So it's the charger that's heating up rather than the phone? Just to be clear, since the thread title suggested it was the phone but your first 2 sentences here suggest it's the charger.

If the charger is getting hot enough that it stops charging that is wrong (and very unutual). Is it an original manufacturer charger or a third-party one? Either way if the charger is getting too hot that suggests the problem is there: they ought to be able to run at their maximum output without getting actually hot (warm, sure, transformers will get warm when they are passing a lot of current, but it shouldn't be hot).

But the phone should control the charging speed, and the phone doesn't know the charger's temperature. So if the charger is getting hot and charging stops that suggests a thermal cut-out - in which case it's good that it has one, bad that it's activating.

 

I agree, need to know whether iris the phone or charger getting hot.

Either way, my first troubleshooting might be to disable fast cable charging in phone settings (device care / battery / three dot menu / settings / fast cable charging=> off). Or maybe try a different charger if that is already disabled.

 

Chargers can be hot but not excessively hot. Personally I would try another charger to compare...

If you have a laser temperature gauge you should be able to know the inside temperature of both chargers.

 

I am having this same problem. When charging the phone up from under half the adaptive fast charger gets so hot it kicks off and back on over and over. The phone hardly even gets warm. This is my 2nd s9 plus. Bought the first one new on eBay and it done this, returned that one because it was a carrier phone (I wanted unlocked) the 2nd one I bought also does the same thing. I don’t know if it could be from using an extension cord? I have only charged it a couple times so I’m not sure yet. I will plug it directly into the wall next charge and see what happens.