Buck Converter Tutorial - Buck Topology Working, Advantages, Applications

Understanding Buck Topology:

DC-DC converters are broadly classified as Non-isolated converters and Isolated converters.

1. Non Isolated converter (Don't have dielectric isolation between input and output)
2. Isolated converter (Have dielectric isolation between input and output)

The majority of the topologies are all derived from the three non-isolated versions called the Buck Converter, the Boost Converterand the Buck-boost Converter.

Buck converter: An Introduction

• Buck regulator is a simple form of the forward-mode type regulator.
• The output voltage is maintained by the controller by varying the duty cycle.
• The buck converter is also known as a step-down converter, since its output voltage must be less than the input voltage.
• Buck converter circuit consists of four components:
A Power MOSFET used as a controllable switch SW ,
A diode D,
An inductor LO, and
A filter capacitor COUT.
• Resistor RL represents a DC load.
• Power MOSFETs are the most commonly used controllable switches in the DC–DC converters because of their high speeds.
• The diode D is called as a freewheeling diode, a flywheel diode, or a catch diode.
• The switching network composed of the transistor and the diode ‘chops’ the dc input voltage VIN and therefore the converter is often called a ‘chopper’, which produces a reduced average voltage.
• The switch S is controlled by a pulse-width modulator and is turned on and off at the switching frequency
fS = 1/T and the duty cycle D defined as
D = ton/T
D = ton /(ton + toff)
where
tON is the time interval when the switch SW is closed and
tOFF is the time interval when the switch SW is open.
• The output voltage VOUT of the buck converter is always lower than the input voltage VIN .
• Therefore, it is a step-down converter. The buck converter ‘bucks’ the voltage to a lower level.
• The buck converter can operate in a continuous conduction mode or in a discontinuous conduction mode, depending on the waveform of the inductor current.
• In CCM the inductor current flows during the entire cycle, whereas in DCM the inductor current flows only during part of the cycle.
• In DCM it falls to zero, remains at zero for some time interval, and then starts to increase.
• Operation at the CCM/DCM boundary is called the critical mode.

Working:

The operation of the buck regulator can be seen by breaking its operation into two periods

1. Power switch is turned ON
2. Power switcch is turned OFF

Switch Turned ON:

• During the switch ON position, a current loop is created that includes the input voltage source, the power switch, the inductor and the load.
• The input voltage is applied to inductor L1 and power is delivered to the output. Inductor current also builds up according to Faraday’s law shown below:-
iL = L (di/dt)
• The current flow is indicated in the above figure.

Switch Turned OFF:

• The property of Inductor is that it trys to oppose sudden change in voltage appears across it. While charging the inductor terminal near to the switch is positive.
• When the switch turns off, the voltage across the inductor reverses (ie, the terminal away from the switch becomes positive). It leads the freewheel diode (Catch diode)D becomes forward biased.
• This allows the energy stored in the inductor to be delivered to the output. This continuous current is then smoothed by output capacitor COUT.
• The current flow during the switch turned OFF is indicated in the below figure

Typical buck converter waveforms are also shown below:

The buck converter is normally always operated in continuous current mode ( inductor current never falls to zero). It ensures that peak currents are lower, and the smoothing capacitor requirements are smaller.

Applications:

• Battery operated portable equipments
• For Unidirectional supplies

• Buck converter is very simple and it requires only one Power switch
• Efficiency of Buck regulator is about 90%
• The cost and size is low
• Line voltage variation has large tolerance

• Slow tansient response
• Normally Input filter is required
• High output ripple

Buck Converter Basics Summary

1. The PWM buck converter is a step-down converter (VOUT < VIN ).
2. It is a transformerless converter, which does not provide dc isolation.
3. It can operate in two modes: CCM or DCM.
4. The peak-to-peak value of the inductor ripple current iL is independent of the dc load current for CCM.
5. The peak-to-peak value of the current through the filter capacitor iC is relatively low and is equal to the peak-to-peak inductor ripple current iL.
6. If the capacitance of the filter capacitor is sufficiently high, the output ripple voltage is determined only by the ESR of the filter capacitor and is independent of the capacitance of the filter capacitor.
7. The minimum value of the inductor is determined by the CCM/DCM boundary, ripple voltage, or ac losses in the inductor and the filter capacitor.
8. A disadvantage of the buck converter is that the input current is pulsating. However, an LC filter can be placed at the converter input to obtain a non-pulsating input current waveform.
9. The corner frequency of the output filter, fO = 1/(2πLC), is independent of the load resistance.
10. It is relatively difficult to drive the transistor because neither the source nor the gate is referenced to ground. Therefore, a transformer or an optical coupler is required in the driver circuit.
11. Buck regulator is a simple form of the forward-mode type regulator. The output voltage is maintained by the controller by varying the duty cycle.
12. The buck converter is also known as a step-down converter, since its output must be less than the input voltage.