EMC protection of USB applications

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.

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 common mode interference.
  • The circuit design (USB filter) is inadequate, the filters affect the signal quality and/or the insertion loss is too low.
  • The interface design (receptacle, housing) is inadequate. Poor ground reduces the shield attenuation of the cable. Filters have poor ground reference.
  • The USB cable is asymmetrical, poorly shielded and has inadequate ground connection. The cable deteriorates the signal quality, radiates signal harmonics and has insufficient shield attenuation towards external interference sources.

The problem: Some of these points cannot be influenced, these include the technical realization of the purchased USB controller or the end user using "cheap" USB cables. So preventive measures have to be met to protect the interface from the effects of external interference, which can lead to the destruction of the USB controller and to limit interference radiation of signals via the cable.

Selection of the protection components

Protection against electrostatic discharge is defined as: protection against ESD pulses in accordance with EN 61000-4-2, protection against surge pulses according to EN 61000-4-5 as well as burst (EFT) pulses in according to EN 61000-4-4. Transient Voltage Suppressor (TVS) diodes have to fulfill these functions. Here it is important: To protect fast data lines like USB against overvoltage, TVS and ceramic ESD suppressors with low capacitance should be used, such that no distortion of the USB signal occurs. For this reason, Würth Elektronik eiSos develops components which are precisely optimized in this respect and are invisible on the data line. TVS diodes have capacitances below 1 pF and ceramic ESD suppressors with capacitances up to 0.2 pF are a good choice for protecting USB ports.

Several questions need to be asked when selecting the components: Is there a supply voltage for rail-to-rail connections (GND < I/O signal < Vcc)? Take a TVS diode. Is there no supply voltage or should ceramic components be preferred? In this case a TVS diode has to be chosen for which the VDD pin can float or ESD suppressors are used, such as the WE-VE "ULC" series from Würth Elektronik. Further questions: What maximum ESD voltage is expected? Should one or two USB ports be protected? Incidentally, if a USB data line is connected to two I/O pins of the TVS diode, this always produces better protection, which is why a TVS diode array should preferably be used.

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