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Get 500W in converter with GaN (Part 1)

Posted: 07 Dec 2015 ?? ?Print Version ?Bookmark and Share

Keywords:data processing? power conversion? GaN? transistors? PCB?

DC-DC "brick" converters are familiar to many engineers, and have wide usage in telecommunications, networking, data centres, and many other applications. This is due in large part to adoption of a common footprint defined by the Distributed-power Open Standards Alliance (DOSA) and generally accepted input/output voltage ranges [1]. These converters provide isolation and voltage step-down, and have become increasingly sophisticated, with features that enable advanced system optimisation and control. They often reside on motherboards where they drive point-of-load converters for processors and memory. Increasing data processing throughput requires more power, more board real estate, or both. Power processing is considered a cost, and information processing a source of profit. Hence, there is continuous pressure to increase power density. However, as the technology has matured, improvements in basic power conversion capability, that is, power density and efficiency, have slowed down to a crawl.

All is not lost, however, as GaN power semiconductors give basic power converter technology a much-needed shot in the arm. GaN transistors already show large improvements over similarly rated silicon devices [2]. This article shows that these improvements are more than good looking datasheets via a design example that demonstrates a complete eight-brick converter design with eGaN FETs and real test results. This converter can deliver more than 500 W using a conventional transformer-isolated, hard-switched, PWM regulated design. It represents a new starting point that can be achieved with GaN C and with room to grow.

This article will appear in two parts. Part 1 covers brick technology, a comparison of eGaN FETs to silicon MOSFETS, a basic overview of the GaN-based eighth-brick design, and experimental results. Part 2 gives a detailed design overview to show how to get the most out of eGaN FETs, along with a number of ways that the design could be improved.

Introduction
At the 100 W to 1 kW level, quarter-brick (Q-brick) and eighth-brick (E-brick) DOSA-compliant converters are commonly used to convert a nominal 48V backplane to a nominal 12V motherboard distribution bus. The main trend has been towards higher power density. Another trend has been an improvement in the regulation of the input bus, which allows a reduced converter input voltage range. This enables further improvement in power density, and in some cases even unregulated converters. For the Q-brick format, this has led to output powers in the 800W range, with output currents approaching 80 A [3].

The E-brick format has not kept pace with the Q-brick. The need for higher power is felt here as well, but the smaller format poses additional challenges. The controls, the gate drives, isolation and spacing requirements, and PCB manufacturing tolerances are nearly the same for both E- and Q-bricks. Since the volume required for this overhead is largely independent of converter size, this places a greater challenge for smaller converters. GaN technology is uniquely suited to address this challenge.

The introduction of commercially available low-voltage GaN FETs dramatically increases the achievable power levels and efficiencies in brick converters. The fourth generation of eGaN FETs have figures of merit (FOMs) up to 14x better than silicon, breaking down a major barrier towards much greater converter power density. This article describes the development of a fully regulated, isolated E-brick converter using eGaN FETs to demonstrate that GaN technology can far exceed the performance of the best silicon MOSFETs [4, 5]. This converter is capable of over 500 W and has a peak efficiency of 96.7%. Full details on the converter, including schematics, bill of materials (BOM) and Gerber files can be found on EPC's website [6]. In this article, we cover the following aspects of this design:
???Existing E-brick technology
???Design goals
???Benefits of 4th-generation eGaN FETs
???Converter design
???Experimental results
???Potential Improvements
???Existing E-brick technology

A commercial example of a high-performance E-brick converter is shown in figure 1. Typical E-brick converters are used as integrated bus converters which convert a nominal input voltage range of 36-75V to an output voltage in the range of approximately 9.6V to 13V. The standard footprint and pinout mean that these converters have been widely adopted for telecom and data centre applications.

Figure 1: An example of a high-performance commercial E-brick converter.

Table 1 shows four examples of state-of-the-art commercially available E-brick converters that use silicon MOSFETs [7, 8, 9, 10]. These examples have the highest power rating presently available, up to 320 W. Each vendor rates power and efficiency under different conditions, so this is an inexact comparison. As one of the methods to achieve the higher power density, the input voltage range has been reduced from the previous generation of E-bricks, and in some cases the output voltage lowered as well.

Table 1: Main specifications for state-of-the-art commercially available E-brick converters.

These converters represent decades of progress and experience with silicon MOSFET-based design, and a great deal of effort is required for relatively small improvements. With the advent of eGaN FETs, it is now possible to leapfrog silicon-based designs.

Design goals
The eGaN FET-based E-brick converter was developed with the following design goals:

???500W output at 12V (42 A output)
???48V to 60V input range (52V nominal)
???Fully regulated
???Isolated
???96% efficient at full load
???DOSA-compliant footprint
???Off-the-shelf parts
The output power was chosen to demonstrate a large step change in output power rather than the incremental changes that are the norm for a long-established technology. The chosen input voltage range is narrow, similar to most of the highest power converters. Requirements include full regulation and isolation, along with a DOSA-compliant footprint. Thermal limitations demand an efficiency > 96% at the full load current. Finally, the converter uses only off-the-shelf components, so that it is clear that the benefits are derived from the use of eGaN FETs.

Benefits of 4th-generation eGaN FETs
eGaN FETs have many benefits over silicon MOSFETs that make them particularly well suited to E-brick applications. These include small size, reduced gate charge, lower parasitic capacitances and inductances, lower gate drive voltage, zero reverse recovery, lower specific RDS(on), and faster switching [11]. The FETs chosen for the E-brick demo board are the 4th generation regular pitch eGaN FETs from EPC, shown in table 2. The table also shows the relaxed pitch FETs, which are targeted towards designs that still require wider tolerance PCB manufacturing processes. These FETs are available in a range of voltages from 30V to 200V. As can be seen in the table, this selection of the available large-area FETs have very low RDS(on) values, a requirement for intermediate bus converters (IBCs).

Table 2: Examples of large-die eGaN FET selection suitable for brick-type converters.


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