Common mistakes while using the TVS diodes

 

There are several sections in the electronics designs which play a crucial role in the functionality of the circuit. Along with functionality there is also a need protect our circuits against external noise. One of the common issue seen is the damage to the electrical circuit with the ESD and Electrical Transients. These enter the circuit at the points like the connectors where the system tries to communicate to the external world. So, protection mechanism employed at the input of the connector is very important. Protection against ESD/Transients can be achieved by TVS diode which is a kind of Zener diode.

The protection must always be included in such a way that it should not interfere with the normal functionality of the board. The need is to protect when ESD/EFT event occurs, have a least leakage when there is no ESD/EFT. Leakage can be minimized but cannot be avoided.

Considering above explanation, it is important we avoid below mistakes while using TVS diodes:

• We might have selected a perfect fit TVS diode for the application but our role doesn't end there. Placing the component on the PCB at the right place is very important. TVS diode must be placed very close to the connector. We should never allow the transient to travel long on the board.
• We all know that TVS diode is placed parallel to the line. This meant there is a tap from the main power/signal line to the TVS diode. This tap must be as small as possible. Longer tap leads to more inductance which is not desirable.
• One of the common scenario we see these days is the cost reduction and designers are using single package of multiple TVS/ESD diodes and due to space constraint they place this away from the connector. In those scenarios, designers have to trade off between cost and design pass and roll back to single diodes which can be placed conveniently to have best performance.
• We commonly see TVS/ESD ground connection not done for performance. TVS/ESD diodes must be connected to the chassis ground and not common ground. In circuits, where there is no chassis ground, ensure that there is no long path back to connector ground.
• Designers commonly use same TVS diode for the power and signal lines. While this is no harm, the capacitance of the TVS diode is a problem. In signal protection scenarios, it is always important that we use low capacitance diodes especially in the high speed signal scenarios. Yes, protection is important but signal integrity is also important.
• Selecting the right clamping/breakdown/peak voltage is always the focus but the designer has to look at the power dissipation aspect as well for the TVS diode. 
• Most of the designers doesn't look at the power dissipation and heat sink requirements and see that the TVS blow up during the compliance tests
• One of the common problem designers face is understanding the specifications, peak voltage, working voltage, clamping voltage, breakdown voltage. Without proper understanding of these hard to find a fit-in TVS diode for a given application. Remember that manufacturers have a different way of mentioning them which complicates the things.
• Bi-directional and uni-directional are used interchangeably by various designers. There is no need to use Bi-directional diode all the time. Bi-directional diode can be used if a negative swing is accepted and not the case in the circuits.
• A never ending search by the designers finding a diode where breakdown voltage same as maximum operating voltage of their circuits and ultimately diode failing at the right moment. Designers have to understand that they should not select a TVS diode with their maximum operating voltage of circuit same as breakdown voltage of TVS. 
• Designers must not just have the operating conditions of the circuit while selecting the protection diodes but they also must keep the compliance requirements of the board in mind.