Basic characteristic of TVS
TVS (Transient Voltage Suppressor) is an electrical device which is used to protect the electrical system from the ESD (electrostatic discharge) damage. The symbol of TVS is the same as Zener Diode, and the TVS are operated at reverse-biased region. Figure 1 shows the basic characteristic of TVS. As a transient voltage input to the TVS, the TVS will be trigger on and clamp the transient voltage at VCL. But when the input signals are normal signals, the TVS will be in an off state.
There are basic parameters of TVS(Figure 2 and Table 1)
(1) VRWM(Reverse Work Max Voltage) : The max work voltage of TVS.
(2) ILeak(Reverse Leakage Current) : The leakage current of the TVS operated at VRWM.
(3) VBV(Breakdown Voltage) : The breakdown voltage of TVS.
(4) Rd(Dynamic Resistor) : As the TVS are trigger on, the TVS will behave as a small resistor, and the resistor is called dynamic resistor.
(5) Vclamp(Clamping Voltage):As the TVS are trigger on, the voltage on the TVS is called clamping voltage.
TVS on the electrical system
The main function of TVS is protect the electrical system from the damage of high voltage and high energy pulse such as ESD (Electrostatic Discharge), Surge and EFT (Electrical Fast Transient). Figure 3 shows the ESD current path with a TVS on the electrical system. As the ESD pulse input the connector, the TVS will be trigger on and the ESD current will be short to the system GND. The TVS will clamp the voltage at Vclamp, hence the internal circuit will be biased at Vclamp when the system are zapped with ESD pulse. Figure 4 shows the reality case of TVS used on the electrical system. In order to minimize parasitic inductance in the board traces, all path lengths connected to the TVS should be kept as short as possible. The TVS should be place near the input terminals or connectors to restrict transient coupling.
The clamping voltage of TVS
The TVS will be trigger on and the ESD current will be short to the system GND. And the TVS will clamp the voltage at Vclamp, hence the internal circuit will be biased at Vclamp when the system are zapped with ESD pulse. For this reason, the clamping voltage of TVS will be a very important parameter. In order to verify the importance of clamping voltage, we chose two different TVS. Figure 5 shows the clamping voltage of this two TVS, and named TVS1 and TVS2. From the Figure 5, the dynamic resistor of TVS1 is 0.25 ohm. And the dynamic resistor of TVS2 is 0.4 ohm. Due to the lower dynamic resistor, TVS have a lower clamping voltage than TVS2. In order to verify the protection ability of this two TVS, we used this two TVS in a computer motherboard to protect the USB (Universal Serial Bus) port. And then we do the system level ESD test to the USB port. The table 2 shows the result of this ESD test. The USB port with TVS1 can sustain 3.5kV system ESD test, which is better than USB port with TVS2. Because the TVS 1 have a lower clamping voltage than TVS2.
How to measure the clamping voltage
The clamping voltage of TVS is a very important parameter, and we usually used the TLP (Transmission Line Pulsing) system to measure the clamping voltage. Figure 6 shows the basic principle of TLP system. The TLP system will stress a series of square wave on the TVS. Every time a square wave stress on the TVS, the TLP system will measure a voltage value and current value of TVS. After a series square waves stress on TVS, we can have the TLP voltage-current curve. And the TLP voltage is the clamping voltage of TVS.