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I am working on a rectifier design. For that I am analyzing the formula for capacitor selection. I came across a video reference.

enter image description here

  • Io: average output current
  • Vo: Average output voltage
  • Vc: capacitor voltage
  • Im: peak current
  • α:duration of peak current
  • π: half cycle duration
  • Vr: ripple voltage
  • Vm1,Vm2:capacitor peak voltage

I understood that for capacitor selection the formula is:

c=(π-α)/π * Io/2fVr

I don't understood how Vm2=Vm1 * Cosα.

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  • \$\begingroup\$ Simply follow the cosine curve in that picture, left from the peak Vm1, for angle alpha. It's right there in the picture. \$\endgroup\$ – user_1818839 Sep 24 '20 at 11:13
  • \$\begingroup\$ thanks for help @Brian Drummond \$\endgroup\$ – power machines Sep 24 '20 at 11:16
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    \$\begingroup\$ You can also have a look at a paper I published on the bulk capacitor selection for ac-dc converters. \$\endgroup\$ – Verbal Kint Sep 24 '20 at 11:26
  • \$\begingroup\$ @ VerbalKint i will refer this paper for calculations thanks \$\endgroup\$ – power machines Sep 24 '20 at 11:58
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Simple trigonometry:

\$V_{m1}\$ is the voltage at \$t = \pi/2\$. and \$V_{m2}\$ is the voltage at \$t = \pi/2 - \alpha\$.

\$\sin (\pi/2-\alpha) = \cos\alpha\$

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  • \$\begingroup\$ thanks for help @RohatKılıç \$\endgroup\$ – power machines Sep 24 '20 at 11:15
  • \$\begingroup\$ You can accept my answer so that the question does not remain open. \$\endgroup\$ – Rohat Kılıç Sep 24 '20 at 11:17
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If alpha is 90 degrees, then the stored voltage has decayed down to zero volts.

This is consistent with VM2 = VM1 * cos(90).

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  • \$\begingroup\$ thanks for help@analogsystemsrf \$\endgroup\$ – power machines Sep 24 '20 at 11:16

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