Analysis of Misunderstandings in the Selection and Calculation of TVS Tube Clamping Voltage

一、Clarification of core concepts

1.1 Definition of key parameters

Typical misunderstanding: Mistakenly believing that VRWM≈Vc (measured for a certain 5.5V VRWM device, Vc=9.8V at 10A)

二、Selection Misunderstandings and correction solutions

2.1 Misconception 1: Directly matching the working voltage

• Incorrect practice: Selecting a TVS with VRWM=3.3V for a 3.3V system
• Failure mechanism: Vc can actually reach 5.8V > IC tolerance limit (4.5V)

• Correct formula： 

•  Case: A certain MCU can withstand a voltage of 6V. TVS with Vc≤5.1V (such as ESD5V0U1U) should be selected.

2.2 Misconception 2: Ignoring Dynamic Impedance (Rdyn)

• Misconception: Believing that Vc is a fixed value

• Actual relationship：

• Measured data：

  Model	Rdyn(Ω)	When Ipp=5A, Vc	Vc when Ipp=10A
  SMAJ5.0CA	1.2	8.2V	14.2V
  SMBJ5.0CA	0.8	7.0V	11.8V

  
  

2.3 Misconception 3: Temperature drift is not calculated

• Temperature coefficient formula：

• Key parameters：

• Silicon-based TVS：α_T ≈ 0.1%/℃
• Automotive electronics case: At 125℃, Vc increases by 12% compared to 25℃

三、Engineering selection process

四、Actual measurement verification method

4.1 Double-pulse test method

IEC61000-4-5Standard waveform：

• First pulse: 8/20μs waveform (simulated lightning strike)

• Secondary pulse: Repeated impact at an interval of 300ms

• Failure criterion: If the Vc fluctuation is greater than ±5%, it is determined that the thermal stability is insufficient

4.2 Infrared thermal imaging positioning

• Safety threshold: ΔT < 40K (@Rated Ipp)
• Dangerous case: A TVS device experienced a temperature rise of 82K at 15A, causing the package to crack

五、Classic Rectification cases

5.1 The industrial RS485 port has failed

• Original selection: SMAJ6.5CA (nominal Vc=10.5V)
• Actual measurement issue: At 12A surge, Vc=14.3V > the tolerance limit of the optocoupler (12V)
• Solution: Replace with P6KE6.8CA (Rdyn=0.5Ω, Vc=9.8V@12A)

5.2 ESD damage to the on-board camera

• Incorrect design: TVS with VRWM=12V (Vc=19V) was selected
• IC actual tolerance: 15V (compliant with ISO10605 requirements)
• Rectification plan: Replace with AUTOESD15D (Vc=12.8V@8A)