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SM72295: How to test SM72295 functionality

Michelle Moore
Prodigy 40 points
Part Number: SM72295
Other Parts Discussed in Thread: SM72442, SM72485

Hello,

I am an undergrad EE student building a solar charge controller that will power a 12.8V, 6Ah LiFePo battery from a 50W, 17 Vmpp solar panel by integrating the SM72442 and SM72295 to implement MPPT. I have been referencing the SM3320 SolarMagic and AN-2124 datasheets for my design. Attached below are the modifications I have made to the recommended application circuit for the SM72295. Both the solar panel and battery sizes are subject to change if necessary. 

I am trying to test the operation of the SM72295 individually in each of its three modes (buck,boost,buck-boost) to verify that the circuit is operational before connecting it to the SM72442 and microcontroller. I would appreciate assistance with the following questions:

Could you please give an example for input signals required for LIA, HIA, HIB, LIB for testing these modes? I need assistance in determining the proper duty cycles and combination of switches that should be driven to test each mode.  I have tried driving LIA,HIB with various duty cycles (D) and driving HIA,LIB with various duty cycles (1-D) at different input voltages as seen in the operation modes figure below, but I have not been able to see any output on the LOA,LOB,HOA,HOB pins for any input voltages up to 17V. How would I implement this using a function generator for testing? How can the on/off switches for buck and boost modes be effectively driven? 

What input/output voltages would be best for testing to simulate the input voltage from the solar panel? I have powered VDD with 5V, and VCCA/VCCB with 10V. 

Do dead-times need to be implemented in testing/ does the SM72295 implement hysterisis? Is there any other important start-up functionality that I am missing?

Thank you very much for your help with testing this design. Any recommendations for changes in part sizes, solar panel/battery sizes are greatly appreciated. I apologize for my lack of experience in this field, however, I am ready to learn from your responses.

  • 0
    TI__Genius 12990 points

    Hi Michelle,

    Thanks for reaching out on SM3320 and welcome to e2e.

    Your understanding of the duty cycles for dc/dc is correct. The MPPT controller, SM72442, should control the dc/dc converter. Examples of these signals and duty cycles can be found in Figure 5,6,7,8 from the reference design report below. If not using the controller and applying a FG to SM72295 directly, dead-time does need to be there. First achieve the required SM72295 input signals, make sure the frequencies are sync'd and add sufficient dead-time. One important part of directly driving SM72295 for a buck-boost configuration is that SM72295 cant do 100% duty cycle because there is no floating supply for the HB-HS bootstrap cap. In order to replenish the high-side supply, HB-HS, the low-side of the respective half-bridge needs to be briefly turned on to replenish the high-side supply (which can be seen in Figure5-8).

    SM72295 has all the required input, supply and protection hysteresis to prevent chattering. During start up, the circuitry in Figure 9 will help achieve a larger duty cycle to achieve current regulation.

    If no output is happening from SM72295, make sure the 10V power supply from SM72485 to power SM72295's VCCA/VCCB pin is present as well as the 5V for VDD from SM27738. Check out the schematic in Figure 20. The 10k resistors you show on the gate should actually be 5 or 10ohm. The 10k value should be for the gate to source resistors (so the FET remains off if no drive is applied).

    Does this help answer some of your questions, feel free to follow up with more questions.

    Thanks,