Things you must know about RJ45 connectors: RJ45 Gigabit PoE transmission × RJ45 surge protection solution × RJ45 neutral salt spray test
1. Gigabit Ethernet Power Supply (PoE) Principle
1. Gigabit Ethernet Power Supply (PoE) Interface – Technology, Signal
Power over Ethernet (PoE) is generally applicable to systems with a maximum supply voltage of 57 VDC and a user-side power of up to 73 W. The voltage is > 42 V when turned on. The voltage is between 36 and 57 V during normal operation, with a typical value of 48 V
However, PoE has different power levels, and their names or abbreviations are also different:
·IEEE 802.3af (PoE) provides 15 W output power, or up to 12.95 W at the terminal device.
·IEEE 802.3at (PoE+) provides 30 W output power, or up to 25.5 W at the terminal device.
·IEEE 802.3bt (4PPoE) provides 90 W output power, and the terminal device can reach up to 71.3 W.
·IEEE 802.3bu (PoDL) for single-pair Ethernet
Table 1: Overview of the most important characteristic data of the Ethernet standards and the associated classes
PoE systems consist of Power Sourcing Equipment (PSE) and Powered Devices (PD, loads), with a maximum recommended cable length of 100 meters. Due to the small conductor cross-section, long cable length, and low system voltage, there is significant power loss in the cable, which can lead to low system efficiency. For example, at Class 4, a PD can handle 25.5 W of power, with a line loop resistance of up to 12.5 Ω at 100 meters, and a maximum current of 600 mA allowed.
This results in a power loss of up to 4.5 W in the cable, and an efficiency of only 82%!
PoE is specified in the IEEE 802.3af-2003 standard (IEEE 802.3-2005 Section 33) or the 2009 update IEEE 802.3at. Depending on the system, different power delivery technologies are used.
·Data pair: powered by the center tap pair of the primary and secondary coils;
·Idle pair: powered by the wiring group of the idle pins directly or by transformer isolation;
In traditional 10BASE-T and 100BASE-TX Ethernet, only two of the four pairs are used for data transmission. The other two idle pairs can be used for PoE (power supply). Data is transmitted through one path and power is transmitted through another path, which corresponds to “idle pair power supply”. When PoE was first introduced, it was the safest way (see Table 2 above), that is, to transmit data and power simultaneously through one cable.
Table 2: Wire configurations in 10BASE-T, 100BASE-TX, and 1000BASE-T (Gigabit Ethernet) Ethernet cables
For 1000BASE-T (Gigabit Ethernet), all four pairs are used for data transmission. In this case, data and power are transmitted over the same pairs (see Table 2 below), which therefore correspond to “data pairs”. This approach is possible here because for Ethernet over twisted pair cables, differential data transmission is carried out over each pair and is decoupled by a transformer. The signal transmission itself does not differ from non-PoE transmission; the data rate and signal amplitude are the same.
Table 3: Power-on sequence and related voltage ranges
2. Gigabit Ethernet interface with PoE interface structure
Compliant with IEEE 802.3at standard (PoE+), the powered device (PD) power is up to 25.5 W. Figure 1 shows the basic circuit of the PoE+ system.
Figure 1: Basic circuit for a system compliant with IEEE 802.3at or PoE+
DC power and load connections are available from the center tap of the transformer on the PSE and PD sides. Each pair of wires operates in common mode through the center tap as one side of the DC power (positive or negative), so two pairs of wires are required to complete the circuit. The polarity of the DC power is not important because the rectification is done on the powered device (PD) side. The powered device must be powered using one of the following two pairs of wires: spare wire pairs 4-5 and 7-8, or data wire pairs 1-2 and 3-6.
3. Power-on process, PoE detection
Before the PSE (power supply equipment) supplies power, the terminal device must be classified. This can avoid damage to terminal devices that do not support PoE, and by classifying the PD (powered device), the power provided by the PSE is limited to the necessary range, thereby minimizing damage. The PSE’s power source uses a classification current and a low voltage to determine whether the end device supports PoE power supply and which class it belongs to. Therefore, depending on the end device, an information exchange (handshake process) is required between the power source and the end device, whereby the end device communicates its PD class. In order to distinguish between PoE-enabled and non-PoE-enabled end devices in the first step, a method based on resistance-based discovery of whether PoE power supply is supported is used in PoE power sources. PoE-enabled end devices are equipped with an input circuit containing passive components for this purpose. The PSE current source checks the internal resistance of the PD circuit using a measurement circuit. If the resistance is between 19 kΩ and 26.5 kΩ and the line capacitance is ≤ 150 nF, the power source is activated. In the second detection phase, the performance class is determined (Table 1). In this phase, the PD gradually increases the voltage until it signals which of the four performance classes defined in the 802.3af standard it belongs to. The system then provides the correct power supply. This detection process takes a total of about one second. To prevent damage to end devices, the PSE automatically turns off power to the associated ports once the PD is removed from the LAN. Figure 2 graphically shows the power-up process, and Table 3 shows the power-up steps, associated processes, and voltage ranges.
Figure 2: Power-up sequence of operation between PSE and PD
Table 4 shows the breakdown of the classes (classified according to Table 3) and the loop current range between PSE and PD required to detect or assign a class.
The grey line (i.e., the middle value) is ignored by the classification system.
Table 4: Classification (based on Table 3) and the corresponding necessary range for loop current between PSE and PD; intermediate values are ignored; classification current = defined load resistance through PD
802.3bt (PoE++) introduced two new PoE types (Type 3 and Type 4) and four additional classes in September 2018. The standard is fully backward compatible with previous PoE standards and can be used successfully with older Type 1 and Type 2 devices. The output power is increased to 90 W – 100 W with currents of 600 mA – 960 mA. In this case, the power supply requires all four pairs to limit line losses. In order to reduce line losses between PSE and PD and achieve high data rates, high demands are placed on the cable; an overview is shown in Table 5.
Table 5: Overview of PoE standards, including the relevant power of each port, the wire pairs used, and the cable category
II. RJ45 surge protection scheme
RJ45 modules are used for interconnection between physical (PHY) chips. As shown in Figure 1, RJ45 has two combinations, one is discrete, the network port transformer and the RJ45 connector are separate, and the other is the network port transformer and RJ45 integrated together.
Figure 1: Two main forms of RJ45
Let’s take the discrete RJ45 100M network circuit as an example. Figure 2 shows a typical 100M Ethernet circuit.
Bob Smith circuit
The Bob Smith circuit is used to improve the transmission quality of network signals and reduce interference design. Its main functions are as follows
1) Common mode suppression
The Bob Smith circuit provides a low-impedance return path for the common mode noise on the signal line
2) Impedance matching
In order to achieve good impedance matching and reduce echo interference, the middle tap of the secondary coil is generally pulled down to ground through a 75Ω resistor.
3) Surge protection
Surge protection is divided into common mode protection and differential mode protection. According to the IEC61000-4-5 lightning surge requirements, the common mode requires 4KV and the differential mode requires 2KV.
Common mode protection
Surge discharge path on the signal line: RJ45 → transformer → center tap → 75Ω resistor → capacitor → ground; the transformer, resistor, and capacitor in this path need to be able to withstand 4KV surge impact;
Surge discharge path on the NC line: RJ45 → 75Ω resistor → capacitor → ground: the resistor and capacitor are required to withstand 4KV surge impact
PS: For the unused pins of RJ45, the Bob Smith circuit must also be connected to achieve signal impedance matching and suppress external radiation interference.
Differential mode protection
As shown in the figure above, the differential mode surge discharge path requires the network transformer itself to withstand 2KV surges. At the same time, the differential mode will be coupled to the PHY end through the transformer, so the PHY end is required to withstand 2KV impacts. Usually, a bidirectional TVS device or other protective measures are placed near the PHY on the data line.
RJ45 protection circuit
Outdoor Ethernet is prone to lightning strikes. The voltage and overcurrent generated by lightning surges can damage Ethernet-related devices. Therefore, some applications will provide additional lightning protection for the RJ45 interface. As shown in the figure below, ceramic gas discharge tubes, ESD and TVS devices are added. The primary coil and the secondary coil cannot be grounded together. There needs to be an isolation area in the middle. Copper is prohibited on the PCB. Magnetic beads are required for the signal ground and sheild.
3. Analysis of the relationship between neutral salt spray test and gold plating requirements for RJ45 connectors
1. Core requirements for salt spray test of RJ45 connectors
As the core means of evaluating the environmental adaptability of RJ45 connectors, the neutral salt spray test (NSS) directly determines the reliability of the connector in a salty and humid environment. According to international standards GB/T 10125 and ASTM B117, the salt spray exposure time of RJ45 connectors needs to be set in accordance with the severity of the application scenario and associated with specific gold plating layer structure requirements:
Consumer electronics/ordinary commercial applications: The corrosion risk in the working environment is low, the thickness of the gold plating layer must be ≥0.5μm, and the thickness of the nickel bottom layer must be ≥3μm. This configuration must pass a 24-48 hour salt spray test, requiring the contact resistance change to be ≤20% after the test, and no substrate corrosion on the plating surface (slight discoloration is allowed).
Industrial control/outdoor equipment: Facing temperature and humidity fluctuations and chemical pollution, the gold plating layer must be increased to ≥1.0μm and the nickel bottom layer must be ≥5μm. The test duration is extended to 48-96 hours, and the functional resistance is required to remain stable after 192 hours.
Automotive electronics/marine equipment: It needs to withstand extreme corrosion such as deicing salt and high salt spray, and use composite coatings (such as nickel + palladium + gold) or gold layer ≥ 1.5μm. The test requires 96-240 hours of rigorous verification, and some scenarios need to superimpose CASS (copper accelerated acetate spray) test.
The core indicators for determining failure include: electrical performance (contact resistance increase > 20%), mechanical integrity (plating peeling or blistering), and substrate corrosion (green rust visible on copper alloy). For example, if the industrial-grade RJ45 has a sudden change in contact resistance after 96 hours of testing, it indicates that the failure of the nickel barrier layer has caused the corrosion of the underlying copper to spread.
2. Quantitative relationship between gold plating parameters and salt spray durability
2.1 Anti-corrosion mechanism of gold layer thickness and porosity
The protective performance of the gold plating layer does not increase linearly, and its impermeability depends on the balance between thickness and porosity. When the gold layer is less than 0.3μm, the electroplating crystallization is discontinuous to form dense pores, and the Cl⁻ ions in the salt spray can penetrate to the bottom nickel/copper interface to cause electrochemical corrosion. When the thickness is increased to more than 0.5μm, the porosity is significantly reduced; when it reaches 1.0μm, the porosity can be controlled at ≤5/cm², and the corrosion risk is greatly reduced. However, too thick a gold layer (>2.0μm) will increase the cost and may cause brittle cracking due to internal stress.
Typical effects of gold plating process defects:
Impurity contamination: organic impurities (such as additive decomposition products) cause the gold layer to bloom, and metal impurities (Fe²⁺, Cu²⁺) reduce the current efficiency, resulting in loose and porous coating.
Current density inaccuracy: incorrect amplitude setting or imbalance of vibration electroplating parameters, resulting in local crystallization roughness (visual redness), accelerating salt spray penetration.
Aging of plating solution: After long-term use, the concentration of cobalt/nickel ions fluctuates, changing the ratio of hard gold (Au-Co/Au-Ni) alloy and reducing density.
2.2 The key role of nickel bottom layer
The nickel layer plays a dual role in the gold-plated structure: mechanical support layer and corrosion barrier layer. When the thickness is ≥3μm, it can effectively block the ion diffusion between the copper substrate and the gold layer; when it is increased to more than 5μm, even if there are trace pores in the gold layer, the passivation properties of nickel can still delay the corrosion of the substrate. Neutral salt spray test shows that the gold-plated copper alloy without nickel layer has red rust within 24 hours, while the sample with 5μm nickel layer has only slight discoloration at the edge after 96 hours.
Table: Correspondence between RJ45 connector gold plating parameters and salt spray test performance
3. Key influence of salt spray test conditions on results
3.1 Temperature, humidity and sedimentation control
Salt spray corrosion is essentially an electrochemical reaction. The reaction rate increases by 2-3 times for every 10°C increase in temperature. The standard NSS test requires a constant temperature of 35±2°C. If the deviation is to 40°C, the equivalent actual corrosion amount of a 96-hour test can reach 168 hours. The sedimentation amount needs to be strictly controlled at 1.0-2.0ml/80cm²·h. Insufficient sedimentation will underestimate the corrosiveness, while excessive sedimentation will cause the liquid film to thicken and accelerate oxygen diffusion corrosion.
3.2 Salt water concentration and pH value
The NaCl concentration needs to be maintained at 5% (mass ratio) to simulate the real marine atmosphere. When the concentration is greater than 5%, the decrease in oxygen solubility will reduce the corrosion rate of steel; but for copper alloys, the corrosion rate continues to increase. The pH value is a sensitive parameter: when the pH drops from 7.0 to 3.5 (such as due to acidification caused by CO₂ dissolution), the corrosion rate increases by 7-8 times. Therefore, the pH needs to be monitored daily during the test and adjusted to neutral with NaOH/HCl.
3.3 Sample placement angle
If the RJ45 connector is placed horizontally (0°), the amount of salt spray deposition on the upper surface is 1.8 times that when it is placed vertically, resulting in excessive corrosion. According to GB/T 2423.17, it is recommended to place it at a 30° tilt to make the corrosion distribution closer to the actual working conditions.
