I. Application of External Network Transformer for 10/100M Ethernet Interface

Taking Ebitech's NS1 Ethernet module as an example. The NS1 module can connect to an external 10/100M Ethernet physical interface and supports the interconnection design method of RJ45 connector and network transformer.

The network transformer and RJ45 connector are combined to form a standard 10/100M Ethernet physical interface, which is then connected to the NS1 module. RX+ and RX- are routed as differential lines with 100-ohm differential impedance matching, and TX+ and TX- are also routed as differential lines with 100-ohm differential impedance matching. The hardware design reference circuit diagram is as follows:

II. Application of the Built-in Network Transformer in the 10/100M Ethernet Interface

The RJ45 interface incorporates a 2KV electromagnetic isolation network transformer. The network data receive pins of the NS1 module are directly AC-coupled to the data receive pins of the Ethernet physical interface, serving as the data transmission channel within the system. RX+ and RX- are routed as differential lines with 100-ohm differential impedance matching, as are TX+ and TX-. The hardware design reference circuit diagram is shown below.

III. Function and Connection Method of the Center Tap (CT Pin) of a Network Transformer

In many Ethernet devices, a network transformer is typically added when connecting to an RJ45 connector via a PHY chip. In wired LANs, computers, routers, servers, and other devices are connected using twisted-pair (UTP) cables. Since the connecting cables and twisted-pair cables can be very long, sometimes reaching tens of meters, directly connecting electronic devices in different locations can cause many problems. To ensure the safety of the network card's integrated circuit chip, reduce bit errors caused by external EMI, and suppress electromagnetic noise emitted into the air from inside the computer, a transformer is added at the connection point between the network card and the UTP cable. This solves the above problems. This transformer is the network transformer, as shown in the diagram below. Currently, most transformers are integrated into the network port (such as Hanren's HR91105A), and while they are relatively expensive, their high integration and small footprint make them popular among hardware engineers. The design and use of the center tap (CT pin) of the network transformer will be explained below.

1. What is the function of the center tap (CT pin) of a network transformer?

(1) To reduce common-mode current and voltage on the cable by providing a low-impedance return path for common-mode noise on the differential line (similar to the function of a Choke).

(2) To provide a DC bias voltage or power source for some transceivers.

2. We can see that some transformer center taps are connected to power supplies (3.3V, 2.5V, and 1.8V are available), while others are connected to a capacitor to ground. Why is this?

(1) This mainly depends on the chip used. Different chips have different connection methods, determined by the UTP port drive type of the PHY chip. There are two types of drive types: voltage drive and current drive. Voltage drive requires connection to power supplies (3.3V, 2.5V, and 1.8V are available); current drive simply connects a capacitor to ground. Therefore, the connection method of the center tap is closely related to the PHY for different chips. Refer to the chip's datasheet and reference design for details.

(2) It is important to note that the center tap must not be connected incorrectly. Incorrect connection will lead to extremely unstable network signals, or even complete failure.

(3) For example, the commonly used PHY chip LAN8720AI-CP-TR is voltage-driven, while the PHY chip KSZ8081RNBIA-TR is current-driven.