TPS23521EVM-001: Surge Tests with TPS23521EVM-001 Evaluation Board, Failure of Diode D2

Sir / Madam,

We are using TPS23521EVM-001 unmodified along with a Power Supply targetted for compliance and did a surge test.

The Power Entry connector has a TVS Diode AK15-066C-Y for surge protection, the link to the datasheet is below.

https://www.littelfuse.com/media?resourcetype=datasheets&itemid=45126c2f-2329-4cc0-accd-1466b7be46df&filename=littelfuse-tvs-diode-ak15-y-datasheet

I am referring to the Section 3 : Description which says "The 200-V absolute maximum rating makes it easier to survive lightning surge tests (IEC61000-4-5")

The surge test -500V, +500V, -1KV, +1KV surge did not affect the board.

However, -2KV surge test caused the circled Diode (D2) of to fail as a short, circled below.

Refer the picture of Figure 1, page 3 of the document SLVUB67.

When this shorted diode was removed, we confirm there was no other shorting components.

I understand that this diode appears to be a "Freewheeling" diode for the inductor L1.

Below is the summary of the setup and outcome.

Below is the capture for the surge waveform at the instance for failure :

I would like to know what caused the Diode D2 failed to a short ?

I would like to know what the exact purpose of L1 and D2 are ? Can L1 be replaced with a short and D2 be replaced with a second TVS diode ?

Thanks,

Rajesh