Hot Swap Controllers：Semiconductors That Provide In-circuit Protection and Control Functions

Hot-swapping is defined as the insertion or removal of a board in a backplane running with power. Hot-plugging technology has been widely used in telecom servers, USB interfaces, firewire (firewire), and CompactPCI. This technique allows a failed board to be replaced while maintaining the voltage of the system backplane and ensuring that other functioning boards in the system remain operational. The biggest risk when hot swapping on a working backplane is that the capacitors on the board create a low-impedance path to the power supply, which can trigger large inrush currents. The inrush current can damage the capacitors, wires, and connectors on the board. In addition, the system voltage may drop below the system reset threshold due to the inrush current, causing other boards connected to the backplane to reset for no apparent reason.

Functional Features of Hot Swap Controllers

The hot plug controller limits the inrush current by controlling an external FET (see Figure 1). In addition, this controller can limit the current in the event of an output short to ground or a large load transient. Designers of FETs usually assume that as long as the FET can withstand the DC current load and the maximum input voltage, this is sufficient. However, if the controller fails and it is the only device that can control the current, then this type of controller will not ensure that the FET is within safe operating limits under any operating conditions. This article will compare two types of controllers, one with current-limited control capability only, and a hot-plug controller that can have both power and current-limited control capability, such as National Semiconductor's LM5069.

Figure 1 LM5069 hot-swap controller

Working principle of Hot Swap Controllers

Hot-swappable functionality is very important in power supply design. In applications with fault-tolerant power supply architectures, hot-swapping is required to meet the zero downtime requirement. During the hot-plugging process, the hot-plugging function should avoid significant fluctuations in voltage and current.

Hot-swapping was first introduced in the server sector to improve the ease of use of servers. In the computers we normally use, there is usually a USB interface, which enables hot-plugging. Without hot-swapping, even if the disk is damaged without causing data loss, the user still needs to temporarily shut down the system so that the hard disk can be replaced.

With hot-swappable technology, you can simply turn on the connection switch or turn the handle to remove the hard drive and the system will still be able to run normally without interruption.

According to the definition of hot swapping, hot swapping should include both power hot swapping and signal hot swapping.

To achieve hot swapping of power supplies means that the transient inrush current on the power bus is controlled to a relatively low level. When replaceable power modules are added to the power supply hot-swap design, the transient inrush current on the power supply bus is limited to a lower level, while not causing the entire system voltage to drop, avoiding the hazards of the hot swap process to the power supply system, thus realizing the purpose of power supply hot swap.

As can be seen, the power supply hot swap function is mainly achieved by current limiting. There are two main methods to achieve this: one is the PTC resistor (positive temperature coefficient thermistor) current limiting, the PTC resistor relies on its own current heat to change the impedance, thereby reducing the magnitude of the current, its disadvantage is the slow response time, and long time use will affect the service life; the other is the MOS tube on and off method, this method is fast response time, long service life.

The purpose of hot-swapping is to properly isolate the signal lines from the system processor and to terminate all communication before the module is powered down. When power modules are not designed to be hot-swapped, transient voltages are generated on the signal lines, which can cause abnormal system communications. When a signal hot-plug design is incorporated, the transient voltage is limited to a reasonable level and the system processor is notified in advance to terminate the communication task, enabling the signal to be hot-plugged. Currently, the signal hot swap function is mainly implemented by connecting a buffer in series with the signal line.

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