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TPS4811-Q1: PU/PD can't turn on external MOSFET without Pre-charge function

Lesheng Cao
Prodigy 90 points
Part Number: TPS4811-Q1

Hi,

The circuits is almost the same with TPS4811-Q1 EVM board, I pasted as below: 

TPS4811 should turn on MOSFET Q60,Q47 (Q13, Q14 not loaded) when INP signal is high, the testing was set as below:

I input a DC voltage of 48V from P_BAT connector,

I measured Voltage level of INP is about 2.7V, INP_G voltage level is about 2.7V.

After I powered on the DC supply, I supposed the MOSFET should be turned on because INP/INP_G  is 2.7V high, but sometimes I found the MOSFETs wasn't turned on, the voltage level of PU/PD is about 39V,sometime the MOSFET was turned on successfully, the voltage level of PU/PD is about 60V. the failure mostly occured on the first time I powered on the DC supply, if I continued to turn off/on DC power supply, I found  MOSFET can be turned on.( this phenomenon occured with PRE-CHARGE function enable);

If I disable pre-charge function by pull down the signal of INP_G to low, the other setting and operation is the same with above, the MOSFET was not turned on at all, I captured the signal waveform of PU/PD, it looks as below:

there is a pulse after I powered on the DC supply, then PU/PD goes to low.

Would you advise what I can do to turn on the MOSFET without PRE-CHARGE function enable?

Why MOSFET sometime was failed to turn on even I enable Pre-charge function?

Thanks

Kelvin

  • 0
    TI__Guru* 95801 points

    Hi Kelvin,

    Any capacitance on the output would trigger SCP and turns-off the MOSFETs. 

    Do you have any output capacitance ? If yes, how much is the value? Can you try by removing the output capacitance

    Best Regards,

    Rakesh

  • 0 in reply to Rakesh
    Prodigy 90 points

    Yes, there are capacitors at the output of 48VDC( Q47), 48VDC is the inputs source of 2 DC-DC controllers, so there are many capacitors at the inputs of the DC-DC controller, but the total capacitance is not big, is about 40uf,  there are lots of capacitors at the DC-DC controllers output, the output capacitance is big, the total is about 400uf. also 48VDC will connect to other board, the capacitance of connected board is un-know, during testing I didn't connect to other board.

    Because there are so many capacitors on the board, I can't remove them one by one, what is the value of total capacitance will trigger the SCP from your experiences? what can I do to NOT trigger SCP? should I always enable pre-charge function?

    additionally, even I enabled pre-charge function, sometime it still can't turn on the MOSFET, maybe one failure if  tried ten times, what caused this failure?

    I changed resistance value of R124/R125 to smaller, seems I got a little improvement, but I am not sure resistor is the root cause.

    Thanks

    Kelvin

  • 0 in reply to Lesheng Cao
    TI__Guru* 95801 points

    Hi Kelvin,

    TPS4811 turns-ON the main FETs in <1us. Because of large dv/dt, even 1uF of output capacitor can also trigger SCP. 

    Always, enable pre-charge path and then switch-over to the main path. To avoid SCP trigger during startup, we can also add RC network on the main FETs as shown below

    During inrush current control via pre-charge path, lot of power dissipation happens in the pre-charge resistor. Large value resistor or high power rated resistor needs to be used if Cout is higher.

    Best Regards

    Rakesh

  • 0 in reply to Rakesh
    Prodigy 90 points

    Hi Rakesh,

    Thanks for your advice, my application may require main MOSFET turn on very quickly <1ms, what RC value combination would you advice?

    Thanks again!

    Kelvin

  • 0 in reply to Lesheng Cao
    TI__Guru* 95801 points

    Hi Kelvin,

    Please try with R1= 100kΩ, R2 = 10Ω and C1=10nF

    Best Regards,

    Rakesh

  • 0 in reply to Rakesh
    Prodigy 90 points

    Hi Rakesh,

    In my design circuit, I didn't add the components of R2,C1, so there is no place to add two components right now, I want to reduce the inrush current, do you think it can help by increasing the R1 value.

    Increasing the R1 value to reduce the inrush current, so that it doesn't trigger the OVP.

    Thanks and best regards

    Kelvin

  • 0 in reply to Lesheng Cao
    TI__Guru* 95801 points

    For inrush current control, you need to add at least R1 and C1

  • 0 in reply to Rakesh
    Prodigy 90 points

    Got it ,thanks

  • 0 in reply to Rakesh
    Prodigy 90 points

    Can it avoid SCP by increase the resistor of below? by increasing the R14 value to increase the margin of the current value that will trigger the SCP?

  • +1 in reply to Lesheng Cao
    TI__Guru* 95801 points

    yes, you can increase SCP threshold by increasing R14.