How 3D Printing Could Add Efficiency to EV Power Electronics

A new approach to inverter design using additive engineering promises advances for EV propulsion.

Bus bar connections (top surface) link directly to the new heat transfer lattice within the cold plate. (University of Bath)

“Think production!” Perhaps that advice should be posted on the wall of every design office, R&D lab and advanced technology center in the auto industry. Although obeying that warning is clearly not cost effective in some instances, others are ostensibly perfect to take their place in volume manufacture. An example is 3D printing (aka additive manufacturing or AM), but despite a broadening scope that now embraces rapid prototyping and tooling by entire houses, it could do better in series production of auto components, particularly in the new world of EVs.

Prof. Peter Wilson said SiC devices offer more opportunity to improve inverter performance – but that system designers are often unable to take full advantage of their potential because their ideas cannot be manufactured using conventional techniques. (University of Bath)

Prof. Peter Wilson of the UK’s newly established Institute for Advanced Automotive Propulsion Systems (IAAPS) at the University of Bath, noted the growing adoption of high-speed SiC (silicon carbide and other wide-band gap) semiconductor devices demonstrates the benefits that 3D printing could have in the production of EV inverters: “SiC devices offer so much opportunity to improve inverter performance. But system designers are often unable to take full advantage of their potential because their ideas cannot be manufactured using conventional techniques.

“Additive manufacturing lets us design in 3D without these constraints and we see great benefit in applying the technique to EV inverters. These have a significant impact on how hard you can drive other components of the EV powertrain, so even a modest advance can create a virtuous circle of improvements in efficiency, packaging and energy density for tomorrow’s EVs.”

Overcoming constraints

Additive manufacturing could bring significant benefits to EV inverter design, noted the UK university research team. Without it, the single piece triangular element in the center of the inverter assembly illustrated, would be challenging to achieve cost-effectively using conventional manufacturing techniques, they state. (University of Bath)
More compact inverter assembly is made possible via application of additive manufacturing. (University of Bath)
Oliver Holt’s design points the way to making the inverter significantly more power dense. (University of Bath)

To prove this, Wilson is leading a multi-disciplinary research team that is completing the first phase of a project that looks at the achievability of 3D printing select inverter components. This could allow key constraints to be overcome – including thermal management, electrical noise and packaging volume. The aim is achieving significant design improvements that can be manufactured in volume. Although it is early days, the inverter project is looking promising, he said.

At present, inverters are typically designed via 2D processes, stacking flat component boards. At the bottom of the stack is a thick aluminum cold plate heat-sink that is typically liquid cooled. “The efficiency, reliability and performance of SiC switches falls significantly as temperature rises, so the ability of this plate to extract heat from the inverter is critical,” Wilson said. “A conventional cold plate has machined cooling channels. But these inhibit the creation of intricate shapes and you can’t specify very thin walls between the channels. As well as making it less effective, that means the plate is heavier and bulkier than you need and introduces additional thermal resistance.”

This is where 3D printing comes in. Oliver Holt, a member of Wilson’s specialist team, has designed a plate with a complex lattice internal structure constructed of walls less than 1-mm thick. The more efficient design will allow the power electronics to carry up to 10% higher current, making the inverter significantly more power dense. “The manufacturing solution would be to 3D print just the complex shapes and drop them into a conventionally-manufactured plate,” Holt said. “We only employ the more expensive technique where it allows us to do something that adds a lot of value.”

The next challenge is packaging, which is also linked to how quickly the SiC switches can be instructed to turn on and off. Electro-Magnetic Interference (EMI) is created not only by a high switching frequency, but also by the rate of change of voltage (dV/dt) in the transition from conducting to not-conducting. Detailing the finer points, Holt explained: “EMI is made significantly worse when the distance between switches and the gate drivers, together with the area of the connection, is large.”

This is because even such a small inductance leads to damped-resonant ringing of the switches as they transition. The traditional solution is to operate the switches at a lower speed, meaning the designer is unable to take full advantage of the SiC technology. Added Holt: “3D printing facilitates manufacture of the connections with highly optimized 3D geometries, which allows the gate drivers to be moved closer to the switches, improving packaging as well as reducing the loop inductance that can limit the achievable switch performance.”

Simplified cooling

What all this delivers, Wilson said, is a reduction in switching losses that translates to both an increase in converter efficiency – especially at low loads – and an increase in inverter power without upgrading the cooling system, or a simplified cooling system for the same power: “It’s a virtuous circle enabled by additive manufacturing’s ability to let us design in 3D. Because the new approach eclipses conventional manufacturing constraints, it delivers the ability to fully exploit proven wideband gap semiconductor devices that will also allow so many other systems in the EV powertrain to work more efficiently – permitting designers to apply it to production.”