JFET Design Spotlight: Maximum Power Point Test Controller

“My daughter, during her graduate studies in renewable energy applications, once needed a tester for solar panels to find their maximum power point under real-world conditions. So, this is what dads do.

The resulting tester demonstrates how different semiconductors can be selected for highly specialized tasks. For instance, the JFETs used here serve three uniquely different functions. J3 is used as a variable-resistor gain element to regulate the control oscillator output level. J2 is used for polarity-control switching of a phase demodulator. J1 functions as a floating integrator reset switch, where charge injection is referred to the op-amp integrator’s low-Z output.

Further down the line, a pair of LS312 bipolar duals are arranged in a classic National Semiconductor multiplier circuit for power calculation. Matching here is essential for achieving reasonable multiplier accuracy.

The design objective is to apply an increasing low-level, low-frequency sine current load to the solar panel up to the point where the panel voltage begins to dip under load. An 88 Hz frequency was chosen because it is non-synchronous to nearly everything, while also being slow enough to avoid panel charge trailing effects (slew-rate related), minimizing gating error during demodulation.

As long as increasing current results in increasing power, the circuit behaves as a positive feedback loop. A phase demodulator feeds the signal through an integrator until an increase in current produces a decrease in power level. At that point, the integrated phase demodulator signal reverses and becomes negative feedback. The balance point between increasing and decreasing current load on the panel occurs at the maximum product of current and voltage — the Maximum Power Point.

Power per unit area of the panel is normalized to the 1 kW/m² standard, where quantum efficiency can then be determined using a separate radiant energy measurement alongside the panel. That portion of the circuit is not shown, as it was added later as an upgrade.

Of course, a few op-amps and a PIC processor could also perform these functions — but that’s not who I am.

To this day, integrated and discrete components remain complementary elements in control systems and analog signal processing, and that will foreseeably continue well into the future." - Kirkwood Rough

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