Buck converter based regulator

Question

Buck converters are extensively used in switching voltage regulator.

I think it's essential for a buck converter to be closed loop to behave as a voltage regulator, otherwise for open loop it's just a converter which can't regulate the voltage.

Could you please confirm on my understanding?

For example, for some step change in load current the buck converter would create the transients and settle down at some value other than nominal output value while the closed loop design reclaims the original nominal value for output voltage. Is this statement correct?

Answer 1

Operation mode matters.

While running at CCM (continuous conduction mode), a step load change doesn't change the output voltage, even if the converter runs in open loop (i.e. no regulation). Because the output voltage is set by the input voltage ^V_I and duty cycle ^D: V_O = V_I D. If these are kept unchanged then the output voltage doesn't change.

At DCM everything is different because the DC transfer characteristics (i.e. the relation between the output voltage and the input voltage) becomes load-dependent. So, any change in load will change the output voltage accordingly.

I think it's Essential...

Depending on the design and loading conditions the operational mode of a buck can dynamically switch between DCM and CCM, or stay at DCM regardless, or stay at CCM regardless. So it's not essential to close to loop, unless regulation of output (output voltage, output current, or even output power) against input voltage and load-state changes is a must. For example, regardless of the operation mode, any input voltage change reflects to the output voltage, so if the input voltage is kept rock-steady and if the design guarantees CCM operation across the whole loading range then closing the loop becomes fairly unnecessary.

NOTE: All of the information given in this answer basically neglects the parasitic elements (e.g. DC resistance of the inductor, on-resistance of the MOSFET, ESR of the output capacitance, etc) and their effects (e.g. the drop in the output voltage caused by the inductor's DC resistance).

Answer 2

I think it's Essential for buck converter to be closed loop to behave as a voltage regulator, otherwise for open loop it's just a converter which can't regulate the voltage. Could you please confirm on my understanding?

No, it's not essential...

You can make a synchronous buck regulator that does a pretty good job of producing a fairly stable output. A sync-buck is just a power square wave feeding an LC low-pass filter so, if you know the input voltage and, have pretty decent MOSFETs to create the power square wave, you can adjust MOSFET duty cycle to keep the output voltage constant based on feed-forward techniques (not closed-loop): -

Image from this answer.

For e.g. for some step change in load current the buck converter would create the transients and settle down at some value other than nominal output value while the closed loop design reclaims the original nominal value for output voltage. Is this statement correct?

So, if you use decent MOSFETs even fairly heavy load changes won't disrupt the output accuracy too much. Of course, the icing on this particular cake might be to use some negative feedback as well but, it's not always 100% necessary.

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Unlike non-synchronous buck regulators (whose voltage transfer ratio changes at low load currents), the sync-buck always keeps in an equivalent of continuous conduction mode and thus, it stays in regulation from zero load current to maximum load current.