Limit inrush current of 120V Switch Mode Regulator Input Capacitor

[frasdoge]

Hello,

I've asked a question on this same design in another post, but to summarise again I would like to create a 120V-5V switch mode converter using the LTC3895. The 120V is sourced from a battery.

The rest of the design is coming along, however I have concerns about the bulk decoupling caps used at the input of the switch-mode controller, which currently feeds directly from the battery. While the controller itself provides a soft start mechanism for its output to ramp up, the input capacitor according to LTSpice sims is liable to exceed 600A when starting the simulation with voltage sources set to 0V (simming plugging in the 120V battery).

I've not really worked with high voltage designs before so I would like to ask for advice on solutions.

Firstly, would high voltage rated caps handle the inrush current natively without any intervention?  I've come across a few solutions like simply placing an NTC resistor at the input, or even adding some soft-start mechanism with some resistors, capacitors, and a P channel MOSFET. To me that seems a little weird given the controller already has its own soft-start mechanism, and that I would need to implement my own just for one input cap. I also feel like going no input cap even for a battery is a poor idea.

[Andy Chee]

Normally the controller's built-in soft-start only limits current to the output capacitor.  It does nothing to control current to the input bulk capacitor.

See here for more information.  Although mains related, the principles apply to your situation as well.

https://www.sound-au.com/articles/inrush.htm
https://www.sound-au.com/articles/thermistor-selection.htm

[Ian.M]

Worse, unless you manage the inrush, the input capacitance can ring with the wiring inductance, transiently overvoltaging  the switching regulator.   Electrolytic caps normally have sufficient ESR to damp the ringing considerably, but high K ceramics can and do ring to over double the supply voltage, if you don't have some arrangement to limit inrush or to clamp their ringing.

[MathWizard]

Pretty much all computer PSU's I worked on, have a NTC in series right before the bulk capacitor (the cap is usually rated 400-500V), I can find one later to see what they are rated for, but yeah it's only a few ohms at 25C, and hundreds of mOhms at operating temps.

Now they also usually sense overall current, by having ~0.1ohms between local GND, and the neg terminal of the rectifier.

That reminds me I replaced a fuse in a stereo PSU with a 1ohm resistor, for inrush limiting, I better put a fuse back with an NTC.

And my "new" 2005 stereo, it has 2x 10mF caps after the rectifier. I think they usually have some in-rush limiting, but I better check the schematic. I'm afraid they are going to explode with the cover off (I'm modding it a bit). Their tops are a bit bulged, I better find some pictures of them to compare with. I don't really want to have to pull the PSU board out to de-cap it.