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:
- Long ground pins make contact; basic electrical safety and ESD protection becomes available.
- Long (or medium) pre-charge pins make contact; decoupling capacitors start to charge up.
- Real time delay of tens of milliseconds.
- Short power/signal pins make contact.
- Connector becomes fully seated; power-on reset signal asserted within component
- Soft-start circuit starts to apply power to the component.
- Real time delay of tens of milliseconds.
- Soft-start circuit completes sequence; power-on reset circuit deasserted
- Component begins normal operation.
Hot-swap power circuits can now be purchased commercially in specially designed ASICs called hot-swap power managers (HSPMs).
Read more about this topic: Hot Swapping
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