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On your marks

ORing systems keep redundant power supplies ready to spring into action.

High availability systems are becoming more widely used. Obvious examples include server systems and other high end computer installations. But there are also many applications in the telecommunications and networking sectors.
Providing and protecting the power supply to these systems is an important part of the design process. One of the traditional ways of accomplishing this is ORing – preserving the redundant bus in the event of a power source fault.
The standard approach to ORing is to use diodes. It’s a simple solution, but it does have a number of drawbacks. Apart from being bulky, the technique is not very efficient, which means a lot of heat can be generated. This in turn brings thermal management – heatsinks or fans – on to the design agenda. But diodes are a simple solution to another problem: reverse current.
Another approach is so called active ORing. In this method, a mosfet and controller are applied, bringing a huge jump in efficiency and a consequent reduction in the need for thermal management. Along with greater efficiency, active ORing solutions generally come in high density surface mount formats. However, the mosfet controller must have a rapid response if there is a fault.

Graham Pitcher

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