How Buck-Boost Circuits Work

We all have heard a lot about buck and boost circuits and know that basically these circuits are used in SMPS designs for stepping up or stepping down a given voltage at the input. The interesting thing about this technology is that it allows the above functions with negligible heat generation which results in a extremely efficient conversions.

What is Buck-Boost, How it Works

I have explained the concept in the first section without involving much technicalities so that it becomes easier to understand what's exactly buck boost concept even to a newbie.

Among the three fundamental topologies named buck, boost, and buck-boost, the third one is more popular since it allows both the functions (buck boost) to be used through a single configuration just by altering the input pulses.

In the buck-boost topology we primarily have an electronic switching component which can be in the form of a transistor or a mosfet. This component is switched via a pulsating signal from an integrated oscillator circuit.

Apart from the above switching component, the circuit has an inductor,  a diode and a capacitor as the main ingredients.

All these parts are arranged in the form that may be witnessed in the following diagram:

Referring to the above buck boost diagram, the mosfet is the part which receives the pulses which forces it to operate under two conditions: ON state and OFF state.

During ON state the input current gets a clear path through the mosfet and instantly tries to make it's way across the inductor since the diode is positioned in the reversed biased state.

The inductor on account of its inherent property tries to restrict the sudden infliction of current and in an compensating response stores some quantity of current in it.

Now as soon as the mosfet is switched OFF it goes under the OFF state blocking any passage of the input current.

Again the inductor is unable to cope with this sudden change of current from a given magnitude to zero, and in a response to compensate this, it kick backs its stored current via the diode across the output of the circuit.

In the process the current also gets stored in the capacitor.

During the next ON state of the mosfet, the cycle is repeated as above however with no current available from the inductor, the capacitor discharges the stored energy into the output which helps in keeping the output stable to the optimized degree.

You may be wondering what factor decides the BUCK or the BOOST results at the output? It's quite simple, it depends on how long the mosfet is allowed to stay in the ON state or in the OFF state.

With an increase in the mosfets ON time, the circuit starts getting transformed into a Boost converter while with the mosfets OFF time exceeding its ON time results in the circuit behaving like a Buck converter.

Thus the input to the mosfet can be made through an optimized PWM circuit for getting the required transitions across the same circuit.

Exploring Buck/Boost Topology in SMPS Circuits More Technically:

As discussed in the above section the three fundamental topologies which are popularly used with switch mode power supplies are the buck, boost, and the buck boosts.

These are basically non-isolated in which the input power stage shares a common base with the output power section. Of course we could also find isolated versions although pretty rare.

The above expressed three topologies can be distinguished uniquely depending upon their exclusive properties.The properties may be identified as the steady state voltage conversion ratios, the nature of the input and output currents and the character of the output voltage ripple as well.

Additionally the frequency response of the duty cycle to the output voltage execution can be considered as one of the important properties.