faster turn off of the passing fet of LT4356-1

Hi, all

     I am using an RC (10ohm, 47nF) on the gate drive of LT4356-1 to limit the inrush current.

     But I am seeing the capacitor 47nF is discharging very slowly probably due to no drain path. But if I put a scope probe on the gate pin, then it is discharging quite quickly.

     Due to the residue voltage on the cap, the inrush current control is not as effective during fast on-off cycles. I saw the below thread, Zach was saying 'A drain resistor in parallel with the capacitor will speed-up the turn-off of the FET which will help with rapid on/off connections.'

https://ez.analog.com/power/f/q-a/97105/lt4356-inrush-current-issue-with-ltc4015

    Did he mean adding a parallel resistor to the capacitor (in my case 47nF)? But the gate drive strength can be as low as 4uA. The resistor has to be fairly huge. Is my thinking correct?

    Or is there any other recommendation?

    Thank you.

Dyu

Parents Reply
  • Hi, Fil,

    Thanks for coming back to this topic. Without the RC, the inrush current is around ~60A peak 100us. With the 47nF in place, I captured the inrush current plot against the output voltage after the pass FET as below. Channel 1 is output voltage and channel 4 is the inrush current with a conversion rate of 1V/A. Therefore it is reduced to around 1.6A peak 2ms at the first peak. 

      

    At the output i have around 300uF of ceramic cap as shown below. 

    I think this 1.6A peak is still acceptable by the customer. therefore i am planning to reduce the 47nF.

    Any idea?

    Dyu

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