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Statement 1: The current at which the Buck SMPS moves into Discontinuous Conduction Mode is not fixed and changes due to different factors.

From what I understood, a Buck SMPS enters discontinuous mode when the output current reaches below the critical current which is equal to half the output ripple current? How can it then enter discontinious mode at different currents and what are these different factors that effect this current

Statement 2: For a fixed output current From a Vout/Vin VS duty cycle plot, You can obtain two different graphs of the SMPS in discontinuous and continuous mode. You can then see when it will enter discontinuous mode

I am currently at home self teaching myself this and do not have access to do this but If I did how exactly would I push the SMPS into discontinuos and continuous when the output current is fixed?

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  • \$\begingroup\$ Varying the switching frequency, input voltage or output voltage set point will have an effect on when the buck will enter DCM. \$\endgroup\$
    – John D
    Feb 24, 2021 at 0:41
  • \$\begingroup\$ Synchronous buck regulators don't really enter discontinuous mode. \$\endgroup\$
    – Andy aka
    Feb 24, 2021 at 8:43

2 Answers 2

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A buck converters enters DCM when

$$\frac{2Lf}{R_{load}} < 1-D$$

where

f is the switching frequency

D is the duty cycle

L is the buck inductance

One can replace \$R_{load}\$ by \$\frac{V_{out}}{I_{load}}\$ and one gets the condition

$$\frac{2LfI_{load}}{V_{out}} < 1-D$$

How can [a buck converter] then enter discontinious mode at different currents and what are these different factors that effect this current

If the duty cycle changes, for example, due to a change in supply voltage, then the converter may enter DCM even though the load current remains constant.

Also, there are some controllers that may alter the switching frequency, with a similar effect.

If \$V_{in}\$ or \$V_{out}\$ or f changes, then the converter will enter DCM at a different current level.

how exactly would I push the SMPS into discontinuos and continuous when the output current is fixed?

By changing the duty cycle (or frequency) by changing the input voltage.

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There is a dual slope (charge/discharge) with exponential decay to zero inductor current when discontinuous.

Energy is stored when inductor is switch on at the end of the current integrated pulse. Then the current ramp reverses slope as the diode then conducts the current to charge the cap. If the cap voltage charging and R loading depletes all the stored energy from L via diode it discontinues the cap charging until the next cycle.

This is affected by every component value for Rs,L,C and load Rp as well as Vin,Vout for any given duty cycle. It is basically a switched series resonant RLC circuit that must be carefully chosen for impedance and Q to be well regulated for the load and frequency to avoid overshoot on startup.

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