Voltage drop on Solar Cell MPPT

Question

I'm trying to build my own MPP Tracker on Solar Cell, here's my schematics Explanation :

• PA6, PB0, PB1, PB2,PA3 : I/O Pin
• PA 7 : PWM Pin
• PA4 : Used to measure voltage
• PA0 and PA1 : Differential ADC to measure current

The problem with this circuit is, voltage drop on solar cell is too big (about 2V from 5V solar cell). I am sure that there is no problem with the solar cell since it works perfectly using MPP tracker commercial module. I've try changing the transistor several times, and also change it into a P channel trench mosfet PMV65XP, which is the same transistor used by the commercial module (it took me 1 month to get it).
So my question is, what did I do wrong? My assumption is I am using wrong kinds of diode. If yes, what diode I should use? or perhaps did I do something wrong with this arrangement?

The solar cell output is on GND and VCC, used to supply MCU and load Here's one of the test result :

Solar Cell V-open circuit : 4.91V - 0mA
Load resistance : 220 ohm (non capacitive)

Using commercial module :
V and A on solar cell : 4.20 V, 40mA
V and A on MPP output : 3.00 V, 45mA

Using my module :
V and A on solar cell : 2.60 V, 42mA
V and A on MPP output : 2.38 V, 37mA

I've tried to change both diodes from Fast Recovery diode FR104 into Switching diode 1N148, but still there's no significant change

Just for information, at first, I also doubt my algorithm and my code, that's why I put a potentiometer (on PA3) so I can manually adjust the PWM (works on 15kHz). And as for @Maximus suggestion, here is the graph (mine is on the left side, and the commercial on the right):
The graph data is taken by changing PWM duty cycle per 10% and observing the voltage and current of solar cell output (VCC) and MPP output (J1 on circuit). And here is some of the PWM output as suggested by @laptop2d:
It appears that the Vpp of PWM signal is increased along with the Voltage of solar cell (which used as VCC supply of uC)

----Edit----
These is the PV characteristics:

Answer 1

The characteristic of a loaded solar cell is just about a constant current source until you hit the a maximum voltage and then it drops off fairly quickly. The trick with MPPT is to get to the top voltage end of that constant current region.

I believe that you are probably taking just a little too much current. Your algorithm should be seeking a maximum for V*I for the solar cell.

Your current sensing is at the battery end of the circuit it should be looking at the current from the solar cell. I also do not see where you are sensing the solar cell voltage.

Your switching transistor is connected back to front as per your diagram, the body diode will be conducting giving you a straight transfer from input to output with a bit of voltage drop. Allowing for current for the LEDs this explains your input to output characteristic.

You need much bigger smoothing capacitors on the input and output in addition to the ones you show depends on your frequency but >=220uF.

Answer 2

You power the MCU directly from solar cell, you have very low power levels nearly down to working conditions of a MCU and therefore I think the commercial one cares about MCU's power saving functions, you don't.

Attiny MCUs have power saving features. Here you can read about sleep modes of ATTiny85.

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Read "power management and sleep modes" on the attiny85 datasheet. First try the idle mode, which the PWM will work. You can wake it up by PWM's interrupt and make calculations on every cycle and go idle, or may be with a slower timer's interrupt, let you make calculations on every 4-5 step of main PWM. If it is not sufficient, the real sleep mode will do it. But PWM won't work there. You will have to implement it with watchdog timer interrupts.

Answer 3

The entire system need to be checked and you need to have an understanding of each part. It may not be just the components that are having a problem. From the looks of your power numbers you are having two problems (assuming that you were using some kind of constant light source for your testing, if your not using a constant light source you may want to go see a psychologist). I'm also going to assume that the commercial setup is close to the optimal peak power point of your cell.

1. The designed circuit is probably not finding the optimal power point because the voltage number (2.6V in) is not close to the optimal point for the cell and you are drawing less power (109mW vs 168mW of the commercial setup)

2. The designed circuit is burning up too much power. Out of the 109mW that your circuit did get, its consumed 24mW which is ~23% of your power going in or 77% efficient. The circuit can be better than that, DC to DC converters get upwards of 90% efficiency these days.

Suggestions on where to go from here:

1. Vet your MPPT algorithm, this may be difficult. Make sure you know how they work. There are also different algorithms and some are more efficient than others. Another thing you will probably want to do is find some kind of simulation software to have something to compare against. LT spice could potentially be used and is free, and can be finagled to do almost any kind of simulation. I've used it to MPPT algorithm simulations before. Sometimes you have to get creative and use Laplace, B-sources and comparators to get results but it can be done. Even solar cells can be simulated in spice. I'd throw a plug in for simulink too but that is harder to get access.

2. Find out where the power is being burned up in your circuit. This can be done with a scope by checking each component (like a differential voltage with two probes across each component) a power calculation can be done for the FET, inductor and capacitor. Make sure you understand how buck converters work. I suspect that your PWM waveform may need to be optimized, with respect to your capacitor and inductor values. (I hope you have an oscilloscope). Spice could also be a great way to find power problems. Simulate your DC to DC converter with components in spice, see where the power drain is in each component. Then compare your waveforms with the real world and note the differences.

What you are building is complex, you need to understand each piece and make sure it works by designing things first (use the buck equations and make sure everything checks out) and then testing and verifying correctness. If you don't you'll be chasing down problems like components when the whole system has a problem. Also use energy to your advantage, power calculations can go a long way.

Answer 4

It seems that initially you used a different transistor but now you replaced it with the same one. However, you said nothing about removing R12? The point is that if your circuit is not identical to the commercial module, then you should not expect it to perform like it.
Once you have the identical hardware, then the only remaining possibility is the programming of the MCU, so if you still have problems, is because you are not using the same or equivalent algorithm.

Answer 5

This is not the answer you are expecting , but essential to know to understand why your design failed. Always start with specs like these curves before a design begins, otherwise failure is inevitable.

The Load line of the PV at maximal power is the locus of operating points. Your LED loads of 20mA may be overpowering the capacity of your cell, making the controller ineffective.

There are 3 significant variables;

• the power capacity of the PV array V,I
• the ambient temperature
• the solar input power / sq-m or in [W/m] at 25'C or equivalent measurement

[] []3

Efficiency variables in all PV arrays.

For future research

Once you define Voc Isc or Pmax and Pin, then a design can begin. The goal for maximal power transfer can be defined as the matched impedance curve vs T['C], Pin (solar) and Pout max. understand it is a voltage limited current source with a non-linear load line. If you can simulate the load line with all the variables, then you can make it perform as well as the commercial with a MUCH simpler circuit. (like the PV simulator)

Answer 6

I have done your project using Atmega328p (Use atmel studio and native C code. Arduino API's are very slow for your project)

Anyway, I had similar results as yours at first. Here is how to fix it:

• Very important: Input capacitors on your module. If you don't have input capacitors (for your system I'd recommend ~1000-~10000 uF range) whenever you switch on and off that mosfet, you'll be riding on the whole VI curve of the solar panel and if your microcontrollers speed to understand that swing and react to it is slower than the VI swing speed, you'll never catch the MPP. One solution to this is to use a faster microcontroller and write better code with C and inline assembly. This is difficult. Another solution is to use bigger input capacitors! What they do is essentially to lower that VI swing to a level your microcontroller can understand and react fast enough. Remember, this is important as if you don't fix this issue, your project will never work. To understand and evaluate the VI swing, just write code to PWM from duty cycle 0-100 and see how the module input voltage changes. If you see lots of harmonics (ups and downs a.k.a voltage swings) this means you need bigger capacitors.

• Another important issue: Is the solar panel directly supplying ATTiny power? If that's the case, when you PWM the solar panel, you may be getting an LVD on attiny. First power the microcontroller from a good source. Analyze your results and algorithm. Then try to power it using the solar panel and see how things go from there. Because with your current configuration, maybe the microcontroller is shutting itself down? (Provide osilloscope data if you can.)

• What does your microcontroller see (as in voltage) on its input and outputs? Send to PC and observe it. If you can, please provide us a copy. (You'll need a duty-cycle vs voltage/current graph on both input and output). Your only controllabe parameter is the PWM dutcy cycle. So, plot it.

With more data, we can adress your solution better.

Good luck

Answer 7

I know this answer isn't as detailed as the other, but I think you should change your R10 and R11 into a bigger resistor (such as 1k). Because the smaller the resistance, more current it will draw.