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Original question:

LM5060-Q1: Can I use LM5060-Q1 to reduce inrush current?

LM5060-Q1: Turning on a load with a very high capacitance (~5mF)

Keshav Raj
Prodigy 190 points
Part Number: LM5060-Q1
Other Parts Discussed in Thread: BQ76942, LM5060

Hello,

I am having issues finding a solution to switching a load with a very high capacitance (up to 5mF). I attempted to add additional gate capacitance to slow the turn on of the FET. However this solution did not work as the turn on rate fell outside of the SOA of the FET. Trying without a gate capacitance triggers upstream short circuit protections (BQ76942 based BMS).

I am using a sir680dp-t1-re3 as the FET switch with an input supply of 36-42V, my initial attempt used a turn on time of 10ms resulting in 21A of current flowing through the FET destroying it during start up.

A few calculations showed it was quite impossible to charge up the output capacitance while staying with the SOA of the FET used.

A few ideas I have include using a separate switch with a small resistor (~40ohms) to limit the current till the output voltage rises to some delta minus the supply voltage before switching on the main FET with a timeout to protect the resistor. But I'm not sure how many times this will work before the resistor gets damaged due to the surges in power dissipation.

The other option is to use a separate switch in an asynchronous buck topology in constant current mode until the output voltage reaches the delta specified above. But this seems a little over-engineered.

I was wondering if there is another method which might work.

Cheers,

Keshav

  • 0
    TI__Guru 57737 points

    Hi Keshav,

    Do you have a startup time limitation ? If not, try using a higher Gate-GND capacitance to further reduce the inrush current (but this increases the startup time). 

    You can use LM5060 Design Calculator (Rev. C) to calculate the Gate-GND capacitance required for a particular output capacitance and FET used. Please watch the training videos linked in the design calculator before using it. 

  • 0 in reply to Praveen GD
    Prodigy 190 points

    Hi Praveen,

    Thanks for your reply.

    I don't have a startup time limitation. I did consider using a higher Gate-gnd capacitance, but my primary concern was what if there really is a short circuit at the output.

    I have a comparator across a sense resistor upstream to detect a short and signal to a processor to disable the LM5060 when a high current >50A flows through.

    With a larger Gate-GND capacitance, according to the datasheet, the maximum sinking current during normal turn-off is 2mA which would discharge the 330nF capacitor (from the calculator for 500mA soft start current) in ~8ms to ground potential which is likely too slow. (Ideally it should turn off under 100us)

    Would I be able add another FET separately controlled from Gate to ground to accelerate the turn off during short circuit conditions?

    Cheers,

    Keshav

  • +1 in reply to Keshav Raj
    TI__Guru 57737 points

    Hi Keshav,

    For faster discharge of the gate-gnd capacitor, you can consider using a pnp based circuit for dvdt control as shown in the image below. 

     

    Did you have a look at TPS48111-Q1 which is our latest 100-V smart high side driver with protection and diagnostics. This device integrates a pre-charge driver (G) with control input (INP_G). This features enables designs that must drive large capacitive loads. Please check if this device is suitable for your application. 

  • 0 in reply to Praveen GD
    Prodigy 190 points

    Hi Praveen,

    Thanks for your reply.

    I think the PNP based circuit will work perfectly.

    I did take a look at the TPS48111, and it would have been a better solution. But due to availability issues I chose the LM5060 instead.

    Thanks for all your help.

    Keshav