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Analog MPPT Solution: Low Cost and Easy Integration

In modern photovoltaic (PV) system design, the maximum power point tracking (MPPT) circuit and algorithms are implemented digitally (in a microcontroller unit or FPGA), so that those advanced algorithms can be used efficiently to improve tracking performance.

The biggest challenges in building an analog MPPT controller, as described in a recent IEEE paper1 , are implementing the algorithm and storing necessary information (i.e., PV panel voltage VPV and output power PPV ) to find the MPP. By creating a truth table from the perturbation and observation (P&O) algorithm, the algorithm can be implemented with just several logic gates. Simplicity is key to the cost, size, and ease of implementation.

The other alternative is to implement the MPPT with analog circuitry. The benefit of such a solution is that the MPPT can be integrated with a DC-to-DC controller, so that plug-and-play capability can be expected for many low-power PV applications — such as an AC module for residential applications. The IEEE paper includes a demonstration that shows the integration of certain functions into a normal PWM controller chip is the best technique for special applications to reduce complexity and system cost. One of the best examples is the single-phase power factor correction (PFC) chip. This integrated solution has led to simpler system designs and lower costs.

However, finding the real MPP with this approach presents problems. Consequently, only three types of MPPT techniques can find the real MPP and be implemented by analog circuitry: hill-climbing/P&O, ripple correlation control (RCC), and state-based MPPT.

Considering the complexity of state-based MPPT and some drawbacks of the RCC method1 , neither of them are good candidates. The most desirable approach for universal analog MPPT is the P&O method. But implementing the algorithm with simple circuits and storing the values of VPV and PPV in the last perturbation cycle remain challenging for analog MPPT.

Thanks to capacitor-based storage cells, information like VPV and PPV can be stored easily. Each value is stored in its own cell. In one of these cells, as the IEEE paper shows, the storage capacitor voltage is proportional to the PV panel voltage and can be adjusted in each perturbation cycle. The minimum voltage step of perturbation can be set by the amplitude of charge (discharge) current and the time duration of charge (discharge) action.

Compared with analog MPPT solutions, this type of controller can find the real MPP dynamically, and its operation will be independent of converter topologies or their control.

The tracking performance of the controller is validated by both the simulation and experimental results using a single-phase PV inverter prototype with 200W PV panels under different sun irradiation levels and temperatures.

A PV system with an analog MPPT controller is a good integrated analog solution.(Source: See Reference 1)

A PV system with an analog MPPT controller is a good integrated analog solution.
(Source: See Reference 1)

Algorithm
The next step is to create the truth table and simple algorithm1 on which the MPPT operation is based. Only a few logic gates are required to implement the MPPT algorithm.

Storage cell
The P&O method will continue injecting perturbation into the panel voltage and examine the change of output power. Then the values of VPV and PPV in the last perturbation cycle must be stored.

Analog multiplier
For this application, the multiplier works only in the first quadrant. This keeps the complexity and cost of this solution down.

The final system diagram

Circuit diagram of the analog MPPT controller.(Source: See Reference 1)

Circuit diagram of the analog MPPT controller.
(Source: See Reference 1)

This is the circuit diagram of the analog MPPT controller. The PV panel voltage and current need to be sensed and filtered by a low pass filter to remove the switching frequency ripples. An analog multiplier calculates the instantaneous output power of the panel.

As the IEEE paper tells us, this controller can be divided into four blocks.

  • Block I executes the MPPT algorithm and decides the next perturbation direction.
  • Blocks II and IV generate the control signals for the storage cells. In block II, the control signal CMD, combined with CMD-S, will determine a period during which the voltage of the capacitor C_S in the cell “PPV storage” will be updated. Consequently, PPV (k-1) will be equal to the latest PPV after this period.
  • Block III determines the charge (discharge) time duration for the storage cell “VPV _storage.”

The high time duration of the signal “Timer,” as well as the charge (discharge) current in the storage cell, will determine the minimum perturbation step of VPV .

Voila! We have a cost-effective, highly integrated analog MPPT solution.

References:

  1. A New Cost-Effective Analog Maximum Power Point Tracker for PV Systems, Zhigang Liang, IEEE Student Member; Rong Guo, IEEE Student Member; Alex Huang, IEEE Fellow, 2010.

22 comments on “Analog MPPT Solution: Low Cost and Easy Integration

  1. David Maciel Silva
    May 31, 2013

    Steve,

    Great article, texas has an integrated circuit for MPPT function SM72442MTE, below your data sheet:

    http://www.ti.com/lit/ds/symlink/sm72442.pdf

    Indicate some more iten for such application?

  2. bjcoppa
    June 1, 2013

    Interesting post. Surprised there are not more comments. It's funny how much emphasis has been placed on PV solar cell manufacturing and improvements to efficiency.  The real issue for solar is to improve storage so that power generated during the day can be used at night. However, it's a lot easier to just push the envelope and incrementally boost efficiency and get headlines although that doesn't solve the core problem for mainstream power plant implementation.

  3. Vishal Prajapati
    June 1, 2013

    @Maciel, What technique does it use?

    – Hill-climbing/P&O,

    – Ripple Correlation Control (RCC),

    – or State-based MPPT?

  4. Steve Taranovich
    June 1, 2013

    STMicro has the STV1040 http://www.st.com/st-web-ui/static/active/en/resource/technical/document/datasheet/CD00287506.pdf

    Maxim has an integrated MPPT in its MAX17710 (See their comments on Energy Harvesting apps)http://datasheets.maximintegrated.com/en/ds/MAX17710.pdf

    Linear Tech has the LTM8062 http://cds.linear.com/docs/en/datasheet/8062fc.pdf

    Just to name a few—most ICs have the MPPT integrated nowadays

     

  5. Davidled
    June 1, 2013

    Higher efficiency of power is a major concern in the PV. I am curious about power conversion efficiency in this MPPT controller. MPPT controller might be validated using solar array simulation before implementing it.  Second, when reviewing diagram, there are two main blocks which are DC/DC Converter and DC/AC Converter to feeding to AC power loading.  I guess that it can be reduced as one block of DC/AC Converter. Third, PV industry continue to research material to boost power conversion efficiency and Solar cell efficiency such as  Cadmium telluride(CdTe) and Copper Indium gallium selenide (CIGS).

  6. RedDerek
    June 2, 2013

    I have been watching the solar PV improvements for over 20 years. I remember when low-teens percentage in efficiency was touted. Now it is upper teens and pushing to the low 20's – not accounting for space applications here. A friend has asked me about solar and efficiency and I stated the only way to improve solar is by one of a few methods:

    1. collection – add lenses to direct sunlight onto the converting cell.

    2. conversion efficiency in the silicon – more on the processing of the cell.

    3. expand wavelength of conversion – solar only converts a small portion of the electromagnetic radiation frequency. This involves new processes.

    A couple of month ago there was an article that one group has been able to increase efficiency to over 40%. As suspected, it was by method 3 and stacking the conversion cells on top of each other. Though not cost effective, it is a good start.

    MPP is not really solar collection side, but in the conversion from the cell's electron generation. I have not delved into this, since it is not my primary expertise, I do find it quite interesting.

  7. Davidled
    June 2, 2013

    Solar cell efficiencies range is from 6 % to 40 %. 10% to 15 % efficiency is reasonable economical. Also, solar panel size might be overviewed in the PV system. One of key factors is average irradiance, the average amount of solar energy a solar panel receives, which is technically symbolized as Watt per square meter. I guess that MPP is more close to application algorithm in the electronic. To look at the solar collection side, material and thinkenss of silicon should be reviewed.

  8. jbike
    June 3, 2013

    I agree, nice article! It would be interesting to compile a table including all the aforementioned analog IC vendor MPPT solutions illustrating their method as your outline suggests, and their efficiency. A power envelope for each integrated solution would help identify “holes” in the current solution space. I am curious if there are any obvious patent land-mines out there for companies to consider before considering a new implementation? In considering the solar power landscape as a whole, at least to me it all makes perfect sense. The issue is in finding the cash and energy to manufacture and install the necessary capacity. Storing all the energy is not a problem at all, the grid is your storage device. Consider a very large, all electric PV community where each home uses on average 3KW, and contributes 3KW to the local grid. Peak power would at times be much higher than this, for instance when cooking or using the drier, or when the heating/cooling system is running. Imagine now other homes within the community also using an average 3KW. If this community is expanded across multiple time zones, statistically, the grid itself provides the energy storage and it all works. The power companies will stay around and monitor the grid as a whole, poised to start natural gas “peaker plants” as needed, or to isolate areas of the grid for maintenance or repair.

  9. Netcrawl
    June 4, 2013

    Interesting topic! The solar PV matrket is on fire, we have seen a lot of activity in the recent years. I think we have no problem aboout cash and energy( we have those things), major tech advancements continue to come at a rapid rate, increasing genration potential, boosting cell efficiency and driving down manufacturing costs. I think the problem is on government side, their strict regulation and policy. Solar energy could bring great opportunities, its the next hot spot in the energy market.  

  10. Brad Albing
    June 4, 2013

    @jbike – Now if we coud extend the grid (well the PVAs actually) a bit farther in each direction (Pacific and Atlantic Oceans) so we'd get more sunlit time zones, we'd be all set. Since that may not be practical, we'll use those oceans a different way and snag power via wave-powered generators.

  11. Brad Albing
    June 4, 2013

    I'll be interested to see if any of the work being done on the diamond-based semiconductor devices ties in to improved efficiency in the PVAs. Stay tuned for that.

  12. jbike
    June 5, 2013

    Also, I would bet that we use most of our power during the daylight hours. In the evening we turn off almost everything and  A/C systems run less frequently. Your furnace runs more in the evening, however,  most of us use natural gas. Since moving volumes of power across time zones is less efficient, we would just  need to make more. Since this new model is distributed, our grid capacity should be running at fractions of it's capacity meaning that we can afford to move power on that front, provided we can make enough. At noon, the sun has the capacity to provide 1000W/m^2, seems like plenty to me.  Perhaps if homes were equipped with some short term storage (4-5 hours), and if we move some power over a few time zones, and we add other renuables, as you mentioned including wind, and we keep the peaker plants to supplement our distributed system, we would be set!

  13. Dirceu
    June 5, 2013

       The blog claims that one of the main advantages is the cost of implementation with digital gates – and the approach is understandable, since this to be a forum on analog. But also would be interesting to see a comparison with an implementation on a low cost microcontroller such as an ARM Cortex-M0 (0.9 DMIPS/MHz, with 8 or 20 pins). Of course, there is the additional cost for the firmware development.

  14. Steve Taranovich
    June 5, 2013

    No that's taking efficiency to the next level, jbike—all good suggestions

  15. Steve Taranovich
    June 5, 2013

    All good methods that will get us to ROI much faster than today!

  16. Brad Albing
    June 6, 2013

    @Dirceu >>would be interesting to see a comparison with an implementation on a low cost microcontroller – Perhaps you could write such a blog for us?

  17. Brad Albing
    June 6, 2013

    OK – makes sense – might not need to span as many time zones as I thought at first.

  18. Dirceu
    June 7, 2013

    Brad,

       what would be a good challenge, since the architecture of the solution is relatively simple, but the problem is always the free time. Your suggestion is also valid for other participants as well.

  19. Davidled
    June 8, 2013

    Japan has top spot in the PV industry and span a big range of 6.9 -9.4 GW. That will put Japan ahead of China and USA as well as other regions. IHS indicates that Japan's share of the global PV system revenue is increased from 14 percent in 2012 to 24 percent in 2013. PV system of Japan is very costly and good quality.   

  20. Netcrawl
    June 8, 2013

    Innovation is happening in all stages of the PV space, it continue to show steady activty, growing fast, especially in China where its seen a lot of potential. But I think we still lack some innovation here, we need some plenty of room for improvement. 

  21. WKetel
    June 20, 2013

    Most solar power installations feed a definite load, which normally requires a specific  voltage. Even charging batteries, the required voltage does not change much, and is well defined. The result is that instead of needing to compute maximum power, aiming for maximum load current becomes a much simpler method of aiming for maximum power. In addition, using analog calculations makes the task even simpler. The target power level is also predictable, so that an initial start point can be set, instead of the system needing to start from zero each time. 

    One other question that I have is how close to the absolute maximum peak must we really be? Backing off on accuracy just a bit is typically a great cost reducer in most situations, probably here as well.

  22. Steve Taranovich
    June 20, 2013

    Good points WKetel. Cost vs. accuracy is a key compromise in Solar and as long as it is accurate enough for the particlar application—that's what will decide the ratio of the compromise.

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