A buck-boost converter is a type of DC-DC converter that can step down (buck) or step up (boost) the input voltage to provide a regulated output voltage. The converter operates in two main modes: continuous conduction mode (CCM) and discontinuous conduction mode (DCM). These modes refer to the behavior of the inductor current during the switching cycle.
Continuous Conduction Mode (CCM):
In CCM, the inductor current never drops to zero during the entire switching cycle. The inductor current flows continuously, and the output voltage is regulated by adjusting the duty cycle of the converter. Here's how a buck-boost converter operates in continuous conduction mode:
During the ON period of the switch (transistor), energy is stored in the inductor, and the inductor current increases linearly.
During the OFF period of the switch, the inductor current decreases but never reaches zero, as energy is still being delivered to the output.
The output voltage is regulated by adjusting the duty cycle (ON-time to the total switching period) of the switch.
Continuous conduction mode offers better output ripple characteristics and reduced electromagnetic interference but typically requires a more complex control circuit compared to discontinuous conduction mode.
Discontinuous Conduction Mode (DCM):
In DCM, the inductor current drops to zero during some portion of the switching cycle. The inductor current discontinues during the OFF period of the switch, and the output voltage is regulated based on the inductor current and load conditions. Here's how a buck-boost converter operates in discontinuous conduction mode:
During the ON period of the switch, energy is stored in the inductor, and the inductor current increases.
If the load and input voltage conditions are such that the inductor current drops to zero before the next ON period, then the converter operates in DCM.
In DCM, the output voltage is regulated based on the ratio of ON-time and OFF-time of the switch, and the inductor current waveform.
Discontinuous conduction mode is generally simpler to control but can result in higher output voltage ripple and potentially more electromagnetic interference, especially at higher load currents.
The mode of operation, continuous or discontinuous conduction, depends on the input voltage, output voltage, load conditions, and the duty cycle of the switch. Proper design and control are essential to ensure efficient and stable operation in either mode. In practice, some buck-boost converters can switch between continuous and discontinuous conduction modes based on the operating conditions to optimize efficiency and performance.