Oscillator using ad829 works - trying to make it work using 741

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  • #26
NascentOxygen
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Baseline drift as the duty cycle changes sounds like you might have the oscilloscope on AC coupling. Use DC.
 
  • #27
FOIWATER
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Actually, it changes even when I don't trim the pot (ie, when the duty cycle isn't changing)
 
  • #28
NascentOxygen
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Then I'd look for careless earthing. AC at mains frequency getting mixed in with your signal to the CRO causing the display to drift up and down.
 
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  • #29
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Ah so you trying to build a buck converter. If so, can you tell me why you use symmetric power supply in this circuit? For buck converter you don't need symmetric supply, and why you don't use P-channel MOSFET of N-channel with bootstrap capacitor? And finally why you use so many op amp.
 
  • #30
FOIWATER
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OK NascentOxygen I will try to rectify using those hints..

Jony, it could be my lack of understanding. Would you suggest a single sided supply, and a sawtooth oscillator?

I need so many op amps, because I am controlling the output of the buck converter. Two of them are for the oscillator, and four are for the PI controller (one subtractor, one summer, one proportional gain, and one integral gain) finally the LM311 to compare the PI signal to the oscillator signal... I am not sure how else to do it.

I am not sure how bootstrapping would help me..
 
  • #31
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Jony Would you suggest a single sided supply, and a sawtooth oscillator?
Yes, you can use single supply and to simplify the circuit you can use sawtooth oscillator instead of a triangle gen.
I need so many op amps, because I am controlling the output of the buck converter. Two of them are for the oscillator, and four are for the PI controller (one subtractor, one summer, one proportional gain, and one integral gain) finally the LM311 to compare the PI signal to the oscillator signal... I am not sure how else to do it.
All you need is triangle/sawtooth oscillator, error amplifier and the comparator.
I am not sure how bootstrapping would help me..
IR4427 is a low side driver, but in your buck converter IRF520 work as a high side switch.
And this is why you need high side driver. Because now the voltage at MOSFET source cannot reach 28V. The max voltage you will get at source is 5V - Vgs ≈ 2V
So your buck converter will not work as you want. Also L1 value don't look right for such a low oscillator frequency.
 

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  • #32
FOIWATER
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I picked the values for the inductor and capacitor based on the formula ripple = Vo*(1-D)(Ts)^2/8LC Using a output ripple of 1% and a 50% duty cycle. But also the change in output current is given by (Vs-V0)DTs/L So I can size L and then choose C accordingly based on the first formula. I got these formulas from Mohan:Power Electronic converters textbook. I can see how they were derived.

I am not sure what you mean about the mosfet driver being a low side driver vs the mosfet being a high side switch? pardon my lack of knowledge..

As for the abundance of amplifiers - I need to utilize PI control

I really appreciate the replies.
 
  • #33
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I am not sure what you mean about the mosfet driver being a low side driver vs the mosfet being a high side switch? pardon my lack of knowledge..
Simply, in your circuit the mosfet work as source follower and that means that the voltage at mosfet source (left side of a inductor) voltage cannot be larger then 5V-Vgs.
As for the abundance of amplifiers - I need to utilize PI control
But you can build PI controller using only one op amp.

I picked the values for the inductor and capacitor based on the formula ripple = Vo*(1-D)(Ts)^2/8LC Using a output ripple of 1% and a 50% duty cycle. But also the change in output current is given by (Vs-V0)DTs/L So I can size L and then choose C accordingly based on the first formula. I got these formulas from Mohan:Power Electronic converters textbook. I can see how they were derived.
For which Vo =?? and I_load =?
 
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  • #34
FOIWATER
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Vo as 20 v I load as 4 A

I understand about the op amp now.

Sure I understand now, this driver needs a higher voltage, right?

But it's only rated for 20,
 
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  • #35
FOIWATER
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I have a high side driver available. IR2011.

If I replace the driver with IR2011, I can only supply it with 20v according to the specification sheets.

Does this mean, if I want to buck 28 volts to 20, I NEED to use a bootstraping technique?
 
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  • #37
FOIWATER
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I need a high input to turn on the mosfet. This seems like a different scenario, no?

I guess.. a high signal IS technically turning on the mosfet.. but it's a low signal from the driver which accomplishes it?
 
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  • #38
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This circuit will do the job
 

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  • #39
FOIWATER
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I wouldn't feel right using it outright. But I will try my best to learn from it. Thanks, Jony

Do these circuits eliminate the need for a driver??

I am lost here
 
  • #41
FOIWATER
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How about if I use a separate supply for the driver and ground the source of the MOSFET.

If this will work, will 5 volts be enough?
 
  • #42
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You want to use a floating power supply for a MOSFET ?
 
  • #43
FOIWATER
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for the driver, will that work?
 
  • #44
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If you don't want to use this discrete MOSFET driver. I think you should try to use IR2011. You can power it from 12V power supply you already have in your design.
 
  • #45
FOIWATER
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Really.. but as you said, 12 volts to the gate of the MOSFET will not allow me to have 28 volts at the source?

I will have 12 - Vgs only, no?

I don't think I understand drivers
 
  • #46
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But IR2011 has high side driver with external diode and capacitor. And this will boost the gate voltage up to
28V + 12V.
 
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  • #47
FOIWATER
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I did not know this thanks Jony
 

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