# instantaneous power from ONE of your circuits

Discussion in 'Homework Help' started by Martin976, Aug 14, 2016.

1. ### Martin976 Thread Starter New Member

Aug 14, 2016
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The instruction is above the picture attached. Do not have the foggiest idea how to sort this out. What is the relation between the Power and the Time? How to describe the reaction?

I'd be grateful for any hint.

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2. ### WBahn Moderator

Mar 31, 2012
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The graph doesn't indicate what the family of light curves represent or what values of what parameter are associated with each of them.

Aug 14, 2016
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4. ### MrAl Well-Known Member

Jun 17, 2014
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Hello there,

Are your circuits max power tracking or not?

The instantaneous power is:
p(t)=v(t)*i(t)

often just written as:
p=v*i

and at a given operating point it will just be two static quantities multiplied together.

For the solar panel you look at what voltage you have and then multiply that times what current you have. If you are max power tracking then you can find out from the light colored curve, but if not you've got to solve for the voltage and current first.

You never heard of instantaneous power before?

Also, are you allowed to call the converter 100 percent efficient or do you have to apply an efficiency rating to it as well?

5. ### Martin976 Thread Starter New Member

Aug 14, 2016
8
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It ll be easier if I let the whole content out.

What I gonna do is sketching 2 of the chosen circuits of Boost and one of a Buck converter that can be used to achieve Maximum Power Point Tracking (MPPT). When it comes to the Boost converter I've taken this one from the attachment above.

After describing and sketching the waveforms of these circuits I am asked to describe how the instantaneous power from ONE of my 2 circuits above would be, given the irradiation changed linearly from 400 W/m2 to 800 W/m2 over two seconds.

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6. ### Martin976 Thread Starter New Member

Aug 14, 2016
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We can assume converter has 100 percent efficiency. I have heard of instantaneous power before. Any other conclusions?

7. ### MrAl Well-Known Member

Jun 17, 2014
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435
Hi,

Well if you are max power tracking then this should be easier. That's because you can follow the peaks of the power curves and log the current and/or voltage that exists at those points.

Now if you only have to produce the first and last operating points then it is super easy because all you have to do is log the start and end voltages and currents.

If on the other hand the question did mention that they change linearly, so you might have to produce an equation that shows how the voltage, current, both, or power changes with change of incident sunlight.
If you have to do this then since they did say 'linear' you can probably get away with a simple straight line curve fit of the voltage current or power from one extreme to the other, or if you want to get fancy then curve fit a second or third degree polynomial (if needed that is, but notice the locus of peak power points tends to curve a little). Since they did say linear though i would suspect a straight line approximation would be good.

BTW, to read the power peaks look at the light colored curves. Each one of those corresponds to one of the dark colored curves.
The light colored curves are for voltage vs power.
For example for the 1000 w/m^2 the light curve with highest peak is used, and for the current and voltages that produce that curve the uppermost dark curve is used.
For a numerical example, at the max power point for the 1000w/m^2 level of sunlight the peak power is about 250 watts and that occurs with a current of about 8 amps and voltage about 32 volts although those two are not exact just approximate, read from the dark graph quickly.

Also, if your circuit really is max power tracking then it will be drawing enough current to force it's input voltage to 32 volts, and from the curves we see that current will be 8 amps approximate (again at the 1000w/m^2 sunlight level).

Since the converter is assumed to be 100 percent efficient, then you can say that the output has the same power as the input, as long as the load is appropriate for max power tracking.
An appropriate load would be a load that can take either a range of voltages, a range of currents, or both. Typical loads would be the AC power line or a battery, but for example if a resistive load then it must allow both a range of current and a range of voltage (eg no voltage regulators or current regulators allowed if the load is purely resistive).

Last edited: Aug 16, 2016
8. ### MrAl Well-Known Member

Jun 17, 2014
2,214
435
Hello again,

I was wondering how you made out with this.

Oh yeah, i should mention that you might only have to follow the power peaks and log and curve fit those because your question was about the power, so you may not be as concerned with the current and voltage. It's good to know though in case you have to deal with a different kind of converter at some point later.

9. ### DGElder Member

Apr 3, 2016
308
72
I am not a solar PV/MPPT/inverter system expert by any means but........

In general how your boost converter circuit behaves with a change in iradiation of the panel/s depends on the control circuit for which you have provided no information. In this particular case for MPPT you need to lower Vo the required amount in 2 sec. (about 3V according to the curves). Since your circuit, regardless of control circuit, can not lower Vo any faster than the capacitor can be drained by the load you need to know C and the load to know if the required change in Vo is faster or slower than the circuit's capability. If it is slower, then the control circuitry could follow MPPT. If faster then the load and C will determine power output during transition and the amount of time to transition to the new MPP.

Last edited: Aug 18, 2016