Barbara Technology Center.
Transphorm, like Nitres, has a core staff of researchers who all came from Mishra’s lab at UCSB. “We have a good blend of fresh graduates and people who have gone on to get industry experience, and they form the nucleus of this team,” Mishra says. “Arguably—no, I will just withdraw that—it is the best team in the world working on this problem.”
To understand the problem itself requires a bit of a detour into material science and electronics. The basic function of a power converter is to turn DC power into AC, AC into DC, or reduce the 110 volts of energy that come out of a standard U.S. wall socket to something appliances can handle. Decades ago, most converters were actually designed to shed excess energy in the form of heat. Later, switched-mode power supplies were developed that converted power much more efficiently by temporarily storing it in transistors, inductors, capacitors, and other electronic elements.
In today’s switched-mode power supplies, the transistors are made of silicon, which is easy to work with but has a built-in limitation: its narrow bandgap, meaning, loosely, the amount of energy required to get electrons moving. Explains Mishra, “The bandgap reflects the amount of voltage you can place across a certain thickness of silicon, and you can only sustain a certain amount of voltage because beyond that the small bandgap of silicon causes the material to break down”—in a literal puff of smoke. Gallium nitride has a much larger bandgap, three times that of silicon, which has an exponential effect on the amount of voltage transistors made of the material can hold. “The most conservative estimate is 10 times, and if you risk smoking and cracking you can go up to 200, but everyone agrees the benefits are an order of magnitude or more,” says Mishra.
Conveniently, gallium nitride is also a better conductor than silicon when a transistor is switched on, losing less power to resistance—and since it’s so much better at holding a voltage, less material is needed, so transistors can be thinner. (It’s gallium nitride’s wider bandgap, and its stability under high frequencies and high thermal loads, that also make it ideal for applications like LED lighting and the violet laser diodes in Blu-ray players.)
Ingenious power electronics manufacturers have taken silicon-based converters to the material’s absolute limits, Mishra says, but “you can’t change the bandgap—that’s a material constant.” But Mishra says it wasn’t until 2007 or so that he started to become aware of the scale of the efficiency crisis in power conversion. “We are wasting hundreds of
