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POE power supply is unstable Do you know its power?

Technology Exchange December 10, 2019 11:27 Source: Weak Current Industry Network
Many friends have asked many times whether poe power supply is stable? What cable is good for poe power supply? Why is the camera powered by a poe switch still not showing? These problems, in fact, are related to the power loss of POE power supply, which is easy to ignore in the project.
 
First, what is poe power supply
PoE refers to the transmission of data for some IP-based terminals (such as IP telephones, wireless LAN access points AP, network cameras, etc.) without any changes to the existing Ethernet Cat.5 wiring infrastructure. At the same time, it can also provide DC power technology for such equipment.
PoE technology can ensure the normal operation of the existing network and reduce costs while ensuring the safety of the existing structured wiring.
A complete PoE system includes two parts: the power supply device and the power supply device: the power supply device (PSE): an Ethernet switch, router, hub, or other network switching device that supports the POE function; The surveillance system is mainly IPC.
Second, poe power supply standards
The international standard IEEE802.3af established in 2003 requires that the PSE can reach an output power of 15.4W, and the power reaching the powered device is 12.95W, and the power loss is 2.45W.
The international standard IEEE802.3at established in 2009 requires the PSE to reach an output power of 30W, the power reaching the power receiving equipment is 25.5W, and the power loss is 4.5W.
Over time, the power of these two standards has been unable to meet the power requirements of the new higher power PDs. Therefore, the new international standard IEEE802.3bt has two requirements:
The first type: one is to require the PSE to reach an output power of 60W, the power reaching the power receiving device is 51W (this is low data from the table above), and the power loss is 9W.
The second type: the PSE is required to reach an output power of 90W, the power reaching the powered device is 71W, and the power loss is 19W.
According to the above standards, it can be known that with the increase of the power supply, the power loss is not a loss proportional to the power supply, but the loss is getting larger and larger. So how can the PSE loss in practical applications be calculated?
Third, poe power loss
So let's first take a look at how the power loss of the junior high school physics is calculated.
Joule's law is a law that quantifies the conversion of electrical current into heat by conducting current. The content is: the heat generated by the current through the conductor is proportional to the square of the current, proportional to the resistance of the conductor, and proportional to the time it is energized. That is, the staff costs generated during the calculation. The mathematical expression of Joule's law: Q = I²Rt (applicable to all circuits) where Q is the power of the loss P, I is the current, R is the resistance, and t is the time.
In actual use, since PSE and PD work at the same time, the loss is independent of time. It is concluded that in a POE system, the power loss of a network cable is proportional to the square of the current and proportional to the size of the resistance. In short, in order to reduce the power consumption of the network cable, we must try to make the current of the wire as small as possible, so that the resistance of the network cable is small. The significance of reducing the current is particularly important.
So let's take a look at the specific parameters of international standards:
In the IEEE802.3af standard, the resistance of the network cable is 20Ω, the required PSE output voltage is 44V, the current is 0.35A, and the power loss is P = 0.35 * 0.35 * 20 = 2.45W. Similarly, in the IEEE802.3at standard, the resistance of the network cable is 12.5 ?, the required voltage is 50V, the current is 0.6A, and the power loss is P = 0.6 * 0.6 * 12.5 = 4.5W.
Neither standard uses this calculation method. However, when the IEEE802.3bt standard is reached, it cannot be calculated this way. If the voltage is 50V to reach 60W, the power will be 1.2A. At this time, the power loss will be P = 1.2 * 1.2 * 12.5 = 18W, minus the loss and reach the PD device. The power is only 42W.
Fourth, the reason for the power loss of poe
So what is the reason?
Compared with the actual requirement of 51W, it has 9W less power. So what exactly caused the calculation error.


Let's look at the last column of this data chart again, and take a closer look at the current in the original IEEE802.3bt standard is still 0.6A. Looking at the twisted pair power supply, we can see that it uses four pairs of twisted pair power supply (IEEE802.3af, IEEE802. 3at is powered by two pairs of twisted pairs) In this way, this method can be regarded as a parallel circuit. The current of the entire circuit is 1.2A, but the total loss is twice the loss when the two pairs of twisted pairs are powered.
Therefore, the loss P = 0.6 * 0.6 * 12.5 * 2 = 9W. Compared with 2 twisted pairs, this power supply method saves 9W of power, so that the PSE can make the PD device receive when the output power is only 60W. The power can reach 51W.
Therefore, when selecting PSE equipment, we must pay attention to reducing the current and increasing the voltage as much as possible, otherwise the power loss is likely to be too large. The power perception of the PSE equipment alone can be used, but it is not available in practice.
For example
A PD device (such as a camera) requires 12V 12.95W to use. If a 12V2A PSE is used, the output power is 24W. In actual use, when the current is 1A, the loss P = 1 * 1 * 20 = 20W. When the current is 2A, the loss P = 2 * 2 * 20 = 80W,
At this time, the larger the current, the greater the loss. Most of the power has been consumed. Obviously, this PD device cannot receive the power transmitted by the PSE, and the camera will have insufficient power supply and cannot work normally.
This problem is also common in practice. In many cases, it seems that the power supply is large enough to be used, but there is no loss calculation. As a result, the camera cannot work normally due to insufficient power supply, and the cause is always not found.
Five, poe power supply resistance
Of course, the above is the resistance of the network cable when the power supply distance is 100 meters, which is the available power at the maximum power supply distance, but if the actual power supply distance is relatively small, such as only 10 meters, then the resistance is only 2 ?, The corresponding loss is only 10% of the loss of 100 meters, so it is important to fully consider the actual situation when selecting PSE equipment.
100m resistance of super five types of twisted-pair cables of various materials:
1. Copper clad steel network cable: 75-100?
2. Copper clad aluminum network cable: 24-28?
3. Copper-clad silver network cable: 15?
4. Copper-clad copper network cable: 42?
5. Oxygen-free copper network cable: 9.5?
It can be seen that the better the cable, the smaller the resistance. According to the formula Q = I²Rt, that is, the less power is lost during the power supply process, so this is the reason why the cable should be used well. some.
We have mentioned above, the loss power formula, Q = I²Rt. If the poe power supply has less loss from the PSE power supply end to the PD power receiving device, it needs small current and small resistance to make the whole power supply process work well.
@ 安防 展 网 AFzhan

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