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SM72442 / SM72295 related question

Konrad Mitka
Prodigy 30 points
Other Parts Discussed in Thread: SM72295, SM72442

Hi everybody,

I am trying to validate my prototype of MPPT tracker built on SM72442 and SM72295 ICs. The device is set to work with following settings on A6 ADC input:

It works with 180W solar panel (40Voc@25Cdeg and 36V at MPPT @25Cdeg). Hbridge is based on SIR876ADP transistors from Vishay.

Device is set to work in boost without any load.

I have already burnt one mosfet transistor (h-bridge upper left - not acting in boost mode) doing some first measurements on regular power supply without loads. I took identical another board I put them at 20-25V power supply leaving them work hours without any load connected. When I switch it to solar panel (still without load), upper left mosfet burns within 1s (shorted). And again the same. Always the same mosfet.
 All Mosfets are 100V rated. Can you please help me to undertand what is wrong? There is attached schematic.1423.MPPT_TRACKER.pdf

Regards,

Konrad.

  • 0
    TI__Intellectual 1695 points

    Hi Konrad,

    The attachment you had after "input:" didn't show up. Can you please reattach that to the post.

    Also, please give me some time to look through your schematic.

    Thanks,

    Shashank

  • 0 in reply to Shashank Reddi
    Prodigy 30 points

    Hello,

    I will start from system requirements:

    - there will be 6 independed and exactly  similiar MPPT trackers (2 per PCB so schematic is composed of two ones) connected ALL in parallel (D3 not placed, D2 placed) working with 6 solar panels (180W monocristal from Sinopuren company).

    - All system will work with 3kW/230V inverter having on its input 48V and batteries (4x12V in series).
    - The goal is to have it working as well without batteries connected (only with inverter) as long I have no batteries bought for the system.

    Based on that I decided to set the system in the following way:

    - Voltage stabilisation at 57V.

    - Start-up phase with 1.2 boost ratio - this is for the case without battery - Inverter will not wake-up when voltage will be lower than 40V.

    - Overvoltage set on SM72295 for 60.5V.

    - please see all calculations for analog settings in the file below (for SM72442, SM72295 and 10VDCDC) - I thing

    7563.MyMPPT_CALC_21032014.xlsx

    Additional questions to SM72442:

    - Shall I understand that with those setting, when there will be more power from solars than consumed, I will have at the output stabilised voltage around 57V?

    - For the analog settings - given values in datasheets are in the middle of range or upper/lower value? What is tolerance  of voltage to be sure that device will not go out from set mode (example on A6?)

    - For what is 60,2kOhm on one NC input? Can it be 62kOhm as more standard value? I see in your demo boards that you have some logical states there.

    Additional questions to SM72295:

    - VDD voltage in my opinion, is just input not power supply and some guys had problems with this input having short circuit to GND. I believe that ESD cell has been trigggered due to fast slew rates. Can you please elaborate a little here? Shall we add protection resistor in series to avoid destruction?

    - When I tried to use buffered outputs in my application, amplifier started to oscilate - it is solved by not using them, this point is for information.

    To be honest those datasheets for this devices are not so full of information what can allow easy going with hose devices :)

    Regards,

    Konrad Mitka

  • 0 in reply to Konrad Mitka
    TI__Intellectual 1775 points

    Hi Konrad,

    - With the setting you have on A0, the SM72442 will begin ramping down the duty cycle whenever it detects that the output voltage is higher than 57V regardless of the MPPT algorithm. This should keep the output voltage around 57V and works pretty well in typical applications. However there is no direct regulation of the output voltage. Simply, every milli-second or so, the duty cycle will be reduced by around 1% until the AVOUT goes below A0. The regular MPPT algorithm then takes over control of the duty cycle.

    - The values given in the table for A2 and A6 are the ideal values provided that VDDA is 5V and VSSA is 0V.

    The SM72442 samples A2 and A6 through an ADC when it is initially powered up with VDDA and VSSA being the high and low reference (see fig.1 of the datasheet). The voltages given in the table for A2 and A6 are the values for each bin's center. The bin should extend 0.31V up and 0.31V down around the center value (again provided that VDDA is 5V).

    - Those pins are used for specific test modes. 62k is fine, you should be able to use a 5% part.

    - VDD is an input and should not be drawing significant current. If this input appears shorted, I would first check that the level did not go above the abs. max. rating of 7V.

    - The buffered output should not oscillate. What was the frequency of oscillation?

    Best Regards,

    Florent Boico