Why do we allow discontinuous conduction mode DCM Electrical Engineering Stack Exchange

A buck converter operates in discontinuous conduction mode when the inductor current drops to zero during a switching cycle. This is because the diode is blocking the input voltage stacked on top of the output voltage. For the boost of 2 buck boost converter will have at least 10\% less efficiency than the boost converter. Thus, operating boost converter in DCM robs its ability to have low input ripple current. What is the difference between continuous conduction mode and discontinuous conduction mode? In continuous conduction mode (CCM), the current in the inductor of a power converter never drops to zero between switching cycles.
From an engineering perspective, one can expect that a certain amount of energy will be lost in each switching cycle, but the required switching frequency to maintain conduction with a given level of inductance would approach infinity as the load current approaches zero. For any given combination of input voltage, output voltage, and frequency, and inductance, there will be a certain unavoidable difference between the minimum and maximum current levels on the inductor if it's operating in continous mode. According to the values of your converter’s parameters including the L,C,R, input voltage and desired output voltage, and mainly the switching frequency, you should know whether your converter will operate in CCM or DCM. In the discontinuous conduction mode, inductor current is not persistent throughout the complete cycle and reaches zero level earlier even before the end of the period.
The choice between continuous and discontinuous conduction modes depends on the specific requirements of the DC motor application. This causes both the diode and the MOSFET to be in an off state, and is called discontinuous conduction mode (DCM). The document compares continuous conduction mode (CCM) and discontinuous conduction mode (DCM) in flyback converters. CCM Vs DCM that are compared include component stress, output voltage regulation and transient response to step-loads, and efficiency. This document presents a comparison of performance between continuous conduction mode (CCM) and discontinuous conduction mode (DCM) flyback converters. This condition is known as discontinuous conduction mode (DCM) operation.
If the value of ξ is greater than 1, the converter enters the continuous conduction mode. The duty cycle ratio for the buck converter is also dependent on the inductance L, load resistance R, and the switching frequency f. From Figure 3, it is clear that the average value of the inductor current is equal to the area under the load current curve divided by T. The buck converter in discontinuous and continuous conduction modes are in the second-order and first-order systems respectively. This is a single-quadrant converter with the following waveforms for the continuous conduction mode shown in Figure 2. The current in the converter is controlled here by two switches labeled S (MOSFET) and D (Diode).
At very light loads, DCM can actually improve efficiency because some control methods reduce switching activity, which lowers switching losses and quiescent power consumption. At medium and high loads, CCM can be efficient because conduction is smooth and the design can be optimized for low losses. CCM generally produces lower current ripple for a given inductor value, which often translates to lower output voltage ripple. That “flat at zero” interval means energy transfer is not continuous, so the output capacitor has to do more work to maintain a stable voltage. In DCM, the waveform rises and falls, touches zero, and then stays flat at zero until the next switching cycle begins. Many modern converters even encourage DCM or pulse-skipping at light load to improve efficiency.
Ripple, in and of itself, is not problematic, when components can be freely chosen for low loss (less power dissipation per reactive power being cycled), or large value (less reactive power being cycled in the first place). Now, that is not to say that a particular product, such as EV charger, would actually use a BCM converter to do the line-current shaping, there are many other ways to achieve line-current shaping, and many design issues that need to be considered before deciding which method to implement in any given application. Still, I just wanted to mention another possible reason a designer may want to deliberately design a power converter to operate in DCM vs CCM. The picture shows discontinuous current mode but, the slopes are the same should the load current increase and the converter enter CCM.
In most of the applications, the continuous conduction mode is employed. Thus, the size of the transformer required in isolated converters is bigger as compared to the continuous-conduction transformer size to suit the larger flux linkage and the losses. Having the same power through the converter, the requirement of the inductor current is higher in the case of the discontinuous conduction as compared to the continuous conduction mode. There are two levels indicated here towards the two-voltage level for the inductor voltage. Two-level DC buck and boost and buck-boost converters will be discussed further in this article.
The conduction mode describes what happens to the inductor current during one switching cycle. When you study or design a buck converter, you quickly realize that it does not behave the same way at every load.
Hence, DCM is necessary (out of our hands) when the load current is small. CCM is a natural evolution of DCM when the load current reaches a value that exceeds the boundary conduction condition. DCM occurs when the output load current is insufficient to maintain operation in CCM. Sometimes engineering is about designing the highest-performance system you can. This is especially important in battery-powered systems that must deal with a wide range of load current. For a given inductor and switching frequency, there is usually a minimum pulse width, limited by the switching element and/or its drivers.
Understanding these modes is crucial for designing efficient and stable power supplies. They describe how the inductor current behaves during a switching cycle. CCM (Continuous Conduction Mode) and DCM (Discontinuous Conduction Mode) are two fundamental operating modes for switching DC-DC converters (like Buck, Boost, Buck-Boost). Since, under steady-state conditions, the average current through the output capacitor is zero and the average inductor current IL equals the output current Io
Otherwise, if the current goes to zero and remains zero until the next switching cycle, the converter is said to operate under DCM. If the next switching cycle begins before the current goes to zero, the converter is said to operate under CCM. During the off period, as the energy stored in the inductor is transferred to the output capacitor, the inductor current continues to fall until it reaches zero. Unlike CCM, under DCM the inductor current briefly goes to 0 during each switching cycle. When designing a boost converter, choosing between asynchronous and synchronous topologies is a crucial decision. The pros and cons of each operating mode are discussed based on experimental results.
If an efficiency of 80% minimum is required, then -- give or take losses elsewhere -- we're probably done. If we choose DCM, we have on the order of 5VA of reactive power cycled through the inductor. These will generally be custom-made parts, using more expensive materials, and they may still require larger sizes to achieve a given Q factor at the given frequency. Q factor is defined as the ratio of real to reactive power in a component (among other definitions).

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Pub: 04 May 2026 07:28 UTC

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