FAQ on Hot Swap: from “don’t dare do it” to “it’s now OK to do” part 3

Hot-swapping is now a standard feature for many plug-in boards; implementing it requires understanding power-management circuits and components.

There is always more that can be added to ensure proper and reliable performance under arduous and mission-critical conditions.

Q: What further can be done to ensure safe performance and no damage to the board being swapped?
A: Many designers add an electric fuse (e-fuse or e-Fuse) function in parallel with the hot-swap controller in a hybrid arrangement, as shown in Figure 1 (ICs that combine both are available as well). The eFuse will handle stressful events by leveraging the integrated overtemperature protection function in the eFuse.

Q: What are the benefits of adding the fuse?
A: This arrangement ensures multiple things:

• The hot-swap MOSFETs turn on only after the eFuse charges the large output capacitor close to the input voltage. The MOSFETs start up with almost zero voltage across them, eliminating power stress during startup.
• The downstream load is enabled only after the hot swap MOSFETs are fully enhanced so that the FETs can offer a low impedance path (compared to the eFuse) and share the majority of the load current.
• The eFuse endures power stress during all fault conditions, and the hot-swap FETs are not subjected to stress in any condition.

Q: Is the operational sequence of this arrangement complicated?
A: Yes, it is, as seen in the state diagram of the hybrid solution of Figure 2.

Q: Is all this added complexity worth it?
A: Yes, it is, in most cases. The major advantage of a hybrid hot-swap approach is that the hot-swap MOSFET SOA is no longer critical. As a consequence, designers can select the lowest drain-to-source on-resistance (R_DS(ON)) MOSFETs, which are generally cheaper and significantly reduce the number of FETs needed if multiple devices are being used in parallel to handle the load.

Q: Do designers have to design their own hot-swap controller?
A: Absolutely not. There are many ICs available from leading analog/power IC vendors that provide many or all of the functions needed. The user still must size some external components to the specifics of the application.

Q: What else can an advanced hot-swap controller IC do?
A: Some ICs also incorporate the widely used I²C and PMBus interfaces. For example, in addition to control and protection, the Texas Instruments LM5066I supplies real-time power, voltage, current,  temperature, and fault data to the system management host through the I²C/PMBus interface, seen in Figure 3. Using the PMBus-compliant command structure makes it easy to program the device. Precision telemetry enables intelligent power management functions such as efficiency optimization and early fault detection.

The hot-swap controller is a vital piece of today’s modern, never-stop systems. Its design requires careful consideration and understanding of operating conditions and realities. ICs are available that fulfill the role, but designers still need to work through some important details.

References

Understanding Hot Swap: Example of Hot-Swap Circuit Design Process, Analog Devices
ADM1273 High Voltage Positive Hot-Swap Controller and Digital Power Monitor with PMBus, Analog Devices
Application Report SLVA673A, Robust Hot Swap Design, Texas Instruments
Protect against high-current faults using hybrid hot-swap architecture, Texas Instruments
LM5066I 10-V to 80-V hot swap controller with improved current, voltage and power monitoring accuracy, Texas Instruments
TPS1663 eFuse with output power limiting, Texas Instruments
What is Hot Swapping?, Geeks for Geeks

Related EE World content

Enabling a do-it-yourself hot-swap circuit design using a hybrid architecture
Fuses, eFuses, thermistors, and fusible resistors – which and when?
Why use an e-fuse? Part 1
e-Fuses, Part 2: Building or buying an e-Fuse