March 2011

Thin-film common-mode filter with integrated ESD suppression

Ultra-thin protection for mobile devices

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The new TDK TCE1210 series is the world’s first thin-film common-mode filter to offer both high-speed common-mode noise suppression and ESD protection in a single component. It thus reduces the number of components needed, shrinks the mounting area and functionally enhances mobile devices.

Mobile devices are increasingly vulnerable to ESD as the semiconductor process geometries of their ICs continue to shrink. At the same time operation at higher speeds and frequencies demands protection from common-mode noise radiated at high-speed interfaces. The TCE1210 series of thin-film common-mode filters feature integrated ESD suppression enabling them to both suppress common-mode noise and protect vital components from electrostatic discharge at the same time. The new thin-film common-mode filters are smaller and thinner than ever before, measuring in at just 1.25 × 1.0 × 0.60 mm³. This was made possible by the use of leading thin-film process technologies. These filters won the “Cho Monodzukuri 2008 Innovative Parts and Components Award” given by the respected Japanese daily business and industry newspaper publisher Nikkan Kogyo Shimbun in the category of electric and electronic components.

Many high-speed digital interfaces such as USB or HDMI employ differential-mode transmission. Here, the signal is transmitted along two lines with a phase difference of 180 degrees. In theory differential-mode transmission, also known as balanced transmission, offers less noise radiation than single-line transmission systems, and is little affected by interference from radiated noise coming from other systems. In reality, however, if the phases of the two signals are only slightly off, or, if a difference in the pulse width or amplitude occurs, unbalanced components known as skew arise and cause a common-mode noise current, which leads to malfunctions and other problems in a system.

Unless protective measures are taken interface cables connecting digital equipment will be a source of noise. For this reason, a common-mode filter is needed as a component to ensure electromagnetic compatibility (EMC) between the electronic equipment and the interface.

Miniaturization using HDD head technology

The demand for ultra-tiny common-mode filters are especially high in mobile phones and digital cameras, where circuitry miniaturization is being taken to the farthest limits possible. Therefore, engineers applied TDK’s advanced thin-film process technologies originally developed for manufacturing hard disk drive heads in order to laminate a thin-film conductor coil onto a ferrite layer. The result was the TCM series of thin-film common-mode filters that take up less than half the space of conventional filters.

With mobile devices in particular a lot of touching goes on, as cables are frequently plugged in and removed. Therefore, their circuitry phones must also be reliably protected against electrostatic discharges (ESD).

Figure 1: Basic structure of the TDK TCE1210 series of common-mode filters

The TDK TCE1210 series integrates a micro-gap ESD protection element into a common-mode filter using thin-film fabrication technology.

The new TCE1210 series integrates the ESD protection function and common-mode noise suppression in a single component. This was achieved by the development of a micro-gap electrode structure with stable characteristics. Micro-gap ESD suppressors are elements that employ a pair of electrodes enclosed in a glass tube and fabricated made into a chip component – in effect a miniature surge arrester. The great advantage of micro-gap ESD suppressors is their verylow inter-terminal capacitances, which enables them to effectively suppress ESD without distorting the signal at very high data rates.

A micro-gap ESD suppressor is fabricated on a ferrite substrate, on top of which a thin-film coil, insulating layer, and other layers are stacked to form the thin-film common-mode filter (Figure 1).

Figure 2: Radiation noise for a USB 3.0 signal

Noise is reduced by about 15 dB when the TDK TCE1210 series filter is used.

The TCE1210 series clearly meets USB 3.0 requirements with room to spare. Figure 2 shows the measurement data confirming the effect of the common-mode filter. A USB 3.0 signal at 2.5 GHz (5 Gbit/s) was used for the transmission signal. Without the filter, spikes in the noise level occur in the 2 to 3 GHz range. When the TCE1210 series filter is used, noise is reduced by about 15 dB.

Figure 3: Characteristic impedance with a varistor and a TDK TCE1210 common-mode filter

The TDK TCE1210 enables a characteristic impedance of 90 Ω (}7) and thus meets new USB 3.0 requirements.

The effectiveness of the TCE1210 series’ ESD suppression is clearly demonstrated in Figure 3. When a varistor with 1 pF inter-terminal capacitance is used, the characteristic impedance drops to below 80 Ω. With USB 3.0, the new parameters for characteristic impedance rises to 90 ±7 Ω. The characteristic impedance of the TCE1210 series thus meets these requirements, demonstrating that it will have next to no effect on the signal.

Figure 4: ESD absorption with and without the TDK TCE1210 series

The TDK TCE1210 effectively absorbs ESD to uncritical levels.

Moreover, when no ESD countermeasures are taken, a steep spike in the waveform is observed. The TCE1210 series filter has a significant effect in absorbing static electricity (Figure 4).

 

Significantly fewer components needed

The TCE1210 series can greatly reduce the number of components and the mounting area required. With HDMI, for example, which uses differential-mode transmission, four common-mode filters were previously used for one terminal, with two varistors for each filter, thus requiring a total of 12 components. By comparison, only four components need to be mounted when the TCE1210 series filter with integrated ESD suppression is used.

Key data: TDK TCE1210 thin-film common-mode filter
TypeTCE1210-900-2P
Common-mode impedance [Ω]60 min. 90 typ.
Cutoff frequency [GHz]5.0
ESD standardIEC61000-4-2 Level 4 compatible
Clamp voltage (max.) [V]100
Rated voltage (max.) [V]10
Rated current [mA]100
Dimensions [mm³]1.25 × 1.00 × 0.60

Demand for the ability to reduce the number of components and shrink the mounting area for mobile devices such as mobile phones, digital cameras, and portable music players, will grow even stronger as functions become more advanced and miniaturization progresses further. Improved common-mode noise suppression will become critical as transmission signals move into higher frequencies. ESD suppression for mobile devices will be just as critical as semiconductor process geometries continue to shrink. By addressing and meeting these demands the new TCE1210 series helps to improve the performance of mobile devices and make them more reliable.

Basic noise suppression principle of common-mode filters

A common-mode filter is an electronic component with two conducting wires wrapped in the same direction around a ring-shaped core. When a current flows through the coil, the magnetic flux that is generated flows into the core. The signal current also passes through the coil. It however uses a differential mode, in which the outgoing and returning signals travel in opposite directions. The magnetic flux also travels in opposite directions, cancelling out inside the core. For this reason, a common-mode filter has no effect on the signal current.

A common-mode noise current, in contrast, flows in the same direction. therefore the magnetic flux in the core also travels in the same direction, accumulates, and gets stronger. This results in an increase in impedance and the common-mode noise current is prevented from passing through.

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