Circuit Protection for Hot-Swappable Devices

The most well-known hot-swappable devices are hard drives and thumb drives. But outside of these products, there is a whole host of specialized systems that allow hot-swapping with modules, daughter cards, and much more. Hot swapping refers to changing out modules or portions of a product while the product is still operational, and it is a very useful feature for high reliability electronic products in a diverse range of areas.

When a hot-swappable module is inserted into a product and begins to receive power, some circuit protection is needed to ensure both sides of the product can withstand the power surge or inrush into the module. Some basic circuit protection designs can help ensure modules can withstand inrush on their input.

How to Protect Hot-Swap Modules

Hot-swapping refers to the insertion of a module or daughterboard into a product while the product is still receiving and outputting power. The power could be pass-through, meaning provided by the main supply, or it could be generated directly on the main connected board. The connection between two boards could also expose signals between the two components that can be used by either module for communication or configuration.

When a hot-swappable module is connected to a mainboard, it very quickly receives power as the two boards are connected and it will begin to turn on. During the connection approach and subsequent power-on stage, the daughterboard module could experience several problems related to receiving power:

• ESD between exposed leads on mating connectors

• Inrush current surge on the receiving daughterboard

• Overshoot in the daughterboard input voltage during power-on

Just like other situations where an expected surge or ESD instance is expected, hot-swappable boards or systems require some basic circuit protection that will ensure protection against these power issues.

ESD Protection

Due to voltage difference between leads on mating connectors, there could be electrostatic discharge that creates a current pulse into the daughterboard module. Therefore, some ESD protection should be used to prevent damage to components. There are several components that can be used for ESD protection, depending on the level of voltage/current protection required.

A simple but effective approach is to place bidirectional TVS diodes on the power pins on each side of the mating interface. This will dampen any current pulse into either side of the mating interface and prevent it from damaging any of the other components in the system.

Bidirectional TVS diodes can provide protection for these exposed I2C lines on a hot-swap mating connector.

Damped Turn-on

When a connection is created during hot swapping, the sudden change in voltage on the daughterboard load creates a transient spike and often creates overvoltage on the input. The overvoltage then oscillates and eventually damps until the daughterboard receives the nominal DC voltage.This overvoltage could be high enough to interfere with or damage some components, but it may not be large enough to trigger stronger circuit protection like MOVs or TVS diodes.

Implementing a pseudo soft start circuit or components can help with slowly ramp up the input voltage into the connected daughterboard. This would involve placing:

• Large capacitance or an RC circuit to ensure there is a damped voltage response

• A power sequencer or MOSFET triggered by low voltage, which then allows high voltage to pass to the daughterboard module

• For an inductive load, add a power resistor in series to slow down the turn-on time

• Use a MOSFET-based soft-start circuit with an RC circuit that slowly ramps the input voltage

All of these are simple circuits that can be placed on the output side (daughterboard module side) of the system.

Surge Suppression

Some circuits are designed to trigger when a current pulse enters system and thus deliver the current pulse to a ground net. For example, surge protection or triggering components are available as integrated circuits; these ICs can take input power and trigger a MOSFET to suppress/divert current pulses or voltage surges. In some cases, the protecting FET array is internal to the surge suppressor IC; an example is found below.

In some cases, these circuits are built with FET arrays that are external to the IC, which will enable very high current protection and overvoltage protection. These circuits provide pulse/current protection generally up to high values and are very useful for systems that require these levels of protection in general.

No matter what protection circuits you want to put onto hot-swappable modules, you can place and route these circuits with the best PCB design features in OrCAD from Cadence. If you’re ready to take even more control over net logic and board layout, you can graduate to Allegro PCB Designer for a more advanced toolset and additional simulation options for systems analysis. Only Cadence offers a comprehensive set of circuit, IC, and PCB design tools for any application and any level of complexity.

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