November 2011

SMD pulse transformer for LAN applications

Precision space saver

No picture available

In order to convey pulsed signals at high speeds over Ethernet links, stable galvanic isolation must be provided between the input and output. The new TDK ALT4532 series of SMD pulse transformers employs an innovative automated coil winding technique and features very uniform electrical properties with a much smaller footprint.

LAN connectivity is becoming a standard feature not only in PCs, but also in digital devices ranging from digital TVs to multimedia appliances. Conventional pulse transformers for LAN applications consist of a toroidal (ring-shaped) core, on which the primary and secondary coils are wound. Toroidal transformers exhibit lower leakage flux and are able to deliver better performance than other transformers with cores that inherently have an air gap. For this reason, pulse transformers have traditionally been designed as toroidal transformers. Due to their shape, however, the coils are normally hand-wound because automated winding is difficult to implement, which unavoidably results in tolerances between finished units and presents an obstacle to stable quality and mass production.

Despite their rather simple operating principle and fundamental design, pulse transformers are actually quite difficult electronic components to build well. Aspects such as design, choice of core material, and winding method affect the outcome considerably, and uniform electrical characteristics are not easy to achieve in mass production. The TDK ALT series of SMD pulse transformers employs a new design that enables a manufacturing process with automated winding.

The design of the new SMD pulse transformers is based on that of the latest TDK SMD common-mode filters, which are used extensively as noise suppression components. A common-mode filter is similar to a pulse transformer in that it also employs two windings. TDK-EPC’s pioneering approach uses automated winding on a rectangular drum core and then joins it with a flat plate core (Figure 1). The new SMD pulse transformers are the first in the industry to be manufactured with automated winding.

Figure 1: New core design of the TDK ALT4532 SMD pulse transformer

After two automated windings have been applied to a rectangular drum core, a flat plate core is bonded to it, forming the functional equivalent of a toroidal core. The magnetic flux travels through the interior of both cores.

Advanced winding design

In an ideal transformer, the coupling coefficient k between the primary and secondary windings is 1. In practice, however, leakage flux due to the air gap in a transformer core and other factors result in a coefficient k that is smaller than 1. The resulting leakage inductance degrades the performance of the transformer. The design challenge for the TDK ALT4532 SMD pulse transformer was to achieve coupling coefficient as close to 1 as possible. Leakage flux was reduced significantly by reducing the gap at the juncture between the drum core and plate core to less than half of existing solutions.

Moreover, the design of the winding itself is also important in optimizing the coupling coefficient. Although each turn of windings is electrically isolated, the potential difference causes adjacent windings to act like the electrodes of a capacitor and produce an intra-winding capacitance. Another type of parasitic capacitance encountered in transformers is the winding distribution capacitance between the primary and secondary windings. Optimized winding design involves a tradeoff between these parasitic capacitances and increased leakage inductance and requires advanced technical know-how.


Ferrite as the ideal core material

Because pulse waveforms usually cover a very wide frequency range, the choice of the ideal core material is crucial to prevent excessive pulse waveform distortion that can degrade the signal.

For example, a pulse transformer for a 100BASE-T Ethernet connection is required to have an inductance value of at least 350 µH when a DC bias current of 8 mA is applied. The outstanding DC superposition characteristics of ferrite materials, therefore, are highly desirable, since the magnetization curve remains linear even when a DC bias magnetic field is applied. Therefore, a ferrite material is required that offers both high magnetic permeability and high saturation flux density over the entire temperature range existing in a normal LAN environment.

Based on the extensive experience with ferrite materials, a ferrite material was developed whose composition and microstructure are optimized for pulse transformer applications.
The TDK ALT series thus uses a new ferrite material that meets the technical requirements of next-generation high-speed LANs, while enabling a smaller core volume and lower number of windings compared to pulse transformers based on conventional materials. The result is a highly compact SMD pulse transformer with a footprint of only 4.5 × 3.2 mm² (4532 size).


High performance thanks to automated winding

The TDK ALT series of pulse transformers delivers the high reliability and performance required of a pulse transformer for LAN applications in a compact SMD package. As the eye pattern in Figure 2 clearly shows, the signal integrity of the TDK ALT4532 SMD pulse transformer is on the same level as with larger conventional products.

In addition to the automated coil winding, the ALT series also employs automated thermocompression bonding for the terminal electrodes and wires. The continuous and fully automated manufacturing process for the ALT series also results in a more uniform quality in mass production than is the case for conventional products made with semi-automated manufacturing processes, with batch processing being employed for the stages from electrical testing to taping.

Figure 2: Comparison of the eye pattern evaluation in Ethernet applications

Despite its notably smaller package, the TDK ALT4532 SMD pulse transformer delivers the same signal quality as conventional products.

Significant space savings

The pulse transformer is normally integrated in the LAN module along with the common-mode choke coils and other components. With conventional components, complex wiring as well as soldering tasks must often be performed manually when mounting the pulse transformer. These components must be fixed with resin before soldering. As SMD components, the ALT series of transformers can be mounted along with other parts during the reflow soldering stage, greatly simplifying the process and reducing the assembly time.

Moreover, the more compact dimensions contribute to further space savings when combined with noise filtering according to the differential transfer method. Compared to the use of conventional transformers and common-mode choke coils, the overall required footprint can be reduced by about 40 percent when the TDK ALT series is used in combination with the TDK ACM series of common-mode filters (Figure 3). In this way, the TDK ACM series and ALT series are predestined to be designed into devices for which the miniaturization of components is especially important.

Figure 3: Space savings with TDK SMD components (100BASE-TX)

Compared to conventional solutions, the combination of the TDK ALT4532 SMD pulse transformer with the TDK ACM2012 SMD common-mode filter can save up to 40 percent space on the PCB.

Table: Key technical data of the TDK ALT4532 series

Min. inductance [µH]200 (DC bias 100 mA, 100 kHz)
Max. insertion loss [dB]1.5 (0.1 to 100 MHz)
Max. intra-winding capacitance [pF]35 (100 kHz)
Operating temperature range [°C]0 to +70
Dimensions [mm]4.5 × 3.2 × 2.8 (l × w × h)