A Better Power Module

The vast majority of the electronics we interact with use a power module to convert high voltage to low voltage and vice versa, and AC to DC and vice versa. Now that our vehicles are becoming, more and more, something like electronic devices, the power module has become an essential element to making them go. As current flows from battery to motor and from motor to battery, it’s the power module that does all the switching. 

Needless to say, electric car manufacturers want their batteries to be as big as possible, which means other parts need to be as small as possible. This need is significantly more prominent in electric aviation where both weight and volume of the power conversion system need miniaturization. And that includes the power module. But, until now, it seemed that power modules were unlikely to get more efficient or smaller. 

But a group of researchers at the National Renewable Energy Lab (NREL) have used new materials and a new architecture to create an Ultra-Low Inductance Smart power module, or what they call ULIS for short. It can do its switching eight times faster—or shrink to three to four times smaller—than existing power converters. 
 

A leap forward in switching speed

Those standard modules of today, delivering 1200V, 400A, operate efficiently at 20 to 25 kilohertz. They can go faster—up to 50 kilohertz—but at the higher frequencies, they lose so much energy that they become far less effective. ULIS, however, works at 200 kilohertz, and with only the kind of loss today’s modules incur at 25 kilohertz.  

“We broke the record,” said Faisal Khan, NREL’s chief power electronics researcher and the lead researcher on the project. “This high switching frequency provides an edge and can significantly help to reduce the footprint and the weight of the power converter—it’s almost nine times better than the commercially available ones.”

It will cost a lot less to produce. The key to this breakthrough is twofold: a new material and a new structure. Firstly, where other power modules conventionally use a ceramic structure sandwiched by two copper layers, the NREL researchers used a film, made by DuPont, called Temprion.  

“Power modules with ceramics are well studied, well executed, proven, and very reliable,” Khan said. “But the biggest problem is fabrication time and cost.”  

Where ceramic is famously brittle and difficult to manage, Temprion is a simple film that can be cut with scissors and manipulated with common tools. And it’s a mere 25 microns thick, compared to the millimeter thick ceramic layers. More important is its ability to handle temperature swings. A power module undergoes thermal cycling all the time, and the film outperformed ceramics in the lab without the delamination that occurs in ceramics after many hot and cold cycles. 

Despite the ease with which Temprion is cut, working with a new material meant learning how to make a power module essentially from scratch.

“It’s so tricky, it took us almost a year to master it,” he said.

A structural shift to ultra-low inductance

In addition to the new film, ULIS has introduced a novel flux cancellation technique inside the module, allowing it to maintain the parasitic inductance at an extremely low level.  

“In a conventional power module, the current goes from point A to point B,” Khan said. “We did not follow that in ULIS. It uses a very proprietary current routing technique—it hasn’t been shared with anyone.”

Shuofeng Zhao, an electrical engineering researcher at NREL and one of the key ULIS architects, came up with this breakthrough. 

What’s more, the overall shape—a flat octagonal star—reduces manufacturing costs significantly.  

“We had weekly meetings, and we came up with this three-dimensional, cylindrical shaped architecture. It showed promise, but it was an engineering nightmare to build it,” Khan explained. “That’s why we eventually abandoned that idea and moved into a planar, octagonal shaped design that preserves pretty much all the good qualities of the three-dimensional structure, but with the fabrication complexity substantially reduced.”  

NREL’s embedded electronics designer, Sarwar Islam, proposed this idea, as well how to make ULIS a “smart” module. The power module can communicate wirelessly, but Khan and his team are working on a new version that can monitor its own health and alert the user if anything goes wrong. 

Such a power module is sure to be adopted wherever lightweight, compact power conversion or faster switching is needed. That includes drones and electric aviation, as well as fast battery chargers, plasma reactors, and induction heating. But there is another advantage to the ULIS power module, at least for those with sensitive hearing.

“I drive a hybrid electric car: it has an inverter that is probably 10 years old. Its switching frequency is somewhere between five kilohertz to 10 kilohertz or even 15. So you hear something, a whining sound,” Khan said. “If you make it with ULIS, it would be in the 200 kilohertz range. So it would be dead silent.” 

Michael Abrams is a technology writer in Westfield, N.J.