Power supplies are the unsung heroes of electronic devices, ensuring that they receive the precise, stable, and clean DC voltages they need to function correctly. However, the magic behind these power supplies lies in the control and feedback systems that manage the energy conversion process. In this first part of the series, we’ll get into the heart of these systems by exploring the two primary modes of inductor current operation: continuous conduction mode (CCM) and discontinuous conduction mode (DCM).
Understanding these modes is crucial for anyone working with switched-mode power supplies (SMPS), as they directly impact the efficiency and performance of the circuit. Their basic understanding is key to further analyzing what happens if the current in coils falls to zero (DCM) or not (CCM). SMPS circuits can behave in CCM or DCM modes depending on external conditions of voltages, load current, switching frequencies, etc. Let’s uncover the basics, refresh on control schemes, and see how the current in the inductor can make all the difference!
DC-DC Control Schemes Refresh
The three fundamental inductance-based DC-DC control schemes are: buck (step-down), boost (step-up), and inverter (also called buck-boost) (Figure 1).
In order to quickly recognize them, one can observe the way the three elements Switch S, Inductor L, and Diode D are interconnected. Their relative positions and orientation determine the mode: buck, boost, or buck-boost.
The “trick” is to consider the central vertical branch: D, S, or L
- The buck or Step-Down is with the Diode as the vertical element
- The booSt is with a Switch in the vertical element
- The Inverter is with the Inductor as the vertical element
Figure 1: The Three Fundamental DC-DC Topologies: Buck, Boost, and Inverter
Source: self-made
Output Setup and Regulation
Once the topology is known, the output value is set and regulated by controlling the amount of current stored from the input source and released to the load. The balance between these two quantities is made by switching Ton and Toff on and off during suited durations in every clock period. The usual factor used is the ratio between Ton and the clock period, called the duty cycle D = Ton/T = Ton / (Ton + Toff).
There are various ways to do that: pulse-width modulation (PWM), pulse-frequency Modulation (PFM), pulse skip mode, constant Ton, constant Toff, hysteretic mode, etc.
In the regulation path, a feedback circuit compares the output voltage and the switching distribution.
Figure 2: Regulation Principle in SMPS
Source: self-made
As a reminder, the three simple formulas giving the transfer function Vout/Vin versus the duty cycle D as:
- For the Buck: Vout/Vin = D Vout is always lower than Vin
- For the Boost: Vout/Vin = 1/(1-D) Vout is always higher than Vin
- For the inverter: Vout/Vin = - D/(1-D) Vout is always negative vs Vin
BUT! Watch out! Those relations are only valid if you ensure the current stored in the inductor is NEVER depleted during the full period of the switching cycle. And that is not guaranteed at all!
Continuous or Discontinuous Conduction Modes
Let’s examine what happens in the inductance during switch-on (when L is storing current) and switch-off (when L is releasing current) times (Figure 3).
Observe that the current profile in the inductor can be a sort of asymmetric triangular form marked by the charge and discharge phases (and this depends on the switching frequency, the duty cycle, and the inductance values).
When the entire current profile is above 0 A (or the current in the inductor is always positive), we have the standard CCM.
When the current in the inductor can be depleted (the coil current can be 0 for a certain amount of time), the mode is in DCM.
Figure 3: CCM and DCM Behaviors
Source: self-made
Without being a “third” mode, the border between CCM and DCM is when the current is just touching the 0 A, without stating such: that’s the boundary conduction mode (BCM).
BCM is desired because it groups the advantages of both the DCM and CCM modes. A more detailed and in-depth analysis will be presented in the following series.
Conclusion
In this first part of the series, we have refreshed our basic knowledge of the three standard SMPS topologies: buck, boost, and inverter. We observed that the current in the coil can be depleted in each switching cycle, defining the CCM, DCM, and BDM behaviors. Grasping these concepts is essential for anyone working with or designing SMPS systems, as they form the foundation for more advanced control schemes and troubleshooting techniques. Stay tuned for the next blog that focuses on what happens in the coil.
See blogs in the DCM & CCM in SMPS series.