Hot swapping Principle and Application
Hot swapping (hotswap, hotplug, and hotdock) means to insert modules, cards, or connectors to the system without affecting the system operation while the system is conductive. Figure 1 shows the hot swapping process. The system and its power supply are displayed on the left. There is a capacitor at the output end of the power supply, two cards on the right, and the capacitor at the input end of these cards. Before inserting the card into the system, the input capacitor is not charged. when inserting the card into the system, a large instantaneous current will be charged to the input capacitor, such a large instantaneous current may cause abnormal system power supply voltage. Hot swapping aims to control the high instantaneous current at a relatively low and reasonable level. There are several implementation methods. The use of PTC (positive temperature coefficient thermistor) is the simplest method. PTC relies on its own current heating to change the impedance, thus reducing the instantaneous current amplitude. Its disadvantage is that the reaction speed is slow and the service life will be affected after a long period of use. Mos adds some simple method of resistance and capacitance delay lines, which is cost-effective and suitable for low-end applications. The best method is to use a hot swapping chip. Generally, the chip contains a drive MOS design and current detection resistor. In addition to basic hot swapping, it can also provide special functions, such as controlling the current rising rate, power supply management, and status report can improve the system's working status. The hot swapping implementation 2 is shown by concatenating a MOS tube and a current detection resistor between the power supply and the load. The purpose of current resistance detection is to transmit the signal flowing through the mos tube to the control line. the control line then controls the conduction of the mos Tube Based on the current setting and timing circuit. The following uses ucc3915 as an example to describe the output current and voltage in the hot swapping process. Figure 3 shows the output current, output voltage, and real-time capacitor voltage waveform of ucc3915 on the left. You can see that when the output current rises to itrip, the timing capacitor starts charging and the voltage rises, start timing; If the output current exceeds itrip and continues to rise to IMAX (the set maximum value), because the mos tube operates in linear mode, limit the maximum output current to this level without increasing the output current. Therefore, the output current is limited to IMAX. On the other hand, if the voltage of the timing capacitor reaches 1.5 V, the mos tube will be disconnected and the output current will be reduced to 0. After the mos tube is disconnected, the capacitor will be discharged until it drops to 0.5 V. Then the mos tube restarts and the current starts to rise. If the output current is still high, the output current is limited to Imax. After timing, the circuit will cut off the mos tube and the circuit will work according to this principle. There are two voltage and current waveforms in the right figure. One is the voltage and current waveform without hot swapping, and the other is the voltage and current waveform with hot swapping. When the hot swapping function is not added, the instantaneous current amplitude is very large, and the high instantaneous current amplitude causes the system voltage to decrease by about 1 V. When the hot swapping function is added, at this moment, the current is limited to a low level, with little impact on the system voltage, so as to achieve hot swapping. Benefits of adding hot swapping to the system include: 1) remove damaged modules when the system is started, and perform updates or expansion without affecting system operations; 2) as the reliability of hot swapping parts is improved, they can also be used as electrical breaker, and because hot swapping can be automatically restored, many hot swapping chips provide the signal of power supply to the system, so that the system can perform fault analysis, thus reducing the cost. Hot swapping is suitable for high-reliability systems, such as communication power systems and server power systems. It can also be used for power supply of storage equipment, therefore, these devices need to replace or update storage devices when the system is continuously powered. Hot swapping is also suitable for power systems with small sizes but high reliability requirements, including some major specifications, such as PCI, PCIe, USB, 1394, etc., so it is widely used. When selecting hot swapping chips, TI's hot swapping management chip should consider the following: 1) the working voltage range of the hot swapping management chip is 48 V,-48 V, or less than 12 V of the low voltage, and the current limit; 2) protection mode, you can choose Automatic recovery or lock protection mode; 3) timing Circuit for power-off. 4) performance in other working states. For example, high-speed response is required when the load is short-circuited, and peripheral devices are not damaged when the load is increased. When hot swapping starts, the current rising rate can be controlled to reduce noise and impact level; 5) Mos tube or current detection resistance power consumption. TI has launched many hot swapping products, and added some special features to improve the working power of hot swapping. This can make the hot swapping MOS tube work in the security zone, thus improving product reliability and reducing costs. Another feature is DI/dt (current rising rate), which can reduce noise and impact on circuit parts. TI's hot swapping products are mainly divided into two categories: High-Voltage Hot swapping products, for 48 V,-48 V or 24 V applications; Low-Voltage Hot swapping products, for 3 V to 15 V applications. For high-voltage hot swapping products, there are two types: + 48v products and? 48 V product, as shown in figure 4. The 48 V products include tps2490 and 2491. The working voltage of this product ranges from 9 V to 80 V and contains a unique function called fixed power setting .? 48v hot swappable control chip, including tps2390, 2391, 2398, and 99, this series for simple hot swappable applications, operating voltage from? 36? 80 V, which is an encapsulation of 8 feet. The second 48v hot swapping is tps2392 and tps2393. Is it a full function? In addition to all the features of the tps2390 series, the 48v hot swapping product also contains the setting of undervoltage and overvoltage, providing two feet for connector detection.
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