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(newbie question) Briefly: In a converter (let's say DC-DC), I want to dictate a voltage to the output of solar panel, on the fly.

As I look around online, for example in some IEEE papers, Buck or Boost converters are used to regulate the second stage: the charging current and voltage. I studied them a bit. But I don't understand: Does changing their duty cycle, regulate their input voltage too? If not, what kind of IC regulates input, based on given duty cycle? Please note that I don't want an on-chip MPPT solution. But to regulate the input of my converter i.e. the output of the solar panel. A microprocessor will specify the voltage of solar panel, through PWM duty cycles.

Going through the products of T.I. or Linear I did not find such an Item.

Solar panel's typical power output would be in the range of 5w...

In this project, it is not important what the charging voltage (second stage) is, actually.

I'll gladly edit, clarify or rephrase the question or title, if you see a problem.

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  • \$\begingroup\$ Charles Cowie gave a very good anwer. For products, most of reseachers used their homemade DCDC for MPPT control, because it is more flexible. You can read my papers: "An Improved Beta Method With Autoscaling Factor for Photovoltaic System", ieeexplore.ieee.org/document/7498594. I gave the components that I used. \$\endgroup\$ – Lecio Jan 23 '17 at 12:46
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Solar panel output voltage and current can not be controlled or dictated. The output is determined by the amount of sunlight received and the characteristics of the connected load. The solar panel output is the voltage and current where the load line intersects the solar panel V vs I curve as shown below. The load is assumed to be a fixed resistor. The buck or boost converter will draw a certain current with a certain input voltage. Depending on the load connected to the converter, in may be able to adjust itself to track the maximum power point. However, it will be limited by the ability of the load to safely and usefully absorb the available power.

enter image description here

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  • \$\begingroup\$ Thank you sir for answering. After reading this answer, I think that I should look for the right setting of converter and battery, instead of looking for input regulated converters. \$\endgroup\$ – Makan Tayebi Nov 25 '16 at 14:05
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If I have understood your question correctly, you have a micro that outputs a PWM signal, and you want a switching converter that regulates its input voltage in proportion to this PWM signal.

First I just want to make sure you understand that, if you are regulating the input voltage / current of a switcher, the output will be what it may. For this reason, the only place that I have seen converters that regulate based on feedback from their input rather than their output are battery chargers. The output current to the battery - assuming it is not too high for safe charging - is a free variable in this case. Anyways, the LT3652 is a battery charger IC that has this functionality out of the box.

The other side of the problem is finding a way to condition the PWM output from your micro to manipulate the behavior of your switcher. The LT3652, for example, works by servoing the the VIN_REG pin to a reference voltage of 2.7V. By using some appropriate op-amp circuits, you could use the filtered PWM signal to introduce an offset on this pin and thereby shift the VIN target.

I've not given all the design details here, by a long shot, but perhaps this gives you a few ideas...

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  • \$\begingroup\$ Thanks for the answer. Yes that is my question. I actually looked at the LT3652, but the evaluation board that it comes in, hard wires the VIN_REG pin to a resistor voltage divider. To change that, I should tweak the board, which is not my first option. Since Microprocessor based MPPTs are not that uncommon, I expected to see more input regulated converters in the market. Is it the case that input regulation is not even necessary for MPPT? \$\endgroup\$ – Makan Tayebi Nov 25 '16 at 13:54
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    \$\begingroup\$ That's an interesting question. Theoretically, input regulation is not necessary to implement an MPPT algorithm of sorts. The micro could monitor the input and feedback to a normal (output regulating) switcher to servo the input to a desired (maximum power) operating point. However, because the relationship between the output and input states are generally not known, and may vary considerably depending on environmental variables, you may find it difficult to design such a loop. Then again, maybe not... I don't know too much about this subject. \$\endgroup\$ – user49628 Nov 28 '16 at 17:02

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