Clamping a 250V transient with a capacitor bank?

[cupoftea]

Hi,
Supposing I have a power supply on a 28V military Bus.
The MIL STD 1275E says it might suffer a 250V transient of energy 2J.

MIL STD 1275E page 3, section 2.2.3
https://www.ti.com/lit/ug/tidudg1/tidudg1.pdf?ts=1672355839311&ref_url=https%3A%2F%2Fwww.google.com%2F

Now suppose I have 20uF of input capacitance in the power supply.
Would I be right in saying that the 20uF input capacitor bank would not be able to quench the 250V transient, because in going from 28V to 250V, ..that only puts 0.62J into the 20uF input capacitor bank?

 

[Easy peasy]

MIL-STD-1275E or MIL-STD-1275D ??

2J is the total available energy from the spike - if your ckt goes high Z then almost no energy is absorbed.

Worst case is of course with the converter sitting at no load.

For a cap, say 20uF, starting at 28V, and charging to 200V the energy absorbed is 0.4 J

In a well designed filter, 200V is all that may exit the filtered end

Large 160V TVS's are starting to look good here - after the filter.

 

[cupoftea]

MIL-STD-1275E or MIL-STD-1275D ??

Thanks, its MIL-STD-1275E (E for Echo)

Large 160V TVS's are starting to look good here - after the filter.

Thanks, My sincere apologies, as i should have said, we cant use 160V TVS's , because they would break over on the 175VDC Surge voltage that lasts for 100ms.

 

[Easy peasy]

Actually an 160V TVS will pass minimal current at 175VDC

--- Updated Dec 30, 2022 ---

You will need some form of clamp at some voltage - as a fast rising transient into an EMC filter at no load can ring up to twice V applied on the filter caps if not well designed - any unloaded LC ckt will do this.