Hot-swap means the removal and replacement of an electronic device or module without powering down or shutting down the system.

Hot-swap is necessary in most critical equipment which are supposed to be fault tolerant and expected to work 24/7. Amongst the common hot swap modules are the power supply units, hard disks, circuit boards and UPS batteries. When a fault occurs in one of the modules such as a power supply unit, the redundant module takes over automatically without interrupting the operations.

The bad unit can then be removed and replaced with a working one or returned after repair, all without interfering with the supply to the load.

Hot-swap controllers

The hot swap controllers are integrated circuits that manage how the electronic cards and boards inserted into live system backplane. The purpose of the controller is to control the inrush current, and isolate the faults.

As the hot swap board is plugged into the system, the controller detects the power, and gently ramps up the board’s capacitors to avoid, sparking at the connectors, system resets, backplane supply glitches, and other associated malfunctions that may be caused by the insertion.

Power management controllers determine when the redundant power unit should start operating. By monitoring the power in each of the units, a drop below the present value will signify a fault and automatically switch the system to the working power unit without interfering with the load.

Design Considerations

Hot-swapping requires some design consideration in terms of mechanical design, connectors, safety, protection, eases of replacing and so on.

Proper mechanical design ensures safety and that the hot swappable components. Guides, holes, notches and pins may be used to guide and ensure proper insertion. The mechanical handles, levers and engagement latches makes it easy to remove, insert and hold the unit firmly.

A typical hot-swap method uses special connectors that have staggered pins to allow some of the pins to be connected before the others and prevent damages. In the staggered-pin design, the longer pins are used for grounding the circuitry first before anything else. The other pins might be the same length but variations may occur in the three pin lengths where the long ones are for the grounding, the middle length for data and the shortest ones for the power. These pins make contacts with the corresponding ones in rapid succession.

In some designs the pins may be of the same length but the angling and mechanical design of the connectors ensures that they do not make contact at the same time.

Reasons for hot-swapping

Hot swapping is applicable to servers, communication equipment, medical and other equipment used for critical applications where shutting down a system cannot be allowed or would cause undesired effects. The swapping is usually done when replacing faulty units, upgrading or repairing equipment.