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TPS2492: Turn MosFet OFF at short circuit

Staffan Johansson1
Prodigy 30 points
Part Number: TPS2492
Other Parts Discussed in Thread: LM5069

Hi

I have a problem with turning transistor OFF at short circuit.

When the fault timer reach 4V and the transistor should be switched off then it just tries but fails to do so.

The current limit is approximately 22A. shunt resistor 2,2mOhm.

Transistor is Infineon IAUT240N08S5N019

The constant power limit is approximately 125W  (I have tried with less and still had the same behaviour).

Is there any suggestions what I shall try to test to solve this issue or may be what the problem is.

attach two oscilloscope pictures to show the problem I have

Yellow: voltage at the source 5V/div

Orange: Current 20A/div

Blue: Voltage at timer capacitor 1V/div

Both pictures shows the same scenario but with different time/div

Best Regards

Staffan Johansson 

  • 0
    TI__Guru** 105361 points
    Hi Johansson,

    Welcome to E2E!
    Can you share your schematic.
    What is the green waveform corresponds to ? Please capture a waveform with Vin, Timer voltage, Current, GATE and send me.

    Best Regards,
    Rakesh
  • 0 in reply to Rakesh
    Prodigy 30 points

    Hi and thank you for your respond.

    I have made some more measurements and adde the part of the schematic that contains the switch.

    Between the switch and VBAT ther is also a "ideal diode". Maybe this should be located after the switch instead. Do you have a opinion on that.

    differences fro schematic in the PCB measured on: C50 10nF, R52 2,2Kohm

    VBAT have hold up capacitor 560uF not seen in this schematic.

    Oscillosope pictures

    Yellow: Gate voltage 5V/div

    Green: Input voltage to the switch (voltage at current sense resistor, same node as VCC on TPS2492) 5V/div

    Orange: Output current 10A/div

    Blue: Voltage at capacitor on TIMER pin  1V/div

    Two pictures at the same occasion but with different time/div

    When the timer voltage reach 4V and expire the transistor try to turn off but fails to do so.

    Best regards

    Staffan Johansson

  • 0 in reply to Staffan Johansson1
    TI__Guru** 105361 points
    Hi Johansson,

    Thanks for the test waveforms. I understood your problem.
    Is it happening at all the input voltage levels?
    Please add schottky diode at the output (like D2 in Figure 19 in the datasheet) and check.
    Can you please check by disabling UVLO and OVP (i.e. with out R45 resistor)


    Best Regards,
    Rakesh
  • 0 in reply to Rakesh
    Prodigy 30 points

    Yes it is happening more or less at all inputvoltages. 

    I have tested without the R45 resistor (disabling UVEN and OV ) and that eliminated the problems with short circuit (and overcurrent)

    The design works as expected without the under and over voltage protection and the transistors can handle the short circuit current and over current.  

    I have tried with decoupling capacitors at UVEN but still had the problem.

    Best Regards

    Staffan

  • 0 in reply to Staffan Johansson1
    TI__Guru** 105361 points
    Hi Staffan,

    Thanks for the update.
    Let me work with the design team and get back to you. Please allow couple of days time.
    thank you!

    Best Regards,
    Rakesh
  • 0 in reply to Rakesh
    TI__Guru** 105361 points
    Hi Staffan,

    I have checked at my end. The problem is with the EN threshold tolerance for this device. The solution to overcome this situation is to set UVLO threshold at slightly lower than Vin threshold. If adjusting UVLO threshold to lower value is a concern, please switch to LM5069 device.

    Thanks for bringing this to our notice. We will be updating the datasheet with a recommendation.

    Best Regards,
    Rakesh