Buck Converter – Circuit and Operation

Buck Converter

A buck converter immediately reduces the applied DC input voltage level. Buck converters are non-isolated DC converters by definition. Non-isolated converters are suited for any circuits on the board that require local conversion. Fax machines, scanners, cell phones, PDAs, laptops, and copiers are all examples of board-level circuits that may need conversion at any point along the circuit. As a result, a buck converter changes the DC level of input voltage to other levels that are necessary. Buck converters are commonly used in low-voltage, low-power applications. Buck converters in multiphase mode may deliver a large current at low voltage. As a result, it's suitable for low-voltage, high-power applications. Both low voltage low power converter and low voltage high power converter will be discussed in this article.

Basic Topology of Buck Converter

A controlled switch, a diode, a capacitor, and controlled drive circuitry make up a simple buck converter. By turning on and off at regular intervals, the switch regulates the flow of input power into the output. The duty cycle refers to the amount of time the switch is turned on throughout the entire period.

The duty cycle D value varies from 0 to 1. When D=0, there is no voltage across the load, but when D=1, there is full input voltage across the load. As a result, buck converters are used when D is larger than 0 but less than 1. The buck converter's basic circuit schematic is shown below.

PWM and PFM are two methods for controlling the average output voltage of a Buck converter. The total switch time T is maintained constant while the switch's turn ON time tonne is adjusted via PWM (Pulse Width Modulation). In PFM, on the other hand, the switching period time T is adjusted while the switch's turn ON time tonne remains constant (Pulse Frequency Modulation).PWM and PFM both have their own set of benefits and drawbacks. However, most of the time, PWM is preferable for buck converter operation.

Working Operation of Buck Converter

The working operation of the buck converter can be explained in two modes. Mode 1 is when the switch is turned ON, while mode 2 is when the switch is turned OFF. Both modes are explicitly discussed here.

Mode 1

When the switch is turned on, the diode becomes reverse-biased to the applied input. As a result, the inductor will carry all of the input currents. As a result, the inductor current equals the DC input current Idc flowing through the circuit.

During the turn-on period, the inductor will charge. The load current Io and the capacitor current Ic are subdivided from this current.

During this time, the voltage differential between the applied DC voltage Vdc and the output voltage Vo is the inductor voltage VLon.

According to the volt second balance, the average voltage across inductor VL is zero. The turn ON time tonne is the product of duty cycle D and total time T, according to the duty cycle equation.

The inductor current-voltage relationship may be used to calculate the rippling current during turn-on mode.

The final result depicts the current slop of the inductor current when it is turned on. The rippling current is seen in the wave shape below, increasing ON time before decreasing with a negative slope.

Mode 2

When you turn off the switch, mode 1 becomes mode 2. The polarity of the inductor reverses in this mode, and it begins to serve as a source. The stored energy in the inductor causes the current to flow in this mode. During this time, the DC supply is turned off. As a result, the current in the circuit continues to flow until the inductor is discharged. With negative polarity, the voltage appearing across the inductor is equal to the load voltage.

The polarity of the inductor changes when the switch is turned off, causing the diode to become forward-biased. During this time, the anode voltage becomes more positive than the cathode voltage, and the anode begins to conduct. The turn-off time toff can be derived from the turn-on-time ton in the duty cycle.

Minimum and Maximum Peak of Inductor Current

Inductor current at a minimum Both ILMIN and IMAX (maximum inductor current) are critical parameters in the design. As a result, the current's lowest and maximum peaks must be identified. According to earlier debates, both ILMIN and IMAX are relatively easy to find. ILMIN is given by

The form of the equation will become as given by placing the values that we mentioned in the previous two ways of operation.

Transfer Function of Buck Converter

For the purpose of determining the transfer function, the buck converter must be regarded in a steady state. This factor will simplify the computations for determining the transfer function. According to the volt second balance, the average voltage across the inductor is zero in steady-state. In addition, in a steady state, the inductor will function as a short circuit to a pure DC.

Where VLon is the voltage across the inductor when it is turned on, and VLoff is the voltage when it is turned off. The outcome of plugging the values of VLon, ton, VLoff, and toff into the preceding equation is