Today I am sharing a case where the common point (main power rail) was not being generated.

When I got the board, I measured all the inductors across the board and found no short circuits. I powered it on directly and found that voltage couldn't even pass the first isolation MOSFET. Because this board is from a very niche brand, I couldn't find schematics or boardviews, so I could only look up its charging chip model to find a similar circuit for reference.

The reason it didn't pass the isolation MOSFET is either a problem with the isolation MOSFET itself, an issue with the charging chip, or insufficient working conditions. Pin 20 VCC has power coming in, and the 6V linear voltage is also present. However, the ACDET pin has 4.2V, which is incorrect. According to the chip's datasheet description, this pin must be between 2.4V and 3.15V; it cannot experience over-voltage or under-voltage. Therefore, the problem is pinpointed right at this series voltage divider!

Don't underestimate the role of this series voltage divider; it will directly prevent the chip from boosting voltage. At first, I thought the chip was bad, but after replacing it with another one, the situation remained the same. Only then did I start paying attention to this specific point... So, beginners must understand the working conditions of the chip; only then will your future repairs be more accurate. (My drawing isn't great, so please just bear with it to understand the principle).

Remember that this chip uses internal boosting. If you encounter a situation where it doesn't boost, first check if the voltage at that voltage divider point is too high or too low, and whether the chip's conditions are met. If it still doesn't boost, only then should you replace the chip (this is my personal experience). Here is a picture showing you the locations of those two resistors.

Without a schematic, you can only use a multimeter to measure and find those two resistors. Finally, I will provide a few internal circuit diagrams for you to look at, and the chip datasheet has also been uploaded. I hope everyone makes a little progress every day...