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LMR38020-Q1: High amount of ripple current causing shoot through

Spencer Waguespack
Prodigy 50 points
Part Number: LMR38020-Q1
Other Parts Discussed in Thread: LMR38020

Hi,

I have a board with the attached schematic for an LMR38020-Q1. This DC/DC converter gets its power from the same rail which has a large 48V compressor which we PWM for a few milliseconds as a means to soft start the compressor in order to extend the compressors life. During this PWM phase there is a large amount of voltage ripple (20V -30V) on top of the 48V battery voltage. This ripple sometimes falls below the falling level of the converters under voltage lockout, Sometimes the under voltage lockout trips and causes the 24V rail to fall to zero and other times it does not. I have also observed 2 of the boards fail in a way that the DC/DC converter burns off its case. This is leading me to believe there is something causing the converter to experience a shoot through event. Is it possible that the logic inside the DC/DC converter somehow gets confused and turns on both the low and high side mosfets?

  • 0
    TI__Guru 56345 points

    Hi Spencer, 

    My guess is that when the large input voltage ripple occurs and the top side FET is turning on, the SW node dv/dt voltage is coupling through the miller capacitor of the low-side FET and causing the FET to turn on at the same time. If the low side FET unexpectedly gets turned on when the high-side FET is on, a shoot through event will occur. I would try to add a snubber circuit on the SW node to see if the issue can be mitigated. 

    Do you have scope shots of VIN and SW? Have you tried adding a bulk capacitance on VIN to limit the voltage ripple?

    Ben

  • 0 in reply to Ben C
    TI__Guru 56345 points

    Hi Spencer, 

    Please let me know if there are any updates. Thanks,

    Ben

  • 0 in reply to Ben C
    Prodigy 50 points

    Hey Ben Sorry for the late reply. This phenomena is very difficult to capture however I think I caught it in this scope capture. For Reference V-Safe is the power being input to the board. Note V-safe is the Voltage input to the converter. 

    If I were to design a snubber for this board what is the capacitance of the high side mosfet for this device. 

  • 0 in reply to Spencer Waguespack
    TI__Guru 56345 points

    Hi Spencer,

    Try adding a resistor in series with the CBOOT capacitor. That should slow down the turn-on of the SW node. Here are some links for your reference on designing an RBOOT resistor.

    https://www.ti.com/lit/an/snvaa73/snvaa73.pdf

    https://www.ti.com/lit/an/slua887a/slua887a.pdf

    Ben

  • 0 in reply to Ben C
    Prodigy 50 points

    Hi Ben Sorry for the confusion I think you miss read what these signals are. Channel 1 (Yellow) is the voltage for the large 48V compressor, Channel 2 (Green) is the Voltage coming into the board other wise referred to as Vsafe, Channel 3 (Blue) is the current going to the compressor, Channel 4 (Red) is the PWM signal to the compressor switch. I think you may have interpreted one of them as the the switch node on the DC to DC converter.

    I think the questions I should be asking are:

    1. Given the large ripple at the input of the DC/DC converter is there anything we can add to protect the converter from being damaged aside from adding more capacitance to the input Power rail.
    2. In addition to the false turn ons from the high DV/DT coupling into the the gate of the highside mosfet, are there any other failure modes we need to be looking out for? In some of my previous testing I have seen this input voltage fall below the under voltage lockout level of the converter and cause the 24V rail to brown out.
  • 0 in reply to Spencer Waguespack
    TI__Guru 56345 points

    Hi Spencer,

    Thanks for the clarification. Below are my responses to your questions:

    Spencer Waguespack said:
    Given the large ripple at the input of the DC/DC converter is there anything we can add to protect the converter from being damaged aside from adding more capacitance to the input Power rail.

    You can add a TVS on VIN to prevent VIN from going above the ABS MAX rating. 

    Spencer Waguespack said:
    In addition to the false turn ons from the high DV/DT coupling into the the gate of the highside mosfet, are there any other failure modes we need to be looking out for? In some of my previous testing I have seen this input voltage fall below the under voltage lockout level of the converter and cause the 24V rail to brown out.

    The example that you mentioned is not caused by the IC. The IC just responds to the transient input voltage. Regarding other failure modes, I can't think of anything at the moment. I think you addressed the high dV/dT on the switch node. 

    Ben

  • +1 in reply to Ben C
    TI__Guru 56345 points

    Hi Spencer,

    If there are no further questions, kindly close the thread by clicking on "resolved". Thanks!

    Ben

  • 0 in reply to Ben C
    Prodigy 50 points

    Hey Ben,

    One other thing:

    What would be The maximum allowable Slew on the input pin for the LMR38020? 

    and If I use a snuber circuit how can I verify that I have solved the problem?

  • 0 in reply to Spencer Waguespack
    TI__Guru 56345 points

    Hi Spencer, 

    Usually the input caps have an affect on the slew rate from the power supply. Also, the slew rate from the power supply is quite "slow" as it charges up the internal power supply capacitors.

    Here is an technical article regarding the design of a snubber circuit.

    https://www.ti.com/lit/ta/ssztbc7/ssztbc7.pdf

    Ben