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LM5050-1: Gate oscillation problem with a PWM load

Tom Hoag
Intellectual 460 points
Part Number: LM5050-1
Other Parts Discussed in Thread: TPS2410, LM5050-2

Greetings to you.　

I am having an oscillation problem on the gate drive pin on the LM5050-1. With a light load the gate drive pin is good. With a 10A load, the gate drive pin is good. However, if I have a 10A load, and the load has a PWM drive of 16khz with any duty cycle, the gate drive pin ramps up and pulls down hard. The ramp up is a much lower rate than the 16khz PWM.　

Please see attached pictures. Power supply is 20V. MOSFET RDSon=3.3 millohms. Circuit used is Figure 24 of data sheet, except output load capacitors = 2000uf. This oscillation problem causes high MOSFET power dissipation. How can I solve this problem? Thank you!

　

  • 0
    TI__Genius 16305 points

    Hi Tom Hoag,

    I am looking into this issue, but would need your help with waveform of IN and OUT captured along with Gate.

    Could you help to get this, so that we can support to resolve this issue quickly.


    Regards,

    Kari.

  • 0 in reply to Karikalan Selvaraj
    Intellectual 460 points

    Thank you for your quick response Kari!

    I have made some measurements and it appears that there is a lot of noise (1V or greater) generated by the PWM switching which drives a motor.  So I believe the LM5050 sees this noise.  As the gate drive ramps up, noise is detected that makes the LM5050 believe the output is higher than the input, so it shuts down.  I am working to reduce the noise.  Attached is one picture that shows the input on top and the output on the bottom.  The small spikes are the 16khz PWM.  Notice how the output ramps up as the gate drive increases.  Vds = 0.45V.  The LM5050 is unable to regulate Vds to 20mv as a result of the noise.  I will report back shortly!

  • 0 in reply to Tom Hoag
    TI__Genius 16305 points

    Hi Tom Hoag,

    Yes, LM5050 gate is turning on/off due to the switching noise on IN and OUT pins.

    IN and OUT pins needs to be filtered so that gate is not turned ON/OFF unnecessarily.

    For the filter recommendation, please refer to the TPS2410 datasheet page 8, section on "FAST COMPARATOR INPUT FILTERING".

    Though it is recommended for TPS2410, basic concept holds good for LM5050-1 as well.

    Regards,
    Kari.

  • 0 in reply to Karikalan Selvaraj
    Intellectual 460 points

    Thank you for the information.  I will add more filtering to my power buss and also try the LM5050 filtering as suggested.  My application does not require the fast switching. 

    Hopefully this will solve my problems. 

  • 0 in reply to Karikalan Selvaraj
    Intellectual 460 points

    Hello Kari,

    Regarding my project with the LM5050-1, I have switched to LM5050-2 as I need very low current to eliminate leakage from other LM5050s in my circuit that may not have power.

    I have added the filtering circuit to solve the problem of high power supply noise.  This seems to work, but only if I add the 1 Megohm resistor to the base circuit.  Or, if I use my scope probe to monitor the gate, then it also works.  But if there is no gate resistor as shown, then the LM5050 does not turn off and allows power to flow back into my battery power supply.  I am not really sure why this happens.  Maybe you have an idea why I need the gate resistor to ground?  Or is there some other problem with my circuit? 

    This is a 20 volt, 40A current application and my RDSon = 1.2 milliohms.

    Thank you!

  • 0 in reply to Tom Hoag
    TI__Genius 16305 points

    Hello Tom,

    R1 in the path from drain of Q1 to OUT pin of LM5050-2 will increase the forward regulation threshold.

    Because OUT pin current of LM5050-2 is 110uA (vs 8uA in LM5050-1). This adds a drop of 19.8mV.

    This changes reverse turn off threshold from -27mV to 46.8mV. Reverse turn current is also changed now.

    Could you reduce R1 from 180 ohm to 20ohm (but can increase the filter capacitor to achieve same filtering).

    Reducing R1 will reduce the drop to 2mV and the gate should turn off and block reverse current flowing into the battery.

    Regards,
    Kari.

  • 0 in reply to Karikalan Selvaraj
    Intellectual 460 points

    That is great Kari!  I changed the resistor to 33 ohms and the capacitor the same at .1uf.  Works great now and no need for the gate pull down resistor.  Thank you!