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BQ 25504 solar cell applicaiton: MPPT doesn't hold voltage at 79% of Voc when light intensity goes up

Karthik Murali
Prodigy 20 points

Hi,

First of all, thanks for assisting me with this issue. 

I'm using the BQ 25504 to harvest power from a photovoltaic array to charge a battery. I've built the typical solar cell application circuit as described in the data sheet. I'm currently using a photodiode array (in a combination of series and parallel configuration) to act as my solar cells. Instead of a battery, I have a 300uF capacity in parallel with a variable resistor (resistor decade box). So the variable resistor is connected to the PIN 14 VBAT and the 300uF capacitor is attached to VSTOR PIN 15. I'm using this variable resistor to measure maximum power output from the chip's circuit. 

At low light intensities (incident on the photodiode array), the MPPT works as it should and the voltage across the photodiodes is around 0.8V (Voc of around 1V). However, as I increase my light intensity, the MPPT stops functioning and the the voltage across the solar cells goes to >90% of Voc. 

The MPPT also stops functioning as i increase the resistance on the variable resistor. 

Why doesn't the MPPT always hold the voltage across the photodiode array at the desired 79%.

  • 0
    TI__Guru**** 159701 points

    Assuming your panel's VOC < VBAT_OV setting, as the power output capability of the solar panel increases (i.e., its output impedance goes down), the hysteretic boost power FET, pulling only up to 200mA from the panel, can't pull enough current to discharge the input capacitor to the MPP.  This means that the IC is pulling all of the power it can from the panel even though the panel can provide more power, even though it is technically not at its MPP.

    If VOC > VBAT_OV and VSTOR = VBAT_OV then the IC has a pull down resistor on VIN_DC that pulls the VIN_DC to down VSTOR.

    I can't explain why increasing what I assume is a variable resistor in series with the panel causes the MPP to stop functioning unless you have increased it so high that the IC can't get enough power to keep VSTOR > 1.8V.

  • 0 in reply to Jeff F
    Prodigy 20 points
    Thanks Jeff. Some follow up details.
    1)
    My panel's Voc is ~1V and its Isc is <10mA. So this is a low power application. So, what you're saying is that the IC doesn't pull enough current to discharge the 4.7 uF (Chvr capacitor) during its cycle? What's a fix for this? Should I decrease my capacitance?

    Related (or perhaps unrelated) question: When my panel is connected to the circuit and my oscilliscope is connected to the panel, I notice the panel's voltage resembles a rectangular waveform. It isn't a constant value. Is this waveform to be expected or should the panel's output voltage remain more or less constant?

    2) My panel's Voc is always < Vbat_ov.

    3) The variable resistor is connected to PIN 14 Vbat (the IC output). Increasing the resistance at the IC's output causes the MPP to stop functioning accurately. Is there an explanation for this?

    Thanks for your help!
  • 0 in reply to Karthik Murali
    TI__Guru**** 159701 points
    With Isc < 10mA and VOC <VBT_OV, what I said in the previous post does not apply.
    Regarding 1, VIN_DC should be a small amplitude triangular waveform. I suspect the VREF_SAMP capacitor is discharging due either to it having too low of resistance or a parasitic resistor created from residual solder flux. There are some datasheet plots in the applications section that show VIN_DC during operation. Unless you a JFET scope probe with >>10Mohm impedance, you cannot attach a scope probe to VREF_SAMP or it cause the same problem that I suspect.
    Regarding 3, the variable resistor is working as a load? So, you are increasing the resistance and therefore descreasing the load. What is the voltage on VSTOR? If you reach VBAT_OV setting, the IC stops functioning.