Oe Project.docx

BUCK CONVERTER Assignment – 4 (OE)

Sheen Gurkha (16)

Introduction Buck converter is a converter in which dc voltages are step down to desired level by high frequency switching of semiconductor switches such as MOSFET. This type of converter is also called a step down converter. If we talk about regulated supply, then it is not so much difficult in ac side but in dc side it is so much difficult and this is only possible with high frequency switching of semiconductor switches. This type of converter is mainly used in switch mode power supplies and dc motor control system. The basic operation of the buck converter has the current in an inductor controlled by two switches (usually a transistor and a diode). In the idealized converter, all the components are considered to be perfect. Specifically, the switch and the diode have zero voltage drop when on and zero current flow when off, and the inductor has zero series resistance. Further, it is assumed that the input and output voltages do not change over the course of a cycle (this would imply the output capacitance as being infinite).

Operation Below, a model of a basic step-down converter is used to explain the circuit operation. By gaining an understanding of the properties of current pathways and nodes from the basic operation, standards for selection of peripheral components and matters demanding attention will become clear. In the diagrams, we replace the high-side transistor and low-side diode with switches to explain operation schematically. The circuit principles are the same as those of diode rectification in a DC/DC converter, but the high voltage obtained by rectifying an AC voltage is directly switched to perform step-down voltage conversion, and so the transistor and diode acting as switches must withstand high voltages, for example 600 V or so.



When the high-side switch (the transistor) turns on, a current IL flows in the inductor L, and energy is stored



At this time, the low-side switch (the diode) is turned off



The inductor current IL is expressed by the following equation （ton： ON-time）



When the high-side switch (the transistor) turns off, the energy stored in the inductor is output through the low-side switch (the diode)



At this time, the high-side switch (the transistor) is OFF



The inductor current IL is expressed by the following equation （toff： OFF time）

Discontinuous Mode and Continuous Mode In switching operation, there are two modes, a discontinuous mode and a continuous mode. They are compared in the following table. The "operation" item for comparison is the waveform of the currents flowing in the primary windings and secondary windings of the transformer. In discontinuous mode, there is a period in which the inductor current IL is interrupted, hence the name, discontinuous mode. In contrast, in continuous mode there is no period in which the inductor current is zero. In each mode, arrows indicate the tendencies for the inductor, the rectifying diode, the switching transistor, and the efficiency; an upward arrow "↑" means an increase, and a downward arrow "↓" indicates a decrease. In the case of the continuous mode, when the switches are ON, a reverse current flows during the reverse recovery time (trr) of the rectifying diode, and losses occur due to this reverse current. In low-voltage switching DC/DC conversion, the reverse voltage of the rectifying diode is low and the reverse current is also small, and so generally the continuous mode is used, giving priority to reducing the output ripple voltage and harmonics. However, in AC/DC conversion, the diode reverse voltage is high and a large reverse current flows, and so discontinuous mode, in which a reverse current does not flow and losses are reduced, is generally used. However, the peak current becomes large, and when the load is large, sometimes operation in continuous mode is preferred. Each mode has its advantages and disadvantages, but in general, the discontinuous mode is used up to about 50 to 60 W. At output powers above this, a decision is made taking into account the size of the transformer that can be accommodated and other factors. In this design example, the discontinuous mode is used.

Circuit Diagram A buck converter is the one which converts the DC voltage level of an input source to a lower value and shift the current level of the source to a higher value at the output. A simple circuit diagram of a buck converter is shown in the figure below,

The switch in the circuit is the main component and it controls the voltage level at the output, and the on and off states of the switch are shown in the figure below,

Simulation The red waveform is the voltage level across the output whereas the green waveform implies the input voltage level. As we have discussed in the introduction part the voltage level of output should be lower than the input thus the simulation gives the same results. Converting the voltage markers to current markers and the current at the output will be higher as shown in the figure below,