GaN-on-Si half-bridge circuits developed

Energy-efficient power electronics for energy conversion and transmission are becoming increasingly important as future-oriented and sustainable energy concepts, such as electric mobility or power supply via renewable energies, depend on them. For this purpose, the Fraunhofer Institute for Applied Solid State Physics IAF has developed a user-friendly, highly integrated GaN voltage converter in a compact package that is extremely resource-efficient and can be used in a modular fashion.

For years, power semiconductors have been used in voltage converters to meet the increasing demands on energy supply and usage. However, it is not only the semiconductor materials themselves that are decisive, but also the packaging and design of the components; the more compact and efficient the packaging is, the more resource-efficiently they work. However, a compact system design is often difficult to reconcile with the discrete standard components that dominate the GaN power electronics market. Most of the time, the critical conductor loops between transistors and power supply have to be individually designed and wired as circuits from discrete components.

A user-friendly alternative is now being offered by scientists at Fraunhofer IAF. They have embedded their innovative gallium nitride-based integrated power circuits (GaN power ICs) as a half-bridge in a printed circuit board (PCB), which features all critical wiring, including gate and DC link capacitors, within the package. The result is a compact and efficient voltage converter for 600 V applications that enables a reliable and modular system design, thus facilitating design and production processes.

Monolithically integrated GaN power ICs

Fraunhofer IAF has many years of experience in the monolithic integration of power electronic GaN chip technologies. In 2019, its scientists managed to monolithically integrate current and temperature sensors, 600 V power transistors, freewheeling diodes and gate drivers in a single GaN power IC. The semiconductor material gallium nitride was deposited on low-cost silicon substrate (GaN-on-Si), making the chip technology suitable for cost-efficient mass production — and therefore very attractive to industry.

The high integration density of the GaN power ICs of Fraunhofer IAF not only enables a higher switching frequency and therefore a higher performance density than comparable circuits, but also higher reliability and compactness due to the integrated sensor technology. With their GaN power ICs in a half-bridge circuit, the researchers have already achieved DC-DC efficiencies of over 98.8% at 350 V, and have demonstrated a high switching frequency of 40 MHz in continuous operation at 250 V and resonant operation.

X-ray image of the half-bridge circuit and circuit diagram of the integrated GaN power ICs. Image ©Fraunhofer IAF

Highly integrated half-bridge circuits via PCB embedding

“GaN-on-Si technology allows monolithically integrated circuits for half-bridge converters, but does not solve the wiring problem to external capacitors. However, these critical connections to gate drivers and DC link capacitors are essential for clean and efficient switching behaviour,” said Fraunhofer IAF researcher Stefan Mönch.

“With our goal of a perfect voltage converter in mind, we were looking to find the optimal highly integrated packaging technology for our GaN power ICs.” For this purpose, Mönch and his team processed their ICs with a thick copper electroplating on both sides, making them suitable for PCB embedding.

This adaptation of the metallisation made it possible for the company Würth Elektronik CBT to build chips using ET Microvia Embedding Technology. Together with the project partners Bosch and the University of Stuttgart, the researchers were able to design a PCB package that is only 12 mm wide and 0.4 mm flat. It integrates two monolithic GaN power ICs as a half-bridge and provides the critical decupling capacities for the gate driver and DC link capacitors already on the package. The embedding technology eliminates the need for bonding wires, which also minimises parasitic inductances. The critical connections between GaN IC and capacitors are thus already optimised and no longer need to be elaborately designed by users. The result is a user-friendly solution that provides all critical components of a voltage converter already optimised in one package.

Top image caption: PCB-embedded GaN-on-Si half-bridge circuit including gate and DC link capacitors. Image ©Fraunhofer IAF

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