Application of LM5070 single chip in Ethernet feeding system

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Application of LM5070 single chip in Ethernet feeding system [Copy link]

Power over Ethernet (PoE) is a transmission technology that uses the same standard Ethernet cable to feed power and data. Internet (IP) phones, CCTV security cameras, wireless LAN nodes, and even some musical instruments can be powered through the Ethernet power feeding system. This article introduces the design of the Ethernet power feeding flyback converter from a practical application perspective. This converter design uses the LM5070 single-chip, high-performance Ethernet power feeding solution from National Semiconductor, which can provide all the necessary functions for the design of an efficient Ethernet power feeding system.
Since the RJ-45 power connector is widely used around the world, electronic devices using Ethernet power feeding technology around the world are compatible, and electronic devices that meet the Ethernet power feeding requirements have the advantages of flexible management and remote power on/off.
As long as the powered device (PD) uses Ethernet power feeding technology, it can obtain power supply through the existing data cable. The power supply equipment (PSE) accurately measures the powered device in advance to ensure that the device meets the relevant standards. If the powered device meets the standards, the power supply equipment will provide it with 48 volts (V) power.
Operation mode of powered devices compatible with IEEE standards
The Institute of Electrical and Electronics Engineers (IEEE) of the United States adopted technical standards for Ethernet power feeding products in June 2003. The IEEE802.3af technical standard has many advantages, such as stipulating that legacy devices and devices that meet Ethernet power feeding requirements can also be powered in a safe manner, the original structure of the cable can be preserved, and the data quality will not be degraded when power is supplied. The IEEE society has formulated relevant standards for this technology. For example, the power supply network must use standard Category 5e (Cat 5e) Ethernet cables containing two or four pairs of twisted copper wires, and can transmit 48 volts of DC power. The power supply is divided into 4 categories, divided by the power supply available per port, with the minimum power supply being 4W and the maximum power supply being 15.4W.
The power supply equipment will first measure the impedance of the cable (characteristic confirmation) in order to determine whether the powered device is actually connected through such a test. If the impedance is between 23.75kΩ and 26.25kΩ, the power supply equipment can determine that the powered device is actually connected.
If the voltage exceeds 23 volts, the relevant technical standards require the use of undervoltage lockout (UVLO) function to avoid bad characteristic confirmation resistors and to ensure that there is enough waiting time before the input capacitor is fully charged and the regulated output begins to be provided until the powered device is fully powered. On the other hand, the power supply device can also obtain the classification current of the powered device so that it knows the type of power supply required by the connected device (see Figure 2).
Once the undervoltage lockout is released, the interface part of the LM5070 chip will charge the load capacitor, which is the input capacitor that must be provided for the switch mode power supply (SMPS). In order to control this charging process, the current flowing into the main power field effect transistor is specified not to exceed 420mA, and the programmable surge current is limited to the range of 100mA to 375mA.
Typical Ethernet feeding DC/DC converter design:
Because the LM5070 controller chip uses highly integrated technology, it can not only support system design that complies with the IEEE802.3af standard, but also improve the performance of the system with very few external components. Figure 3 shows a non-isolated flyback output power supply with minimal external components. A few important basic components, such as a switching MOSFET (Si4848, 150V/3A), a transformer with primary and secondary windings, a diode to provide rectification for the output voltage, and input and output capacitors, support this power supply. The flyback continuous mode circuit layout is more suitable for
a fast pulse width modulation (PWM) current mode controller because this controller can control the input current and set the limit for it, and can also stabilize the output voltage in the same circuit. The transient response of the line and current can be determined by adjusting the duty cycle of the power transistor. The duty cycle depends on the output voltage error and the sawtooth waveform generated by the primary winding inductor current flowing into the external current sensing resistor.
The controller can set the current limit for the input current per cycle by comparing the current sensing signal with the internal reference voltage. If the duty cycle exceeds 50%, subharmonic oscillation will become a potential instability factor, but as long as internal slope compensation is provided for the current ramp signal, even if the duty cycle exceeds 50%, the instability problem will not occur.
Figure 4 shows an isolated Ethernet power feeding solution with programmable level settings and programmable surge protection current. In addition, to enhance the fault current protection function, this solution also adds 3 different current limiting and overheating protection functions:
① The programmable controller current over-current protection function can suppress the current spike generated during startup.
② If the DC current flowing into the main power MOSFET rises to more than 375mA, the current limiting amplifier can saturate the field effect transistor and increase the drain (RTN). As long as RTN rises above 2.5V wrt VEE and PowerOK=0, the soft start will be triggered and the chip will continue to try to restart.
③ The overheating limit function provides an additional safety guarantee to prevent system errors or other events that exceed safety warnings.
At the same time, Coilcraft has developed a number of transformers using surface mount packaging for the LM5070 chip to ensure that the system can fully utilize its efficiency.