Electrical – Calculation of capacitance parallel to Bidirectional TVS diode

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How to find the value of suitable capacitor, parallel across the Bidirectional TVS diode? Is there any formula to calculate?

Best Answer

They are all different but the general rule of thumb is capacitance rises as power handling capability increases for pulse power rating

here comparing one TVS diode to another diode. ( aka ESD diode) enter image description here

enter image description here - where Ppw is the pulse power rating where \$ Ppw= Ippm*Vf= Ippm^2*ESR\$

  • comparing Ppw / Cr ratios in [Watts/pF] or ESR*C=T time constant in [ps] , which is an indication of reaction time.

I picked two TVS diodes from Panasonic at random with a widely different capabilities. ( by orders of magnitude) using the 0V capacitance for these 402 SMD's

Another relationship shown in Varistor tables is peak current @8/20us (risetime/width) two times ESR * pF

enter image description here

The power dissipation per pF is in the same range and shown above as 1.8W/pF in spite of significantly different part fabrication, vendor and size.

  • The relationship is due to body size, Rja thermal resistance, chip size and junction capacitance
  • I estimated Pd steady power dissipation package thermal rating even though used in pulse protection using the std 8/20us IEC/UL Lightning pulse. .
    • this is just a quick estimate for relative figures of merit (FoM), not a rigorous equation

Side note

  • you can do the same RdsOn * Ciss =T comparison for MOSFETs easily to compare and understand why several stages of MOSFETs are needed to go from logic level to 1000A due to this ESR * Ciss = T , in order to maintain a max acceptable transition time T, of a half bridge switch.
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