Protection of USB 2.0 Applications Series: ESD Device Tests and Layout Recommendations for USB Ports

Welcome back to our final edition of the USB 2.0 Applications Series!

If you’ve been following along, you now know that USB-Interface is the most distributed PC interface in the world with a variety of common industry-applications.

Let’s conclude our series with a look at testing the protection level of ESD devices and recommended layouts for single and double USB ports.

Testing the Protection Level of ESD Protection Devices

The easiest way to measure the protection level is to apply an ESD pulse to your electronic circuit with the ESD protection device, and measure the peak voltage of this ESD pulse before and after the protection device. This way carries out different problems.

Due to high frequency and wide range of ESD pulse (few MHz to some GHZ), high frequency refractions will occur during the measurement. The absolute peak voltage and the spike voltages during the refractions are an indicator for better and worse protection, but you cannot indicate the definitive clamping voltage. By the way, this is not a reliable and repeatable measurement!

Engineers familiar with semiconductor products do know the TLP (transmission line pulsing) measuring method. The TLP method is an accurate measurement system — due to all measurements are done in a 50-System — allowing repeatable and very accurate measurements.

Shown above, the defined current impulse (left) is charged into the protection component, and the resulting voltage across it is measured (middle). This procedure is repeated with increasing TLP currents. As a result, you get the TLP curve (right).

This measurement can be done for VDD and I/O pins. The lower the voltage you measure, the better the protection device and the reliability of your electronic circuit is.

Thanks to the built in snapback technology, Wurth Electronics TVS diodes have the lowest ESD clamping voltage on the market. Compared to competitors’ products, the WE-TVS do outperform them clearly! This you can see with a short view at the TLP curves.

Recommended Layout for Single USB Port

The two differential signal lines (D+ and D-) are routed from connector to TVS-Diode (WE-TVS 824 011) and via a common mode choke (WE-CNSW 744 232 090) to the USB controller as shown above.

This results in an awesome ESD protection and EMI suppression of both data lines. VBUS is routed like the signal line, but instead of the common mode choke a chip bead ferrite (WE-CBF 742 792 641) is used. After the chip bead you may add a capacitor and a second chip bead as well to get the highest possible EMI suppression effect.

For very sensible IC’s and/or high reliable applications, you can get an optimized ESD suppression effect if you double contact a four-fold TVS array (WE-TVS 824 015) as shown above.

Designers preferring single channel components can also use the ESD Suppressors WE-VE. The connection has to be carried out from D+ / D- to GND. The other parts are connected in same way as shown above.

Recommended Layout for Double USB Port

According to single USB port protection the routing is very similar. The use here is exactly the same parts as for single port protection. The protection level will also be the same.

For an easy design of interfaces Wurth Electronics has launched a special Interface Design Kit. This design kit includes a design guide for USB 1.0 to USB 2.0, CAN, Ethernet, VGA, DVI, RS232 and RS485 interfaces and all the components used. These are ESD Suppressors, SMD Common Mode Chokes, Chip Bead Ferrites, LAN Transformers, and the corresponding Connectors.

In total are 35 different passive components with 235 parts and 4 evaluations boards in.

The color scheme makes it easy to locate the suitable parts for your application. Just follow the specific application color and arrange your needed parts.

For each application you find a simple block schematic where you see how to place the different components to get the best result. Give it a try!

Thanks for Following our Protection of USB 2.0 Applications Series!

In case you missed it, read part one of our USB 2.0 Applications Series.

To request a quote or to order samples of products to protect your USB ports, click here.