How to calculate POE power loss

How to calculate the power loss of the POE, let's first look at how the power loss of the conductor in the junior high school 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 must be 1.2A. At this time, the power loss is P = 1.2 * 1.2 * 12.5 = 18W, minus the loss to reach PD The power of the device is only 42W.