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SM72445: Buck converter design with SM72445

Frieder Rabl
Prodigy 120 points
Part Number: SM72445

Hi community,

I'm starting to work on a project concerning a power optimizer for solar modules. I checked on the SM72445 and have a few basic questions before I start to work on the design.

Firstly, the SM72445 is controlling a four switch buck boost converter. Is there a possibility to let it controll a simple buck converter?

Secondly, the Evaluationboard was once the Solarmagic power optimizer. What is the advantage of using a buck-boost design? The only thing I can think of is to reduce stringcurrent. But it comes with double the components and more losses compared to a simple buck design. Am I missing something here?

Thirdly, are all the possible adjustments that can be made mentioned in the datasheet of SM72445. Or can other changes be made. For example to force the converter to activate Panelmode under given circumstances.

Fourthly, are other components of the TI-family possible to use for such an application?

Thank you for your help!

Best regards,

Frieder

  • 0
    TI__Guru* 78220 points
    Hi Frieder,

    Thank you for your interests. The SM72445 was developed 10 years ago for the Solarmagic power optimizer and it can be used to control a simple buck converter. However, the operating range would be limited.

    The buck and boost modes were chosen to offer a freedom in multi panel configurations, whether in series or parallel connections. The buck and boost modes allows any input voltage and output voltage combinations.

    I don't fully understand your third question, but you can make adjustments if they do not cause conflict. Since the panel mode is activated by VIN is close to VOUT, you may force the input or output voltage sense pin to the same to enter the panel mode. However, in this case, MPPT is not functioning.

    To your 4th question: This is the only hard coded digital device in TI for the MPPT. You may also consider TI's C2000 or MSP430 and use software to achieve MPPT and produce PWM control signal.

    Thanks,
    Youhao Xi, Applications Engineering
  • 0 in reply to Youhao Xi
    Prodigy 120 points
    Hi Youhao Xi,

    thank you for your quick and revealing reply. I have a few further questions.

    1.Can you be more specific concerning the operating range of a buck converter. What limitations would I have to expect apart from Vin > Vout? And I guess it's not done with just disconnecting the PWM pins of the two then needless switches, but rather would need some programming, right?

    2.Basically it makes sense to have buck/boost only when you need to achive a certain output voltage. Or when you want to pretend that your stringvoltage drops beneath the specification of an inverter. Maybe it's a plus to keep the current in the string low do reduce losses there. When you say multi panel configurations, do you mean panels of different Isc and Voc? Why would a buck boost be necessary there if both can be stepped down to the same voltage/current?

    3. So only hardware modifications can be made, besides the adjustements in registers described in the datasheet?

    4.Resolved

    Thank you!
    I appreciate your help a lot!
  • 0 in reply to Frieder Rabl
    TI__Guru* 78220 points
    Hi Frieder,

    1. My impression when the buck duty cycle is greater than 96.5%, namely the Vin become 3.5% greater than Vout, the controller will move out of the pure buck operation.

    2. Yes I meant different panels that may have different Isc and Voc. The buck boost can relate any Vin and Vout voltage levels without affecting the power delivering. When the output is the same, the input voltage, which is the mppt operating voltage of the panel, may be different. If you can make sure all your input voltage is higher than the output voltage by at least >3.5%, the you just need a buck.

    3. These requires the hardware modification. I don't remember any programming affect this matter.

    Thanks,
    Youhao
  • 0 in reply to Youhao Xi
    Prodigy 120 points

    Hi Youhao Xi,

    2. What you described is mainly for parallel configurations I guess.

    1./2. I did some testing of the evaluation board with four panels of the same model and each panel had a Solarmagic Evaluationboard connected. I noticed that the current in the string chosen by the boards is lower than the MPP current of the panels. So they are using their boost mode. Let's assume I configure the board to be only a bucktype, would the control decide for another, higher current in the string and still do MPP-Tracking, or would this cause the controller to fail/ work faulty? There is a inverter included, so I assume it would just choose another current where the control is working fine. But still I wonder if there are situations where the missing boosttype will make troubles?

    Kind regards and thank you alot!
    Frieder

  • 0 in reply to Frieder Rabl
    TI__Guru* 78220 points
    Hi Frieder,

    I believer your put the four module outputs in series, right? It may be a case that some panel may not operating in the mppt if you only populate and use the buck leg. It is because, if one panel lost power, the rest modules need to make up the total voltage and that may fall into the boost mode but your modules are not able to run boost. If your inverter can adjust voltage to a lower level, it may not be a problem, but I am afraid it may not be that simple. Therefore, I suggest you to use all four switches to avoid any problem like what I mentioned above.

    Thanks,
    Youhao Xi, Applications Engineering