Buck-Boost Converter

What is Buck-Boost Converter?

We can define Buck-Boost Converter as a type of DC – DC converter whose generated output voltage that is either higher or smaller than the applied input voltage.

Buck-Boost Converter is alike a flyback converter, but difference is that it uses inductor and flyback converter uses transformer.

Buck-Boost Converter is a combination of Buck Converter and Boost Converter.

Below figure shows a Buck-Boost Converter Module;

Buck-Boost Converter Module

To know more about Buck Converter please visit the article, "What is Buck Converter".

There are two different topologies of buck-boost converter. Inverting topology and non-inverting topology. In detail both are explained below;

The inverting topology

Like buck converter and boost converter the inverting buck-boost converter is like a switched-mode power supply and its circuit topology is same like buck converter and boost converter.

The polarity of the output voltage is opposite as compare to the applied input voltage.

For simple or basic inverting buck-boost converter we can see or get a negative output voltage with respect to the ground.

The duty cycle of the switching transistor or MOSFET, the output voltage of buck-boost converter is adjustable or changeable.

The driving circuitry becomes complicate, when we see that the switching component does not have a terminal at ground. Which can be a disadvantage.

But, this disadvantage is of no importance. If the power supply is isolated from the load circuit because at this condition the power supply and polarity of diode can be upturned or reversed.

At the reversed condition, the switch can be on either the supply side or at the ground side.

Working of inverting topology

An Inverting buck-boost converter is shown in below figure. It is the schematic of a basic inverting buck-boost converter which is working in continuous conduction mode (CCM).

Inverting Buck-Boost Converter

We can see an output capacitor, in the power stage a metal-oxide semiconductor field-effect transistor (MOSFET) is present, a diode, and an inductor is present.

Anode of the diode is connected to the load.

Anode of the diode is also connected to negative terminal of electrolytic capacitor.

When the MOSFET (M1) is ON, the voltage through the inductor (L1) is Vin, and the current in the inductor ramps up with the rate that is proportional to the applied input voltage Vin.

This results in storing energy in the inductor L1. While M1 is ON, the output capacitor C1 supplies the entire load current.

When the M1 is OFF, the diode (D1) is forward-biased and the inductor current ramps down at a rate proportional to output voltage Vout.

While M1 is OFF, energy is transferred from the inductor L1 to the output load and output capacitor C1.

The non-inverting topology

Below figure shows a non-inverting buck-boost converter. Here we can see a buck (step-down) converter is joint with a boost (step-up) converter.

Non-Inverting Buck-Boost Converter

The output polarity of this type of converter is same like the polarity of the applied input. But the output level may vary i.e. may be higher or lower depending on the design and component selection.

We can see in the figure that there is only one inductor is used even there is buck mode and boost mode is combined.

This single inductor controlled by switches instead of diodes. At a time only one switch is in ON condition.

If it uses many inductors with only one switch same like seen in Cuk or SEPIC topologies, then it is called as "four-switch buck-boost converter".

The popular non-inverting buck-boost topologies are; Zeta, SEPIC and two-switch buck-boost converter.

All these above mentioned buck boost topologies generate positive output but these topologies have extra power components and less efficiency as compared to a basic inverting buck-boost converter.

Working of non-inverting topology

We can see in figure of non-inverting buck-boost conductor; two high frequency switching MOSFETS are used along with the two diodes and these diodes have a low forward junction voltage when it conducts.

Operation of buck converter

We can understand the operation of buck-boost inductor on the basis of inductor's "reluctance", which allows quick change in current (current across inductor).

Initially current through the inductor is zero. This means at the initial stage when MOSFET is not working i.e. switch is open and there’s no current across the inductor i.e. nothing is charged.

When the MOSFET switch is first closed, the blocking diode stops current from flowing through it as it is reversed biased, so the current passes through the inductor.

Initially the Inductor will keep the current low by dropping most of the source voltage, as the inductor doesn't like fast current change. With the time the inductor allows the current to increase slowly with the decrease in voltage drop. By this inductor will store energy i.e. magnetic field.

Let see the operation of buck converter in step wise in detail;

Below figure shows the mode of operation of buck converter when MOSFET M1 is ON. In this mode the high frequency square wave generated by IC keep the MOSFET M1 ON and OFF. MOSFET M2 is turned off in this case.

Buck converter operation when M1 is ON

When the gate terminal of MOSFET M1 is high, M1 starts to work and makes current to flow though inductor L1. This current charges L1, capacitor C1 and supply further to the connected load.

The diode D1 is turned off because of the positive voltage on its cathode i.e. it is reversed biased.

Below figure shows the mode of operation of buck converter i.e. current flow when MOSFET M1 is OFF. Now the inductor L1 is fully charged and now it is the only source of the current.

Buck converter operation when M1 is OFF

The magnetic field generated across the inductor L1 starts to collapse and generates back E.M.F. this E.M.F. opposites the voltage polarity across L1.

This polarity change turns on the diode D1 and current flows through the diode D2 and further to the connected load.

Further as the current across inductor L1 decreases, the charge stored in capacitor C1 during the ON time of MOSFET M1, now adds to the current flowing across the load. This additional current from C1 keeps output voltage (Vout) constant during the M1 OFF time.

This gives an advantage of less ripple at the output.

Operation as a Boost Converter

Below figure shows the mode of operation of boost converter when MOSFET M1 is continuous ON. In this mode the high frequency square wave generated by IC is applied to the MOSFET M2 gate terminal.

Boost converter operation when M2 is ON

When MOSFET M2 is ON and it’s conducting, the input current passes across the inductor L1 and through M2, and further current flow to the negative terminal of supply. This current charges or creates magnetic field around inductor L1.

Also in this step the diode D2 not conducts i.e. current not flows across the diode D2 because high speed conducting MOSFET M2 keeps anode terminal of diode D2 at ground potential.

Now for this ON period, the charge developed by previous oscillating cycles on the capacitor C1 acts as a supply for the load.

The slow discharge of capacitor C1 throughout the ON period and its immediate recharging creates high frequency ripple on the output voltage.

This high frequency ripple is at a potential of approx. Vs + Vout.

OFF-Period of MOSFET M2 can be understand by below figure;

Boost converter operation when M2 is OFF

As the OFF-period of MOSFET M2 starts, the inductor L1 charges and capacitor C1 partially discharges.

As the inductor L1 charged it generates a back E.M.F. The value of this E.M.F. depends on the rate of change of current as MOSFET M2 switches OFF and ON the total inductance. This inductance is hold by the coil (inductor).

So, depending on the circuit design the back E.M.F. can be of any voltage range.

Now the polarity of the voltage across inductor L1 has now reversed and this voltage is added to the input voltage Vs. This gives an output voltage that is equal or greater than the applied input voltage Vs.

Diode D2 is now forward biased i.e. it starts to conduct and so the current further flows through the load.

At the same time this current from diode D2 re-charges the capacitor C1. Which will be further use in next period of MOSFET M2 ON.

Applications of Buck-Boost Converter

We can see Buck-boost (step-down and step-up) converters mostly in the applications like; most of industrial machines or tools where different power supply voltages are needed for machine or tool operation, industrial computer hardware’s and automotive systems.

Battery dependent systems; where at the full charge of battery, the high voltage can’t be supply to the system, then buck regulator circuit works. But as the battery charge comes to end the boost regulator circuit starts to work and boost the available voltage to the required level, so the system works properly.

These types of battery depended systems can be seen in automotive applications i.e. mostly in e-bikes or e-cars.

In the applications mentioned above, the applied input voltage could be either higher or lower than the required output voltage.

Conclusion

By joining these two regulator designs i.e. buck regulator and boost regulator we can have a regulator circuit which can handle a wide range of input voltages. These input voltages may be both higher or lower than that required by the circuit.

Both buck and boost regulator uses same types of components and we have to just re-arrange them, depending on the applied input voltage level.

Power Electronics Talks