Tutorial
Bus Converters
As shown in Fig. 1, a typical Bus Converter delivers an unregulated, stepped down voltage of 9.6 to 14V with a nominal 2000Vdc input-output isolation. This converter is ideal for a loosely regulated 12Vdc Intermediate Bus Architecture to power a variety of downstream non-isolated, point-of-load regulators. These modules are suited for computer servers, enterprise networking equipment and other applications that use a 48V (+10%) input bus.
Cost savings can be achieved in many applications by replacing multiple 48V in, isolated dc-dc converter bricks with low cost, non-isolated POL modules or embedded converters that are fed from the 12V Bus Converter rail. Implementing one central point of isolation eliminates the need for individual isolation at each point of load, allowing reduced costs, greater flexibility and savings on board space.
Bus converters achieve high efficiency by limiting the input range and essentially optimizing for a single input voltage. Bus converters are designed for efficiency. Removing the entire feedback path (reference, error amp, optocoupler, etc.) liberates board area and power. Additional parallel MOSFETs may be added to lower on resistances. MOSFET duty cycles in the power train are set and maintained at 50%, and all components are optimized for the voltages they will actually experience and not the voltages they may experience. Also, for high efficiency most bus converters employ synchronous rectifier outputs.
Bus converter packages come in many sizes, from SIPs and SMTs to quarter-brick, eigth-brick and sixteenth brick modules.