TDK Electronics · TDK Europe

SMD pulse transformer for LAN connectors

November 24, 2014

Space-saving, made-for-manufacture design

TDK has developed compact LAN pulse transformers that enable pulse transformer modules small enough to fit in a standard RJ-45 LAN connector. The SMD components are designed for fully automated manufacturing processes and offer outstanding performance.

Although the use of wireless connectivity with services such as wireless LAN and Bluetooth is rapidly expanding in smartphones, tablets and other mobile devices, wired LANs still have many clear advantages: higher data rates, greater EMI immunity, superior operational security, reliability and stability. As a result, the number of servers and routers that serve ever more LAN ports will continue to grow, as will the multitude of consumer devices such as notebook PCs, digital TVs, and other audiovisual devices that will profit from the benefits of wired LAN connectivity.

Wired LANs need pulse transformers for the transmission of rectangular pulses. Like conventional transformers, pulse transformers consist of a core with a primary and a secondary winding. The galvanic separation serves to protect sensitive ICs and networked devices from DC bias on the line. Because they must transmit pulse waves that contain many different frequency components resulting from the Fourier transform, their cores must feature low losses and prevent pulse waveform distortions across a wide frequency range. To meet these demands, TDK has developed the ALT series of LAN pulse transformers using high-performance ferrite materials for the core.

In order to suppress the intrusion or outflow of common-mode noise, LAN pulse transformers are normally combined with common-mode chokes to form a pulse transformer module. This module, in turn, is embedded in the RJ-45 connector to form a connector module (Figure 1). Thus, for use in standard RJ-45 connectors, LAN pulse transformers must also be very compact.

Figure 1:

Typical LAN interface structure (100BASE-TX):
Two pulse transformers and two common-mode chokes

For performance reasons, pulse transformer modules employ ring cores for both the transformer and common-mode choke. Compared to other core shapes with inherent air gaps, ring cores feature lower leakage flux. In the past, ring cores – even for compact SMD designs – had to be wound manually, which resulted in uneven characteristics across production batches, presenting an obstacle to quality stability in volume production. Moreover, the limits of miniaturization were effectively reached with package size 3232 (3.2 mm × 3.2 mm × 2.9 mm). TDK’s innovative ALT series of LAN pulse transformers, which is designed for fully automated manufacturing processes, solves all of these problems.

Designed for automated manufacturing

As LAN-connected devices have become increasingly popular, the demand for SMD LAN pulse transformers has grown accordingly. To address the performance, quality and miniaturization requirements of the expanding market, TDK introduced an entirely new design that enables the fully automated, high-volume production of SMD common-mode filters and LAN pulse transformers. The pioneering approach, which was first developed for SMD common-mode chokes, uses a coil carrier with a rectangular profile (DR core) that is wound automatically and then bonded to a ferrite plate (SP core), creating the functional equivalent of a ring core (Figure 2).

Figure 2:

Design of the TDK ALT series of LAN transformers:
The TDK ALT series of LAN pulse transformers is designed for fully automated manufacturing processes. After two windings and the terminal electrodes are applied to the rectangular DR core, an SP core is bonded to it with a temperature- and moisture-resistant adhesive, creating the equivalent of a ring core and allowing the magnetic flux to travel through the interior of both cores.

The cores are based on Ni-Zn ferrite material featuring both high magnetic permeability and saturation flux density over the entire temperature range found in the normal LAN environment. By applying this technology to LAN pulse transformers, TDK has achieved a manufacturing breakthrough in terms of the above requirements and also the automated mounting of the components on circuit boards. The ALT series not only utilizes automated winding for the coils, it also employs automated thermo-compression bonding for the connector electrodes and wires. These fully automated processes are key enablers for the mass production of products with uniform and stable characteristics and quality.

High performance in a space-saving component

The first TDK SMD pulse transformer introduced was the ALT4532 with a footprint of 4.5 mm × 3.2 mm and an insertion height of 2.8 mm. A low-profile type is also available with an insertion height of just 2.2 mm. In the meantime, the ALT series of pulse transformers has been extended with the miniaturized ALT3232 LAN pulse transformer, which measures in at just 3.2 mm × 3.2 mm × 2.8 mm. In combination with the miniaturized ACM series of common-mode chokes, these compact dimensions enable the design of even smaller pulse transformer modules. Thus, the mounting space of a module based on two ALT4532 transformers plus two ACM2012 common-mode chokes is around 30 percent smaller than a conventional pulse transformer module for 100BASE-TX (TDK TLA series) with a mounting space of 86 mm² (Figure 3). If the pulse transformer module is designed with the new ALT3232 plus ACM2012, it needs as little as 50 percent of the space.

Figure 3:

Space-saving solutions with the TDK ALT series of LAN pulse transformers:
The TDK ALT series of pulse transformers and the TDK ACM series of common-mode chokes, which are manufactured using advanced materials and autowinding processes, enable pulse transformer modules that are small enough to be integrated in standard RJ-45 LAN connectors.

Despite the SMD pulse transformer’s compact size, it still demonstrates very low insertion loss. Over the range of 0.1 MHz to 100 MHz, the ALT4532M features an insertion loss of 1.5 dB or below (Figure 4), equalling the characteristics of larger existing products. The insertion loss of the miniaturized ALT3232M does not exceed 2.5 dB over the same range. Thanks to their low intra-winding capacitance of just 35 pF and 25 pF, respectively, the ALT4532M and ALT3232M pulse transformers feature especially low insertion loss values at the high frequencies.

Figure 4:

Typical insertion loss of the TDK ALT series of SMD pulse transformers:
The typical insertion loss of the TDK ALT series of LAN pulse transformers is well below 1 dB over a very broad frequency range.

Integrated solutions for wired and wireless connections

With its SMD pulse transformers and common-mode chokes, TDK has created miniaturized, low-profile components that offer all the benefits of fully automated production processes. Because servers and routers have increasing numbers of LAN ports, TDK will continue to focus on the further miniaturization of the vital connector components with integrated connector modules that support high speeds and provide outstanding reliability. TDK is also developing products for use in factory automation applications under a wide range of temperature conditions. Making full use of its extensive and broad know-how in materials, processes and manufacturing, evaluation and simulation, and device and module technology, TDK will continue to develop advanced LAN application products for the high-speed networks of the next generation that integrate both wired and wireless connections.

Table: Key technical data for the TDK ALT series of LAN pulse transformers
Min. inductance [µH]
(DC bias 8 mA, 100 kHz)
Max. insertion loss [dB]
(0.1 to 100 MHz)
Max. intra-winding capacitance [pF]
(100 kHz)
Operating temperature range [°C]-40 to +85-40 to +85
Dimensions [mm]4.5 × 3.2 × 2.83.2 × 3.2 × 2.8


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