1 Evolution of Hot-Swap Architectures

As documented in [1], data center power architectures are transitioning from traditional power shelves inside the IT rack to power sidecars to meet the power demands of AI. Hot-swap technology—the ability to replace or add hardware components without shutting down a system—has evolved from basic, specialized peripheral support into a critical, high-performance feature for modern data centers and computing. The evolution of the technology spans from simple connector standards to advanced power management, driven by the need for maximum uptime and efficiency.

Early Era (Pre-USB)

Initially, hot swapping was limited to high-availability systems such as mainframe computers, where specialized, expensive hardware enabled the removal of modules without shutdown. The small computer system interface introduced hot swapping for peripherals, although it was not as reliable as today's standards.

The USB Revolution

The introduction of USB normalized hot swapping for consumer electronics, allowing the safe addition and removal of keyboards, mice, printers, cameras, and flash drives while systems were active. The evolution moved from simply powering off a slot (passive) to using active MOSFET controllers that safely manage power-on sequences, protecting both the component and the backplane.

Data Center Growth (12V Era)

As data center downtime became too expensive, hot-swapping functionality became essential to achieve high availability. Hot-swap controllers helped manage inrush current and protected the main bus during live insertion of server blades on a 12V backplane.

The first generation of hot-swap controllers, shown in Figure 1-1, used a simple power-limiting architecture to make sure that the MOSFET stayed within the safe operating area (SOA) during start-up and fault recovery. The controller consisted of a current-sense amplifier that monitored the voltage across the sense resistor to obtain current information, and a voltage-sense circuit to measure the voltage across the FET.

Multiplying the output of the current-sense amplifier and voltage sense circuit gives the power loss in the FET, which must be limited to safeguard the hot-swap FET while managing inrush currents or fault currents in the system.

The second generation of hot-swap controllers, shown in Figure 1-1, offered more sophisticated features such as overtemperature protection, telemetry to use for system diagnostics, and data traffic management between trays in a rack sever. But as the power level of the rack tray increases, the output capacitance also increases; this makes designing with a power limited-based controller impractical given severe SOA stress during startup, which commercial FETs cannot handle. An alternate approach [2] is to limit inrush current with the output voltage slew rate (dv/dt) control circuit shown in Figure 1-1, which reduces the power dissipation during start-up and spreads the dissipation over a longer duration such that the FETs can safely handle the power.

To support heavy data traffic, data center operators adopted faster and more power-hungry CPUs and memories to increase data processing capability. This pushed the power requirements as high as 4kW per tray in the same form factor, thus calling for a high-density protection solution with integrated FETs (also called eFuses), as shown in Figure 1-3. TI's 12V high-current eFuses offer differentiated features [3] such as adjustable overcurrent blanking to support transient demand from the downstream computing load, black-box fault recording, active synchronization, and load sharing during start-up and steady state for unlimited scalability—all in a 4.5mm-by-5mm footprint while supporting 60A of root-mean-square (RMS) current.

Cloud Computing (48V Era)

With the growing demands of cloud computing applications, enterprise servers have become power-intensive, as these servers simultaneously process a large amount of data and storage. The steady-state power rating of each server motherboard is now 5kW to 6kW, which makes power distribution on a 12V rail ineffective. Moving from a 12V to 48V server architecture reduces current by 75% for the same power. This reduction drops resistive power losses (I²R) by a factor of 16, shrinks copper bus-bar and cable requirements by up to 90%, and significantly lowers rack heat dissipation.

The graphics processing units (GPUs) form the predominant load in a 48V server system, where the load amplitude, slew rate and frequency of transient loads are three to four times higher when compared to general servers. This requires a few circuit enhancements, as discussed in [4], when using TI's first-generation LM5066I hot-swap controller to support high-power 48V designs.

TI's next-generation hot-swap controller, the LM5066H2, offers a dual gate-drive architecture (Figure 1-4) that enables a single SOA FET to handle power stresses during start-up and fault conditions. Combined with multiple low drain-to-source on-resistance (R_DS(ON)) FETs for normal load-current operation, the LM5066H2 helps reduce total solution size by 40% for >4kW designs.

The industry's first 48V eFuse, TI's TPS1685, enables enhanced FET protection and ease of scalability to realize high-density high-power designs. Figure 1-5 illustrates the circuit configuration with 48V eFuses.

AI Computing (±400V_DC or 800V_DC Era)

The surge in AI processing power has driven a transition from 12V to 48V architectures in server racks to reduce power losses and improve efficiency. AI-based IT racks use hundreds of processors to rapidly process the volume of computations necessary for AI to function best, thus demanding rack power in the range of 100kW to 1MW in the near future [5]. Distributing this amount of power on a 48V rail results in currents exceeding 20kA, requiring heavy and expensive bus bars that are impractical to implement and necessitating a higher ±400V_DC or 800V_DC bus voltage for power distribution in AI IT server racks. The low-voltage DC architecture requires advanced hot-swap controllers to handle higher-voltage inrush currents and provide accurate telemetry, while taking appropriate safety and insulation measures.

Leading hyperscalers and cloud service providers are currently assessing several facility-level architectures such as ±400V_DC (two wire), ±400V_DC (three wire), and 800V_DC. These architectures directly influence hot-swap protection requirements.

Figure 1-6, Figure 1-7, Figure 1-8, and Figure 1-9 show the various architectures [6], along with the IT rack-side hot-swap protection sockets.