Introduction

The topics covered in this chapter are as follows:

• •  Principle of Step-Up Operation

•  Boost Converter with Resistive Load and EMF Source

•  Boost Converter with Filter and Resistive Load

•  Buck-Boost Converter

 

Principle of Step-Up Operation (Boost Converter)

The circuit diagram of a step up operation of DC-DC converter is shown in Figure 1. When the switch S₁ is closed for time duration t₁, the inductor current rises and the energy is stored in the inductor. If the switch S₁ is openerd for time duration t₂, the energy stored in the inductor is transferred to the load via the didode D₁ and the inductor current falls. The waveform of the inductor current is shown in Figure 2 .

 

Figure 1: General Configuration of a Boost Converter	Figure 2: Inductor current waveform

 

When the switch S₁ is turned on, the voltage across the inductor is

                                                                                                        (1)

The peak to peak ripple current in the inductor is given by

                                                                                                        (2)

The average output voltage is

                                                            (3)

From Equation 3 the following observations can be made:

• • The voltage across the load can be stepped up by varying the duty ratio D

• The minimum output voltage is V_s and is obtained when D = 0

• The converter cannot be switched on continupusly such that D = 1. For values of D tending to unity, the output becomes very sensitive to changes in D