MAX4372 Sense Input Protection: Results

Solar Photovoltaic Panel Maximum Power Point Cloud
Solar Photovoltaic Panel Maximum Power Points

Here’s the result of using the Schottky diode input protection circuit I proposed there. I used 10 and 5 ohm resistors, twice the values shown in that schematic.

The circuit runs a load test and determines the maximum power point (MPP) of a solar photovoltaic panel every minute. The points represent the results of about two hours of winter-afternoon sunlight.

The test program applies an increasing load in steps of 10 mA (it’s a small panel, OK?) and records the corresponding panel voltage. One combination of current and voltage extracts the most power from the panel; that’s the MPP.

Obviously, the MPP varies with the amount of sunlight falling on the panel, so the result of the test is a cloud of points. That’s what you see in the graph: the highest points represent the most intense sunshine, the lower points come from shadows and changing sun angle.

The load is applied through a current sink that draws 100 mA per volt, as generated from a microcontroller PWM output. That’s pretty well calibrated by twiddling a gain pot, so I think it’s quite close.

The graph shows that the 10 mA steps recorded by the now-well-protected MAX4372 high-side current amp are low by about 10%, regardless of the absolute current level. That’s more error than predicted by the SPICE model (even with the larger resistors) and may represent contributions from something other than the protection network.

However, it does look as though a simple calibration routine could compensate for the error. That’s a simple matter of software, right?

Most important of all, the MAX4372 has survived the usual mistreatment. The load test is essentially DC, where the inductor counts as a piece of wire. Under those conditions, the combination of a stiff voltage source and an imposed load exceeding 600 mA produces a lethal differential voltage across the current-sense resistor. The diode clamps that voltage to 300 mV or so, which is enough to protect the MAX4372.

Rumor from my source at Maxim says the protection circuitry inside the MAX4372 can withstand maybe 50 mA, so the high-value external resistor approach (without the diode) may be the better way to go. Getting rid of the nonlinear diode should be a win…

Update: A different plot shows a different result. I think the offset comes from something other than the protection circuitry.