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Survivability of electrolytic capacitors to overvoltage?

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treez

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On page 74 of the following App note concerning a 25W offline LED driver, there is a scope shot showing a peak voltage of 841V which appears across a 4.7uF, 450V electrolytic capacitor (C3).
The LED driver circuit is said to survive this and remain operational.
Can electrolytic capacitors always be expected to survive such overvoltage?

The schematic is on page 7…

LED driver app note:
https://led-driver.power.com/design...gn-examples/der-429-18-25-w-isolated-flyback/
 

The better specified capacitors will have a surge rating, usually expressed as a porcentage of the maximum continuous rating, over a period of time (1 minute if a remember correctly)

But you are mentioning almost twice the rated voltage.
Unless the particular capacitor vendor specifies this, I would not assume that every capacitor could withstand it.
 
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Thanks, i agree, thats a heck of an overvoltage for that electrolytic to withstand.
Also, in real life, i would think it would need one heck of a mains transient to take that electrolytic capacitor voltage up to 841V?
For a typical 60us mains transient, that represents a power of 22.3kW.

I doubt the capacitor would survive very long after such an event?
 

Hmmmm
Perhaps it would lose some useful life.
 
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How does this Vdrain voltage affect C3? In the diagram they are not connected?
 
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Thanks, the transinet comes from the mains, ie via C3.
As you know, V(drain)_OFF = Vin+v(reflected).......plus the drain spike of course.
 

I am sure the DC bus is stiff enough and there are enough lossy components that C3 won't see any damage from this voltage spike.
 

Such a voltage overshoot, if the Cap really does see it, will not do the capacitor any good. But, what really shortens the life of electrolytic capcitors is violation of its ripple current specifications. Not all electrolytics are the same, and those designed for SMPS and other applications typically have higher ripple current allowance than the "generic" ones. I would make careful measurement of your circuit to determine ripple current from the voltage ripple on the cap and choose a capacitor that is rated for at least 50% -100% higher for best reliability/longevity.
 
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