# Electronic – Factors that effect what current a Synchronous buck enters the Discontinuous mode

buckswitch-mode-power-supply

# 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?

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.