Protection of TVS Layout and System against ESD


The constant evolution of advanced chip manufacturing process leads inevitably to the growing vulnerability of chips to ESD/EOS interference, resulting in decreased resistance of connectors, such as USB, HDMI and LAN, to interference energy like ESD/EOS. Adding a TVS element to connectors has become a trend in system product design. The following are parameters that need attention in selecting a TVS element. 

There are a few parameters that needs attention when selecting an appropriate TVS element, including: (1) working voltage (VRWM), where the working voltage of the TVS cannot be lower than that of signals in order to keep your signals free from interference in a normal working condition; (2) capacitance, where a suitable size of capacitance should be selected to prevent signals from being compromised, since excessive parasitic capacitance of TVS element causes problems in high-speed signal transmission; and (3) clamping voltage, which is a very important parameter for TVS protection elements, as the lower the clamping voltage, the better protection for system against ESD/EOS. 

TVS’s clamping voltage is an important parameter for protection against ESD/EOS as mentioned above. However, if there is no suitable layout in system circuitry to accommodate TVS, the TVS cannot do its job protecting against ESD/EOS energy impact even if the best TVS element is selected, and the end result is system damage. Next, we are going to talk about how TVS layout affects system protection. 

2 suggestions are proposed for the layout: 

(1)   The TVS should be placed as close to connector as possible As shown in Figure 1, when the TVS is located close to connector (I/O), the majority of incoming energy is dissipated to ground very quickly through TVS, which is a better way to locate a TVS element; on the other hand, when the TVS is placed a distance from connector, the chance is greater for the incoming energy to generate signal interference, which interrupts system operations before TVS can provide its protection.  

Figure 1 

(A) Incoming energy bypass when the energy hits the TVS element in the circuit 

(B) Difference in interference between how far and how close the TVS is from the port as the incoming energy comes in 

(2) Locating a TVS on a branch should be avoided PCB is a piece of prime property. For this, a branch is added to the single wiring for connection to TVS for the convenience to locate it, as shown in Figure 2 (left). A branch like this may prevent TVS from working its best to protect the system. When usually wiring a TVS, it is suggested to have the signal wiring to be protection straight through the TVS, and a branch is not recommended. We are going to simulate system wiring design with actual measurements to show how TVS branch affects the protection the TVS provides. 

Figure 2 TVS wiring

Figure 3 is an experiment test board. The experiment is performed with AZ1143-04F. On the far left is the signal wiring position. Various TVS positions are designed on the signal wiring to simulate the changes in system protection provided by TVS as the TVS is located at different distances from the signals. On the board from left to right, there are Position 1 through Position 4. A TLP system measures the difference in clamping voltage due to different TVS locations from the signal wiring, and the measurements are used to demonstrate the changes in TVS protection. To focus on the impact of distance between signal wiring and TVS on the clamping voltage, the test board is designed to have GND closet to the TVS in order to reduce the influence of ground circuit on the experiment. 

Figure 3 Experiment test board

The TLP measurement results (Figure 4) show that the clamping voltage measured at Position 1 is the lowest at 8.6V and that at Position 4 is the highest at 12.2V, both with the same TLP current at 16A. As the TVS is located farther away from the circuit to be protected, the clamping voltage starts to increase in the chips in the back, indicating decreasing protection. 

Figure 4 Clamping voltages measured on the experiment circuit at Positions 1-4 using TLP system

This experiment proves that the TVS layout has a significant impact on system protection in the system circuit design. Clearly for designing circuit protection, the protection element should be located close to connector and the TVS should be placed directly on the signals to be protected, as this allows TVS to provide all the protection it can for system protection against ESD/EOS and, thus, improves system resistance to external ESD/EOS interference. 

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