POE power supply problem

bigbat

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I found a few articles about POE power supply on the Internet, but I was confused after reading them. I have a few questions to ask the experts

The standard mentions that the POE management device PSE will first send out a low voltage when the device is connected to detect whether it is a PD device. If it is a PD device, it will start to supply power. When the PD detects and inputs a voltage of 2.7V to 10.1V, it must be connected to a 25KΩ load. If the input range of the PD is 14.5V to 20.5V, the PD will be connected to a constant current load, which will display the power consumed by the PD.

Question 1: At the beginning, the device has no power. How can the circuit start the detection circuit? Is it only started by the voltage of 2.7V~10.1V?

Question 2: How to respond when selecting power supply according to the power of the device, such as 1/4, 1/2, or 1/1 power supply type?

qwqwqw2088

This sentence should be included in all the information. PSE is the manager of the protocol power supply process. Before powering on, PSE uses a 2.8V-10V detection voltage to detect whether there is a PD connected. In specific implementation, two voltages between 2.8V and 10V (with an interval of 1V or more) are sent to the network link, and then calculations are performed based on the two different current values obtained (ΔV/ΔI).

bigbat

qwqwqw2088 posted on 2020-1-3 08:49 This sentence should be included in the information. PSE is the manager of the protocol power supply process. Before powering on, PSE uses a 2.8V-10V detection voltage to detect whether a PD is connected. The specific implementation...

If we use method B, how will the voltages of the 45 and 78 lines change? Can we first connect the circuit to a 25K resistor and then switch it back to 48V DC-DC after a few seconds?

达普POE交换机

This is the power supply classification process captured by an oscilloscope of a POE switch

达普POE交换机

Question 1: After being powered on, the POE switch will continue to transmit detection voltage at fixed intervals until it detects a resistance of 24.9KΩ. If no resistance is detected, it will continue to transmit.

bigbat

Dapu POE switch published on 2020-1-3 11:38 This is the power supply classification process of the POE switch captured by an oscilloscope

Thank you, I don't understand? How does the voltage change and how does the PD operate? I want to make a POE device with a very low power consumption of 0.1W. As I just said, can my PD device be connected to a 25KΩ resistor at the beginning, and then switch to 48V DC/DC after a delay of 1S? If so, please recommend a chip that is feasible.

qwqwqw2088

Please refer to here (may need to be reviewed)

Working process of PoE power supply

bigbat

qwqwqw2088 posted on 2020-1-3 12:03 Please refer to here (may need to review) PoE power supply working process

Thank you, I have read the information you gave me. From the market, there are non-standard equipment and standard equipment. Some people say that the POE of small factories is generally non-standard, that is, they do not test, directly output a 48V voltage, and the equipment can be connected. If it is standard, it must respond to the test. Then I have my question: first connect a 24.9K resistor, delay 100uS to start the DC-DC device. Or more simply, directly connect a 24.9K ohm resistor in parallel to the DC/DC input end, can it work?

bigbat

In fact, there is a very simple way, that is, there is a separator that supports 12V or 9V output modules. It can be used, but I think it is better to make a circuit myself so that people can see it more comfortably.

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达普POE交换机

bigbat posted on 2020-1-3 12:02 Thank you, I don’t understand? How does the voltage change, how does the PD operate? I just want to make a POE device with a very low power consumption of only 0.1W. What I just said...

It requires a protocol chip and then a DCDC step-down chip. Our factory specializes in the production of POE modules. The principle is relatively simple, but it is still difficult to achieve good performance, life, ripple, etc.

bigbat

I made a PD, not a PSE, and my circuit can work in principle. Ripple and other issues are related to the DC/DC of the PD and the power supply of the PSE. There is basically no problem with the power supply of the digital circuit!

达普POE交换机

We all make PSE PD. PD does have ripple problems, but some devices have high requirements and some have low requirements. If your device has low requirements, the filtering does not need to be so good. The POE module we usually use is the PD end.

bigbat

EEPROM is much more convenient than flash, so we need to make an algorithm balance. The simple algorithm is not to delete or rewrite, but just keep storing data. If it is full, start from the beginning.