The buck-boost converter is a type of DC-DC power converter that can increase (boost) or decrease (buck) the input voltage as needed. This type of converter is particularly useful in applications where the input voltage can vary above and below the desired output voltage. However, the polarity of the output voltage is reversed compared to that of the input.
A disadvantage of this converter is that its switch does not have a terminal connected to zero, thus complicating its control. You can use the Fly-back converter if you want to avoid this problem.
Compared to Buck and Boost converters, the main differences are:
The buck-boost converter operates in two main phases: the energy storage phase and the energy transfer phase.
– When the switch (often a transistor) is closed, current flows through the inductor and the switch, storing energy in the inductor in the form of a magnetic field.
– During this phase, the diode is blocked and the charge is supplied by the capacitor.
– When the switch opens, the energy stored in the inductor is released. The inductor forces current to flow through the diode and supply the load.
– The output voltage can be set by adjusting the duty cycle of the switch.
The buck-boost converter operates in discontinuous conduction when the current demanded by the load is low, and operates in continuous conduction for higher currents. The limit between continuous and discontinuous conduction is reached when the current in the inductor cancels out just at the moment of switching.
Switch (S): Typically, a power transistor, such as a MOSFET, regulates the flow of inductor current. The operating modes and output voltage of the converter depend on the ON and OFF states of the switch.
Diode (D): When the switch is in the OFF position, it allows current to flow in one direction only, from the inductor to the output. The output capacitor is prevented from discharging to the input source by the diode.
Inductor (L): stores energy during the ON state of the switch and releases it to the output during the OFF state. The inductor is essential for smoothing the output voltage and current waveforms.
Capacitor (C): this component filters and smoothes the output voltage waveform by storing and releasing energy. It helps maintain a stable output voltage by attenuating voltage ripple and transient reactions.
Input and output filters: These are optional components, usually capacitors or inductor-capacitor (LC) combinations, used to reduce electromagnetic interference (EMI) and noise at the converter’s input and output.