In the telephone era, - 48V power supply shall be used to feed the fixed telephone at the office end. It is assumed that the link from the convergence point of the telephone office to the user is filled with water vapor, and the insulation of various terminal points and cables is poor. This is a very common situation, such as being located in some wet pipe lanes or manholes. 48V is enough to electrolyze water. The electrolysis process is oxygen evolution at the positive electrode and hydrogen evolution at the negative electrode. The oxidation reaction of positive metal materials will be quite serious. Due to the existence of grounding, if the negative electrode is grounded, there is no potential between the negative electrode and the earth. If the positive electrode has a + 48V potential, the positive electrode will oxidize rapidly after oxygen evolution. On the contrary, the positive electrode is grounded, there is no potential difference between the positive electrode and the earth, and hydrogen evolution occurs after water electrolysis at the negative electrode. As for the return path of leakage current, the earth or positive cable can be used, depending on the return impedance. In short, the corrosion of positive grounding to metal conductors such as cables is much less than that of negative grounding. It seems reasonable to use negative power system for communication power supply.
By the way, why use 48V voltage? Generally, no matter how high the voltage is, it is not safe enough, the requirements for cable insulation grade are high, and breakdown is easy to occur in humid environment. The voltage is low and the current is large, which is not suitable. In addition, considering the uninterrupted power supply system using lead-acid batteries, 48V is exactly 12 2V battery units. For small capacity systems, only 4 12V batteries are needed. The number of strings is reasonable, and the battery pack is easy to balance. In short, it is the result of comprehensive consideration of all parties.
-In the 48V power supply system, the AC mains power is converted to DC 54.5-55v by switching power supply through low distribution to float charge the battery and output to the equipment for power supply at the same time. Simple and clear.
The voltage of about 54V is mainly determined by the floating charge voltage of lead-acid battery, about 2.2V per grid. Considering the charging and discharging platform of the battery, the actual allowable input voltage range of the equipment is generally wide, at least - 40V ~ - 57V. The general limit is - 36V ~ - 62V. Of course, lithium batteries have also been used in the communication industry in recent years, which is unknown.
For data centers in the IT industry, ac110 or 220 power supply is commonly used, and - 48V power supply equipment can hardly be seen. The main reason is that it equipment evolved from computers and has no history of using DC power supply. In addition, the power density of IT equipment is increasing day by day. It is common that the power density is 200 watts per u or 5000 watts per frame. In recent years, each high-density data center is 9kw, and some equipment is designed with a power capacity of more than 10kW in a chassis of about 15u. At such a density, it is also difficult to realize power wiring and terminals with hundreds of amperes of overcurrent capacity by using 48V power supply voltage.
When using AC power supply, as a reliability requirement, it is always necessary to deploy an uninterruptible power supply system. The problem caused by this is that the energy efficiency is low. The AC mains power is converted by AC-DC to about 300-400v DC, floating charge the battery pack (or maintain the flywheel operation for flywheel UPS), and then DC-AC inverter to the equipment. AC-DC (APFC) / DC-DC conversion is performed again for the equipment power supply to the target voltage. Even if the efficiency of each transformation reaches a high level of 92%, the overall efficiency is less than 85%; In addition, the construction cost of UPS is high; It is difficult to connect to the grid in the AC system at the same time.
In recent years, high-voltage DC power supply is a hot trend in the data center energy industry. Using about 240-270 DC power supply has low construction cost and high energy efficiency, which means that the operation expenditure is reduced. With the popularization of IT equipment supporting HVDC power supply, it should be a development direction.
Bread here means two things:
(1) Why is the voltage - 48V (- 36 ~ - 72V)?
(2) Why is the polarity a negative source (i.e., a positive ground)?
Why is the voltage value 48?
Let's start with the first question. The use of - 48V power supply is caused by historical reasons. The earliest communication network is the telephone network, and the telephone is powered by the Telecommunications Bureau. 48V is selected to increase the distance from the user to the end office as much as possible under the prevailing conditions (36V is the safe voltage, which is more than too unsafe). Later, in order to be compatible with early equipment and reduce cost, the office communication equipment still used - 48V power supply. Through the media, you may also know that there are equipment using - 24V power supply system, which is used by some modern internal equipment for design convenience. Generally, the output voltage of the power supply is 26.8v. Generally, the equipment is required to work normally within ± 20% of the voltage fluctuation range. For - 48V system equipment, the working voltage range is required to be - 38.4v ~ 57.6v, but we generally require the working range to be - 36V ~ - 72V. It is mainly considered that - 48V system equipment should be compatible with - 60V power supply system, which requires - 48 ~ - 72V. In this way, the working voltage range is required to be about - 36V ~ - 72V.
Incidentally, - 48V power supply system is only the communication power supply standard adopted by China and most countries. Not all countries use this standard. For example, Russia will use - 60V power supply system, and some countries use - 24V power supply system. If products are to be sold in these regions, these different standards must be taken into account. The standards of mains power are also different all over the world. For example, 220V mains power system is adopted in China and Europe, and 110V mains power is adopted in the United States and Japan.
Why choose - 48V as the supply voltage instead of + 48V?
Firstly, the positive power supply should be grounded, which can effectively avoid noise and other interference. The selection of positive grounding can minimize the corrosion of grounding devices.
The negative power supply system is adopted, and the positive grounding is only conventional. There was a saying that there are a lot of negative charges in the air. According to electrochemical knowledge, positive grounding can adsorb negative ions in the air, so as to protect the shell of telecommunication equipment from corrosion. In fact, this statement is not very right. Galvanic cell reaction and electrolytic reaction will lead to equipment rust, but because they exist in micro form on the equipment, they have little impact. For example, the networks of non communication systems are negatively grounded (such as the computer you are using), but they are not rusty. In addition, the - 48V is internally isolated by DC / DC. The negative electrode of DC / DC output is grounded, and the single board is not corroded or rusted. So no matter which pole is grounded, it is the same.
As for the shell grounding (PGND) of the equipment, this is for the purpose of protection. The accumulated charge on the equipment will be quickly discharged to the ground, so as not to damage the equipment and staff. Our products basically use - 48V power supply system, and the measured actual voltage is - 53.5v. This is because for the sake of reliability, the communication equipment is equipped with a backup battery (- 48V). In order to ensure the reliable charging of the battery, the power supply voltage needs to be slightly higher than the battery voltage.
