We have applied 500V EFT voltage across all the terminals to the SMPS. That time we have found that power supply went to continuous hiccup mode (ON-OFF). Load was around 200W.

So to debug, added straps in all main places. at last we found that while applying EFT, input voltage to the power supply is varying between 29V to 98V.

Mine is telecom power supply, and it's working range is between 36V to 74V. So obviously because of this input variation, my power supply is went to hiccup mode.


Questions:
1. Why this input voltage is varying this much while applying EFT to SMPS?
2. If this variation is common means, what will be the workaround for this problem?

Kindly help me on this.


Notes:
1. Variation in the input voltage is in terms of ms.
2. This hiccup condition is not happening with lesser load (<150W)

Lots of possibilities here, in absence of a system diagram.

Where is the input voltage coming from? I think you are indicating that the 29 to 98 V a relatively low-frequency oscillation?

Does the hiccup continue after the EFT is removed?

Perhaps you have a power supply interaction instability that is kicked off by the transient. That might be suggested by the higher power level instability, since it doesn't happen at lower power.