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SM72445: How to test AN-2241 SM72445 Evaluation Board

user6207070
Prodigy 20 points
Part Number: SM72445
Other Parts Discussed in Thread: LM5117, LM5145,

Hi,
I'm David.

I was testing the AN-2241 Evaluation Board. I followed the testing steps which were in the datasheet.
There were 6 steps I found.

First is Connect S.A.S. to input.

Second is Connect Electronic load to output. 

Third is Turn on SAS Verify proper startup pattern (see Figure 4).

Fourth  is Set Electronic load to 2A and turn on Electronic load.

Fifth is Wait two minutes and verify that SAS is operating at Maximum Power Point. If output voltage is close to 40V, increase Electronic load current.

Last is  Vary Electronic load current and verify that the SAS is operating at Maximum Power Point in step-up (lower load current) and step-down (higher load current) operation.

The problem was that when the load was open and then turned to 2A. The circuit would become to Boost mode. I wanted to use this EVM to operating in Buck mode. Could my idea is feasible?

 my idea is feasible?

I saw someone ask SM72245 how to set the mode . the answer was when the Vout>Vin the mode operated in the boost mode and when the Vin>Vout the mode operated in the buck mode.
So i was wondering how to let my circuit always in the buck mode. Could you give me others testing steps? 


Thank you !

  • 0
    TI__Guru* 75370 points

    Hi user,

    The mode is automatically decided by the SM72245.  To always stay in the buck mode, you need to have the Vin to be at least 2V higher than your Vout.  If you just want to operate in the buck mode, the SM72245 is not the right part for you.  Please chose a buck controller instead, like the LM5117 or LM5145.

    Best Regards,

    Youhao Xi, Applications Engineering

  • 0 in reply to Youhao Xi
    Prodigy 20 points

    Hi Youhao Xi,

    Thanks for you answer. But I still have some problem.

    I was wondered that why the input voltage divider is not the same magnification as the output voltage divider. The SM72445 determined the operation mode by the input and output.

    If the feedback magnification isn't the same value,I don't know that the circuit should operate in which mode. At the AN-2241, the input feedback magnification is 2/22 and the output feedback magnification is 2/26.8. When the circuit is operating in panel mode. The output feedback voltage is small than input feedback voltage,and then I think the circuit may operating in buck mode. But in fact, when the circuit start MPPT function, the circuit operate in boost mode and make the output voltage at 40V which is setted by A0. So I still don't know  what may determine the operation mode. Is that determine by the A0 or relation of input and output voltage.

    The PV panel characteristic is,the open voltage is 21.9V and the Vmpp is 17.6V . whether I have to set the maximum output voltage to less than 17.6V to let my circuit operate in buck mode. 

    The picture is my last question.

    I was testing the EVM board. I set the curve parameter.Voc=35V Isc=3.333A Vmp=28V Imp=3A 
    and I follow the operation steps. Set the electronic load to 2A and turn off the load. the circuit start at no load,and then turn on the load the ouput voltage may increase to 40V after 2 minutes.
    the status at output is 40V,2A. and I increase output current to let the input match to max power. After the input voltage and current is close to maxpoint,the point will fastly go to a low voltage and high current. 
    and i will hear some noise from the inductor at the status of low voltage and high current.
    i doubt why the circuit don't trace the maximum power point.

    Thanks.

  • 0 in reply to user6207070
    TI__Guru* 75370 points

    The VIN and VOUT sense if just for reference and the actual value does not matter much. It is because the MPPT is based on perturbation-observation algorithm and it changes the duty cycle by a step to see whether the intake power from the panel is increasing or decreasing.  Setting of the VIN sense please refer to the SM72445 datasheet. AVOUT is does not affect the MPPT but it sets the max Vout threshold, therefore it has a different divider.  

    These are discussed in the datasheet in details. Please refer to Page 8.

    To run the test with the EVM, you need to use the PV simulator or the real PV.  Usually Imp=0.8 x Isc, and Vmp=0.8x Voc.  You should also use the CV mode on your e-load instead of CC mode.  The MPPT is extracting the max power but you just create a conflict by turning off your load. 

    Thanks,

    Youhao Xi.