Isolated Full Bridge Converters
Editor's note: Previously EDN had published Chapter one of "GaN Transistors for Efficient Power Conversion", published by Power Conversion Publications and Chapter five "Buck converters". Now let's look at Chapter six regarding "Isolated full bridge converters". This chapter examines the various aspects of these type of modular devices and the added benefits of using eGaN FET's in the design architecture.
Isolated Full Bridge Converters
Isolated full bridge converters are widely used in servers and telecommunication systems and are available in a variety of standard sizes, input and output voltage ranges. Their modularity, power density, reliability and versatility has simplified and to some extent commoditized the isolated power supply market.
A straightforward topology that we can use to explore the capabilities of eGaN FETs in isolated DC-DC converters is a full bridge primary side and a synchronous rectifier secondary side. Two test vehicles were chosen; a fully regulated eighth brick format with a nominal 48 VIN and 12 VOUT, and a PoE-PSE half brick format with a nominal 48 VIN and 53 VOUT. In each case it is shown that eGaN FETs allow the user to significantly improve efficiency, and therefore power output and cost, while maintaining the required size constraint.
As these types of converters are of a defined size, designers are motivated to come up with innovative ideas to increase the converters’ output power and power density. For example, within the spectrum of eighth brick converters there are numerous input and output voltage configurations, topologies and output range tolerances (e.g., regulated, semi-regulated, un-regulated), and they all have a very similar maximum power loss at full power; between 12-14 W. This is a physical limit based on the fixed volume of the converter and the common method of heat extraction. Thus, for an eighth brick converter that is 90% efficient (ƞ= 0.9) at full load, the maximum output power, assuming 14 W loss, will be:
If the efficiency can be improved by just 2%, the output power is increased to 160 W. That is 28% more output power.
As will be shown in chapter 7, it is possible to reduce the power loss in the magnetic components by increasing the operating frequency. However, this is not normally done because the increase in the silicon MOSFET switching losses outweighs the potential improvement. For that reason, the operating frequency is typically reduced to the point where the magnetic structure size is maximized within the overall brick size constraints.
Comparing Isolated Brick Converters
Figure 6.1: Comparison of eighth brick and quarter brick efficiencies [1, 2].
Even when limiting our comparison to regulated 12 V output, eighth brick converters, there are still a significant number of variations between commercial designs. Over time, advances in devices, materials, construction and other innovations have enabled greater and greater output power. Even so, the efficiency achieved in a specific brick converter can easily be improved simply by allowing the converter to increase in size. This increase in efficiency with size is shown in figure 6.1 by comparing eighth brick and quarter brick efficiency for the same generation products.
The eGaN FET-based eighth brick converter developed for this study is not necessarily an
optimal solution. The design goal was to deliberately push the operating frequency much higher than current commercial systems to show that eGaN devices could enable someone skilled in power supply design can take advantage of the superior switching characteristics to develop next-generation products.
A Fully Regulated eGaN® FET-Based Eighth Brick Converter
For the 48 V to 12 V eGaN FET-based eighth brick converter, a phase-shifted full bridge (PSFB) converter with a full bridge synchronous rectifier (FBSR) topology was chosen as shown in figure 6.2 (A more complete schematic is shown in the figure 6.3). The objective was to show that, due to their relatively small device size, a significant number of eGaN FETs can be used within the restrictive eighth brick size limitations.
The choice of transformer turns ratio (6:3) means that, at 75 VIN, the secondary side winding voltage is 38 V, thus too high to use 40 V devices, and therefore 100 V devices were used on both the primary and secondary sides. For optimal performance, the Texas Instruments LM5113 half bridge driver, designed for eGaN FETs, was chosen. The actual prototype is shown in figure 6.4 and is compared side by side to a similar  silicon-based converter. The significant amount of unfilled PCB area (green space) could certainly be further exploited to improve efficiency.
Figure 6.2: 180 W eighth brick fully regulated, phase shifted full bridge (PSFB) topology, with full bridge synchronous rectification (FBSR) using eGaN FETs.
Figure 6.3: eGaN FET-based 180 W 1/8th brick schematic.
Figure 6.4: Comparison between the eGaN FET-based eighth brick converter (lower image) and
the silicon based converter (upper image)  showing top and bottom views (scale in inches).