Question about Power-Input-Protection, Star-Grounding and - Page 1
Forum section: electronics / beginners

author: ChrisB_FX
author_rank: Newbie
timestamp: March 23, 2022, 09:59:59 pm
content: Hi everyone, this is my first post on this forum. While I have designed several PCBs before (some even working) I would still call me a novice.I'm currently designing a pcb to control some LED-strips via an esp32 and I have several questions (I'm loosely following this blog post:https://dubiouscreations.com/2020/08/16/building-an-esp32-light-controller/which was really helpful).Questions:- What kind of power-input protection should I consider? A Schottky-diode for polarity protection and a smd-fuse? How do I dimension them correctly, is 2x of the expected current a good value? (Should I use a fast blow or a slow blow fuse?)- Do I need star-grounding for the pwm-switching mosfets (I found contradicting information on the internets on this one)? Or is my design ok? I tried to keep the ground plane for the mosfets separate.- If a connector is rated for 3A is it really ok to use it for that current, or do I have to include a safety margin and if so how much?- Do I need those extra capacitors near the output connectors? I've included 56uF ones because I've read stories of people needing those.I plan to use this pcb for a bedside lamp and a ceiling lamp I want to build, so I don't want to burn the house down if I make a mistake.The board will use different power inputs depending on the lamp:- USB 5V <2.5A, mainly for programming and later maybe for some 5V LED-strips I have lying around- 12V <3A- 24V <1AThe LED-strips can be directly powered by the 12/24V input, or by the onboard buck-converter which delivers 5V (2.5A max) (which also powers the esp32 by another converter), the selection is made with a simple jumper. The selection between 5V from USB or from the buck-converter is done by the LTC4415 (https://www.analog.com/en/products/ltc4415.html) ic which also limits the current to 2.5A.For the mosfets I'm gonna use PMV40UN2 (https://assets.nexperia.com/documents/data-sheet/PMV40UN2.pdf), i never had to choose a mosfet by myself (this took a LONG time to find). Would be cool if someone could confirm my choice?Thanks already for reading this post.

author: bob91343
author_rank: Super Contributor
timestamp: March 23, 2022, 11:23:21 pm
content: You ask good questions but all the answers can be figured out. For instance, star ground reduces interference so if you aren't worried about that, don't bother with star ground. All ground planes have resistance and inductance so if that can cause trouble, find another way. The best connection to a ground plane is at a single point.As for fuses, decide what sort of failure you want to protect against. Then create that failure and see how much current flows. A one Ampere fuse is worthless if the worst fault current will be 100 mA.When it comes to bypass you have to decide why you want it. Protection from instability, or from external interference, or what else? Then see what reactance will help. In addition you need to realize that a capacitor is only capacitive over a range of frequencies, so if your unwanted signal is 100 MHz, an electrolytic capacitor probably won't help much.To sum up, there is no substitute for knowing what you are doing and why.

author: JustMeHere
author_rank: Frequent Contributor
timestamp: March 24, 2022, 01:16:01 am
content: With the price of 4 layer boards being so low, go with a ground and power plane. No need to worry about star ground. Be careful with through hole components creating slots in the planes. These boards are much easier to route.Regarding power input protection, read this (sorry for ugly link):https://www.google.com/url?sa=t&source=web&rct=j&url=https://www.ti.com/lit/snva681&ved=2ahUKEwihw_LvyN32AhWkmeAKHa8DBikQFnoECAQQAQ&usg=AOvVaw2uofhW07kWuRPOPl0RsSBG

author: tooki
author_rank: Super Contributor
timestamp: March 26, 2022, 02:19:54 pm
content: I’d be concerned about the USB data lines. USB is really particular, so unless they’re carefully impedance- and length-controlled, you should be aiming to keep those traces as short as possible. The UART to the MCU is nowhere near as sensitive.