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# TPS5420: Photovoltaic Panel IV Behavior

Part Number: TPS5420

I am using the TPS5420 as a step-down regulator for a solar panel, with the solar panel connected directly to the input of the TPS5420. I am trying to get a grasp for the expected behavior.  I have identified that the output power of the TPS5420 (set at 14V) is equal to P = V*I = 14 * (14 - V_diode - V_battery) / R, and the solar cells (modeled as a current source in the schematic), will operate at a power equal to the power output of the TPS5420 plus any power losses.

With this in mind, as the battery depletes in charge starting from 14V, the power demand on the output of the TPS5420 increases. This can be visualized by starting at Voc on the PV curve and moving upwards until the power demand reaches Pmp minus your power losses. What I am wondering, is what will the TPS5420 do once this point is reached? The solar panel can not provide any more power, so either the voltage or current needs to drop. I suspect the voltage will drop, meaning the TPS5420  will no longer be able to output at its setpoint and the output voltage will decrease in order to keep at maximum power and satisfy Ohm's law over the output resistor, but I am not sure!

Fig 1: Simplified Schematic

Fig 2: PV IV Curve

• Matthew,

Your intuition is correct. Your solar panel will output max voltage at no load and 0V at max load. So starting at VOC and increasing the load will cause the voltage of the solar panel to drop.

Solar panels output max power at PMP usually around VOUT = 80% of VOC. So as current demand exceeds the current at PMP, solar panel voltage will start to fall pretty quickly depending on the load profile. This is why we make chips with dedicated MPPT (maximum power point tracking) which will monitor the solar panel voltage and only load to 80%.

-Sam

• Thanks for the response! In regards to the behavior of the TPS5420, can I expect that once the load reaches PMP, the TPS5420 will abruptly stop regulating and drop its output voltage (such that the TPS5420 output voltage is equal to the solar panel output voltage) in order to maintain the maximum current draw from the  solar panel between Isc and Imp (depending on the battery voltage and resistor value from the schematic above)?

• Matthew,

The TPS5420 will regulate its output voltage but cannot output a voltage higher than the input. So yes, the TPS5420 output voltage will drop if the input voltage gets too close to the output voltage.

It's also tricky if the load is a battery. A battery will hold the output voltage at the battery voltage so the TPS5420 may hit overcurrent limit. And the battery voltage pre-bias on the output may cause issues but I assume you're considering both of these cases since you have a resistor and diode in your schematic.

-Sam