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Original question:

TPS40210: TPS40210 soft start consult

TPS40210-Q1: when start up will skip switch pulse sometimes

Irene Qin
Intellectual 1620 points
Part Number: TPS40210-Q1
Other Parts Discussed in Thread: TPS40210

Hi team

My customer is using TPS40210-Q1 as fly-back to power IGBT drive.

One quick question about soft-start , when start up the SW will start to switch pulse skip sometimes as below:

Vin is 16V, Vfb is 5V; F=125kHz

abnormal waveform:

yellow:GDRV ;Green:Vsense; Blue：VDD; Purple ：Vfb（5V）

As you can seen when start up, the switch frequency is not stable at 125kHz.(will skip pulse  ) 

I've check there is no overcurrent when start up, Vsense max is 90mV

normal waveform:

yellow:GDRV ;Green:Vsense; Blue：VDD; Purple ：Vfb（5V）

As you can seen when start up, the switch frequency is stable at 125kHz.(no skip pulse  ) 

May I ask what reason may cause this kind of skip pulse?

Also does VSEE in datasheet is one of the IN+ of TPS40210 internal error amplifier ?

Looking forward for your response.

irene

  • 0
    TI__Intellectual 1620 points

    Update the abnormal waveform regularity:

    1.Yellow :GDRV ,we found that pulse will  skip in multiples of 8， eg：skip 8*pulse or 16*pulse

    2.Blue ：SS；Purple：FB；when start up, Vss is around 300mV, FB is around 40mV, with abnormal GDRV switch , Vss will increase ,but FB will keep at 40mV for a while,

    The GDRV on time is minmium on time 80ns

  • 0
    TI__Mastermind 19290 points

    Hi Irene,

    Thanks for using the e2e forum.

    During softstart, the TPS40210 device will regulate the output voltage according to the rising slope of the SS voltage. During this time, the device regulates in open loop and the compensation network is not active yet. Once the softstart is completed, the device will switch over from the SS reference to the fixed internal reference of the error amplifier and the regulation will continue in closed loop.
    As there is only open loop regulation during softstart, it is possible that the output voltage overshoots over the rising slope reference. In this case, the device will stop switching and waits for the output to drop again, or the rising SS slope catching up, and then continues to switch.

    Because of this, a hiccup behavior may be observed.

    Please let me know if further explanations are necessary.
    Best regards,
    Niklas