How to get pure sinusoidal AC output from DC input using PWM inverter

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How to get pure sinusoidal AC output from DC input using PWM inverter

By Suman Majhi

What is Inverter?

Inverter is an electrical equipment which is used to convert dc supply into AC supply. there are two different types of inverter used in the electrical field. they are given below,

1) square wave inverter
2) sine wave inverter

here we will going to see about how to convert pure dc into pure ac sinusoidal wave. these process involved different steps. so we will briefly discuss about each steps.


Square wave inverter


 Generally all inverter produced square wave ac output only. because square wave inverter is not required any addition component to convert ac from dc supply. even cost wise its really cheaper then pure sine wave inverter.

working of square wave inverter


 The working of square wave inverter is  explained through below diagram.


let us consider full bridge rectifier, S1,S2,S3 and S4 are switches and resistance is load. let switch s1 & s2 keep it on and switch s3 & s4 keep it open. then current from the battery flow from battery negative to switch s2 and load and switch s1to battery positive terminal.  let switch s3 & s4 keep it on and switch s1 & s2 keep it off. then current flow from battery to load is opposite direction. this is the working of square wave inverter.

we all know our home appliances all required 50/60 Hz frequency. that means we need to switch on this above circuit 120 times per second to get this frequency. it is not possible to making these witches ON/OFF continuously by manual or mechanically. that’s why we used semiconductor switches that is MOSFET. the transistor MOSFET is operating continuously  1000 times ON/OFF per second  with help of control signal we can easily do the  on/off the transistor.


Disadvantages of square wave inverter

This square wave inverter produced the square output that’s why if you connect electrical equipment in square wave inverter it will produce humming noise
because of this square wave output we can not use this inverter into sensitive electrical equipment like laptop, mobile charger, etc..
it will produce heat up in the electrical equipment.


Sinusoidal or sine wave inverter

The sinusoidal inverter produced the pure sine wave output. the problem of square wave inverter is over come by adding PWM pulse width modulation technique in the existing inverter.


Working sine wave inverter

The working of sinusoidal wave inverter is same like square wave inverter. they upgraded the square wave inverter with PWM technique. as per this new technique the DC voltage is converted into different width of pulse of sign wave.



The average of pulse will be come pure sinusoidal wave. practically it can be achieved by adding two comparator to the same inverter circuit.

one comparator using normal sign wave and other comparator using inverted sign wave. the first comparator connected between switch S1 & S4 and second comparator is connected between switch S3 & S2.  switch S1 & S4 determine the voltage level at point A and switch S3& S2determine the voltage level at point B. more over the each comparator having one side NOT gate. the purpose of NOT gate is we can not switch ON S1 & S4 at a time. same thing switch S3 & S2 can not turn ON at a time. comparator check the each signal with out put signal,
Vsine  > Vtrain output is 1
Vsine <  Vtrain output is 0
so the resultant output is connected to load through inductor and capacitor passive components to get the smooth current and voltage purpose.

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