Transient and Surge Protection

Question

It seems that when choosing ESD/TVS diodes, MOVS and GDTs that while their clamping voltage is rated in Volts, their protection is rated in Watts or Joules.

For example, the LittleFuse SMCJ250CA is 405V Clamping and 3.7A Current. On the front page it states "1500W peak pulse power capability at 10/1000μs".

This makes sense I suppose.

But then the standards I have been reading only talk about transient/surge voltages. For example: 1kV Surge at 1,2us/50us (8/20).

Since the standards I have read are only referencing voltage not current or power. How do I use this information to select a protection device which talks about Watts? Is there some standard current values to assume?

Thanks

Answer 1

For example: 1kV Surge at 1,2us/50us (8/20).

That sounds like EN 61000-4-5 (indirect lightning surges) and, if so then the output impedance of that surge will be specified. In most situations, this will be 2 Ω hence, the peak current into a short circuit could be as high as 1 kV / 2 Ω = 500 amps.

The worst case power delivery is when the suppression device looks about 2 Ω - it will dissipate a peak power of half the volts squared divided by 2 Ω. That's 125 kW. OK, it's only for a short duration but, your device (SMCJ250CA) or, any device in that range is incapable of surviving that for a 50 μs surge duration: -

You need to have a much heftier TVS to survive EN 61000-4-5 if you are trying to protect a power port (2 Ω source impedance) from surges.

choosing ESD/TVS diodes

If you are only interested in ESD then you might find it works but I'm not going to guess what specification you might be considering.