Welcome to the dynamic and essential world of power electronics, where the buck converter plays a pivotal role in efficient voltage regulation. In this segment of our series, we will explore the intricate details of continuous conduction mode (CCM) and discontinuous conduction mode (DCM) in step-down converters. Join us as we learn more about the operational characteristics and practical implications of CCM and DCM, uncovering the secrets to maximizing the performance and efficiency of your buck converters.
Buck Converter in CCM Operation
Figure 1: Step-Down in CCM Operation
Source: self-made
Let’s suppose we are in CCM. During the time the switch is ON, the inductor (L) gets charged up with power from Vin. The rate of this charging is determined by (Vin - Vout)/L. When the switch turns OFF, the inductor starts to discharge at a rate that's set by Vout/L. At steady state, currents must be balanced; otherwise, there will be energy accumulation.
Solving the equation, we find that Vout = d*Vin.
Since d is between 0 and 1, we have Vout < Vin.
Ideally, if everything is perfect, the output remains stable and isn’t affected by the load, the size of the inductance, or the switching frequency. But, for example, if the load starts to require more current than usual, the inductor can run out of stored power before the next cycle begins. That's when the converter stops operating in CCM and switches to DCM. When that happens, our simple Vout = d*Vin doesn't hold up anymore.
Buck Converter in DCM Operation
Figure 2: Step-Down in DCM Operation
Source: self-made
Here’s how a basic buck converter behaves in DCM mode. In situations where the current in the inductor falls to zero and remains there for a while, Vout is not simply d*Vin. After the inductor has fully discharged, which is after Toff1, it then stays at zero for Toff2. The challenge is that we cannot directly control Toff1 and Toff2 separately. However, we can control their combined total, Toff. This is influenced by how quickly the inductor discharges, which depends a lot on the load that's connected to the output. The heavier the load, the quicker the inductor discharges, making Toff1 shorter and Toff2 longer. Unfortunately, these are not factors we can directly manage.
Conditions to Ensure a Buck is in DCM or in CCM
We know that the same circuit can operate in DCM or CCM modes, depending on the values of the load resistance (R), the inductance (L), the switching period (T), and the switching duty cycle (d). Depending on the mode running, the Vout behavior differs. For example, in CCM Vout = d*Vin and in DCM Vout =kVin/2 * [-1 + (1+2/k)1/2] with k=Rd²T/2L. Now, the important question is, how do we know a circuit is in DCM or CCM? And, for a given circuit, how do we ensure the operation will be in a chosen mode for the entire time?
Figure 3: Buck Converter: When Is it in DCM or CCM?
Source: self-made
Buck Converter in BCM Operation
The boundary conduction mode (BCM) is a state that sits right between DCM and CCM. It happens when the inductor's current drops to zero just as a new cycle is starting. This isn't considered a separate mode but rather a blend of DCM and CCM modes. In this mode, the off time Toff2 is essentially zero, which means the current hits zero, but it doesn’t stay there. To achieve BCM, certain conditions must be met that tie together the inductance (L), the load resistance (R), the switching period (T), and the duty cycle (d). The formula leading to BCM defines a critical inductance (Lcrit), which depends on these factors: L = R*(Vin-Vout)*d*T/2 Vout. This is known as the critical inductance for the circuit to operate in BCM.
By equaling the peak current charged during Ton and discharged from there until zero during Toff, the Lcrit value can be easily computed. If the actual inductance in use is higher than this critical value (L > Lcrit), the system will operate in CCM. If it's lower (L < Lcrit), the system will be in DCM. For a circuit designer who wants to ensure the circuit consistently operates in either CCM or DCM, the inductance (L) must be selected with a margin that is significantly above or below the critical value, potentially two or three times greater or smaller. This deliberate sizing of L provides a buffer to firmly keep the circuit within the desired conduction mode.
Figure 4: Buck Converter in BCM Operation
Source: self-made
Conclusion
The buck converter's performance and efficiency are deeply influenced by whether it operates in CCM or DCM. Understanding the nuances of these modes is essential. CCM offers a straightforward and stable output, while DCM introduces complexity but can be advantageous in certain scenarios. BCM serves as a transitional state, providing a bridge between the two. By carefully selecting the inductance (L) and other parameters, you can ensure that your buck converter operates consistently in the desired mode, thereby optimizing its performance for your specific needs. Whether you're designing a new circuit or troubleshooting an existing one, this knowledge will be invaluable in your journey through the fascinating world of power electronics.
See blogs in the DCM and CCM in SMPS series.