# Is discontinuous conduction mode of a buck converter more efficient than continuous conduction mode

buck

Can someone explain why you need less duty cycle % to achieve the same output voltage in the discontinuous mode of operation in a buck converter?

This indicates to me that it's more efficient. If the current in the inductor goes to 0, then the diode turns off and there are no more losses associated with that freewheeling diode.

Also, the upper switch seems to turn on with less losses.

It should be the more efficient mode of operation.

Is this true? If so, what are the disadvantages that make it seem like it should be avoided at most times?

It seems that it isn't possible to accurately control the output voltage just using open loop in DCM.

Can someone explain why that in discontinuous mode of operation in a buck converter, you need less duty cycle % to achieve the same output voltage?

Let's say you have a buck converter capable of operating in both modes of operation. You will find that if it's operating in DCM, the load current has to be low. As the load current rises you reach a point (it's called the boundary point) where the converter is operating on-the-cusp of CCM. Above this point it fully operates in CCM.

So, low current loads are serviced in DCM and higher current loads are serviced in CCM. And, importantly, in CCM the output voltage is fully defined by the duty cycle whereas in DCM, the output voltage is defined by both the duty cycle and the load current.

Also it seems that it isn't possible to accurately control the output voltage just using open loop in DCM

So, if the current taken by the load is low, you MUST have a very low duty cycle or your output voltage rises out of regulation. This is because in DCM, the converter is a power regulator; in CCM the converter is a voltage regulator. Low output powers therefore require low duty cycle and, the point you may be missing is that it is impossible to force CCM on very light loads. Hence, in DCM you cannot run in open-loop without massive changes in output voltage as the load current changes.

This indicates to me that it (DCM) is more efficient?

DCM services low power/low current loads and CCM cannot do that. CCM services high power/high current loads and DCM cannot do that. Hence DCM cannot be more efficient on high load currents and CCM cannot be more efficient on low load currents.

what are the disadvantages that make it (DCM) seem like it should be avoided at most times?

• Higher ripple voltage on the output i.e. it produces a noisier output voltage
• Higher ripple current on the input