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Minimum dc link capacitor voltage in offline smps?

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Flyback

Well-Known Member
Hello,
I am designing a 30W offline flyback for 230VAC, 50Hz mains.

I know that the input power is 35.2 Watts.
Supposing I select a 22uF , 400V electrolytic capacitor to go after the mains diode bridge rectifier.
How can I calculate the minimum voltage that will occur at the DC link with this size of capacitance?

I have found the equation 3 on page 2 of the following.....

https://www.fairchildsemi.com/an/AN/AN-4137.pdf

....However, I do not know what is the "Dch" value that goes into this equation.
It seems that its a chicken and egg situation.....you can't find the minimum DC link voltage until you know the bridge diode conduction time, and you can't calculate the bridge diode conduction time until you know that minimum DC link voltage.

Do you know of a better equation than this?..one that allows the calculation of minimum DC link voltage for a given capacitor value?
 

MikeMl

Well-Known Member
Most Helpful Member
Try simulation with LTSpice. Run the sim over several input cycles.

Even this simple example shows capacitor voltage, load and diode current, that would be hard to calculate by hand.
 

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Flyback

Well-Known Member
Thanks, I will certainly do the simulation to check the calculation, but I would like to calculate it aswell. Just to show the gaffer I have been thorough and don't rely totally on simulations.
 

MikeMl

Well-Known Member
Most Helpful Member
Thanks, I will certainly do the simulation to check the calculation, but I would like to calculate it aswell. Just to show the gaffer I have been thorough and don't rely totally on simulations.
Gee, I would think you would like to show the gaffer that you know how to use available tools...:happy:
 
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