For flyback SMPS, how do we know that our input capacitor value is sufficient for some peak power requirement?

I understand that bigger input capacitor will have more advantage on this peak power. These capacitors are local energy buffers that can quickly supply high currents to the load when it needs it, but how minimum do we need to meet this peak power?

Schematic reference:


Assume that the transformer have enough primary current,

Example if the flyback SMPS is designed at rated power 16W, and it has peak power requirement at 80W, therefore I need to change capacitor value from 47uF to 270uF?

Below are some calculation that I made & some reference from Infineon Design Guide


Is this the correct way?

Please share if you've any better way to do it.

UPDATE : 29-MAY-2020

Thanks to @Verbal Kint for very useful comment.

By referring to some equation in Bulk Capacitor Calculations, C.Basso, I think the estimation of the input capacitor value for certain peak power requirement will be better due to it able to check the minimum bulk valley voltage at low line while delivering 94W.

Below are my new calculations:


  1. Vin
  2. Vpeak
  3. Vmin
  4. Pout

By using equation below, I will get 116.4uF


After that, selecting a normalized value for the bulk capacitor. For this example, I choose 150uF, and check value of Vmin when the equation below equal to zero.




Remark: For charge/discharge curve, Vcmin = 84V is based on td = 5.957ms, Vpeak = 120.2V, Cbulk = 150uF

From graph above, during Y=0, Vmin = 68V, and Vbulk = 52V

Based on that, for peak power 94W at 85V minimum voltage input, with 150uF capacitor, the capacitor DC voltage will discharge from 120V to 68V within 6ms.

Just one new question, how do we relate peak duration, example the peak duration requirement is 100ms into the calculation above?

  • 2
    \$\begingroup\$ What is the peak duration? Also, going from 18 W nominal to 94 W, it is quite a peak Cyrano would say! Depending on the recurrence and the duration of the peak, I would not calculate the input capacitor for steady state (as the sheet implies) but I would look at the very minimum bulk valley voltage at low line while delivering 94 W. Depending on the minimum operating voltage of your converter, you can perhaps tolerate an increase of the bulk ripple voltage down to 50 V dc for the valley worst case and it helps reducing the effective capacitance you need. \$\endgroup\$ – Verbal Kint May 22 '20 at 7:50
  • \$\begingroup\$ @VerbalKint, after read your paper cbasso.pagesperso-orange.fr/Downloads/Papers/…, I think it should able improve my estimation. Thank you so much! I updated my calculation as above. Just one comment, how do we relate the peak duration, example the peak duration requirement is 100ms into the calculation above? \$\endgroup\$ – Lutz Fi May 29 '20 at 4:09

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Browse other questions tagged or ask your own question.