Solar Charge Controller Reference Designs Finds the MPPT Sweet Spot

This post sponsored by Texas Instruments.

Photovoltaics are the top energy harvesting technology in use today, and the promise of clean, renewable, and dependable “free” energy without moving parts is enticing enough to make business and homeowners take notice. Given the volatility of the world’s energy suppliers and the ups and downs of oil, gas, and petroleum, if a company or country can hedge its dependence on oil, it is one step closer to energy independence.

We see evidence of this with many innovative and forward-looking companies. For example, FedEx has a 2.6 million Kilowatt hour rooftop array at its New Jersey distribution hub that produces 2.42 megawatts of power covering a 3.3 acre rooftop. The 12,400-panel array is one of five FedEx solar projects, and this one array provides up to 30 percent of the hub’s annual energy needs. Walmart, too, is investing in solar energy on rooftops for stores and facilities, realizing that the low cost of solar cells, the high cost of oil, and federal incentives make now a good time to invest in future infrastructure planning.

Interestingly, the construction of such facilities is split as to the architectural nuances of PV installations – some opting to use grid-tied solutions, and others selecting non-interruptible power solutions. Older solar architectures that use panels, batteries, charge controllers, and inverters are giving way to the simpler grid-tied inverter architecture. Many find this a much more attractive option, since it does not require costly, heavy, and bulky batteries that carry with them a lot of maintenance, upkeep, and the expense of needing to be replaced every so often.

The efficiency and effectiveness of grid-tied systems are more scrutinized since energy is not wasted on the inefficient maintaining and charging of batteries. Instead, the cost of inverters has been dropping as more highly efficient designs allow each solar panel to be connected to its own inverter. But, grid-tied systems do not typically provide any benefit if the utilities go down, opening the door for such instances where battery-based systems can really shine.

In either case, maximum energy extraction from the solar panel involves finding the point where maximum current is flowing at the maximum voltage. Operating the charge controller at this point can be a constantly changing adjustment. Both solar intensity and panel temperature can affect this and the desire is to squeeze every Joule of energy from the panel. As one might imagine, the task of finding the optimum power point is a perfect application for integration onto monolithic chips and that’s just what’s happened.

For instance, take the Texas Instruments Solar Micro-Converter technology as illustrated in the company’s Reference Design. This example shows how a 20 Amp solar charge controller design works with a 96 percent efficiency for use with 12 or 24 Volt panels (actually up to 44 Volts in). It is scalable so it can be modified to operate at 48 Volts and 40 Amps using the larger TO-220 packaged versions of the power FETs like the 40-Volt , N-Channel NexFET (CDS18502Q5B), and the 60-Volt N-Channel NexFET (CDS18532Q5B). This design also takes advantage of the company’s MSP403F5132 mixed-signal microcontroller, which can be used to monitor and control battery charging as well as inverter operations.

Several design resources including an Alternative Energy Applications Overview and Smart Grid Solutions Guide can help educate and illustrate some clever designs and solutions to alternative and renewable energy technologies.

Even hybrid-style designs are illustrated, demonstrating how a single design can accommodate battery charging and management as well as inverter operations and grid-tie functionality. This provides the best of all worlds in a single compact solution (Figure 1).

Figure 1: Note how charge control and inverter control are leveraged to allow local battery banks as well as 120 VAC generation for connection to a grid. This can not only back-feed a meter, but can also provide backup power when the utilities go down. (Source: Texas Instruments)

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Author: Jon Gabay, Hearst Business Media