BYTE.com > Features > 2007
Fundamentals of Power System ORing
By Martin Patoka
March 26, 2007
(Fundamentals of Power System ORing
: Page 1 of 1 )
Fundamentals of Power System ORing
By Martin Patoka,
Many modern devices and systems require power architectures with redundancy, summation of power for capacity, or selection of multiple sources. In this article, these functions generically are referred to as "ORing." Systems using ORing are ubiquitous and varied in size and complexity. This includes systems such as portable devices, blade servers, and telecom switches.
Issues of power combination, selection, hot-plug and bus protection rise once the application requires more than a single power source. Paralleling sources without protection results in interrupted operation due to failures, shorts, hot-plug or removal of one device. While these functions are similar to typical hot swap functions of inrush and fault protection, they are distinctly different in their position and operation. Originally, semiconductor diodes performed this ORing function, and still are the best solution in some applications. MOSFETs became the basis for higher performance solutions as they evolved.
There are a number of situations where power must be combined between sources to power the load. High-power systems like blade servers, or shelf-based telecom systems, may have multiple power supplies combined for flexibility, redundancy or capacity in an N+1 configuration. Typically, these supplies are replaceable while the system is operating (hot-pluggable), and are circuit cards. Another example is an appliance that might run from an AC adapter, USB or battery power.
ORing Architectures
The power combining topology is demonstrated in Figure 1. The diode symbol may be implemented as a semiconductor diode, or a higher-performance functional block. Physically, ORing could be located in several places. ORing could be located within the power supply, if grouped to the left of line B. The ORing also could be located on a backplane or midplane, if located between lines A and B. Finally, the ORing could be on the load, if located to the right of line A.
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