Compatible with standard high power PoE system design

The formulation of IEEE802.3af standard promoted the development of Ethernet

The use of power supply technology, and TI's standard-compatible high-power power management chips TPS23841 and TPS2376H have solved the access problem of network devices within 25W. Based on the above two chips, this paper designs a standard-compatible high-power Ethernet power supply system.
1 Introduction to IEEE802.3af Standard

The IEEE802.3af standard defines a method that allows 48 V DC power to be delivered while transmitting data through Ethernet. It introduces Ethernet power supply (Power over Ethernet, PoE) technology into the existing network infrastructure with a maximum transmission distance of 100m. PoE consists of power supply equipment PSE and powered equipment PD. PSE is divided into two types: termination type and mid-span type. It mainly realizes the detection, power classification (optional), power supply and power off of PD; PD is mainly used as a standard identification device to return various standard-compliant signals and continuous working signals to complete the entire power supply process.

2 PSE system structure composition

PSE mainly realizes the above power management functions. The system consists of hardware and software. The PSE system structure is shown in Figure 1. The 16-bit single-chip microcomputer MSP430F147 reads/writes the internal registers of TPS23841 through the simulated I2C bus to realize the power management function; the working mode of TPS23841 is set through P4.0; the error signal is obtained through P2.7, and a valid reset signal is generated for TPS23841 through P4.2. The necessary signals and power are isolated in the system. MSP430F147 communicates with the power supply monitoring PC through the UART interface after being converted to RS485 level by MAX13085 . When it is not connected to the PC, the system will run autonomously.

Figure 1 PSE system structure diagram

The operating voltage range of TPS23841 is 21.5~57V, which is much larger than the voltage range defined by the IEEE802.3af standard. In other words, TPS23841 can provide power supply within the above voltage range, which enables Ethernet power supply technology to be applied in 24V power supply systems such as medical and industrial. At the same time, the transmission power can be increased to 25W by increasing the input voltage, and the power supply can be transmitted to the remote PD through line pairs 1/2, 3/6 or 4/5, 7/8. In special cases, power can also be transmitted through 4 line pairs at the same time, so that higher transmission power can be obtained. The AC disconnection detection circuit is used to generate an AC disconnection detection signal, and the address setting circuit is used to set the I2C device address of TPS23841. By setting different addresses, multiple TPS23841 can be connected, thereby achieving the purpose of expanding the power supply port. MSP430F147 monitors the system operating temperature in real time through the internal integrated temperature sensor, and starts the fan to dissipate heat when overheating.

3 PSE hardware design

The power supply management function of PSE is mainly implemented by TPS23841. When the TPS23841 is running, the 10 V, 6.3 V and 3.3 V required for internal operation are generated by external power supply. Each power supply port can provide up to 570 mA of current, and each PD can provide up to 25W of power. The TPS23841 integrates 4 independent 15-bit A/D converters to measure the resistance, voltage and current of each port, which can realize the power supply management function more accurately. As shown in Figure 2, the hardware circuit of PSE is mainly composed of TPS23841 and its peripheral circuits. Self-recovery fuses are added to the 4 power supply circuits to realize the overcurrent protection function, and the overvoltage protection function is realized by adding transient voltage suppressors. The circuits led out from pins 50 and 51 generate AC circuit break detection signals, and display circuits are added to each power supply port. The 5-bit address of I2C is set by the external DIP switch S1, and the three-wire I2C bus is connected to become a standard two-wire I2C bus after optical coupling isolation. In the PSE system, the advanced management function of the power supply is realized by programming the MSP430F147.

Figure 2 PSE hardware schematic

4 Software Design of PSE

The TPS23841 integrates a standard I2C module, and the MSP430F147 implements advanced power management functions through I2C communication. The TPS23841 has three working modes, namely automatic mode (AM), semi-automatic mode (SAM) and power management mode (PMM). In AM mode, the TPS23841 does not need microcontroller control, and automatically implements functions such as PD detection, classification and power supply. In this mode, the DC circuit break detection method is used to detect whether the PD is disconnected; in SAM mode, the TPS23841 runs in AM mode, but can read the contents of all status registers and A/D registers through the I2C bus, and can automatically complete the detection of PD without the participation of the microcontroller; in PMM mode, AC circuit break detection can be performed, and the voltage and current parameters of each PD can be obtained, and the power supply management is more flexible. When the TPS23841 works in SAM and PMM modes, the microcontroller MSP430F147 is required to complete the entire power supply management process. The operation control process of PSE is shown in Figure 3. The control program mainly realizes the functions of system initialization, controlling TPS23841, communicating with PC, and encapsulating and parsing data. When not connected to PC, TPS23841 is set to work in AM mode, and TPS23841 will run autonomously; when connected to PC, the system will set TPS23841 to the corresponding working mode according to the user's requirements. In SAM and PMM modes, the operating parameters of each port can be collected, and the power supply of each port can be controlled in PMM mode. Finally, the system encapsulates the collected data and sends it to PC via RS485.

Figure 3 PSE operation control flow

5 Design and Implementation of PD

TPS2376H is a high-power PD controller compatible with the standard launched by TI. The design principle of PD is shown in Figure 4. The rectifier bridge is used for polarity protection, and the transient voltage suppressor D1 is used to eliminate the damage caused by overvoltage. When designed as a standard PD, the external detection resistor and classification resistor of TPS2376H are set according to the parameters defined by the standard. When designed as a high-power PD, PSE needs to provide a power supply voltage of 52~57V, and the classification resistor of TPS2376H needs to be set to the standby level 4 defined by the standard. At this time, PD can receive the maximum power of 25W transmitted by TPS23841. High-power transmission can be obtained by increasing the transmission voltage. At the same time, it can also be powered by the four pairs of Ethernet lines. The power transmitted is twice the power transmitted by two pairs. Both methods have their own advantages. Two-pair power supply is used in the IEEE802.3af system. One pair is used to transmit current, the other pair is used as a loop, and the other two pairs do not transmit power. It is the simplest implementation method, but the two pairs that are not powered cannot be well utilized, which is a loss. Four-pair power supply reduces the impedance of the cable, but increases the current balance problem of the line pair. Different isolation transformer impedance, cables, connectors and rectifier bridges on the PD side will cause current imbalance in the power supply line pair. The PD side can solve this problem through two independent DC/DC converters. In the 24V power supply system, in order to prevent misclassification, the PSE should choose non-classification operation. The LT1074HVIT is used at the back end of the PD to convert the voltage into the 5V or 3.3V voltage required for the device to work. The input voltage of the LT1074HVIT is 10~60V, the maximum output current is 5A, and the output voltage can be set according to Vout=2.21×(1+R1/R2).

Figure 4 PD design schematic

6 Application of PoE in RFID System

When designing RFID application system, the power supply of reader must be considered. For readers based on Ethernet communication, the use of Ethernet power supply technology will greatly reduce the cost of readers. In practical applications, the PSE system and switch are integrated to design a terminated PSE (i.e. PoESwitch), making the application more flexible and convenient. A typical application is shown in Figure 5.

Figure 5 Application of PoESwitch in RFID system

When PoESwitch detects that the reader and wireless access point are legitimate PDs, it will perform optional classification operations and then supply power to them. At the same time, the PSE terminal monitoring software on the monitoring PC will monitor each power supply port.

Conclusion

This design uses the MSP430F147 microcontroller and Ethernet power management TPS23841 and TPS2376H to develop a high-power PoE system compatible with the IEEE802.3af standard. This system can also be used in medical and industrial power supply environments with a power supply voltage of 21.5 to 57V and a maximum power of 25W, with good compatibility.