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SM72442: MPPT mode not behaving correctly

Elyka Abello
Prodigy 20 points
Part Number: SM72442

Hi, I'm using SM72442 to supply power to a battery charger (LTC4020). My circuit has 6 different solar panels so I used one SM72442 for each one, but now I'm testing just one of them. You can see the schematic and the PCB design attached.

My device is always in panel mode (I've checked PM out pin and I'm getting the square waveform). I'm testing wit a solar array simulator as input (Agilent E4350B) and a DC electronic load (Maynuo M970), I've tried with different solar array curves (Voc=2.65V , Vmp=2.35V, Isc=0.4613A, Imp=0.4456A and Voc= 4.71V, Vmp=4.1V, Isc=0.474A, Imp= 0.456A); I've tried with different modes of the load (constant current, constant power), I've tried changing load power, changing solar array curve, and only got it into MPPT mode twice, once for each curve. I verified the outputs for the H-bridge driver en the second curve case and it was working in Boost mode. In every cases, the power at the output is way below MPP of the set curve. In both cases, when I reduced the load current below the 125mA threshold, it began the PM mode but then it didn't coma back to MPPT mode even if I increase the current again or change the input curve. Can anyone tell me what I'm doing wrong? 

I also have some questions about theoretical behavior of the SM72442:

1) The voltage set in A0 is the maximum output voltage, as it is written in the datasheet, or is it the initial output voltage set and then it can then be changed through the I2C communication, as is written in AN-2121?

2) Through pin A2, one can set the exiting panel mode power variation. Is this Input power variation or output power variation? I've tried both and my controller still in panel mode.

3) Is the start-up circuitry shown in Figure 9 of AN-2121 always required when  panel voltage is lower than output voltage? I've set my maximum output voltage (trough A0) to 55V. It was intended actually to be a maximum output voltage limitation, not a regulated voltage at the output. But if it is the set voltage at the output, maybe that is why my design is not working well, I don't have the start up circuitry.

Thank you in advance for any comment on this issue.

MPPTsch.PDF

MPPTpcbTOP.PDF

MPPTpcbBOT.PDF

  • 0
    TI__Guru* 78470 points
    Hi Elyka,

    Thank you for using the SM72442. We will investigate this and get back to you by next Wednesday.

    Thanks,
    Youhao Xi, Applications Engineering
  • 0 in reply to Youhao Xi
    Prodigy 20 points

    Thank you for your reply. 

    After writing the post, I changed Rb1 (R517 in the schematic) value to 3.3k Ohm, in order to set Vout_max=8.5V. Assuming that AN-2121 is right in my two former questions, my Vout_max of 55V without the start-up circuit won't let SM72442 to start MPPT mode. I succeeded on gettin MPPT mode, although it enters MPPT mode after more than 2 minutes, instead of only 1 minute as the datasheet says. I performed more test and now it seems to work in MPPT mode most of the time; even so, I got PM mode a couple of times without apparent reason.

    Now I'm having problems with the output regulation. With my new Rb1, Vout_max is supposed to be set to 8.5V; nevertheless, I'm getting voltages of 2.5V~3.2V, 1.5~7.3 and 1.2~10.82, depending on the SAS curve set, which actually meet my requirement, but It confused me about the value setting in A0, what exactly I'm I setting through that pin?

     Even in I'm getting MPPT mode (verified from PM output=0V), I'm having problem understanding what is exactly doin the SM72442 to harvest my solar array power. This might be an analysis or test setting problem rather than a design or IC problem, but It still be useful some professional help. I'm attaching my results tables. For curves 5 and 8 I used the electronic load in constant power mode; for curve 11, I used it in constant current mode. For curve 11 it can be seen that the solar array moves its working point through all the I-V curve, so whats the advantage here of using SM72442? For curves 5 and 8 it is worst; the working point of the solar array is always in the unstable zone. How can I interpret these results?

    Thank you in advance,

    Elyka

    MPPT-Results.xlsx

  • 0 in reply to Elyka Abello
    TI__Guru* 78470 points
    Hi Elyka,

    I took a look of your schematic, and what you saw may be caused by two factors: (1) output capacitor is too small to have remarkable ripple voltage, and (2) MPPT resolution issue owing to small signals. The MPPT is based calculating and comparing incremental power between the MPPT steps. If the output ripple is large, it may defeat the MPPT algorithm. Please add more output capacitor to reduce the ripple voltage first, and see if you get any improvement.

    Thanks,
    Youhao
  • 0 in reply to Youhao Xi
    TI__Guru* 78470 points
    Hi Elyka,

    May I assume this issue is resolved, and close this thread? You can always re-open this thread by following a new post.

    Thanks,
    Youhao