UCC25800-Q1: Maximum current during soft start

Olli Pokkinen

Part Number: UCC25800-Q1
Other Parts Discussed in Thread: UCC25800,

Tool/software:

Hi,

I'm designing a ~2..3W supply with UCC25800: Vin=24v transformer turns ratio 1:1, fsw 500kHz, voltage doubler rectifier. I wonder how much output capacitance is permitted so that the mosfets won't get too hot during soft start? Is it enough to stay below maximum OCP2, ie. below 5A peak? I suppose TSD is not fast enough to react within soft start period? For instance, with 30uF output capacitance, according to simulation, SW pin current looks like this during SS:

BR,
Olli

7 months ago

0 Steven Mappus1 7 months ago

TI__Mastermind 33115 points

I don't like to see such high inrush current for any bias supply. Your sim result seems close but still less than OCP2 (5A during soft start). Compare your 30μF sim result to a case where the output cap is <<30μF. Maybe also consider the soft start (dV/dt) of the input supply to the UCC25800-Q1.

Steve

0 Olli Pokkinen 7 months ago in reply to Steven Mappus1

Prodigy 40 points

I agree it's not nice to have such high inrush and would be nice to have longer soft start time. The inrush will certainly be smaller if I decrease the capacitance but I might need quite big amount, so the question is that do you have any means to estimate what is safe amount? I suppose TI should have some kind of understanding on thermal impedance from mosfet junction to case? dV/dt limit to the input is not possible in my case due to space constraints.

0 Steven Mappus1 7 months ago in reply to Olli Pokkinen

TI__Mastermind 33115 points

Internal MOSFETs are protected by OCP and OTP. With such large capacitance you are close to OCP2. If we use OCP2=5A as the limit, you could plot your output voltage in your sim, measure the dVout/dt, then calculate C=I*dt/dVout, input current is approximately output current since your transformer is 1:1.

Steve

0 Olli Pokkinen 7 months ago in reply to Steven Mappus1

Prodigy 40 points

Ok, so just to be clear, for my initial question "Is it enough to stay below maximum OCP2, ie. below 5A peak?" the answer is "yes"? So mosfets are protected by OCP2 and TSD even with high output capacitance and inrush current close to 5A?