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Mains transient lasting 2 seconds?

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cupoftea

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Hi,
We have been asked to bring up a 500W PFC’d full bridge SMPS for a customer.
115VAC, VOUT=28, PFC=200KHZ, Fullbridge=250kHz, PFC chip = UCC3818.
Its hard to get past their project managers and to speak to their actual engineers.
Their spec says "VAC input = 105-124VAC".
However, it also says that the VAC input may go up to 172VAC for 2 seconds, and the PSU should be able to “withstand” this. So, the spec says “Input transient withstand = 172VAC”.
Do you know what on earth could make the VAC input do this?.....would it be a smooth rise up to 172VAC, or a brick wall rise to it?
I don’t see the point of having the PFC Vout set to 275Vdc (which they have), when the peak of 124VAC is just 175V…..I believe we should change PFC vout to 235Vdc. Would you agree?
(We can bypass the PFC sense resistor and PFC inductor with diodes for any surge currents)
 

A PFC DC rail of 200V is reasonable, as is bypass diodes to handle surges, but the caps should be rated to at least 250VDC ( + surge if you can buy them ) to avoid catastrophy.
 
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