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Design Articles

Integrated MPPT Charge Controller and LED Driver

Traditional charge controllers seldom take into consideration the loading on a solar panel and thus don't derive the complete available energy from the panel. This is where maximum power point tracking (MPPT) charge controllers come into the picture.

By Anshul Gulati, Staff Applications Engineer, and Srinivas NVNS, Senior Applications Engineer, Cypress Semiconductor

solar cells

Solar charge controllers form an interface between solar panels and batteries in a solar power system. These controllers regulate the charge provided by the solar panels to the batteries. Development efforts in the area of charge controller technology have continually aimed at enhancing system performance and reducing system cost. Maximum Power Point Tracker (MPPT) based charge control systems are an outcome of these efforts. These systems aim to maximize system performance across continuously varying operating conditions.

Introduction

With diminishing availability of fossil-based fuels used for generation of electricity, mankind has been on the lookout for alternative sources of energy. Sustainable or renewable sources of energy are sources that meet the present energy demands, reduce dependency on fossil fuel sources, and contribute to world energy security. According to the American Council for an Energy-Efficient Economy, renewable energy and energy efficiency are the ‘twin pillars’ of sustainable energy policy [1[. The sun has been a source of energy from times eternal and will continue to be so well beyond the projected dominion of humanity. Solar power is the generation of electricity using the radiation from the sun. The International Energy Agency (IEA) classifies solar power as the second generation of sustainable energy technologies that is growing in usage but still requires substantial R&D efforts to achieve general and competitive deployment [2].

Improved harvest and use of energy from available sources to provide for general consumption is an important aspect of energy efficiency. While sources such as solar power, wind power, bio mass et al have been identified as sustainable sources of energy, efforts are underway to increase energy efficiency that slow down growth in energy demands. This in turn gives the necessary time to enhance the development of alternative clean sources of energy.  This article will address one such method to increase energy efficiency by harvesting more energy from solar modules by improved charge collection and storage methods.

Solar power, photo voltaic panels and charge controllers

Earth receives 3.85 million exajoules [3] of energy per year at the surface, about 50% of total incident solar radiation or insolation. Of this, only a miniscule fraction is used for regular energy demands. Solar energy can be a long term viable option if it is properly harnessed and put to use.

Solar panels are silicon-based devices that convert this available solar energy to electricity by a process known as the photoelectric effect. Solar panel efficiencies range from 15% - 20% with the best of panels converting solar energy to electricity with 25% efficiency. The operation of solar panels are affected by available solar radiation and operating conditions.

Photovoltaic panels are verified for performance from an I-V curve with I representing the terminal current and V representing the terminal voltage. Figure 1 shows a typical I-V characteristic of a solar panel. It is non-linear and varies with irradiation and solar cell’s temperature. The available current capability of a solar panel is directly proportional to the solar radiation incident on the panel; the open circuit voltage is inversely proportional to the panel operating temperature.

figure 1
Figure 1: I-V characteristic of a solar panel

Solar energy is not available at night and sometimes intermittently during the day. Hence, it becomes imperative to store captured energy for continuous availability. In the case of systems connected to the local electric grid, energy can be transferred to the grid rather than stored. During the night, the grid supplies the required energy. Standalone PV systems typically use rechargeable batteries to store energy.

Solar charge controllers form the interface between batteries and solar modules. These controllers regulate the charge provided by the solar panels to the batteries. Batteries that are overcharged or deeply discharged will have their life significantly reduced. This is especially true with lead acid batteries, traditionally used in solar power systems. Modern charge controllers use PWM-based techniques to regulate charging voltage/current. This type of controller allows for careful charging of the batteries without any electrical or chemical stress on the battery.

Traditionally charge controllers connect the battery to the solar panel and monitor the charging voltage and current. These devices seldom take into consideration the loading on the panel and do not derive the complete available energy from the panel. This is where maximum power point tracking (MPPT) charge controllers come into the picture.

Maximum Power Point Tracking (MPPT) controllers

The maximum power transfer theorem states that maximum power transfers to a load from a source when the load impedance matches the source impedance. This is the premise on which the MPPT controllers operate.  There are at least ten algorithms available today that implement MPPT. MPPT is an electronic system and should not be confused with mechanical trackers that direct the solar panels towards the sun.

Solar panels are characterized by an operating voltage (Vmp) and an operating current (Imp), at which point the panel provides maximum power to its load. The ratio of this voltage to the current is called the dynamic impedance. The goal of the MPPT controllers is to keep the operating voltage and current of a solar panel close to the (Vmp, Imp) point. This maximizes the battery charging current.

figure 2
Figure 2: I-V characteristic of a solar panel showing maximum power point

The load on a solar charge controller is the battery. Load impedance matching is done by modulating the charge current to the battery. This in effect changes the load impedance that the panel sees at its terminals. This can be done in different ways, of which four of the most commonly used methods are:

  1. Perturbation and observe – The method involves modifying the operating voltage or current of the photovoltaic panel until maximum power is obtained from it. If increasing the voltage of a module increases the power output, the system increases the operating voltage until the power output begins to decrease. Once this happens, the voltage is decreased to get back to the maximum power output value. This process continues until the maximum power point is reached. The power output value oscillates around a maximum power value. Perturb and observe is the most commonly used method. This method will fail under rapidly varying conditions of insolation and is not agnostic to local maxima.
  2. Incremental conductance – Incremental conductance is a technique that makes use of the fact that the slope of the power-voltage curve is zero at the maximum power point. The slope of the power-voltage curve is positive at the left of the maximum and negative at the right of the maximum. After the maximum is set, the system maintains that point until a change in conductance occurs.
  3. Load current maximization – When a battery is connected to the solar panel, maximizing the power of the panel maximizes the output power. For a given battery voltage, this effectively means that the battery charging current is maximized. This concept can effectively track maximum power point. The advantage of this method is that the control requires with one feedback – the load current.
  4. Source voltage maximization – This method is similar to (3) but varies the source current from the panel, looking at the voltage of the panel until it is maximized. The advantage, again, is that the control will need only one feedback – the source voltage.

The factors that determine the use of an algorithm is the availability of voltage and current sensors, cost of implementation and convergence speed. With the availability of high-speed microcontrollers with better analog and digital capabilities and higher integration at a lower cost, it is now easier to implement all methods with good speed of convergence. The variation of short circuit current and open circuit voltage with irradiation and temperature can also be compensated with such digital systems.

These controllers are typically DC-DC step up/down converters depending on the application. Streetlights for example, use the step down approach to convert solar power to electricity and store them in batteries.

Advantages of implementing MPPT

Figure 2 shows the V-I (bold trace) and P-V (dotted trace) characteristics of a typical 75 W panel at 25 °C and 1000W/m2 of irradiance. A conventional charge controller charges a battery by placing it directly across the solar module. This causes the panel to operate at the battery voltage, thus delivering lower power than what it can actually deliver.

Instead of connecting the battery directly to the photovoltaic modules, MPPT solar charge controller modulates the battery charging current. This operates the module at the voltage where it is capable of producing a maximum power of 75 W. This can be done regardless of the value of battery voltage. If the battery voltage is 12 V, and the battery is directly connected to the panel, the panel can supply only 4.5 A of current to the battery. The voltage at which the maximum power is drawn from the solar module is typically around 17 V; the maximum current the module can deliver is 4.5 A. If the panel is operated at this voltage, there is an increase in battery charging current of up to 1.875 A. This significantly improves the ampere-hours delivered to the battery. The greater the difference in the module voltage at which it delivers maximum power and the battery voltage, greater is the increase in the battery charging current. This is more evident in climates where the temperature is low and abundant sunshine is available. Studies have shown that the current increase can be approximately between 25% - 30%. The increase in charging current translates to faster charging times, and high availability of energy storage systems. Further, since more power is harvested from a solar panel using MPPT, it effectively means the panel can be smaller for a given application; bringing down initial setup costs.

MPPT solar charge controllers

The advantages of MPPT controllers can be perceived better if they are integrated with the battery-charging algorithm. Battery charging controllers control charging current and prevent the battery from overcharging. Batteries, if overcharged, degrade faster due to internal generation of gas at high terminal voltages. These controllers can also prevent deep discharge of battery that can be damaging if operated for extended periods. This is usually done by disconnecting the load at a low voltage set point. If implemented in a digital controller, fault monitoring and metering can also be easily added as additional functionality. It also becomes easy to add temperature compensation for changing set points of overcharge voltage, under voltage load disconnect voltage, or float voltage thresholds.

MPPT solar charge controllers have the dual advantages of tracking maximum power of the panel and a better battery-charging regime. This ensures that the batteries charge to their full capacity quicker than conventional charging methods thereby enhancing battery availability and autonomy.

Integrated MPPT solar charge controller and LED driver using PowerPSoC

High Brightness LEDs (HBLEDs) are the emerging trend in the world of lighting and illumination. These LEDs have the advantages of longer life, low maintenance costs, and higher efficiency. A power driver is required to maintain constant current through the LEDs. Typically, this will be a DC-DC step up/down converter.

figure 3
Figure 3: Cypress's integrated solar charge controller and LED driver

Figure 3 shows a block diagram of an MPPT-based system designed to extract and deliver maximum power from a solar panel based on an integrated MPPT solar charge controller and LED driver built on Cypress’ programmable system on a chip (PSoC) devices. Onboard resources such as analog and digital peripherals, hysteretic controllers, gate drivers, pulse width modulators allow for tight integration of signal measurement, conditioning, and control. Figure 3 shows the simplified block diagram of the solution. It can work across panel sizes and with varying battery loads and charging conditions.

The PowerPSoC family incorporates Programmable System-on-Chip (PSoC) technology with the best-in-class power electronic controllers and switching devices to create easy to use power system-on-chip solutions for lighting applications. It is an ideal platform to create lighting solutions and is designed to replace the microcontroller, system ICs and discrete components required for driving high brightness LEDs. 

The MPPT algorithm is flexible and robust in searching the peak power, operating by taking voltage and current feedback from the panel and adjust the control signals to operate the panel at its peak power over different operating conditions.  PowerPSoC also generates the necessary control signal to drive a synchronous buck converter that converts the solar panel power to charge the battery. The system also controls the battery charging process and drives the LEDs. The algorithm has three distinct phases - Test, Park and Track (TPT)

  1. Test Phase – This phase is for estimating the maximum power that the input panel can supply.
  2. Park Phase – At the end of this phase, the controller operating parameters are fine tuned to extract the true maximum power from the panel.
  3. Track Phase – Once parked, the system continuously tracks the maximum power point for any change.

Development efforts in the area of solar charge controllers have continually aimed at enhancing system performance and reducing system cost. The ability to integrate efficient energy harvesting methods, simplified yet effective battery charging algorithms and LED driving into a single intelligent device greatly enhances the advantages associated with each of the functions. System designers will find it easy to design such systems, while reducing the time to market.

About the Authors

Anshul Gulati holds a Bachelors degree in Electrical and Electronics from BITS – Pilani, India. She has a over 7 years of experience in embedded systems design. Her interests include firmware development, digital design, and firmware/hardware co-design. She is currently working on developing reference designs based on Cypress’ PowerPSoC family of controllers.

Srinivas NVNS holds a Master’s degree in Power and Control from Indian Institute of Technology Kanpur, India. He enjoys working in power electronics and embedded systems design. He is currently working on PowerPSoC solutions.

Footnotes

[1] The Twin Pillars of Sustainable Energy: Synergies between Energy Efficiency and Renewable Energy Technology and Policy

[2] Renewables in global energy supply: An IEA facts sheet

[3] 1 exajoule = 1018 joules

Cypress Semiconductor
San Jose, CA
(408) 943-2600
www.cypress.com

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