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In the previous column of this series, some of the design considerations for the traditional buck converter were presented. In this installment, more complex hard switching converters used for isolated DC to DC power conversion will be discussed. The isolated DC to DC bus converter is widely used in computing and telecommunication systems as part of the intermediate bus architecture (IBA) approach. It is available in a variety of standard sizes, input and output voltage ranges, and topologies as shown in figure 1. Their modularity, power density, reliability, and versatility has simplified and to some extent commoditized the isolated power supply market. As these “brick” converters are of strictly defined sizes, designers are forever coming up with innovative ideas to increase their output power (and power density). Although these ideas are numerous and varied, they are all related to system efficiency, as the maximum power loss for a given standard size is fixed based on the surface area of the converter and method of heat extraction.
Consider an eighth brick converter as an example – although there are numerous input and output voltage configurations, topologies and output range tolerances (regulated, semiregulated, and unregulated), they all have very similar maximum power loss numbers at full power (i.e. between 12-14 W). This is a physical limit based on the fixed surface area of the converter and the method of heat extraction. Thus, for an eighth brick converter that is 93% efficient at full load, the maximum output power (assuming 14 W loss) will be about 186 W. If the efficiency can be improved by just 1%, the output power would increase to 220 W. That’s an 18% increase in output power!
Hard Switching Intermediate Bus Converters The majority of today’s bus converters use traditional hard switching bridge topologies operating at a low frequency range (150 kHz to 250kHz) to maximize efficiency. At these lower switching frequencies, the isolation transformer and output inductor are very bulky, occupying a large portion of the board area. To improve their power density, the operating frequency must be increased to be able to process more power through the inductor and transformer. As switching frequency increases, however, the losses from MOSFET body diode conduction, reverse recovery, and switching increase significantly, limiting the converters output power capability. Because of this, power density improvements have come from changes in design optimization and topology changes with actual switching frequencies decreasing, rather than increasing.
Eighth Brick Converter with eGaN FETs
Figure 1: Intermediate bus architecture (IBA) showing voltage ranges for bus converters.
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To showcase the improvements that eGaN FETs offer in this design space, a high switching frequency eGaN FET based eighth brick bus converter was constructed. For the eighth brick converter, we chose a full-bridge primary side converter with a full-bridge synchronous rectifier as shown in Figure 2. This demonstrates the ability to use a large number of eGaN FETs and drivers within the limited eighth-brick footprint. The actual converter is shown in Figure 3 and compared side by side to a similar MOSFET-based converter. To the skilled designer, the significant amount of ‘green’ space (unfilled PCB area) could be further exploited to improve efficiency.