LM5030: Startup issue due to output capacitor inrush current

Thread Starter


Joined May 3, 2021
Hi, I was designing dc-dc converter using LM5030 , the output specifications are

Output 1: 13V, 4.9A

Output 2: 13V, 3.8 A

the primary input voltage is 74V and the estimated primary continuous current is 4.3A

so for overcurrent protection is was going to set the current sense resistor connected to current sense pin to stop the ic for current above 5 A on the primary side

The issue is as per the simulation the inrush current on the output capacitor was 22 A so the corresponding increase in current demand would be reflected on the primary side which would exceed the set current limit hence would stop the ic and hence the converter wouldn't work at all

Any solution to this?


Joined Aug 7, 2020
How about a circuit diagram of how you have it configured? The datasheet application circuit only shows one output.


Joined Oct 7, 2019
the inrush current on the output capacitor was 22 A
The purpose of the current limit circuit is to limit the current into C3+C4. Also make the "SS" capacitor much bigger to limit start up current.

Many things I don't understand.
The data sheet shows power MOSFETs and there are none.
Why two 13 volt outputs? They share a common ground so why not one output?
OUTPUT 1 has L9 while OUTPUT 2 has no inductor. They work in a very different modes. OUT2 will not regulate.
U3-VIN probably should go to C2 not L7.

Are you using LT SPICE?


Joined Aug 7, 2020
I was curious as to where a 22A surge could come from if the transformer was being driven by the MOSFET drivers, which have a current limit of 1.5A and an output resistance of about 2.5Ω.


Joined Aug 7, 2020
That opto is bizarre. Could Texas (or NatSemi) not find a single-channel opto-isolator in the entire factory? And even if they did use half of a two-channel, why put it in the application note?
They are obviously trying to get more interest in the LM3411 - because anyone else would have used a TL431.
I think someone somewhere makes a combined TR431 and opto-isolator.