What is an MPPT?

In applications where photovoltaic arrays are used to provide energy, maximum power trackers are used to correct for the variations in the current-voltage characteristics of the solar cells. As you can see from the typical silicon cell I-V curve on the right, as the output potential of the string rises, the string will produce significantly less current. The current-voltage curve will move and deform depending upon temperature, illumination, and consistency of cell quality in the string. The figure on the right is an idealized curve with no deformations due to cell damage or bypass diodes kicking in. For the array to be able to put out the maximum possible amount of power, either the operating voltage or current needs to be carefully controlled. This so-called maximum power point is seldom located at the same voltage the main system is operating at, and even if the two were equal initially, the power point would quickly move as lighting conditions and temperature change. Hence, a device is needed that finds the maximum power point and converts that voltage to a voltage equal to the system voltage.

The devices that perform this function are known as Maximum Power Point Trackers, also called MPPTs or trackers. Most current designs consist of three basic components: a switchmode converter, a control and tracking section, and an auxiliary power supply. The switchmode converter is the core of the entire supply. This allows energy at one potential to be drawn, stored as magnetic energy in an inductor, and then released at a different potential. By setting up the switchmode section in various different topologies, either high-to-low (buck converter) or low-to-high (boost) voltage converters can be constructed. Normally, the goal of a switchmode power supply is to provide a constant output voltage or current. In maximum power point trackers, the goal is to provide a fixed input voltage and/or current, such that the array is held at the maximum power point, while allowing the output to match the battery voltage.

What's Commercially Available?

While photovoltaics have been in use for many years now, MPPTs have usually been confined to higher-end applications, such as communications satellites and solar vehicle racing where the size of the array is somewhat limited and cost is not much of an issue in extracting every bit of energy possible from that array. However, with the improvements in electronics over the past five or ten years, there's now absolutely no reason that the average consumer can't take full advantage of these devices to improve the efficiency of any PV array. So, several commercial firms have begun producing MPPTs for consumer use. The ones that I know of are:

The MPT-150 by Brusa Eletronik of Switzerland
Fire Wind & Rain makes tracking charge controllers
Northern Arizona Wind & Sun also manufactures MPPTs
Solar Converters markets a line of low-voltage MPPTs
And finally, AERL makes really awesome buck (step-down) MPPTs

So Why are You Crazy Enough to Design Your Own?

For Team PrISUm, Iowa State University's Solar Vehicle Racing Team, reliable, efficient, specialized trackers are an important part of an overall winning car design. The commercial trackers that are currently available do not meet all of the needs of the harsh mobile environment of solar vehicle racing. Most respond erratically to fast changing array conditions, such as passing under the narrow shadows of tree branches or the quick-moving shadows of passing cars. Team PrISUm's experience has also been that the currently available trackers, including both those developed previously here at Iowa State University and commercial models, are problematic on even the best of days and are far from ideal for use on a modern solar-electric race vehicle. Common problems included destroyed output stages due to the lack of built-in overvoltage protection, unreliable startup in the morning, lack of telemetry data available directly from the tracker, reliability problems in a such a rugged environment, and inefficient operation.

The goal of this project is to develop a maximum power point tracker explicitly for use on solar race vehicles that are equally or more efficient compared to commercial tracker models, are more cost-effective than models produced to date, and meet Team PrISUm's application-specific requirements. Because of the specialized application that these trackers are intended for (management of the arrays on PrISUm Phoenix, Iowa State University's 5th solar race vehicle), the design requirements tend to be unique and overrated for most other typical applications. Plus there's always the point that I like a good challenge and needed a senior design project. So, we set forth with the following design objectives for our new MPPT:

Cost
Commercial units of the grade we required were in the range of $1000-1200 each. On a typical solar car construction budget, buying eight of these is a big expenditure.

High efficiency
Commercial trackers are available with switching efficiencies approaching 98%. In order for our trackers to be feasible for use as an alternative to commercial units, the new design needs to meet this same throughput efficiency. In addition, by using low power design techniques on the control side of the tracker, it should be possible to lower the quiescent power drawn off the array to keep the power trackers running. In turn, this will equate to more net usable energy into the car. Simulations of power consumption in the circuit has reinforced the notion of 98%+ efficiency.

Electrical characteristics
It was decided that the next trackers should operate on array voltages from 0-120V and main system voltages from 0-150V at up to 10A of output current, such that no matter how far the system fluctuates, the electronics are designed to withstand it.

Track power point with an algorithm suitable for mobile arrays
As discussed earlier, fast-moving shadows can cause problems with many available trackers because they are mainly designed for stationary applications. Fast shadows cause these trackers to lose the maximum power point momentarily, and the time lost in seeking it again, because the point has moved away quickly and then moved back to the original position, equates to energy lost while the array is off power point. On the other hand, if lighting conditions do change, the tracker needs to respond within a short amount of time to the change to avoid energy loss.

High reliability under adverse conditions
Solar racing vehicles are the epitomes of adverse operating conditions. Under typical operating conditions, temperatures inside the car can reach a sustained 50°C. Mud and water entering the shell is common. Large amounts of shock due to imperfections in the highway and stiff suspension can destroy electronic hardware quickly. Components are often accidentally thrown around and abused during maintenance. Also, system voltages and loads can change very rapidly, causing high voltage spikes from line inductance. All these add together to create a unique environment where custom and commercial electronics packages alike are extremely likely to fail if not designed explicitly for the conditions at hand.

Specialization
Because of the fact that these trackers will be used on a vehicle which will face 10 days of unpredictably changing conditions in Sunrayce 99, as well as many days of service in general use, flexibility is a must. Digitally-configurable options for tracking speed, output voltage limiting, bus address configuration, and output current limiting are essential such that the tracker can work effectively with whatever system voltage and battery management hardware is implemented on the final race vehicle.

RDB-compliant device
RDB is a proprietary protocol that uses a pseudo-RS-485 transmission structure with a unique protocol developed by members of the Team PrISUm electrical team (mainly Einy). All of the devices on the car communicate with the telemetry system via RDB. The telemetry data is then sent to the chase van via radio modems so that all of the car's systems can be monitored and their performance logged for later evaluation. The tracker must be able to communicate fully on RDB. To learn more about RDB, check out it's website over here.

Handheld monitor
Having a tracker that can be specialized, an interface is needed to set all of these customization variables. With a handheld device that is plugged into the MPPT this could be accomplished. Also, due to the highly mobile needs it is not feasible to have a computer near the vehicle at all times to monitor tracker operation. A mobile handheld unit would be capable of displaying all information about the current status of the tracker.

Introduction to Building Your Own

Isn't that really why we're all here? Because some crazy guy out there is giving away the plans and the source code for maximum power point trackers? Well, don't get too excited, it's still a long, painful road to actually building one.

I do promise, however, that in the links that follow is more than enough information to actually produce one of these critters. And yes, I promise they do work - PrISUm Phoenix used four of them each and every day throughout Sunrayce 1999, and we finished fifth. The only problems were, day after day, water. That's right, good old wet, liquid, filled with ions, corrodes things instantly, muddy water. For those of you who don't know, Sunrayce '99 was nicknamed Rainrayce by the end, just because it rained each and every day. And, just if you're wondering what water does to a tracker, having that in your power sections is a Bad Thing(tm). Other than that warning to avoid wet stuff, here you go - I've tried to break up the stuff by general category so it's easier to find and just download what you want.

Hardware Documents

Software Downloads and Documents

Project-Related Documents