Let's see the below waveform
This wave is switching between 0 volt and 5 volts. From the diagram it is obvious, that the voltage is at 5 volts for exactly as long as it is at 0volt, then an appropriate device connected to its output will see the average voltage and think it is being fed 2.5volt exactly half of 5 volts.
So by changing the width of the positive pulse, we can change the average voltage in the same way, if the switches keep the voltage at 5 for 3 times as long as at 0volt, the average will be 3/4 of 5volt = 3.75 volts.
see the top diagram where we have vary the on-time width.
if the output pulse of 5v volt remains on only 25% of the overall time, then the average is 25% off 5 volt that is 1.25 volt. So By changing the on time of the pulse, we can alter the average voltage. So we are doing pulse width modulation. I said earlier that the output had to feed an appropriate device. A device such as a motor will act in response to the average, so PWM is usual for motor control.
Now how to generate the PWM waveform. It’s a very easy Task. You have to generate a triangular waveform using a circuit and then compare it to a dc voltage where you control the duty cycle of PWM. As in the below diagram as triangular wave voltage increases the required voltage the output goes high and when the triangular wave voltage decreases the required dc voltage the output goes low. When the require dc voltage which we are comparing with triangular is at midpoint magnitude of the triangular wave we will have PWM of the equal duty cycle of on and off time. I hope you understand PWM.
So by changing the width of the positive pulse, we can change the average voltage in the same way, if the switches keep the voltage at 5 for 3 times as long as at 0volt, the average will be 3/4 of 5volt = 3.75 volts.
see the top diagram where we have vary the on-time width.
if the output pulse of 5v volt remains on only 25% of the overall time, then the average is 25% off 5 volt that is 1.25 volt. So By changing the on time of the pulse, we can alter the average voltage. So we are doing pulse width modulation. I said earlier that the output had to feed an appropriate device. A device such as a motor will act in response to the average, so PWM is usual for motor control.
Now how to generate the PWM waveform. It’s a very easy Task. You have to generate a triangular waveform using a circuit and then compare it to a dc voltage where you control the duty cycle of PWM. As in the below diagram as triangular wave voltage increases the required voltage the output goes high and when the triangular wave voltage decreases the required dc voltage the output goes low. When the require dc voltage which we are comparing with triangular is at midpoint magnitude of the triangular wave we will have PWM of the equal duty cycle of on and off time. I hope you understand PWM.
There are PWM IC are also available where there is no need for separate triangular wave generator, comparator, and the reference voltage. We just have to change the value of PWM like Varying the resister value.SG3525 is a voltage control PWM IC which I had used for dc-dc converter in my inverter project.
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