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TPS61200 Question | App Note: SLVA345A

A Train
Prodigy 535 points
Other Parts Discussed in Thread: TPS61200, OPA379

I have a customer with the following question: 

In working on the MPPT hardware, I am looking at doing something like in the App Note SLVA345A, Figure 2.  However,...

(1) The tps61200 is only unidirectional, correct?  I.e. there is no condition during which my output can discharge back into the tps61200's output pin and toward the Vin source?

(2) Regarding the MPPT function, is the R4 divider at OPA379 + terminal input supposed to be set for the PV cells MPP, or for a low voltage cutout threshold?

(3) Finally, this report states the MPPT circuit was tested with a variety of solar cells.  Can a table of a couple examples be included?  For example, the PV cell in Figure 1: 0.6Vmax, 0.5V MPP, say 0.4Vmin (operating), what was the R4 ratio for this cell?  Although, this cell was obviously a fair size, and the tps61200 isn't capable of delivering all the current it can supply to a load, right?  This actually brings up another questions:

(4) the 'short circuit' current protection would also act as a over-current protection (in the case of the 5A PV cell capability), correct?  What will it limit the input / output currents to, ~1.3A device current rating??

 

Can you help??

  • 0
    TI__Prodigy 80 points

    Hello  Mr. Train,

    The TPS61200 is a typical boost converter with the advantage of running from 0.5V supplies or even less. If the device is disabled the load (battery) is disconnected from the input. Nevertheless the connected battery supplies the MPP circuit if no power is coming from the input.

    With R4 you can control the reference voltage. This means if the solar cell voltage is above that value the MPP circuit does nothing. If the voltagedecreases below that value the MPP circuit starts decreasing the output voltage of the converter to a output power that can be delivered from the input. It keeps the solar cell voltage at the voltage you set by R4.

    The R4 value is always close to the MPP value. There is a small variation but this was the reason for putting a poti to that point. So, you can fine tune the solar cell to the best output power regarding light condition and cell temperature.

    The internal current limit limits the current through the main switch or rather through the inductor. That means it limits the input current minus current ripple. To give you the right picture. TPS61200 has 1.3A switches, therefore the inductor should also have a current rating in this range. Generating ~4V of output voltage from just 0.5V is equal to a conversion ratio of 8. Adding an efficiency of over all 70% you end with a conversion ratio of 11.5. Calculating your output current means [1.3 - (Ripple_Current/2)] /11.5. Assuming a ripple current of about 200mA this lgives you an output current of about 100mA (MAX).

    Regards Andreas

     

  • 0 in reply to Andreas_66
    Prodigy 10 points

    Maybe I am thinking of this too simplistically, but it would seem the MPPT OpAmp may not be setup ideally… Specifically, the OpAmp feedback/output is connected directly to the resistive voltage divider,… seemingly causing conflict in the FB node voltage being dominated by the OpAmp output rather than the Vout divider.  In my initial simulations of the circuit, this seems to be the case in steady state,... which I did not expect because of the cap in the feedback.  Are there more detailed equations for how this setup is expected to inject current into Vfb node over the cell range (i.e. 0.3 - Vmppt range)?

    Thank you,

    Matt

  • 0 in reply to ZTech
    TI__Guru**** 150820 points

    Yes, the point of the MPP circuit (the op amp circuit) is to override the external feedback resistors and produce a different output voltage.

    From the app note: "The operational amplifier output is connected to the feedback pin of the TPS61200 to control the output voltage of the converter depending on the supplied input voltage of the solar cell. The operational amplifier is working mainly as a comparator which compares the voltage at the solar cell to a voltage reference."

    As the cell voltage drops due to drawing more power than the maximum power point level would provide, the op amp begins to go high on the output.  This adds more current to the FB pin, which causes the TPS61200 to regulate Vout to a lower voltage.  This reduces the output power and thus reduces the input power, which is the power drawn from the solar cell.  The solar cell voltage will then begin to rise back up to the MPPT.

    To generate equations for computing the resistors on the FB pin, simply do a KCL (Kirchhoff's current law) on the FB pin: (Vout - Vfb) / R1 = Vfb / R2 + (Vfb - Vopamp) / R8