Switching power supplies - pulse width modulation work

Thread Starter

anditechnovire

Joined Dec 24, 2019
93
Please can someone explain to me how pulse width modulation work with mosfet in a smps circuit.
Oscillator ics in smps (like viper22A) have an internal mosfet control by pwm.
So please what is the output waveform of a transistor or mosfet that is driven with pwm, and why must pwm be given to a transistor first to switch.
Since the flyback transformer needs a varying voltage, then why not feed the pwm directly to the transformer.
It making me find difficulty understanding the difference between square wave and pwm.
Is there anything am not getting right?
 

Delta prime

Joined Nov 15, 2019
336
It making me find difficulty understanding the difference between square wave and pwm
It means modulating/varying the width of the pulse (Not the frequency).in a PWM signal is that the time-period and hence the frequency is always fixed. Only the ON time and OFF time of the pulse (duty cycle) varies. By this technique, we can modulate the given voltage.The one difference between a square wave signal and PWM signal is that the square wave signal has same ON and OFF time (50% duty cycle), whereas a PWM signal has a variable duty cycle
 

Papabravo

Joined Feb 24, 2006
13,727
It means modulating/varying the width of the pulse (Not the frequency).in a PWM signal is that the time-period and hence the frequency is always fixed. Only the ON time and OFF time of the pulse (duty cycle) varies. By this technique, we can modulate the given voltage.The one difference between a square wave signal and PWM signal is that the square wave signal has same ON and OFF time (50% duty cycle), whereas a PWM signal has a variable duty cycle
I disagree. IMHO a square wave can have any fixed non-zero ON/OFF ratio. The only real requirement for what we call a square wave is that rise time and fall time are very much less than the period. The reason for having the PWM is that we are (in a buck converter) switching a higher input voltage onto an inductor, which will supply current from the collapsing magnetic field to the load as the voltage drops. When the voltage drops it is time to recharge the inductor again, and so it goes. Applying the control voltage to the output would not do the job, especially if it was lower than the output.
 
Last edited:

Tonyr1084

Joined Sep 24, 2015
4,535
In PWM, there's a square(ish) wave. It has a straight "UP" side and a straight "DOWN" side when transitioning from high to low (on to off) (1 to 0) and vice versa. By modulating the width of the pulse (the up side) you modulate the amount of average voltage. Where a square wave is at (lets say) 12 volts when on and 0V when off, if the on time pulse were modulated to 50% of the cycle you'd get an average of 6V. If it's on time is 10% then the average voltage is 1.2V. At 90% of the cycle you would get an average of 10.8V.

When we refer to a square wave - in the truest sense of the word, square means equal sides. So the on time will equal the off time, thus, 50%. But nobody calls it a rectangular wave where you don't have a true square wave. So PWM modulates the amount of on time to control the average voltage.
 

Papabravo

Joined Feb 24, 2006
13,727
Yes,yes,of coarse,the poor chap seemed to have trouble understanding the nomenclature .I was easying my way in. I think you scared him away :mad:
I dunno, he asked what seemed like a straightforward question. Most people have trouble grappling with processes they cannot see, hear, touch, or feel. You get the hang of it after a while. I still don't get art. I mean I appreciate it, but I have absolutely no facility for it.
 

Thread Starter

anditechnovire

Joined Dec 24, 2019
93
It means modulating/varying the width of the pulse (Not the frequency).in a PWM signal is that the time-period and hence the frequency is always fixed. Only the ON time and OFF time of the pulse (duty cycle) varies. By this technique, we can modulate the given voltage.The one difference between a square wave signal and PWM signal is that the square wave signal has same ON and OFF time (50% duty cycle), whereas a PWM signal has a variable duty cycle
Ok thanks, but please what about the first question that is related to pwm and smps. Can we connect a pwm directly to a flyback transformer primarys without first switching the pwm with a transistor or mosfet.
 

Thread Starter

anditechnovire

Joined Dec 24, 2019
93
I disagree. IMHO a square wave can have any fixed non-zero ON/OFF ratio. The only real requirement for what we call a square wave is that rise time and fall time are very much less than the period. The reason for having the PWM is that we are (in a buck converter) switching a higher input voltage onto an inductor, which will supply current from the collapsing magnetic field to the load as the voltage drops. When the voltage drops it is time to recharge the inductor again, and so it goes. Applying the control voltage to the output would not do the job, especially if it was lower than the output.
It means modulating/varying the width of the pulse (Not the frequency).in a PWM signal is that the time-period and hence the frequency is always fixed. Only the ON time and OFF time of the pulse (duty cycle) varies. By this technique, we can modulate the given voltage.The one difference between a square wave signal and PWM signal is that the square wave signal has same ON and OFF time (50% duty cycle), whereas a PWM signal has a variable duty cycle
Ok thanks, but please what about the first question that is related to pwm and smps. Can we connect a pwm directly to a flyback transformer primarys without first switching the pwm with a transistor or mosfet
 

Thread Starter

anditechnovire

Joined Dec 24, 2019
93
I disagree. IMHO a square wave can have any fixed non-zero ON/OFF ratio. The only real requirement for what we call a square wave is that rise time and fall time are very much less than the period. The reason for having the PWM is that we are (in a buck converter) switching a higher input voltage onto an inductor, which will supply current from the collapsing magnetic field to the load as the voltage drops. When the voltage drops it is time to recharge the inductor again, and so it goes. Applying the control voltage to the output would not do the job, especially if it was lower than the output.
Ok thanks, but please what about the first question that is related to pwm and smps. Can we connect a pwm directly to a flyback transformer primarys without first switching the pwm with a transistor or mosfet.
 

Thread Starter

anditechnovire

Joined Dec 24, 2019
93
In PWM, there's a square(ish) wave. It has a straight "UP" side and a straight "DOWN" side when transitioning from high to low (on to off) (1 to 0) and vice versa. By modulating the width of the pulse (the up side) you modulate the amount of average voltage. Where a square wave is at (lets say) 12 volts when on and 0V when off, if the on time pulse were modulated to 50% of the cycle you'd get an average of 6V. If it's on time is 10% then the average voltage is 1.2V. At 90% of the cycle you would get an average of 10.8V.

When we refer to a square wave - in the truest sense of the word, square means equal sides. So the on time will equal the off time, thus, 50%. But nobody calls it a rectangular wave where you don't have a true square wave. So PWM modulates the amount of on time to control the average voltage.
Ok thanks, but please what about the first question that is related to pwm and smps. Can we connect a pwm directly to a flyback transformer primarys without first switching the pwm with a transistor or mosfet
 

Papabravo

Joined Feb 24, 2006
13,727
Ok thanks, but please what about the first question that is related to pwm and smps. Can we connect a pwm directly to a flyback transformer primarys without first switching the pwm with a transistor or mosfet
You can do anything you want to do. Will it accomplish your goals? I can't answer that question without seeing a schematic diagram, annotated with component values and part numbers.
 

ElectricSpidey

Joined Dec 2, 2017
1,156
Generally speaking the circuit used to produce PWM is only a "signal" then that signal is passed to a "switch" that can handle the current and voltage needed by the load.
 
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