#01 How TV works
TVS (transient voltage suppressors), that is, transient voltage suppressors, also known as avalanche breakdown diodes. This is an integrated device with one PN junction or several PN junctions, manufactured using semiconductor technology. TVs are divided into single-sided and double-sided. Single-sided TVs are usually used in DC power circuits, and double-sided TVs are used in AC voltage circuits. As shown in Figure 1, when connected to a DC circuit, unidirectional TVS are connected in parallel in the circuit in the reverse direction. When the circuit is operating normally, the TVS is in a shutdown state (high resistance state), which does not affect the normal operation of the circuit. When an abnormal overvoltage occurs in the circuit and the breakdown voltage of the TVS (avalanche) is reached,
Figure 1. How the TV works
#02 TV selection process
Before choosing TVs, we must first understand the ultimate goal of choosing：
1. Suitable voltage can protect the circuit after cascade；
2. The transfer capacitance of the TVs shown cannot affect the circuit；
3. TVs have enough power reserve to meet the testing standards, and cannot be hung in front of the fuse. The selection process can be carried out according to the following steps：
(1) Select the maximum operating voltage of TVs Vrmw；
(2) Select TVS VC terminal voltage；
(3) Select TV power；
(4) Estimate the effect of leakage current IR；
(5) Evaluate the effect of connection capacitance；
A. Select the maximum operating voltage of TVs Vrmw；
Under the normal operating conditions of the circuit, the TVs should not work, that is, they are in the off state, so the cut-off voltage of the TVs must be greater than the maximum operating voltage of the protected circuit. In this way, we can ensure that the TVs will not affect the operation of the circuit during the normal operation of the circuit. However, the operating voltage of the TVs also determines the clamping voltage of the TVs. When the cut-off voltage exceeds the normal operating voltage of the line, the operating voltage of TVs must not be selected too high. If it is too high, the clamp voltage will be higher. Therefore, when choosing Vrwm, the operating voltage of the protected circuit and the circuit tolerance of the subsequent stage should be fully considered. Vrwm must be greater than the operating voltage, otherwise the operating voltage will be greater than Vrwm,
Vrwm≈1.1~1.2*VCC;--------where VCC is the highest operating voltage of the circuit.
B. Select to select TVS VC clamp voltage；
The TVS terminal voltage must be less than the maximum transient protective voltage that the post-stage protected circuit can withstand. VC is proportional to the flashover voltage TVS and IPP. For TVs with the same power level, the higher the breakdown voltage, the higher the VC. The maximum clamp voltage Vc of the selected TVs cannot exceed the maximum voltage that the protected circuit can withstand. Otherwise, when the TVS clamp is at Vc, it will damage the circuit. Vc can refer to the following formula：
VC <Vmax;-----Where Vmax is the highest voltage the circuit can handle.
C. Select the Pppm (or Ipp) power of TVs；
The rated transient power of TVs must be greater than the maximum transient overvoltage that can occur in the circuit. Theoretically, the higher the power of TVs, the better, and they can withstand more impact energy and frequency, but the higher the power, the more bundled TVs, and therefore the power of TVs can meet the requirements. For TVs with different power levels, the VC value for TVs with the same characteristics, the voltage is the same, but the IPP is different. Therefore, Pppm is proportional to Ippm. The more Ippm, the more Pppm. For a particular circuit, there are corresponding test requirements. If the maximum test current in the real circuit is Iactual, then Iactual can be evaluated as：
Iactual=Uactual/Ri；--------- Where Uactual is the test voltage and Ri is the test internal resistance.
TVs must pass the test, so the actual circuit requires that the minimum power consumption of TVs at a 10/1000 µs waveform is:-------Where di/dt is the waveform conversion factor if other waveforms are the actual test waveform waveform such as 8/20us waveform, then it is recommended to use di/dt, if the test waveform is 10/1000us, then the actual TVS selection should leave a certain margin, and the Pppm power selection for TVS should be Pppm > Pactual .
D. Estimate impact based on connection capacitance and leakage current of selected TVs
When TVS is used to protect high-speed I/O ports, analog sampling, and low-power equipment, the effects of connection capacitance and leakage current must be considered. The smaller these two parameters, the better.
#03 TV selection distance
All the data is boring, let's take real cases as examples. The normal operating voltage of the VCC circuit is 24V, the maximum operating voltage Vmax is 26V, the maximum transient voltage that the circuit can withstand after the cascade is 50V, the test waveform of the experiment is 8 /20μs, the test voltage is 500V, the total internal resistance of the test power supply and the static resistance of the PPTC is 2Ω. Select the appropriate TVs based on the above information.
1) Select the highest operating voltage of TVs
Vrmw≈1.1~1.2*VCC=26~28V
2) Select select TVS clamp voltage
VC＜Vmax=50V
3) Calculate the actual power of the test signal
Actual=Vc**=50*（500/3）*1/2= 4166W
According to the calculation results, TVS 5.0 SMDJ26A can be selected. Since this TVS is used in the power port, the connection capacitance and leakage current are basically negligible.
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