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TPS92200: Heater Control

Kok Khuan P'ng
Prodigy 40 points
Part Number: TPS92200

Hi TI,

We are investigating to use TPS92200D2 to control 24 Ohm heater resistor with schematic shown below.

Output R&N measured at C124 to GND when PWM is at 50%:

Output R&N measured at C124 to GND when PWM is at 100%:

Do you have an idea where 145kHz ripple comes from? How to solve this ripple?

Regards,
Kok Khuan

  • 0
    TI__Mastermind 23190 points

    Hi Kok,

    I am wondering if the maximum duty cycle of TPS92200 is triggered and the device works in PFM (Pulse Frequency Modulation) when PWM = 100%, as your output voltage (around 23.4V in the above figure) is too close to the input voltage 24V. Actually, you set the maximum current of TPS92200 at 0.099 / 0.075 = 1.32A. Ideally this will generate 1.32A x 24ohm = 31.68V with your heater resistor, which is larger than the input voltage and this Buck LED driver definitely cannot output such high current as you expected.

    Could you probe the SW waveform under PWM = 100% to verify my assumption?

    Best Regards,

    Steven

  • 0 in reply to Steven Li
    Prodigy 40 points

    Hi Steven,

    Below is the SW waveform together with the output R&N. I believe this can be solved by lowering the heater resistance so that the output voltage is having some margin from input voltage when the output current is at the required maximum rating.

    There is another oscillation at around 1.6kHz, which can be solved by placing the 10uF 50V capacitor across output and FB instead of placing it across output and GND. I believe this is related to loop stability. May we have more detailed elaboration on this?

    Regards,
    Kok Khuan

  • 0 in reply to Kok Khuan P'ng
    TI__Mastermind 23190 points

    Hi Kok,

    Kok Khuan P'ng said:
    There is another oscillation at around 1.6kHz, which can be solved by placing the 10uF 50V capacitor across output and FB instead of placing it across output and GND.

    Does this 1.6kHz oscillation also exist when the load is LED string? Do you modify all the output capacitors across the load or do you use a combination of capacitors across the load and from output to GND? Could you show me the different connection you use that can solve this oscillation?

    Kok Khuan P'ng said:
    I believe this is related to loop stability. May we have more detailed elaboration on this?

    For measuring loop stability, you can check this E2E thread: TPS92200D2EVM: How to evaluate the loop response - Power management forum - Power management - TI E2E support forums

    Best Regards,

    Steven

  • 0 in reply to Steven Li
    Prodigy 40 points

    Hi Steven,

    The solution for 1.6kHz oscillation is to change the red capacitor connection from GND to FB as shown in picture below.

    May we know why by doing this can solve the 1.6kHz oscillation?

    Regards,
    Kok Khuan

  • 0 in reply to Kok Khuan P'ng
    TI__Mastermind 23190 points

    Hi Kok,

    Kok Khuan P'ng said:
    May we know why by doing this can solve the 1.6kHz oscillation?

    It looks like the 1.6kHz oscillation may be caused by the loop instability. It would be better if you could measure the loop response for the two different connections and let me know the results.

    Best Regards,

    Steven