EMC protection of USB applications: Page 2 of 4

April 17, 2015 //By Jochen Baier, Würth Elektronik eiSos
EMC protection of USB applications
The USB port has revolutionized PC peripheral devices and is becoming increasingly popular, for example in acquiring measurement data or installing software updates on machines. As a bus system for data transmission, it is found wherever mobile devices have to be connected. Although the connectors used in daily life may seem robust, those who develop USB applications must still not neglect protection of their interface.

Don't forget the power supply

For an end-to-end EMC-compliant design, it is important to also filter the power supply (VBUS). Many developers neglect this point and wonder why their product does not pass the tests in the EMC laboratory. Two optimized designs for one or two USB ports are presented here. Two USB lines can be protected with a TVS diode. All four signal lines, as well as the common power supply, are well protected against ESD. Further optimization is achieved by setting up an LC filter with a current-compensated data line choke and capacitors to filter common-mode and differential-mode interference at the input. Excellent suppression is achieved at the power supply with a WE-CBF series SMD ferrite. (Figure 1)

Figure 1: Two-port USB port with ESD protection

Single channel protection components, such as the WE-VE series ESD suppressor must always be connected from the signal line to ground. No low capacitance ESD suppressor has to be used to protect the power supply, a normal SMD varistor is quite sufficient here. This can absorb higher energies and higher peak currents and so is the first choice in design. (Figure 2)

Figure 2: In contrast to shielding the data line, no low capacitance ESD suppressor is necessary for the power supply.

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