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Determining the cost savings GaN offers power supplies

Determining the cost savings GaN offers power supplies

mkasper
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Wide bandgap (WBG) technologies such as gallium nitride (GaN) transistors have, thanks to their superlative figure-of-merit (FoM) values, been shown to be superior in performance to the silicon (Si) MOSFETs currently in use in switching power supplies. However, these benefits come at a price due to GaN’s higher price. While a GaN-based design may reduce losses by a fifth over a Si-based solution that already attains an efficiency of 97.6%, can the higher capital expenditure (CAPEX) be justified?

The method of multi-objective Pareto analysis can be used to find answers. By systematically considering multiple degrees of freedom in a switched power supply design, the optimal solution for efficiency, power density, and total cost of ownership (TCO) can be determined. This allows a scientific basis for decision making that considers differing topologies, configurations, electricity costs, and power usage effectiveness (PUE), rather than ‘gut feel’ and rough calculations in a spreadsheet. It can also include control losses, the cooling system, and the costs for casing, components, and PCB manufacturing, as was the case in the example discussed here.

The design chosen for evaluation included a totem-pole power factor correction (PFC) stage together with an inductor-inductor-capacitor (LLC) topology switching converter. The GaN-based design used continuous conduction mode (CCM) with a fixed switching frequency for PFC modulation. The silicon-based solution used triangular current mode (TCM) for soft switching. To simulate a telecom power supply's typical usage conditions, the load was defined at 50% for a 230 Vin and 54 Vout.

Pareto analysis

The analysis took a range of factors into account from the number of totem-pole HF legs, EMI stages, and switching frequency (AC/DC and DC/DC stages), as well as the value of passive components and design of the inductors, transformer, and capacitors. The result generated is termed a ‘Pareto surface’ that describes optimal designs according to the selected design goals.
The optimization results show that the circuits using GaN consistently deliver higher efficiency for the same power density or a higher power density for the same efficiency (Figure 1).

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Figure 1: Pareto-optimal highest-efficient GaN and Si designs compared.

The GaN approaches show a clear 0.4% to 0.8% efficiency improvement across the board, while a power density improvement of over 15 W/inch3 is also attainable at particular efficiencies. However, how this translates into TCO remains unanswered.

Determining designs with optimal TCO

To consider the financial implications, the TCO is plotted against efficiency. To maintain the legibility of the results, the analysis is restricted to a single power density. For example, by focusing on designs of 80 W/inch3, and normalizing to the silicon design with the lowest initial costs (not necessarily the lowest TCO), a 100% line can be generated. From here, the Pareto results for the GaN and silicon designs can be considered side-by-side. In this particular example, the GaN-based solution that offers an efficiency of 97.35% also delivers a TCO improvement of 13% over the best silicon-based alternative (Figure 2).

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Figure 2: The most efficient 80 W/inch3 density GaN-based design can deliver a 13% TCO improvement versus the most efficient Si-based solution.

When reviewed in more detail, it turns out that the reduction in heat generated by the higher-efficiency design leads to lower electricity costs. It also led to a decrease in cost for the cooling system implementation. This outweighed the higher initial costs of the GaN devices over the silicon-based designs over the application's lifetime.

Making the appropriate justification

Like with any new technology, it can be challenging to fully understand the impact technical improvements of a single component will have on the business case. This is especially difficult in the case of power supplies where so much has already been achieved over the years, and efficiencies are already above a very respectable 95%. The Pareto analysis approach shows that significant TCO savings can be provided to customers when using GaN, even if a higher Cap-Ex is required. It should also not be forgotten that, as the market shifts towards higher densities and volumes for GaN increase, prices come down, and the GaN advantage will grow further.

To learn more about how Infineon’s GaN portfolio and solutions can help your power design, go to www.infineon.com/gan.

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