The MOS tube suddenly heats up and charges intermittently. What's going on?

In a battery charging circuit, when charging 12V, the MOS tube is very hot. What's going on?

Share this MOS tube heating problem that appeared online.

Before this, the charging was complete. When I checked the charging status later, the current stopped halfway through, and then continued after a while. It was never fully charged, but there was no problem with the voltage and no large current flowed through the chip.

During the inspection, it was found that the problem was poor welding, that is, the MOS tube pad was only in contact with the ground in a small area, and the contact with the heat dissipation pad was insufficient, causing the MOS tube to heat up rapidly during charging.

So why is charging intermittent?

It is still because of the heat. The heat will cause the thermistor of the chip to work. When the temperature is too high, the chip here, that is, the MOS tube, will start overheating protection. When the temperature accumulates to a certain level, the MOS tube will be turned off and charging will stop.

After the chip stops charging, the temperature of the MOS tube drops and the overheat protection limit disappears, then charging starts again, and this cycle repeats. This is why it charges intermittently.