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Offline SMPS: Minimum DC link voltage?


Aug 3, 2009
Aug 3, 2009
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.....

....However, I do not know what is the "Dch" value that goes into this equation.
It seems that its a chicken and egg 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? that allows the calculation of minimum DC link voltage for a given capacitor value?

...Ok, in the end I plotted vpksin2pift from 0ms to 5ms.....and then for each one, calculated the energy and the corresponding power, and then read it off when the power was the input power......this gave for 22uF a min dc link volts of 279V for 36w input power and 230vac mains.
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