Ir2153 + uc3843 = low side modulation idea

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expressel

New member
Friends, look at this idea:

You can change IR2153's duty cycle if you control the low side signal via transistor and UC3843.

Just feed 100khz pwm from UC3843 to the base of a BC547 in emitter follower configuration and feed IR2153's low side signal (50khz) to the collector.
The result is 50khz with changable duty cycle.

Simple modulation :adoration:

You dont need to modulate the high side signal too. Just only the low side because the high side current cant flow if you dont fully discharge the capacitor with the low side FET.

I don't know if I explained it well or not.

I haven't tried but Im sure it will work.


 
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Silvio

Silvio

Well-known member
@ Expessel I do not see the reason for this thus using two chips I would rather use an SG3525 which is properly made for the job and the duty cycle can be varied to control the output voltage.

Experimenting is good in a way as it helps us learn different ways of controlling smps. I think with the modulation you are talking of will also vary the output frequency. Did you consider this? Varying output frequency also varies the magnetic flux density at low frequency magnetic flux rises in relation with number of turns in the transformer. From what I can see on your scope shot it seems that there is a missing pulse so if the frequency is 50Khz then with the missing pulse becomes 25Khz. Do you think this will reduce the output voltage?

Why not try it out and see what happens.
 
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expressel

New member
The duty cycle can be varied with this method too.

I think with the modulation you are talking of will also vary the output frequency
Click to expand...

No, it will not vary the freq. See this scope shot. The freq stays 50khz, the duty goes 25% because 50% of 50% is 25%
If you apply 25% 100khz signal, you will get 0.25*50% = 12.5% duty cycle.

Why not try it out and see what happens.
Click to expand...

I will try it soon.



 
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expressel

New member
Look, in Multisim I can change the duty cycle of the osc.



Duty varies, frequency stays 50khz. :)

You can vary it from 0% to 50% when your 100khz source varies from 0 to 100%

For voltage regulation, you can vary only the low side FET's duty cycle.

Varying the low side varies the discharge of the half-bridge capacitor.

When you don't discharge it (0% low side duty) the capacitor don't pass current to flow through the transformer and this equals 0V output voltage.

When you discharge it fully (50% low side duty) the capacitor passes current and this equals maximum output voltage

This time, I think, I explained it well :)

Look what happens when you add delay (simulating when the two oscillators are out of sync like in real life the two ICs aren't synced)

 
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Silvio

Silvio

Well-known member
@ Expressel I am really impressed keep up the good work. Just one question, how are you going to synchronize the oscillators?
 
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expressel

New member
I can sync them with a simple turn on delay circuit: RC, comparator and potentiometer. When the capacitor charges to Vref (the potentiometer), the comparator turns on the IR2153 or the TL494.

Even I think, they should start synced without delay circuit. If their oscillators are similar (they should be, i dont know).
 
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