TVS Calculation and Selection (03)

TVS  Calculation and Selection (03)

TVS selection process

After understanding the basic parameters of TVS, we begin the process of selecting the most important TVS. Before choosing TVS, we first need to understand the ultimate goal of choice: 1. Suitable voltage can protect the subsequent circuit; 2. The junction capacitance of the introduced TVS should not affect the circuit; 3. TVS has sufficient power margin, meets testing standards, and cannot be hung before the fuse. The selection process can be carried out as follows:

Select the highest working voltage Vrmw for TVS;

Select TVS clamp voltage VC;

Select the power of TVS;

Evaluate the impact of leakage current Ir;

Evaluate the impact of junction capacitance; Select the highest working voltage Vrmw for TVS; Under normal working conditions of the circuit, TVS should not work, that is, it is in the cut-off state, so the cut-off voltage of TVS should be greater than the highest working voltage of the protected circuit. This can ensure that TVS will not affect the circuit operation under normal circuit operation. However, the working voltage of TVS also determines the clamping voltage of TVS. When the cut-off voltage is greater than the normal working voltage of the line, the working voltage of TVS cannot be selected too high. If it is too high, the clamping voltage will also be higher. Therefore, when selecting Vrwm, it is necessary to comprehensively consider the working voltage of the protected circuit and the bearing capacity of the subsequent circuit. Require Vrwm to be greater than the working voltage, otherwise working voltage greater than Vrwm will cause TVS reverse leakage current to increase, close to conduction, or avalanche breakdown, affecting normal circuit operation. Overall, Vrwm can refer to the following formula: Vrwm ≈ 1.1~1.2 * VCC-------- VCC is the highest operating voltage of the circuit.

Select TVS clamp voltage VC; The clamping voltage of TVS should be less than the maximum transient safety voltage that the protected circuit can withstand, and VC is proportional to the avalanche breakdown voltage and IPP of TVS. For TVS of the same power level, the higher the breakdown voltage V C, the higher the breakdown voltage V. The maximum clamping voltage Vc of the selected TVS cannot be greater than the maximum voltage that the protected circuit can withstand. Otherwise, when the TVS clamp is in Vc, it will cause damage to the circuit. Vc can refer to the following formula:

<Vmax----- Where Vmax is the highest voltage that the circuit can withstand

Select the power Pppm (or Ipp) of TVS; The rated transient power of TVS products should be greater than the maximum transient surge power that may occur in the circuit. In theory, the higher the power of TVS, the better it can withstand more impact energy and frequency. However, the higher the power, the larger the packaging of TVS, and the higher the price. Therefore, the power of TVS can meet the requirements. For TVS of different power levels, the V C value of TVS with the same voltage specification is the same, but the I PP is different. Therefore, Pppm is directly proportional to Ipm, and the larger the Ipm, the greater the Pppm.

For a certain circuit, there are corresponding testing requirements. If the maximum test current in the actual circuit is actual, then actual can be estimated as: Actual=Uactual/Ri--------- Among them, Uactual is the test voltage, and Ri is the test internal resistance.

To pass the test, 10/1000 is required in the actual circuit μ The minimum power P actual of TVS under waveform s is: ------ where di/dt is the waveform conversion coefficient. If the actual test waveform is another waveform, such as 8/20 μ S-waveform, it is recommended to take di/dt, for example, the test waveform is 10/1000 μ s. In actual selection, there should be a certain margin for TVS, and the power PPMP selection of TVS should follow Pppm>Pactual.

Evaluate the impact based on the junction capacitance and leakage current of the selected TVS. If TVS is used in high-speed IO port protection, analog signal sampling, and low-power equipment, it is necessary to consider the impact of junction capacitance and leakage current. The smaller the parameters of both, the better