A chopper is a static circuit that converts fixed DC input voltage to a variable dc output voltage. Normally output voltage of chopper is unregulated. Dc Converters can be used as switching mode regulators to convert dc voltage, normally unregulated voltage to a regulated dc output voltage. By using Pulse Width Modulation (PWM) this regulation is achieved.

The three basic types of dc converters which are using switching mode regulators are

• Buck Converter

• Boost Converter

• Buck Boost Converter

 

Boost Converter

As the name implies Boost converter is a dc – dc converter using switch mode regulator to step up the voltage while step down output current.

Boost Converter Circuit Diagram

In the above figure, we have seen that inductor is connected in series with input source this leads to source current be constant. And Capacitor is connected parallel to load this leads to constant voltage across the load.

Boost converter working

Let us say that switching frequency of switch is F and correspondingly time period is T  =  1/F. 

Let us say that switch is on for T_on time and off for T_off time.

So total time period   T   =   T_on + T_off

Duty Ratio (D):- It is the ratio of on time to total time period.

 

So according to above discussion we can divide the circuit operation in two mode. 

• Mode- I, when switch is on, during T_on time period

• Mode – II when switch is off, during T_off time period

 

When the switch is on the current path is shown in the given figure. During this time period, inductor stores energy and voltage across the inductor is equal to the source voltage.

 For mathematical equations apply Krichof’s Voltage Law (KVL) in the given figure. After applying you will get

 

              VS   =   VL         

            ......................      (1)

ΔI = Peak to Peak ripple current

When the switch is off the current path is shown in the given figure. During this time, inductor discharge energy through the diode. But inductor current cannot go down instantaneously this current is forced to flow through the diode, capacitor and load until the switch is on for next cycle. As inductor discharge the polarity of the inductor voltage becomes reversed.

During next cycle when the switch is on, inductor current changes from I₁ to I_2.

After applying KVL in the given circuit you will get

 

Vo  =   Vs  +  VL

VL   =   Vo   - Vs

Consider the circuit is lossless, so that the energy stores in the inductor during on time and energy releases the inductor during off time is equal.

So 

Vs.Ton  -  (Vo-Vs). Toff  = 0

Vs.T  =  Vo.Toff

       ...........         (2)

As we assume circuit is lossless so that input power is equal to output power.

P_in  =  P_o

V_s.I_s  =  V_o.I_o

Boost Converter Design using MATLAB Simulink

1.  Open a new MATLAB Simulink model.

2.  Select the following components from MATLAB library.

Dc voltage source :->  Go to Library >> Simscape >> Electrical >> Specialized Power System >> Sources >> Select Dc Voltage Source.

MOSFET (any switch) and Diode :->  Go to Library >> Simscape >> Electrical >> Specialized Power System >> Power Electronics >> Select MOSFET and Diode

 

Three RLC branches Modified accordingly as per circuit diagram

Go to Library >> Simscape >> Electrical >> Specialized Power System >> Passives >> Select Series RLC Branch

Note

• Don’t select series RLC Load.

• By double click on each RLC branch select resistance, capacitance and inductance.

Current Measurement and Voltage Measurement

Go to Library >> Simscape >> Electrical >> Specialized Power System >> Select Two Voltage Measurement and Current Measurement.

 

Pulse Generator :-> Go to Library >> Simulink >> Sources >> Select Pulse Generator.

 

Scope :-> Go to Library >> Simulink >> Sinks >> Select Two Scopes

 

Powergui block :-> Go to Library >> Simscape >> Electrical >> Specialized Power System >> Select Powergui Block.

Display :-> Go to Library >> Simulink >> Sinks >> Select Display. To display your values

 

3.  Connect this block according to the circuit diagram.

4.  Assign values for different components as per requirements by double clicking on them.

5.  Select stop time 0.2 Sec. and then run your Simulink model.

Some important questions from internet

   1)  5V to 10V  DC-DC Converter

In this converter desired output voltage V_o is 10V and supply voltage V_s is 5V. So from Above discussion we can say that equation 2 will help to get this desired value.

Vo  =  Vs / (1 - D)    

 

10V  =  5 / (1 - D)

From above discussion we can say that by making duty ratio 0.5 we will get 10V output from 5V supply voltage.

For given values, values of inductance and capacitance set accordingly to get smooth current and voltage waveforms.

Related Terms

#_Rectifier

#_Diode

#_SCR(Silicon Controlled Rectifier)