EMC Protection of USB Applications

April 17, 2015 // By Jochen Baier, Würth Elektronik eiSos
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.

Concerns regarding the susceptibility of USB ports are even raised in Intel's "High Speed USB Platform Design Guidelines". Intel recommends the use of current-compensated chokes for EMI suppression and further components for protection against electrostatic discharge. Electronics are exposed to electrostatic discharge. ESD pulses have voltages of up to 30 kV and and are therefore hazardous for all types of integrated circuits. Some current ICs are "safe" against ESD, but this safety is only guaranteed for a small selection of the potential threats. Daily practice shows: Additional protection is indispensable. Only with external protection is the complete board ESD-free and highly dependable products can be developed. Dedicated suppression measures are equally imperative. Wireless connected electronic devices are to be found in all spheres of life and their number is growing continuously.

It is important to make one's own products immune to interference radiation. Only if the anticipated forms of interference are considered, can the necessary suppression components be immediately integrated in the design and development time reduced. As we know, the radiated interference of one's own product also has to stay below a certain level. This is very precisely evaluated by EMC test laboratories. If the product fails this test, the costs for reworking very soon exceed the costs of suppression components many times over.

Actually interference immune

Differential mode data transmission offers a significant advantage over the simple coaxial cable when it comes to the effect of interference on the USB. In the case of the inductive interference effect (magnetic field), the twisting of the wires achieves compensation of the interference effect. With symmetrization of the partial inductances of the respective twisted wire, the interference influences compensate each other. This interference immunity can be compromised in practice.

  • The inputs/outputs of the USB controller are insufficiently symmetrical, the USB signal displays common mode interference.
  • The layout is not HF/EMC compatible, parasitic capacitances and the lack of wave impedance matching generates
Design category: