Limiting inrush current for a 40v to 5v buck stepdown convertor - Page 1
Forum section: electronics / beginners

author: WinterBreeze
author_rank: Newbie
timestamp: April 13, 2025, 08:50:44 pm
content: Greetings-I am trying to build a simple noise filter circuit around a step down buck converter (https://www.mateksys.com/?portfolio=mbec12s) to convert 40v battery to 5v for Arduino that will draw no more than 1A from the 5v output or 250mA from the 40v input.Given that I used large upfront filter capacitor, I used Method 5: MOSFET and Delay Network Circuit here (https://www.tech-sparks.com/how-to-limit-inrush-current/). The circuit works on LTspice.I used SI2392ADS mosfet that is supposed to be able to handle 2A current and 60v. However, the mosfet is blown every time I connect to the 40v battery.Could you please offer me some tips?

author: Smokey
author_rank: Super Contributor
timestamp: April 13, 2025, 09:05:52 pm
content: Simulate the inrush.Watch the power in the fet. Check that against the SOA for the fet.You probably have the turn on slope way too slow and it's just dissipating way too much power. You just need enough slope to knock down the peak current.Also I'm curious why you need so much input capacitance. Your supply is not very demanding. My intuition says you have overcomplicated things, unless you have a really good reason to need that much capacitance in a filter.

author: incf
author_rank: Frequent Contributor
timestamp: April 13, 2025, 09:40:14 pm
content: I'll second smoky on the total capacitance question.Specifying the right capacitors and switching frequency would let you cut it by a factor of 100.

author: WinterBreeze
author_rank: Newbie
timestamp: April 13, 2025, 09:50:29 pm
content: Thank you all for the helpful tips. I see what my problem is now. From the LTSpice, the power to the Mofset will peak to 7w! Yes, that must be why I managed to blow 5 of them and a few long nights of soldering de-soldering.As to why I need this much capacitance, it is a longer story. I hope to make a semi auto gokart using the 40v battery and adding the ultrasound sensors to detect obstacles (https://www.instructables.com/Hacking-Automotive-Ultrasonic-Sensors/). The ultrasound sensors are from an old GM car (ebay). The code and the rest of the circuit work as expected, meaning the Ardunio can command the ultrasound sensors to send out ultrasound and detect distance to obstacles. However, when the BLDC motors are turned on (or the BLDC controller is turned on), the ultrasound sensors didn’t return the electronic signal as expected (I verified this using the oscilloscope.) Tried as I may, I still couldn’t get it to work. I do see a lot of voltage noise from the 40v battery input to the buck convert when the motor control turns on. So I am trying to add a lot of filters, even including the common mode choke, to try to stabilize the 5v and another 9v feeding to the ultrasound sensor. Still no luck so far.The Brosche ultrasound sensors are supposed to be automotive grade and capable of resisting electrical noises although the exact communication protocol is proprietary. I have also thought about whether the BLDC motor itself generates a lot of ultrasound noise confusing the Brosche sensor. That was not the case however. When they don’t share ground, meaning powering the Arduino and the Brosche sensors using a separate battery from the motors, the ultrasound sensors works fine even when the motors are running.I don’t have any electrical engineering background other than from websites. If you have tips for me to try, I will definitely appreciate it. Thank you again!

author: Benta
author_rank: Super Contributor
timestamp: April 13, 2025, 10:48:54 pm
content: I think you need to rethink the layout of your gokart.Think about EVs.They have a very big power battery for propulsion, but still carry a normal 12 V battery for all the control/management.I think you'll need to split your design the same way. It may be possible to "filter" your way out of the issues, but I have grave doubts.

author: WinterBreeze
author_rank: Newbie
timestamp: April 13, 2025, 11:00:16 pm
content: Thank you for your comments. I haven't thought of that EV having separate batteries. Do the 12v battery share ground with lithium battery for propulsion? I would thought they share ground plane and to charge the 12v battery using the lithium. I don’t have a EV otherwise I would test it myself. In my situation, if the two batteries that I used shared a ground wire, the ultrasound doesn’t work. I hope to use one Arduino to control both the ultrasound sensor and the BLDC motors.

author: Andy Chee
author_rank: Super Contributor
timestamp: April 14, 2025, 12:51:10 am
content: Consider using a flyback topology instead of buck.By using a flyback topology, you can take advantage of using transformer turns ratio to assist in bringing down the voltage.