Furnish Full-On Uptime in High Availability Systems with Hot-Swap Controllers

Key Takeaways

• Critical applications and services that rely on high availability systems are provided full-on uptime, even under maintenance and failures, with the help of hot-swap controllers.

• Additional features of commercially available hot-swap controllers include timers, level shifting, surge suppression, current limiting, dual response to overcurrent scenarios, variable current regulation, load current monitoring, integrated charge pumping capability, output voltage slew rate control, temperature sensing, and inrush current control. 

• Hot-swap controllers work as the electronic switchgear in high availability systems.

It is hard to imagine spending a day without using some form of technology.

In today's society, we are so accustomed to connectivity that it is hard to imagine losing it, even for a moment. Many critical enterprises rely heavily on this connectivity, and losing it would be catastrophic for them, and for those who rely on these enterprises’ services. 

High availability systems rely mostly on data centers, servers, and cloud computers. The critical applications and services that are built on these high availability systems are provided full-on uptime, even under maintenance and failures, with the help of hot-swap controllers. They are analogous to electrical switchgear, as they protect the system from short circuits and overcurrents.  

Hot-Swap Controllers in High Availability Systems

Hot-swap controllers are circuits that are employed in PCBs and plug-in circuits to increase their reliability and useful life. In high availability infrastructure, the maintenance, upgrades, updates, and power quality issues in the system should not interrupt its uptime. The hot-swap controller makes the replacement and repair possible when the system is live and running. This process of hot-swapping also prevents the circuit failures from short-circuits, undervoltage, overvoltage, and overcurrent.

Generally, hot-swap controllers are IC-based circuit protection systems. They monitor the variations in the power supply and latch the respective switches appropriately to protect the critical circuits from damage and unsafe operation. Some of the additional features of commercially available hot-swap controllers include timers, level shifting, surge suppression, current limiting, dual response to overcurrent scenarios, variable current regulation, load current monitoring, integrated charge pumping capability, output voltage slew rate control, temperature sensing, and inrush current control. Hot-swap controllers supervise the safe functioning of circuits under abrupt power up and down conditions. 

Hot-Swap Controller Operations

Depending on the power supply rail of the sensitive circuit, the selection of hot-swap controller topology changes. For example, in telecommunication and data communication systems, the voltage supply rating is -48 V and +12 V, respectively. You cannot use the same hot-swap controller for ensuring reliability and power management in both of these circuits. The current trend is to use a low-side hot-swap controller in -48V systems and a high-side hot-swap controller for +12V systems. In both of these topologies, an N-channel MOSFET is connected externally to the hot-swap controller IC. The hot-swap controller is connected between the input power supply rail and the critical circuit (load) as shown in the figure above. 

Hot-swap controllers can be used in a system to suppress surge, prevent undervoltage, or to limit the inrush current. Together, the internal and external circuitry supports the purpose of the hot-swap controller IC. For each power quality issue, detection requires pairs of resistors, capacitors, MOSFET switches, and a gate-drive circuit. 

Inrush/Overcurrent Protection

Typically, to check the inrush current or overcurrent to the circuit, a resistor is connected between the input supply and load. The internal current sense amplifier of the hot-swap controller monitors the current and the voltage across the resistor, and is amplified by its gain. The amplified voltage is compared with a reference voltage and if it crosses the limit, the overcurrent event is alerted. The resistance value, current sense amplifier gain, and reference voltage are the parameters that influence the current limit threshold in hot-swap controllers. The drive circuit of the pass MOSFET is controlled by the hot-swap controller. It is the hot-swap controller that decides whether the MOSFET switch needs to be switched on or off. The controlling variable of the MOSFET drive circuit is the current through the resistor. 

Output Undervoltage Protection

The undervoltage protection provided by the hot-swap controller is completely based on the power good threshold input to the IC.  The power good threshold input to the IC is from a voltage divider circuit. A voltage divider circuit made of two resistors is used to supply voltage to the power good threshold input pin. The power-on reset output pin gets activated and deactivated by evaluating the power good threshold input with a reference voltage. When the power good threshold input is less than the reference, the power-on reset pin is low and is in deactivated mode. When the threshold input goes beyond the reference and its tolerance band, a capacitor connected to the ground is charged to the threshold reference voltage, and the power-on reset output pin is activated for one timing cycle—alarming an undervoltage condition. A filter is used on the threshold input pin to prevent the false activation of the power-on reset output pin. 

Input Overvoltage Protection

The input supply transients are not acceptable in critical and sensitive circuits. The circuits need to be monitored and protected from overvoltage events. The overvoltage timer and the crowbar circuit in the hot-swap controller IC get triggered for an overvoltage pin input that is greater than the threshold voltage value. In this power quality problem, detection also involves a capacitor. Whenever the input to the overvoltage pin crosses threshold voltage, the capacitor in the crowbar timer circuit starts charging. The timer is activated at a particular capacitor voltage at which the crowbar circuit current increases. This current toggles the gate drive to the external MOSFET and the switch is turned off. The MOSFET switch operates as a circuit breaker under the hot-swap controller overvoltage protection scheme. The MOSFET switch trips the circuit, so to restart the hot-swap controller operation,  the gate drive logic toggle is required.

If a business relies on high availability systems, a hot-swap controller is necessary to ensure continuity under any circumstance of damage or failure. Power management, prevention of data loss, increased reliability of system operation, and uninterrupted uptime are some of the advantages of a hot-swap controller. If you aspire for zero-downtime in your critical circuits, incorporate hot-swap controllers that work as electronic switchgear to secure the system.