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LTspice: plotting the integral of a waveform

Discussion in 'Homework Help' started by mikewax, Nov 9, 2016.

  1. mikewax

    mikewax Member

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    THANX guys for ur assistance with my spice problems :)

    now i've made and plotted a working circuit, a DC buck convertor, with LTspice and started tweaking it around. But one of the traces on my plot shows a math function that is not producing the right #s and i can't figure out why. The current thru R1 is supposed to be equal to the instantaneous average of the current through V2.
    The equation for V is the integral of the (green) current thru V2, summed over a Δt of 22u, divided by 22u. So the white trace at the top is supposed to be parallel with the green one underneath. And at the beginning, it is. But the deviation looks very linear, like there's some kind of bias goin on in B1. I have zoomed in systematically at each 10ms interval and compared the average of the two traces. the top one is definitely deviating and has the wrong slope. anyone know what the problem could be?

    buck.jpg
     

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  2. alec_t

    alec_t Well-Known Member Most Helpful Member

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    Your formula for B1 is incorrect. Try this instead:
    v=-idt(i(v2)+delay(i(v2),22u))/44u
     
  3. Les Jones

    Les Jones Well-Known Member

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    A 10 mF (10000 uf) capacitor looks a very large value on the output of a switching regulator.

    Les.
     
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  4. dave

    Dave New Member

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  5. mikewax

    mikewax Member

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    no that didn't work either. it just gave the same results divided by 2. something must be wrong with my equation but god knows what.
     
  6. mikewax

    mikewax Member

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    yeah that's because the purpose of the circuit is to charge a supercap.
     
  7. alec_t

    alec_t Well-Known Member Most Helpful Member

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    It works for me. Did you notice the sign change?
     
  8. mikewax

    mikewax Member

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    well i don't know what i'm not seeing, because when i use the formula for B1, V = -idt(I(V2) + delay(I(V2), 10u))/20u, what i'm getting is this:
    buck.jpg
    and the numbers show the same thing:
    avi1: AVG(i(v2))=-0.00927139 FROM 0.01 TO 0.0101 <--- 10ms
    avr1: i(r1)=23.9429 at 0.01 <-----V = -idt(I(V2) + delay(I(V2), 10u))/20u
    avi2: AVG(i(v2))=-0.00904411 FROM 0.02 TO 0.0201 <--- 20ms
    avr2: i(r1)=33.2628 at 0.02
    avi3: AVG(i(v2))=-0.00918508 FROM 0.03 TO 0.0301 <--- 30ms
    avr3: i(r1)=42.5975 at 0.03

    when i use this formula for B1, V = idt(I(V2) - delay(I(V2), 10u))/10u, i get this:
    buck.jpg
    which looks fairly accurate at a scale of 40ms, but at 400ms, it looks like this:
    buck.jpg

    so neither formula seems to be giving the average of the curve for the current I(V2) which is what i need to be able to do the circuit. it seems like such a simple thing, a basic integral. but it just ain't workin.
    thanx alec
     
  9. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi Mike,
    Its an accumulation of transient tolerance errors in the LTS maths, to demonstrate the effect of these tolerances try changing Trtol [ Transient error tolerance] in LTS Tools.

    The Default value is 1, try 7, you will see that I(R1) curve error is an an artefact of the LTS maths.

    Why are you charging a super cap in this way.?

    E
     
  10. alec_t

    alec_t Well-Known Member Most Helpful Member

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    That's what fooled me. I didn't have the patience to run the sim beyond 40mS :).
     
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  11. alec_t

    alec_t Well-Known Member Most Helpful Member

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    I note that the average obtained using your formula gives wildly different results depending on the value chosen for the sample interval. (Not surprising, since the samples are being taken at non-random instants related to the switching of V1). Change the '10u' to '40u', for example, and see. For a true average value I reckon the formula would be V=idt(I(V2))/time.
     
  12. mikewax

    mikewax Member

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    ok i had to google that. so these transient errors are coming out of my integral equation? and they accumulate? in spite of the fact that the data that i'm integrating over remains essentially constant? when i changed the Trtol to 7 i got this:
    buck.jpg the red line is holding pretty constant at about .009, while my equation just walks away. this **** is over my head.

    i work for a rich tinkerer. He grabbed a webcam, stuck it on a motorized gimbal, and said "this thing plugs into the usb port. every 60 seconds the motor has to run for 1 second at 5v, 800mA. And put limit switches to make it reverse every 200 degrees of rotation"
    right away sir....
     
  13. mikewax

    mikewax Member

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    WOW. you're really on to something. cause when i changed Δt to 40, i got this:
    buck.jpg
    that's WAY better.

    OK i'm gonna keep tweakin the numbers. increasing Δt and Trtol brought the error way down. i don't really understand this, but i'm gonna keep workin n maybe i will.
    THANX fellas. now i can make progress again. :))))))
     

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