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I am trying to calculate the duty cycle of the buck converter as in the figure for discontinuous conduction mode of operation. I have worked to the math and have got the following graph as in the picture- Buck Converter circuit Discontinuous conduction mode buck converter waveforms of current and voltages

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The Duty cycle depends on the load current. How we find out the on time if we do not have the load current?

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You can't figure out how long your switch is going to have to be on without knowing what current the load draws!

Remember, the physical idea of your converter is "charge the field in an inductor, exactly as much as necessary to supply the right output voltage".

A load takes energy from that field, so you'll have to replenish that, and that's exactly what you do during your on time. So, if less energy is taken, then there's less on-time.

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  • \$\begingroup\$ Exactly! You need to understand your load's V/I characteristic +1 \$\endgroup\$
    – Andy aka
    Commented May 1, 2020 at 15:11
  • \$\begingroup\$ @Andyaka I'd even go one step further and say: any voltage regulator is exactly that, a regulator, and hence, a control system. In the case of the buck converter, the only control element is the on-time - and hence, it has to react to the changes in load conditions – not only if the load has a fixed V/I characteristic, but also in a dynamic sense. \$\endgroup\$
    – Marcus Müller
    Commented May 1, 2020 at 15:20
  • \$\begingroup\$ what if the discontinuity is due to the extremely low ON time period? @MarcusMüller \$\endgroup\$
    – Ishani Engineeŕ
    Commented May 1, 2020 at 15:21
  • \$\begingroup\$ my answer applies! \$\endgroup\$
    – Marcus Müller
    Commented May 1, 2020 at 15:22
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    \$\begingroup\$ If you are not sure about your loading changes go with a synchronous buck converter because the output is largely load-independent. \$\endgroup\$
    – Andy aka
    Commented May 1, 2020 at 15:28

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