Electromagnetic Compatibility - 4

For beginners in electronics or in that case engineers at the junior level, looking at the different options available for the protection at the input gives lot of headaches in component selection. For that case, first let us look at the scenarios where protection is to be implemented. Designers inorder to comply to different EMI/EMC standards across different domains of electronics have to protect their circuitry against the following cases:

1. Over Voltage

2. Reverse Polarity

3. Inrush Current

4. Electrostatic Discharge (ESD)

5. Electrical Fast Transients (EFT)

6. Electrical Surges

7. Lighting strikes

8. Radiated RF

9. Conducted RF

10. Short circuit protection

11. Ingress protection

Now, when we talk of circuit design, designers have to ensure that none of these cases affect their circuit functionality. When electronics are built, designers take their systems to accredited labs like TUV and get their systems tested for compliance. The compliance tests and standards depend on the domain in which end units are intended to be used.

For over voltage, reverse polarity, inrush current, short circuit protection implementation is straight forward. But for ESD, EFT, Lightning and surges the protection mechanisms are little confusing.

1. When your systems has to pass ESD, using TVS diode is the option. TVS diode is like Zener diode but with different construction of p-n junction. Many designers tend to confuse with TVS and Zener diode. The Zener diode is intended to be used with regulating the voltage and allows low power transients to scape through. TVS diode operates with large avalanches and power transients. It is always the energy difference (in terms of Joules) that we talk about. For low voltage clamping applications, zener is the best while for high energy pulsed applications, TVS is needed. The impedance is high for TVS diodes and this is the reason they are not so usable at low power applications. So, for your circuit to pass ESD tests, TVS diode on the terminals which are exposed outside (Example, USB connector, power conenctor, etc) the systems is necessary. TVS diode could be bi-directional as well as uni-directional. Uni-directional TVS diodes helps to eliminate ESD on one side while bi-directional TVS diodes eliminate ESD on main and ground lines. 

2. When it comes to much higher energy, in terms of Joules, that try to interfere with the circuit through the I/O terminals, TVS diodes may not be usable. In this case, we have to move towards MOV (Metal Oxide Varistor). The MOV and TVS diode does the same, clamp to a certain level and divert the high energy (to ground) away from the circuit. For protecting the circuit against strikes which arise out of lighting, transients occuring becasue of fast load switching (EFT/Surges), MOV is to be preffered. Here again, based on the domain and standards applicable, designer have to take a call on the protection.