Hot Swapping - Power Electronics

Power Electronics

The DC power supplies to a hot-swap component are usually pre-charged by dedicated long pins that make contact before the main power pins. These pre-charge pins are protected by a circuit that limits the inrush current to an acceptable value that cannot damage the pins nor disturb the supply voltage to adjacent slots. The pre-charge circuit might be a simple series resistor, a negative temperature coefficient (NTC) resistor, or a current-limiter circuit. Further protection can be provided by a "soft-start" circuit that provides a managed ramp-up of the internal DC supply voltages within the component.

A typical sequence for a hot-swap component being plugged into a slot could be as follows:

1. Long ground pins make contact; basic electrical safety and ESD protection becomes available.
2. Long (or medium) pre-charge pins make contact; decoupling capacitors start to charge up.
3. Real time delay of tens of milliseconds.
4. Short power/signal pins make contact.
5. Connector becomes fully seated; power-on reset signal asserted within component
6. Soft-start circuit starts to apply power to the component.
7. Real time delay of tens of milliseconds.
8. Soft-start circuit completes sequence; power-on reset circuit deasserted
9. Component begins normal operation.

Hot-swap power circuits can now be purchased commercially in specially designed ASICs called hot-swap power managers (HSPMs).