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Original question:

SM72445: Inquiry about SM72445 EVM gerber file

SM72445: The MPPT operation of SM72445

Feng Xu
Prodigy 130 points
Part Number: SM72445

Hi

I have a question about the MPPT operation of SM72445.

A maximum power point tracking (MPPT) algorithm monitors the input current and voltage and controls the PWM duty cycle to maximize energy harvested from the photovoltaic module.

It controls the PWM duty cycle to adjust the input impedance of the power stage,so that the PV module can output maximum power.

But, what if the duty cycle controlled by MPPT  comes into conflict with the regulator's duty cycle ?

In boost mode Vout=1/(1-D)*Vin and in buck mode Vout=D*Vin.

How does the power stage keep its output voltage stable ?

  • 0
    TI__Mastermind 21720 points
    Hi Feng,

    The engineer who supports this device is out of the office. He will respond in the next couple of days.

    Thanks,

    Garrett
  • 0 in reply to Garrett Roecker
    Prodigy 130 points
    Hi Garrett,
    Can you help me ?
    Thanks!
  • 0 in reply to Feng Xu
    TI__Guru* 76695 points

    Hi Feng,

    Thank you for your interests in the  SM72445.  Sorry for the delay;  I am currently on a business trip.

    The controller can operate in Buck, Boost, and Buck-boost, namely it can settle to any duty cycle, to establish the relationship between the MPPT panel voltage Vin and the output voltage.  The output voltage can be a fixed voltage like a battery, or can be varying like a resistor.  The algorithm finds the MPPT Vin point, and adjusts the duty cycle to extract the max power from the panel to the output.  Unlike the regular dc-dc controllers that produces a constant voltage, this controller produces a “constant power” no matter what the output rail is.  

    The algorithm is based perturbation and observation mechanism.  It increases the duty cycle a step and evaluates the power. if power increases, it will increase the duty again.  If power is deceased, it will inverse the duty cycle change.  It continues this perturbation and observation., and the result is settling at the max power point.

    The controller sense the input and output voltage to decide the operating mode.  For instance, the MPPT voltage is found to be Vin1, and your output voltage is Vo.  If Vin1 >Vo, then the controller will operate in buck at a duty cycle of Vo/Vin1.

    If Vin1<Vo, the controller will operate in boost at a duty cycle of (Vo-Vin1)/Vo.

    If Vin1 is close to Vo, the controller will operate in the buck-boost mode, at appropriate duty cycle.

    If your output is a resistor,  the algorithm will adjust the duty to settle to a Vo that can supply the max power to that resistor.  And the operating mode (buck or boost or buck-boost) will be determined by the controller according to the relationships mentioned above.

    Hope this clarifies.

    Thanks,

    Youhao Xi, Applications Engineering

  • 0 in reply to Youhao Xi
    Prodigy 130 points
    Hi Youhao Xi,
    Thank you very much for your detailed answer!
    There's something else I don't quite understand.Hope you can help me.

    If 3 panels are in parallel,and each of them is equipped with MPPT ,although each one panel has reach its max power point, but how the output voltage of them will be?

    Thank you.
  • 0 in reply to Feng Xu
    TI__Guru* 76695 points
    Hi Feng,

    It will help you to understand these issues if you treat the circuit as a constant power. The three panels will produce W1, W2, W3 power. When they are connected in parallel, the total power will be W1+W2+W3, and their output voltage will be the same. If the output is a battery, the voltage will be the battery voltage. If the output is a resistor, the voltage will be sqrt (R x (W1+W2+W3)).

    Similarly if you put the output in series. Total power still W1+W2+W3, and the total current across the load resistor would be Iout=sqrt( (W1+W2+W3)/R). This Iout will flow through each output of the three circuit, therefore, the individual MPPT circuit's output voltage will be:

    Wi /Iout, where i=1,2, or 3.

    Hope this clarifies.

    Thanks,
    Youhao Xi, Applications Engineering