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  #1  
Old 08-25-2010, 10:38 PM
Lee_831 Lee_831 is offline
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Default Low-Side Current Sensing for MPPT

Hi Guys,

I'm new to this forum but it has already been a great help!

My name is Lee and I'm doing an MSC project in MPPT technologies. I'm looking for some help with the design..
Apologies if this material has been covered before!

I have coded what I believe to be a Perturb and Observe algorithm in C, to be run on a PIC16F877A Micro-controller, which I intend to hook up to a DC Buck Converter.

My last concern is with measuring the current flow in the circuit, it has been suggested that I use a low-side current sensing op-amp. I'm not a particularly analogue-minded person, and could do with some help!

I'm looking to have a shunt resistor of around the order 10 Ohms, and use a 741 as the op-amp, and I'm feeding the op-amp output into a PIC16F877A's ADC input, which has a reference voltage of 5v.

At maximum sunlight, the cell produces about 1.5mA at 5v, and at this level I'd like the output of the 741 to be at the 5v maximum. Does this sound feasible? There seems very little in the way of tutorials on how to design these circuits.

I very much appreciate your help,
Lee
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Old 08-25-2010, 10:57 PM
tom66 tom66 is offline
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I'm not sure about your main question, but be aware that your LM741 won't be able to put out +5V with a +5V input. It also cannot go to ground.
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Old 08-25-2010, 11:03 PM
Lee_831 Lee_831 is offline
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Hi,
Thanks for your quick reply,

My main question is how do I design a low-side current sensor circuit, ie what are the values of resistor / capacitor that I should use on the inputs and as the feedback resistance?

Would you recommend an alternative op-amp for this application?

Thanks,
Lee
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Old 08-25-2010, 11:06 PM
tom66 tom66 is offline
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Well, if you need to go to zero and you can afford to power the op-amp from about +8V, an LM358 will do the job fine. But if you need a rail-to-rail op-amp, that can go up to +5V with a +5V input, the choices are many and I'm not certain of an industry standard part.
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Old 08-25-2010, 11:08 PM
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retched retched is offline
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Here is some MPPT info:

Read these:
http://circuits.linear.com/339

http://www.ti.com/litv/pdf/slva378

Also, attached is a PDF with some good info on the perturb and observe method. Check out the circuit to see how they do it.

Also attached is a app note that may help.
Attached Files
File Type: pdf Design Ideas - Solar Array MPPT.pdf (307.7 KB, 134 views)
File Type: pdf OnSemi-MPPT-AppNote.PDF (451.1 KB, 110 views)
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Old 08-26-2010, 01:09 AM
sage.radachowsky sage.radachowsky is offline
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Here's an idea. This is a simple non-inverting amplifier. The current over the sense resistor is ramped up and the output of the amplifier multiplies it by 250 times.



However, the devil is in the details. Getting an actual op amp to work this well may be tricky. This model is an idealized op amp.

You'll need either (A) true rail-to-rail input or (B) a negative power rail for the op amp.

You'll also need to use an op amp with a very low offset error, or trim one if it has the nulling inputs. Otherwise your very low voltages over the 10 Ohm resistor will be overshadowed by the offset error.
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File Type: png CurrentSense.png (49.1 KB, 161 views)
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