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USB Port Protection

Jochen Baier, Head of Technical Marketing at Würth Elektronik eiSos discusses the Protection of USB 2.0 Applications. The USB-Interface might be the most distributed PC interface in the world. The usage in industry applications is more and more common. Let’s have a closer look to the special environmental conditions of industry applications.

There are real concerns regarding the robustness against EMI and ESD as written in Intel’s “High-Speed USB Platform Design Guidelines”. Intel recommends the usage of a common mode choke for EMI suppressions and another component for protection against ESD pulses.

Würth Elektronik offers the following types of products:
- The WE-CNSW common mode choke was developed for EMI suppression on high-speed signal lines.
- For ESD suppression the TVS diode arrays WE-TVS with very low capacitances (<2pF) or the ESD. Suppressors WE-VE (capacitances down to 0.05pF) are your preferred choice.

But for outstanding EMI behaviour, it is absolutely necessary to protect the power supply (Vbus) as well. A lot of design engineers forget this important point and wonder themselves when their product don’t pass all the tests in EMI lab.

Two typical schematics for optimised protection of one or two USB ports are shown below:

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With one TVS diode array WE-TVS you can fully protect two USB lines. All four signal lines as well as the common power supply are well protected. For further optimization, i.e. to filter incoming common mode noise as well as differential mode noise from USB line, a LC filter is built with the common mode choke WE-CNSW and the capacitors.

For power supply lines very good noise suppressions can be reached with a chip bead ferrite, like the Würth Elektronik WE-CBF series. Single line protection components like ESD suppressors series WE-VE also have to be connected from signal line to GND. For power line protection it is not necessary to use a low cap ESD suppressor, you might prefer a standard SMD varistor which can withstand higher surges and higher transient energies.

Single line protection components like ESD suppressors series WE-VE also have to be connected from signal line to GND. For power line protection it is not necessary to use a low cap ESD suppressor, you might prefer a standard SMD varistor which can withstand higher surges and higher transient energies.

About TVS Diodes
Würth Elektronik launched the TVS diodes array series called WE-TVS. These TVS diodes arrays include three main functions:
- protection against ESD pulses according to EN 61000-4-2
- protection against surge pulses according to EN 61000-4-5
- protection against EFT pulses according to EN 61000-4-4

WE-TVS series are high performance designed TVS diode arrays which include surge rated diodes.  They are the best choice to protect high-speed data lines like USB 2.0, DVI or LAN from over-voltages. The WETVS series exceeds the requirements outlined in EN 61000-4-2. Due to their ultra low capacitance (<2.0 pF) they are nearly invisible on the signal lines.

 About ESD Suppressors

The ESD suppressors are special raised varistors with a low and defined intrinsic capacitance. Würth Elektronik has three different series:
- WE-VE standard series: Capacitances of 1pF to 120pF and voltages from 5V to 24V
- WE-VE “ULC” series: Capacitance of 0.2pF, suitable for signal lines up to 12V
- WE-VE femtoF series: Capacitance of 0.05pF and rated voltages of 6V, 14V and 26V
For the first two series are also four-fold array packages available.

 Why ESD Protection?
Since the first electronic developments, all the parts are subjected to electrostatic discharge. ESD events have peak voltages up to 30kV and therewith they are very dangerous for all kind of integrated circuits. However, some state-of-the-art ICs are protected against ESD pulses but real world conditions show daily that an additional protection device is indispensable. On the one hand to have the whole board ESD free and on the other hand to develop more reliable products to satisfy the needs of your customers.

Why EMI Suppression?
With the increase in electronic/electrical products and the need to have wireless connectivity in today’s global markets, there is an increased requirement to ensure that your product is immune to the impacts caused by both the intentional and un-intentional RF environment. Only if you consider now which EMI phenomena impacts on the product, will you be able to design in the appropriate protection and reduce time to market.

In addition, the product must not generate excessive levels of EMI and this is typically assessed by an EMC Test Laboratory, and should the product fail then costs to implement remedial work will increase aggressively.

To read the full application note click on the Protection of USB applications 2.0 below.

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11 Mar 2015, 7:42