CCM to DCM in a Buck converter

A non synchronous buck converter will enter DCM when the inductor current goes to 0 A ( Iout  < 1/2 inductor p-p ripple current).  So a larger inductor value will decrease the ripple current and lower the CCM/DCM threshold.  Most synchronous buck converters have forced CCM mode ( internal FETs can sink current allowing inductor current to go negative).  Some have DCM emulation or pulse skipping eco-mode that act like DCM.

JohnTucker said:

A non synchronous buck converter will enter DCM when the inductor current goes to 0 A ( Iout  < 1/2 inductor p-p ripple current).  So a larger inductor value will decrease the ripple current and lower the CCM/DCM threshold.  Most synchronous buck converters have forced CCM mode ( internal FETs can sink current allowing inductor current to go negative).  Some have DCM emulation or pulse skipping eco-mode that act like DCM.

Jon, can you name a TI part that can go from CCM to DCM.

What about the Inductor preventing CCM TO DCM transition.

 a general purpose non synchronous buck such as TPS5430, TPS54160, TPS54331 can transition from CCM to DCM when the load current is less than 1/2 the P-P inductor ripple current.  The inductor ripple current is inversely proprtional to the inductance.  So larger inductor will have lower CCM to DCM threshold.  I'm sure there are many other devices as well.  Can I ask what is your concern?

I don't have any practical concern. I overheard a conversation saying the inductor prevents the dcdc converter from going from ccm to dcm, because of it's property(no sudden changes in current). I got curious.

Hi Hitesh.

As a general rule, the greater is the inductance, the lower is the current ripple, the lighter will be the load for which you will switch from CCM to DCM...

It's up to you to decide at which load condition your step down will run in DCM rather than CCM selecting a proper inductance value...

If you are running in peak current mode control you will not change anything in terms of closed loop transfer function, so...

Bye

Paolo

"If you are running in peak current mode control you will not change anything in terms of closed loop transfer function"  This is technically true for ideal peak current mode control.  In that case the inductor is seen as an ideal current source and does not affect the loop.  In practical cases, the control scheme will probably employ some slope compensation.  In that case, the inductor value does affect the power stage gain and phase characteristics.  The more slope compensation that is applied, the greater the effect.

You're right John, but I was speaking from zero-poles point fo view, not about slope compensation...

Have a nice day

Paolo