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TVS and inductive kickback

TVS and inductive kickback

TVS and inductive kickback

Hi everyone,
I am using a TVS to protect an SSR (no zero cross circuitry) from a soleniod valve.  The specifications of the three parts in question are:

SSR:  2A continous - 100 V max
TVS: (SMBJ51CA) Bi-directional - 82 V max clamp - 7.3 A max peak pulse current
Solenoid: 1.1 A max continous - 24 VAC

My calculation for the peak pulse power is Ppeak=Imax x Vclamp = 1.1 x 82 =  90.2 W

The maximum time for the inductive kickback transient is 1ms and soleniod is only turned on every 1 minute or so.  The average power dissipated by the TVS is 90.2 x .001 /  60 = 0.0015Wavg.  This tells me that all is well since the continous power dissipation of such a part is well into the 1W area.

Am I in the clear here? Or did I miss something?


RE: TVS and inductive kickback

The TVS shall have its energy dissipation rating in Wattseconds or Joules available, to be on the safe side, since it makes a difference if:
Energy dissipation = 82 V x 7.3 A is for 1 second = 598.6 Wattseconds (or Joules), and for 1 milisecond
Energy dissipation = 82 V x 7.3 A x 0.001 second = 0.5986 Wattseconds (of Joules).
Therefore the TVS may be rated at approximately 600 Wattseconds in the first case and 0.6 Wattseconds in the second case. Obviously, the 0.6 Wattsecond rated TVS would not be able to withstand 600 Wattseconds, and it would be damaged (e.g. by the thermal runaway). The energy rating of TVS affects its physical size. The bigger energy dissipation is needed, the bigger TVS will be. Also, the peak voltage rating for the TVS is supposed to be available, in general.

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