We'll explore what happens in the coil during the three key conduction modes—CCM, DCM, and BCM. Whether you're an engineer or a tech enthusiast, understanding these modes is crucial for optimizing power conversion efficiency and reliability. Let's get started!
Figure 1: Inductor Current Profiles in CCM, DCM, and BCM
Source: self-made
In CCM, the inductor current stays above zero all the time, while in DCM, the current can fall to zero for a certain amount of time in every cycle. BCM occurs when this time is just at zero.
Inductance Principle
Figure 2: Inductance Definition
Source : self-made
The coil is the central element of all the inductive SMPS designs. The voltage-current characteristic of an ideal inductance is that the voltage developed on a coil is proportional to the variation of current on it. The proportional factor defines the inductance value in Henries.
The derivative form can be reversed, and we can writethat the current variation developed in the coil is the applied voltage divided by L:
∆I = VL/L * t
This indicates that if a constant voltage is applied to a coil, the current is a linear slope versus time. Since in SMPS, the coil is often switched from Vin to 0 and vice-versa, the current form is made by two line segments: one growing and one descending. The growing line has a slope corresponding to VL/L, and the descending line has a negative slope at -V’L/L.
VL and V’L are the voltages across L: they depend on various parameters such as Vin, Vout, Ton, Toff, and L.
Let’s see in more detail when CCM and DCM can appear.
Continuous Conduction Mode (CCM) Analysis
In this mode, the current flowing through the inductor doesn't stop or drop to zero.
The system is designed to charge the inductor during period D1*TS, and then it allows the inductor to discharge during period D2*TS.
After the inductor discharges, an identical cycle begins again, supposing the circuit is in stabilized operation (transients have vanished).
When the system keeps running like this and reaches a steady state, we can easily calculate the average current, IO.
Figure 3: Inductor Current Form in CCM
Source: self-made
The current in this mode moves from I1 and I2 with both I1 and I2 positive. It is easy to calculate the average current constantly stored in the inductance: IO = (I1 + I2)/2.
Notice the entire current diagram can stay positive if charging and discharging are made in such a way. The green segment will have no time to touch zero if the switching time Ts is not too long, the inductor value L is not too small, or the voltages across L are not too large.
The engineer can over or undersize some of the above parameters to ensure the circuit will always operate in CCM.
Discontinuous Conduction Mode (DCM) Analysis
Figure 4: Inductor Current Form in DCM
Source: self-made
In DCM, the current that goes through the coil drops all the way to zero with each cycle, TS.
In this mode, we split the cycle into three parts:
T1 = D1*TS is when the coil gets powered up,
T2 = D2*TS is when the coil lets go of its energy, and the current goes down to zero, and
T3 = D3*TS is the rest period when the coil has no current until the next cycle starts.
How long the current stays at zero during T3 can be influenced by several things, such as switching period (TS), inductor value (L), the load current, the duty cycle (D1), Vin value, and Vout value. Finding the value of the average current IO in this mode is more challenging because of these factors.
Boundary Conduction Mode (BCM) Analysis
Figure 5: Inductor Current Form in BCM
Source: self-made
Looking at both CCM and DCM, there's a special case to point out. BCM is when the inductor's current just hits zero but doesn't stay there (T3 = 0).
Another way to observe this intercept point between CCM and DCM is when the inductor, L, has released all its energy, it coincides perfectly with the start of a new cycle in the switching process.
With a well-designed control scheme, we can design a circuit that does this. By doing so, the circuit will have the benefits of both DCM and CCM modes at the same time.
Conclusion
By now, you should have a solid grasp of the various modes (CCM, DCM, and BCM) and their respective inductor current profiles. Each mode has its unique characteristics and applications, and understanding them is key to creating efficient and reliable power supplies.
Notably, by design and by application, it is possible to influence the SMPS circuit to behave in one of the three profiles. However, it is also true that without careful attention, a circuit originally imagined working in a particular mode can fall into another scheme when conditions change (such as applied voltages, switch frequency, duty cycle, load current, and more). Whether you're fine-tuning an existing design or starting a new project, mastering these conduction modes will be invaluable. Stay tuned for more insights into the fascinating world of power electronics!
See blogs in the DCM & CCM in SMPS series.
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