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TPS92411 low power factor

Miguel Camus
Prodigy 120 points

Hi there!


I've made a board very similar to the 120VAC EVM. It works fine, but the power factor is about 0.76, what looks quite low comparing to the datasheet. The main difference is that the LEDs forward voltage is about 22V@120mA (7x Cree Xlamp MX-6S). Originally the design was for 20Vf but I realized now that it has to be about 22Vf and the resistors (Rset) are slightly different. Any clue about what can it be? I've ordered new Rset resistors to try, but I wonder if it's not the problem.

Thanks in advance,

Miguel

  • 0
    TI__Mastermind 28655 points

    Hello Miguel,

    The Rset and Rsns resistors should not make the difference you are seeing, it should actually work as it is with such little difference in Vf.  If you have the test equipment you can look at the current waveform and the input voltage waveform, preferable after the rectifier bridge (note that the input is not isolated so you have to be careful in how you connect an oscilloscope to the circuit).

    I would guess something is not correct on the board.  Either the current regulator is dropping out, or it is causing distortion or something else is causing the low power factor.  A large line (EMI capacitor could also cause a phase shift.  There are oscilloscope pictures in the user guide to get an idea of what they should look like.  You can see some DC offset for triac dimmer compatibility and the phase shift caused by the EMI capacitor and damper but the power factor is still very high.  You can also run into issues if the current regulating MOSFET is much too large.

  • 0 in reply to Irwin Nederbragt
    Prodigy 120 points

    Hi Irwin,

    Thanks for your answer. I've be researching and made a couple of discoveries.

    I've took a picture comparing the voltage and the current looking really good except for the noise in the mains:

    As you can see are really similar. The only issue is that I've got a noise in the mains input coming from outside that you can see in the picture and I can not  remove from the input. I guess this is altering the PF measuring. I'm using a transformer to measure the voltage from the mains (230VAC to 115VAC).

    The other issue is that the PF measurement device that I own is under reading, I've compared with one good from a friend and goes from 0.76 to 0.83.

    Please if you think there is something else that I can do any help would be great.

    Thanks so much for your help!

    Miguel

  • 0 in reply to Miguel Camus
    TI__Mastermind 28655 points

    Hi Miguel,

    One thing you can try is just placing an incandescent bulb or resistive load to measure the power factor (at similar power level).  If it's low it's the source causing issues or the power factor measurement is not correct.  Another, if you have it, would be to try the EVM and see if it's similar.

    You can run into issues with this circuit running from a transformer.  The only EMI filtering is the capacitor from rectified AC to ground.  When the TPS92411s switch, due to characteristics of the current regulating MOSFET, there can be a di/dt on the input source.  If there is inductance it can cause a dv/dt which can cause the TPS92411s to switch multiple times.

    If you plan on triac dimming you, most likely, will run into issues running from a transformer, its impedance is not low enough to prevent interaction with a triac dimmer.

    Noisy lines are common.  If triac dimming it's a difficult problem to deal with, this circuit or a power supply solution.

  • 0 in reply to Irwin Nederbragt
    Prodigy 120 points

    Dear friends,


    Happy new year to all of you! finally I solved the PF problem, I modified the PCB, basically I made good paths for the ground return and now the PF is above 90%, so I am happy with that. There is another issue, but I'll write it in another post.

    Thanks for your help!


    Miguel