Introduction

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 output voltage is of opposite polarity to that of the input
  • The output voltage can vary from 0 to −∞ (for an ideal converter).

Pros

  • Output voltage flexibility: The converter can produce an output voltage that is either higher or lower than the input voltage.
  • Energy efficiency: Modern buck-boost converters can achieve very high efficiencies, often above 90%, making them ideal for low-power applications.

Cons

  • Design complexity: Compared to buck or boost converters, the buck-boost converter is more complex to design and control.
  • Converter switch: does not have a terminal connected to zero, thus complicating its control.
  • Current Ripples: There may be larger current ripples, requiring additional filtering components to stabilize the output.

How it works

The buck-boost converter operates in two main phases: the energy storage phase and the energy transfer phase.

A. Energy Storage 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.

B. Energy transfer phase

– 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.

C. Continuous and discontinuous conduction


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.

Circuit explanation

We’ll now look at an example circuit to help you better understand how a Buck-Boost converter works:

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.

Conclusion

The Buck-Boost converter is widely used in electronic installations, particularly to supply a low-voltage circuit, after an AC/DC converter. However, the DC-DC converter most commonly used in electronics is the flyback, because it’s easier to operate than the Buck-Boost.