Design the Buck Convertor and its Components: A Comprehensive Guide

A type of DC-DC converter, the buck converter is used to convert a higher voltage to a lower voltage. This popular type of converter is preferred for its high efficiency and feasability. 

The basic operation of a buck converter involves two switches, an inductor, a capacitor, and a diode, and the output voltage is controlled by adjusting the amount of time the switch is closed and the amount of current flowing through the inductor.

In this blog, let us look at how the different components of a DC buck converter are designed.

Where are buck converters used?

Buck converters are used in a wide range of applications, including power supplies for computers, mobile devices, and other electronics. They are also used in automotive applications to regulate voltage for sensors and other electronic components. In renewable energy applications, buck converters are used to convert the high voltage output from solar panels or wind turbines to a lower voltage that can be used to charge batteries or power other devices.

Inductor Designing for the DC-DC Buck Converter

This section will go through the most significant aspects of the inductor that a buck converter requires. This comprises two primary concepts: critical inductance and inductor peak current rating.

Critical Inductance

Inductance critical Lc is the inductance value at which the current in the inductor reaches zero. As a result, it is the most critical requirement for using a buck converter in discontinuous mode. In other words, the inductor value is set below the critical inductance for a buck converter to operate in discontinuous mode. The minimum percentage load is used to determine the requirement. 

Another option is to use the previously described maximum ΔIL to establish the requirement. The inductor value is selected greater than critical inductance while operating a buck converter in CCM mode.

• L < Lc for operating buck converter in DCM
• L > 1.05Lc for operating buck converter in CCM

Using the previously obtained ILmin equation and putting ILmin =0 in the equation, the critical inductance value can be simply determined.

By simplifying the equation, the value of critical inductance can be calculated immediately. After solving the given equation, we get the following answer.

This is the most crucial equation for determining critical inductance, which determines the working mode of the buck converter. When these quantities are selected as given,

• Lc's computed input voltage is Dmax.
• At the lowest output current, Rmax is computed. Vo/Iomin = Rmax is computed and selected in such a way as to keep CCM constant. It's been chosen for a 10% load with CCM. Maximum ΔIL can be used to specify the value of Iomin, resulting in Iomin = ΔIL /2. 
• f is the switching frequency, which the designer generally selects. The critical inductance Lc has an inverse relationship with the switching frequency f. As a result, selecting a high value for f will minimize the size of Lc, resulting in a smaller DC converter.

Current Rating for Switch in Buck Converter

The average current is used to determine the current rating of a switch in a buck converter circuit. The average value of the current can be estimated by sketching the switch current waveform. This is where the average current of the switch is computed. Using KCL, the total inductor current equals the switch and diode currents sum. When turned ON, the inductor current is equal to the switch current, whereas when turned OFF, the inductor current is equal to the diode current. During the turn ON and turn OFF time, the buck converter waveforms of the switch current (Iswitch), diode current (Idiode), and inductor current (IL) are illustrated below:

The average switch current <I switch> is

Diode Design (Shotkey Diode) for Buck Converter

Shotkey diodes are suitable for discharging buck converters on account of their quick recovery time. They are hence ideal for buck converter operations at high frequencies. This section will look at the current and voltage ratings of shotkey diodes for buck converters. 

Voltage rating of the diode for buck converter

The diode's greatest voltage is VPRM or PIV (peak inverse voltage), and it is listed on the datasheet of the component. Peak inverse voltage VPRM is equal to the maximum input voltage (VDCmax) for an ideal switch. VPRM is equal to VDCmax plus the maximum forward voltage drop Vsw across the switch in non-ideal cases.

The value of Vsw is determined at the highest load current. It will be possible to use a safety factor of at least 20% in this computation.

Current Rating of the diode for a DC Buck Converter

The same method is used to calculate the current rating of a diode, as it is used to calculate the current rating of a switch. The current buck converter waveform of the diode is used to compute the average forward diode current. Toff is taken into account in the computation since the diode conducts during the OFF duration of the switch. 

The current waveform of a diode is shown below. It simplifies the process of determining the average value.

Capacitor Designing for Buck Converter

In this section, we will go through the key parameters in a capacitor that allow for it to function in safe mode. Furthermore, the capacitor is constructed in such a way that it performs the required function efficiently. 

Voltage Rating

The capacitor is chosen and built in such a way that the maximum capacitor voltage can tolerate the maximum output voltage. Vcmax is the maximum capacitor voltage that should be, Where word ∆Vo denotes the slope of the output voltage. 

For particle capacitors, the process is different. Since realistic capacitors have an ESR, the expression ∆Vo /2 is thrown off (equivalent series resistance). (ESR * ∆IL) is the contribution of ESR to output voltage ripple. The following strategies can be used to eliminate the contribution of ESR:

1. By lowering the ECR, the voltage ripple disturbance will be reduced. There are two methods to lower the ECR: either by employing capacitors with a low ECR value or by connecting capacitors in parallel.
2. Reduce the value of ∆IL by raising the operating frequency or selecting a larger inductor L value.

Minimum Capacitance According to Voltage

While pure DC current flows into the load, the specified capacitor will offer a conduit for AC ripples of the inductor current. As a result, the capacitor would function as a filter. The current waveform of the capacitor will look like this:

The charge is the product of the area and the slope, as seen in the waveform capacitor current vs. time below.

Q = area. Δ

RMS Current Rating and Minimum Capacitance According to Current

The capacitor is constructed so that the maximum input voltage is equal to the voltage of the standby capacitor. Both should be assumed equal, i.e. Vcmax = VDCmax, but the capacitor has an ESR (Equivalent Series Resistance). The ESR factor influences capacitor loss. For increased efficiency, the ESR can be minimised by connecting capacitors in series or selecting a capacitor with a low ESR.