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TPS40303 Fails to start after a period of reliable operation.

Marc Kelly
Prodigy 30 points
Other Parts Discussed in Thread: TPS40303

We are using a TPS40303 as a 12V to 5V@4A converter in a controller product that is under development.  In a small number of cases (two so far in perhaps 30 PCBs made) the converter fails to start after a couple of weeks of reliable operation.  I have attached a plot of the voltage at pin 2 (EN/SS) which appears to show the controller repeatedly trying to start up.  The four high frequency spikes in each repeat are at about 1.3us period (770kHz).  The cycle repeats about every 16.2 us (61.7kHz).  I note that about 9 in 10 cycles look like the wider ones and about 1 in 10 are somewhat narrower.

We are using the converter in spread spectrum mode to reduce EMI issues.

From a quick diagnostic look it seems the controller is unable to drive the high side FET correctly, replacing the controller beings the system back into operation so indicates the issue is not directly with the FET.

We have a 3R3 resistor on the BOOT pin, and the FET gates are currently not current limited, and have 0R in series. I have seen mention that the BOOT pin resistor could be increased to slow the turn on of the FET, is this likely to be a useful change?

Currently I cannot post the schematic here directly as I do not have permission to show it in an open forum but it is roughly based on your data sheet and application notes. I am able post in private if needed.

Best regards

Marc.

  • 0
    TI__Mastermind 24075 points
    Hi Marc,

    Looks like the High-side gate drive is damaged by voltage spike on SW/BOOT/HDRV pins.
    Can you probe the voltage on SW/BOOT/HDRV pins to check if the voltage is above SPEC?
    Can you send the schematic to below address?
    Q-CHEN@TI.COM

    Thanks
    Qian
  • 0 in reply to Qian Chen
    Prodigy 30 points

    Hi Qian,

    I have probed the broken board, and a working on, the plots are below for comparison. Broken board on the left and Working board on the right.

    HDRV, Broken board startup, and working board startup.

     

    SW. Broken Board and Working Board.

     

    BOOT. Broken Board and Working Board.

     

    The HDRV does seem to not be firing correctly, It never reaches the voltages needed to properly turn the FET on.

    I've forwarded the area of interest in the schematic to your email.

    Regards

    Marc.

  • 0 in reply to Marc Kelly
    Prodigy 30 points

    Just an update in case anyone else suffers the issue.

    On initial power up the Boot PIn was seeing >37 volts for several dozen cycles.

    Under running conditions it was ~27V

    To lower this the High side fet gate drive was moved to 22R, Boot Resistor was moved to a 22R. The snubber mentioned was already in the design and now the turn on pulse is ~27 V for 5 cycles, then settles to ~20V during all loadings.